In-situ measurements in microscale gas flows – conventional sensors or something else?

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In-situ measurements in microscale gas flows – conventional sensors or something else?

Juergen Brandner

To cite this version:

Juergen Brandner. In-situ measurements in microscale gas flows – conventional sensors or something else?. Micromachines, MDPI, 2019, 10 (5), pp.292. �10.3390/mi10050292�. �hal-02415747�

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Micromachines 2018, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/micromachines

Review

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In‐situ measurements in microscale gas flows –

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conventional sensors or something else?

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Juergen J. Brandner ^1,*

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1 Karlsruhe Institute of Technology (KIT), Institute for Microstructure Technology (IMT), Staff Position

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Microstructures and Process Sensors (MPS), Hermann‐von‐Helmholtz‐Platz 1, 76344

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Eggenstein‐Leopoldshafen, Germany; juergen.brandner@kit.edu

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* Correspondence: juergen.brandner@kit.edu; Tel.: +49‐721‐608‐23963

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Received: date; Accepted: date; Published: date

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Abstract:

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Within the last decades miniaturization was a driving force in almost all areas of technology,

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leading to tremendous intensification of systems and processes. Information technology provides

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now data density several orders of magnitude higher than a few years ago, and the smartphone

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technology includes, amongst simple communication, things like internet, video and music

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streaming, but also implementation of global positioning system, environment sensors or

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measurement systems for individual health. So‐called wearables are everywhere, from

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physio‐parameter sensing wrist smart watch up to measurement of heart rate by underwear. This

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trend holds also for gas flow applications, where complex flow arrangements and measurement

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systems formerly designed for macro scale have been transferred into miniaturized versions. Thus,

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those systems took advantage of the increased surface to volume ratio as well as of the improved

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heat and mass transfer behavior of miniaturized equipment. In accordance, disadvantages like gas

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flow mal‐distribution on parallelized mini‐ or micro tubes or channels as well as increased pressure

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losses due to the minimized hydraulic diameters and an increased roughness‐to‐dimension ratio

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have to be taken into account. Furthermore, major problems are arising for measurement and

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control to be implemented for in‐situ and/or in‐operando measurements. Currently, correlated

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measurements are widely discussed to obtain a more comprehensive view to a process by using a

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broad variety of measurement techniques complementing each other. Techniques for correlated

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measurements may include commonly used techniques like thermocouples or pressure sensors as

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well as more complex systems like gas chromatography, mass spectrometry, infrared or ultraviolet

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spectroscopy and many others more. Some of them can be miniaturized, some of them cannot yet.

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Those should, nevertheless, measure at the same location and the same time, preferably in‐situ and

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in‐operando. Therefore, combinations of measurement instruments might be necessary, which will

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provide complementary techniques to access local process information. A recently more

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intensively discussed additional possibility is the application of nuclear magnetic resonance (NMR)

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systems, which might be useful in combination with other, more conventional measurement

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techniques. NMR is currently undergoing a tremendous change from large‐scale to benchtop

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measurement systems, and it will most likely be further miniaturized. NMR allows a multitude of

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different measurements, which are normally covered by several instruments. Additionally, NMR

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can be combined very well with other measurement equipment to perform correlative in‐situ and

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in‐operando measurements. Such combinations of several instruments would allow to retrieve an

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“information cloud” of a process. This paper will present a view to some common measurement

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techniques and the difficulties to apply them on one hand in miniaturized scale, on the other hand

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in correlative mode. Basic suggestions to reach the above mentioned objective by combination of

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different methods including NMR will be given.

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Keywords: miniaturization, gas flows in micro scale, measurement and control, integrated micro

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sensors, advanced measurement technologies

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1. Introduction

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The interest to gas flows in miniaturized systems has grown tremendously in the last couple of

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years. Driven by, i.e., automotive, semiconductor, chemical and pharmaceutical industry, modelling,

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precise measurement and control of the flow of gaseous compounds, mixtures and reactive systems

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through mini‐ and micro‐structured devices gained importance for various applications like heat

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transfer [1‐16], gas‐liquid or gas‐solid contacting [17‐24] as well as chemical reactions [25‐30].

