Electrical Resistance Method of Measuring Moisture in Materials

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Technical Note (National Research Council of Canada. Division of Building Research), 1961-03-01

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DIVISION OF BUILDING RESEARCH

NATIONAL RESEARCH COUNCIL OF CANADA

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NOT FOR PUBLICATION FOR INTERNAL USE

PREPARED BY _ｐｾ

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N.B.H.

PREPARED FOR

SUBJECT

RILEM Seminar by Correspondence

-"Measurement of Moisture in