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Amongst correct design and manufacturing of the miniaturized devices, all the named topics need

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precise measurement and control of the processes taking place inside microstructures. The final

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objective of these efforts is to provide an almost comprehensive description of the process to be able

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to model and simulate as well as to predict by software. The following description will focus on

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measurement and close out control, to make it not too exhaustive.

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While measurement of gaseous flows in macroscale is not trivial but manageable, it turns out to

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be much more problematic in micro scale. There are several reasons for this. The fact that gas is a

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compressible medium per se makes it more complex to measure certain behavior and parameters of

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a flow inside confined or miniaturized systems [31‐35]. Additionally, the scale down of

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conventionally‐sized macro scale tubes or channels into the micro scale makes it more complex to

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measure. This is, on one hand, due to sensors which are simply too large to be inserted into the micro

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devices. Figure 1 (a) & (b) show examples for this. For temperature measurement in macro‐scale

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tubes, a thermocouple is simply located into the flow, measuring the gas flow temperature. This

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changed by scaling down the tube diameter. While Figure 1 (a) shows a mini heat pipe cut open,

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Figure 1 (b) provides a view to a so‐called “micro‐thermocouple”. It is quite obvious that the

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thermocouple will block the inner diameter to a major extend and, therefore, change the fluidic

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behavior totally. Thus, no precise measurement would be possible. More problems occur due to the

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low density of gases, the change in viscosity, the small specific heat capacity and the need for

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increased leak‐tightness of microstructures while handling gases (and here, depending to the gas,

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the acceptable leak rate QL can vary in orders of magnitude!). However, in many cases Micro

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Electromechanical Systems (MEMS) have been applied as measurement tools to be implemented

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into microstructure devices [33]. This is often a practical solution, in other cases it is not, as will be

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shown in the later discussion.

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(a)

(b)

Figure 1. Microstructure fluidic device. (a) Micro heat pipe, opened to see the inside structure. Inner

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diameter is 0.8mm. From: KIT‐IMVT; (b) Miniaturized Type K thermocouple. Although this sensor

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has an outer diameter of 0.4mm only, a major part of the structure in (a) would have been blocked.

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From: www.ninomiya‐ew.co.jp.

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Nevertheless, from 1964 until today, roughly about 45,000 papers have been published on

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miniaturized gas sensors and sensor systems [36]. A large variety of relevant technologies is

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available which will not be described in this paper in details, more can be found in comprehensive

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textbooks or, i.e., in [37].

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The following will present an overview on parameters of gas flows in microstructures to be

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measured as well as measurement methods for these. By no means the presented overview is

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complete. The envisaged field of measurement is very much in move, thus, new technologies and

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improved methods are rapidly developing, especially by improving sensitivity and selectivity of the

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measurements. One of these developments is the use of nanomaterials for sensing opportunities.

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Here, numerous different technologies based on nanostructured materials or nanostructures in

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specific materials have been described and are used now for sensing in gases. Examples are given in

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[38‐47]. The sensitivity and selectivity of nanostructure / nanomaterials sensors has been improved

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significantly in the last years [48‐50].

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Another trend is a combination of sensing elements in micro or nano scale for measurement of

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biological parameters or environment (biosensing, [51‐56]). This field is relatively new, and lots of

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developments are to expect in the next future.

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As mentioned before, an additional point is the correlation of a multitude of measurement

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methods to achieve a more dense “information cloud” and, therefore, reach a better understanding

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of the effects and behavior in gas flows. This leads to the necessity of in‐situ and in‐operando

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combination of several measurement methods with similar (ideally: identical) timely and spatial

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resolution. All measurements have to be taken simultaneously and at the same location to provide

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the highest possible information density of the process taking place at this point. As an example for

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such a process a heterogeneously catalyzed gas phase reaction could be taken. Here, gas flow, gas

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composition, catalyst‐gas‐surface interaction, temperature, pressure and product concentration

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should be measured at the same time and the same location, to name just a few of the parameters of

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such a process.

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The data obtained from each of such monitoring systems will then be correlated to those of all

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others, in time as well as in space, to get a description of an n‐dimensional parameter space as

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comprehensive as possible. Within the overview presented here several possible technologies to

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correlate measurements are very briefly and rudimentarily described. Possibilities, advantages and

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disadvantages of each method are presented, and a possible use in combination with other methods

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is evaluated. This overview is not intended to give a vast description of the measurement methods,

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their advantages and disadvantages, their working principles or underlying functionalities – this is

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left to much more specified papers or textbooks. This collection, without going deep into details of

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the methods themselves, should provide a quick overview for possible correlative measurement

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technologies only. For this objective, the following section deals with the parameters which most

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regularly are measured, as well as with some most common methods for those and with scaling, the

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problem of miniaturization itself [57‐59].

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2. Parameters, measurement methods and scaling

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Numerous parameters of gas flows in micro scale can be measured. However, many of them are

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only useful in specific applications. Amongst those are parameters like chemical reactivity, pH,

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solubility in liquids, speed of sound etc. This publication will focus onto more common parameters

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like temperature (or temperature difference), heat flux, pressure (or pressure difference), mass flow,

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flow velocity, mixture composition and concentration (concentration gradients, respectively) of

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species inside micro scale gas flows.

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The measurement methods can in general be split in electrical measurements using

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conventional wire systems [60, 61], electrical measurements using micro‐electromechanical systems

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(MEMS) [62], spectroscopic measurements or optical measurement methods [63]. There are

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numerous papers on each of those different measurement methods, thus, it shall not be the objective

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of this publication to provide more detailed information on them.

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Wire‐system based methods use conventional sensors which are, in many cases, macroscopic.

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In general, these are thermocouples or thermistors (see below), pressure sensors, mass flow meters

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and similar, which are well known measurement devices in style as well as in behavior. These

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sensors are mounted aside of the process tubing, e.g. thermocouples before and after a possible

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reaction vessel as well as pressure sensors or mass flow meters. An in‐situ or in‐operando monitoring

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is hardly possible.

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MEMS‐based measurements show the possibility to be implemented for in‐situ and in‐operando

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measurements. Pressure sensors, temperature sensors, flow sensors and others more are available of

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the shelf. They are, in general, Si‐based, small, provide a short response time, good reliability and

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long lifetime. Aside, most of those systems are cheap in production. However, they need to be

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coupled with a visualization module or a data logger to make the measurements available for the

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operator. This is regularly done by wiring. There are some cases of wireless MEMS sensors [64].

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However, due to the continuous developments in semiconductor technology, wireless solutions are

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becoming more and more popular, and their use is greatly enhanced by, i.e., radio frequency

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identification (RFID) and near field communication (NFC) technology [65‐69].

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What was mentioned above for wire‐based electrical sensor in principle holds also for optical

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sensors, where either an optical fiber is implemented into the measurement location, or an active

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optical component (light emitting diode LED, laser or other light source) is integrated there – in

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many cases also an optical detector like a photo diode or similar. Examples are presented in [70‐76].

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Most of the measurement methods of the sensors named above have been derived from

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macroscopic standards, adapted and improved for mini scale and then applied and even further

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revised for micro scale; and almost all of them (but the wireless MEMS devices) share the same

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problem of scaling. This holds for all type of sensors, electrical or optical.

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Scaling a measurement method simply down from macro to micro scale will normally not

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generate the desired results. Either is the fluid influenced too strongly by the measurement method

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(as it was shown in the example given in Fig. 1), or the precision of measurement suffers from the

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miniaturization. It is, in many cases, not easy to decide where in the fluid flow to place the sensors to

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obtain correct measurements. A good example is the measurement of temperatures. If a

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MEMS‐based measurement system is placed into the sidewall of a microfluidic system, the wall

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temperature is measured, or maybe the temperature of the gas flow near the wall. The same result

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will be obtained with an optical element placed inside the sidewall of a microfluidic system. The

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temperature in the core of the flow is not determined in any way. Moreover, the MEMS system may

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alter the wall‐gas interaction (because the material might be different), and therefore not even

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provide a representative measurement signal for the sidewall temperature of the gas flow [77]. This

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might not be so much the case for the optical sensor. Fig. 2 shows an example of a silicon MEMS

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sensor system used for temperature measurement inside a microchannel gas flow under ambient or

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slightly rarefied conditions [78].

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Figure 2. Silicon microstructure chip for temperature measurement in a gas flow. The chip is inserted

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into a sealed housing and forms the fourth side of a rectangular microchannel. Numerous

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thermopiles have been implemented into the chip, to measure in combination with precision

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resistors the temperature of a gas flow [78]. www.gasmems.eu

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If the process is well known, the core temperature of the flow might be calculated correctly. Not

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knowing the process precisely, it might be necessary to measure the core flow temperature – which

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is possible with a sensor implemented into the flow. Placing a very thin wired sensor or a thin

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optical fiber inside the flow core may lead to a more precise measurement of the temperature there,

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but also could lead to a deflection of the measurement sensor due to insufficient stiffness, or to

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generate local turbulences and eddies and, therefore, disturb the flow. The same holds for a wireless

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sensor, which has to be mounted and fixed somehow in the flow. In any case, scaling down fluidic

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systems for gas flows into the micro scale needs a very careful consideration of the location and the

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precision of the sensors applied. This holds for all parameters to be measured.

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2.1. Temperature measurement

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One of the most common tasks in gas flows is the measurement of temperatures, as mentioned

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above. At the same time, it is a relatively complex measurement, which is, in many cases,

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underestimated [79‐85]. In the following, some possible methods will be described and evaluated as

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a general example for other parameters to be measured in micro scale.

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2.1.1. Measurements using conventional intrusive sensors

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Conventional sensors in this case means either thermistors or thermocouples. Optical sensors

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shall not be considered here, because they are in this case more delicate to handle and in general

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more expensive [86‐88]. Both thermocouples and thermistors are commercially available in various

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shapes, sizes and forms, which makes them flexible and handy tools. Moreover, the efforts to obtain

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some results are limited. In both cases, wired standard solutions as well as wired MEMS are

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available.

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Thermistors are temperature‐dependent electrical resistors. They can be separated in devices

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with positive temperature coefficient retrieving a higher resistance with increasing temperature, and

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with negative temperature coefficient, which lowers the resistance by increasing temperature. In any

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case, the dependency between the change of resistance and the temperature is very linear (or can be

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linearized in an easy way) [89]. The most common example is the PT100, a platinum resistor with a

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nominal value of 100Ω at standard conditions (variations with 10Ω or 1000Ω are also common). The

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PT‐sensor is used as part of a Wheatstone bridge circuit, which makes a very precise measurement

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possible [90].

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Temperature sensors based on the resistor principle are wide‐spread in any application,

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because they can be manufactured as discrete devices as well as integrated circuits in semiconductor

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technology. Thus, extremely small sensors can be generated using standard semiconductor

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manufacturing processes. This holds, in some cases, also for the second common temperature

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measurement principle, which is thermocouples.

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A thermocouple is an electrical device consisting of two dissimilar electrical conductors

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forming electrical junctions at differing temperatures. A thermocouple produces a

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temperature‐dependent voltage as a result of the thermoelectric effect, and this voltage can be

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interpreted to measure a temperature [78, 91]. Some types of thermocouples can also be produced in

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micro‐ or even nano scale by use of semiconductor technology, others cannot [37]. However, the two

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main disadvantages of thermocouples are their limited signal strength and the need of a reference

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temperature. While the signal strength can be enhanced by creating thermopiles (numerous

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thermocouples circuited in a row, see Fig. 2), the reference temperature need is not to avoid. This fact

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makes it not easy to apply thermocouples in miniaturized equipment, which regularly shows a more

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or less isothermal behavior, short distances in the mm or μm range and very short temperature

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spreading times. Thus, the difference between reference temperature and measured temperature

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might be diminished rapidly.

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Both measurement principles mentioned above are regularly done with wired sensors. Wireless

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temperature sensors, especially in micro scale, are not so widely common yet, but will be more

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common in future [92‐94]. However, all the sensors presented in this subsection need a wire

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connection to the outer environment, thus, sealings and interconnections are necessary. This might

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be, depending on the supervised process, a source of leakage and uncertainty. Therefore, wireless

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measurement would be a better option, as it was mentioned before. Additionally, it was mentioned

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that introducing a sensor into the gas flow would disturb the flow mode or even change the process

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parameters significantly. Thus, non‐intrusive methods have to be used.

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2.1.2. Measurements using non‐intrusive methods

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As the example of the use of conventionally sized resistors or thermocouples (presented in Fig.

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1 (b)) shows, this is, in many cases, not an option, mainly for size reasons as it was described before.

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Thus, it has to be asked which other techniques and methods could be used for such a simple

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measurement task as retrieving the temperature?

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One option is the use of non‐intrusive optical systems (i.e. infrared (IR) radiation, ultra violet

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(UV) fluorescence, others) or ultrasound [95‐97]. Here, surface temperatures are measured,

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providing a non‐intrusive possibility to acquire data on the thermal state of a body. However, these

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methods have several disadvantages. They are not applicable for all cases, since certain materials are

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not transparent for IR radiation (i.e. common glasses, metals etc.) or reflecting or damping‐out

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ultrasound very strongly (i.e. dense liquids, metals etc.). In such cases, a wrong signal, a strongly

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reduced signal or no signal at all can be obtained, leading to a very low signal‐to‐noise ratio (SNR).

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With this, a measurement might be useless. A further disadvantage is that the measured signal

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represents the surface temperature of a device obtained by conduction, convection or radiation from

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the inner side, but not the core temperature. This means, that the inner parts of a body could differ

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significantly in temperature from the outer surface. This cannot be seen and measured by the chosen

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measurement technology. Same holds for other measurement technologies using optical

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fluorescence to measure temperatures [98].

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Another option is Raman spectrometry [99]. With this technique, temperature distributions

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inside of miniaturized structures are well to measure, if the surrounding is transparent for the

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Raman light. All limits named before for IR and ultrasound measurements hold here also, because it

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still is an optical method. Efforts are high, and the Raman systems are expensive and complex to

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handle. If all these drawbacks are given for measurement of an all‐day parameter like temperature,

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does this also hold for other measurement needs like pressure, flow, density or more complex sets of

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parameters like local concentration? And what might be feasible in terms of in‐situ, in‐operando and

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correlated measurements?

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2.2. Measurements of other parameters

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As pointed out in the sections before, measurements of several parameters leads to application

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of different techniques at a single process. The process pressure is measured with pressure

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transducers, in most cases located outside the process vessel. The process flow through tubes or

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vessels is obtained by anemometers or similar systems. Electrochemical sensors are used to measure

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pH or conductivity. Viscosity is obtained with a rheometer (in general, outside of the process run),

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and so on. Additional analytical methods like gas chromatography (GC) [100, 101], infrared (IR)

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spectrometry [102], ultraviolet (UV) spectrometry [103], the above mentioned Raman spectrometry

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[104], mass spectrometry (MS) [105, 106] or Nuclear Magnetic Resonance spectroscopy (NMR)

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[107‐111] are used to determine the components of the mixture, depending on the applicability of the

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respective method to the gas. Visualization of flow processes can be obtained by, i.e., high speed

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videography [112‐114]. An example for a microstructure used for high speed videography is given

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in Figure 3.

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(a)

(b)

Figure 3. Bifurcative microchannel distribution system for liquids and gases. (a) Overview to a major

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part of the system; (b) Magnification of one of the splits from (a). A step between the single incoming

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channel and the two outgoing channels is clearly visible, which is due to manufacturing. From [112].

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All of these systems work, in most cases, independently, are time consuming and costly. In‐situ

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measurements or in‐operando measurements inside of micro scale systems are possible, but rarely

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done, in most cases the analysis is performed offline with a specific sample taken from the process

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flow. A correlation of results of these analytical methods to gain more complete process information,

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as it was proposed in the introduction section, with measurement results of the process parameters

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obtained as described before is at least difficult [115‐117], in many cases it is almost impossible.

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Thus, it might be a good idea to have a combination of several complementary measurement

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and analytical systems which can provide all the information wanted within a single measurement

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campaign at once.

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2.3. Combined and correlated measurement system

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The objective mentioned above, namely to measure in‐situ, in‐operando, simultaneously at the

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same location, can be obtained by combining several measurement and analytical systems.

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However, aside of the tremendous efforts to be undertaken, this will result most likely in a spatial

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correlation only. Due to different measurement time constants of the different sensors and systems, a

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timely correlation is very unlikely to reach. All the methods named above show drawbacks for

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in‐situ, in‐operando, combined and correlated measurements. While Raman spectroscopy will need

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an optical access, IR spectroscopy as well as UV spectroscopy will need additionally specimen active

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in the named wavelength region. In‐situ and in‐operando GC or GC‐MS is possible (see i.e. [118]), but

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in general done in macro scale. To scale those techniques down into micro scale might be feasible but

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is generally not available yet. NMR measurement systems [111] are, in general, big machines with

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super‐cooled magnets and a huge bunch of external equipment, which is one of the major drawbacks

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of this technology. Figure 4 shows an example of a 400 MHz (9.4T) NMR device made by the Bruker

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company.

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However, with an NMR spectrometer like this, a sample can be characterized for various

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parameters in a single measurement campaign, correlating time and location with each of the

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separate measurements taken. The question arising now is whether such a system is able to

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characterize the desired parameters with a spatial and time resolution high enough to be useful in

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micro scale.

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For the spatial resolution, a value of about 10μm is given in literature (see i.e. [119]). The time

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resolution is named to be around 20ms (see i.e. [120]). Assuming this to be valid for a multitude of

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measurement objectives performed by NMR, spatial as well as time resolution given in literature are

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in principle good enough to run NMR systems as well‐chosen measurement and analytical tools for

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numerous applications, reaching from chemistry to materials research, pharmacy and food

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engineering to biomedical applications or physics. However, it has to be considered that many

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measurements will not be done after the 20ms given above, but take much longer and averaging a

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multitude of single measurement shots.

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Thus, beside the drawbacks of size, costs and running efforts for such a device in terms of liquid

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helium and nitrogen for cooling down the magnets etc., it is not clear yet whether all desired

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parameters can be measured in reasonable time, or if even all parameters important for a process to

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be characterized can be measured. Aside of this, not all materials can be used with NMR. The

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technology is very selective, but limited to non‐magnetic materials for devices. If process parameters

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can only be reached using a vessel made of magnetic material NMR measurement methods fail due

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to the magnetic properties of the device. Another drawback is that not all desired parameters can be

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measured directly, some can be retrieved indirectly only.

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While the measurement of temperature with NMR is in some cases precise and simple [121], the

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process pressure can be retrieved by pressure‐sensitive materials only [122]. Viscosity, density, mass

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flow and phase, phase changes or particle content can be measured directly (i.e. [123]), as well as the

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electrochemical potential of compounds (see [124]), concentrations of single mixture compounds

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[125] or the pH of a mixture.

317 318

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Figure 4. 400MHz (9.4 Tesla) NMR system of the Bruker company. www.bruker.com

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Additionally, this technology allows not only to measure different parameters non‐intrusively

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in very small confinements like micro channels but also in living tissues [126, 127]. This is an add‐on

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not provided by a multitude of the other techniques presented before. Another add‐on is the

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possibility to visualize processes by magnetic resonance imaging (MRI, see: [128]). An example of

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this is presented in Figure 5, showing a cut through a human head and imaging the brain.

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Thus, NMR seems to provide the possibility to cover lots of measurement tasks of other

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methods in a single machine, being non‐invasive, non‐intrusive and non‐detrimental to the

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measured object. By all the possibilities provided herewith a discussion is needed on at least two

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topics: is measurement with an NMR correlative? And, moreover, is NMR measurement the best

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possible solution for every measurement task?

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Figure 5. Magnetic resonance tomography (MRT) cut picture of a human head as an example for the

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MRI possibilities. From: Private.

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3. Discussion

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As it was pointed out in section 2, lots of different methods for measurement of process

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parameters are available and explored. Miniaturization of process systems results in major problems

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for scaling them down, as it was mentioned and described before. However, with larger efforts,

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smart design and good planning of experiments, scaling down of measurement systems and

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obtaining precise results of the process parameters is possible to a certain extent, using various

339

methods for process parameter characterization. Thus, miniaturization and scaling is a problem for

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intrusive measurement methods, while precise measurement of the parameter distribution is a

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problem for non‐intrusive methods. This has clearly been described in section 2. In almost all cases,

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measurements are performed sequentially, having each of the measurement and analysis systems

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running after the one before. This type of performance, aside of the difficulties given by scaling,

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allows to achieve good and reliable measurement results, but not to correlate them to each other and

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to obtain a more fundamental understanding of the interactions between process parameters and

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underlying principles. This is shown schematically in Figure 6, in which the current state‐of‐the‐art

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measurement methodology is presented.

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Thus, correlation of the measurement results is the major point to deal with. One of the future

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perspectives of measurement is to obtain correlated information in situ and in operando by as many

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different techniques as possible. Correlated measurement means timely and spatially, and with

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similar resolutions for all applied techniques. This is necessary to gain an exhaustive view to the

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examined process, to understand the underlying principles and actions, and therefore make them

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accessible for modelling and simulation as well as in silico prediction and optimization. The latter

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will lead to major reductions in resource consumption. Figure 7 shows a scheme of a possible future

355

measurement environment, in which a multitude of instruments are involved in parallel, means at

356

the same time and location, measuring in situ and in operando and, therefore, deliver results which

357

can directly be correlated. With this method, a much deeper understanding of links between actions

358

and effects can be achieved in a more appropriate way. Thus, correlative measurement, generation

359

of linked information and interpretation, management and feedback of connected data will be one of

360

the main tasks in future. The measurement structure schematically shown in Figure 7 can support

361

this task.

362

Some of the measurement methods presented above are useful for stand‐alone measurements

363

only, some can be performed simultaneously. However, the methods and techniques described in

364

sections 2.1 and 2.2 can hardly be done in a correlated way. It is possible to measure temperature,

365

pressure and chemical composition of a flow at the same position using, i.e., thermocouples,

366

pressure sensors and Raman spectrometry. To obtain a precise time correlation by using a trigger is

367

difficult, mainly due to different time constants of the measurement methods. Additional

368

visualization of the process adds some more problems, because for high speed videography as

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described in section 2, very high light intensities are necessary, which might cope with other optical

370

measurement methods; therefore a combined high speed videography – Raman measurement is not

371

an option.

372

All this holds for all of the conventional methods described above. Without precise triggering,

373

correlative measurement is most likely impossible; and even with a time trigger, the location of

374

measurement as well as the spatial resolution most likely differs largely.

375

NMR makes it partly easier in this particular point. As described before, in principle a

376

multitude of parameters can be retrieved with NMR, all of them at the same location. Thus, spatial

377

resolution is consistent. However it is doubtful that a timely consistence can be reached because

378

some measurements simply take much longer than others, as pointed out before. Thus, truly

379

correlative measurements in terms of time and space cannot be generated for the most cases, this

380

problem stays. However, the data measured by NMR are to a certain extent per se correlated,

381

because several parameters are measured with the same method and then averaged on a time

382

period, which is a huge advantage compared to conventional measurement equipment. Process

383

pressure has to be measured via pressure‐sensitive materials (see description above and [122]),

384

which makes this a special case to be dealt with. Additionally, with MRI a handy visualization tool is

385

available, which provides even more in‐sight view to processes and flows. Thus, many of the

386

problems attached to conventional measurement systems are less applying for NMR. However, the

387

time correlation problem is kept, and due to the limitation of applicability to non‐magnetic device

388

materials NMR is not a panacea. It is a good toolbox for lots of possible measurement solutions

389

correlating the obtained data directly internally. A future vision would be to retrieve all process

390

parameters at the same time, with the same precision, the same spatial and time resolution, in‐situ

391

and in‐operando, as it was requested. Thus, NMR might be a nice additional tool for certain

392

measurement applications, but it cannot considered to be a stand‐alone tool for generation of

393

correlated data on processes yet. There’s need for combinations of methods, to obtain the desired

394

“information cloud”. Only with this, a reasonable combination of different measurement and

395

analytical tools, more comprehensive descriptions of processes might be feasible.

396 397

398

Figure 6. Current state‐of‐the‐art sequential measurement and analysis method. The timely and

399

locally separated measurement makes it difficult or even impossible to correlate measurement

400

results to each other and retrieve by this deeper insight into the underlying mechanisms.

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Recently ideas for further correlated measurements came up, like MR‐optical combinations, or

402

integration of atomic force microscopy (AFM) and NMR into a single device. Moreover, proposals

403

on combining NMR technology with additive manufacturing, i.e. 3D printing methods, came up.

404

Here, the characterization as well as the measurement would be integrated in a single process,

405

allowing to generate miniaturized systems in a way which has not been thought of ten years ago.

406

407

Figure 7. Future improved parallel measurement and analysis method. The detectors are triggered

408

automatically in a way that they all measure at the same location, and deliver the measured data

409

with the same spatial and time resolution. Data are then transferred to an analysis level, where they

410

are correlated.

411

4. Conclusions

412

Macroscale measurement of gas flow parameters with conventional technology like

413

thermistors, pressure sensors etc. is, by no means, anymore a problem, but scaling those systems

414

down into the mini or micro scale still is. Due to either insufficient size of classical sensors or

415

insufficient possibilities to position micro sensors in the gas flow, the measurement influences the

416

process itself largely. Non‐intrusive methods are possible, but show other limitations, like optical

417

accessibility, the need for high power light exposure or non‐magnetic equipment materials.

418

One of the most interesting measurement problems is to achieve timely and spatially correlated

419

results. By conventional methods involving thermistors, GC, MS, Raman, IR or UV spectroscopy,

420

pressure transducers etc., this is almost impossible. Either the measurement location largely differs,

421

or the time resolution is simply not suitable. Aside, the combination of data from different

422

measurement sources is not trivial. The use of nuclear magnetic resonance systems (NMR), which in

423

principle allow measurement of a multitude of parameters at least at the same location, and in most

424

cases with the same spatial resolution, eases this problem slightly. Here, results can be obtained from

425

the same measurement location, show consistent data format, and can therefore be easily correlated.

426

Even visualization is relatively easily achieved by combining regular NMR with magnetic resonance

427

imaging MRI, which adds some more valuable data to the parameter cloud obtained by the

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measurement. The scale range for NMR / MRI systems reaches from macro to nano scale. Spatial and

473

time resolution is not depending on the scale but on the magnetic system, which is also an advantage

474

compared to other measurement methods.

475

Recently, NMR and MRI systems themselves have been target of miniaturization. Meanwhile,

476

benchtop low field NMR systems up to 80MHz resonance frequency (about 1.88T) are available

477

which can easily ran in a conventional lab environment, used for stationary or flow‐through data

478

achievement. These systems are easy to handle and deliver a bunch of analytical possibilities.

479

Nevertheless, miniaturization went on, and meanwhile developments are reaching into the direction

480

of mini or micro NMR systems [126, 128‐130]. Even wireless systems are under research now, thus, it

481

is to expect that application of miniaturized and integrated NMR in correlative measurements will

482

be greatly enhanced in future.

483

It stays a vision to have measurements of different parameters by NMR with the same time

484

resolution, as well as the possibility to measure all the desired parameters of a process. Thus,

485

combinations of a multitude of analysis instruments will be necessary, ideally within a single

486

system, controlled by a combined measurement electronic system, which triggers internally all

487

methods.

488 489

Acknowledgments

490

The author wish to acknowledge Prof. Dr. Jan Korvink and Dr. Neil MacKinnon of KIT‐IMT for lots

491

of fruitful discussions, which led to some of the thoughts briefly described in this paper.

492 493

Funding

494

This research received funding by the European Commission H2020 Marie Skłodowska‐Curie

495

Actions under the Grant Agreement No. 643095 “MIGRATE”.

496 497

498

499

and conditions of the Creative Commons Attribution (CC BY) license

500

(http://creativecommons.org/licenses/by/4.0/).

501

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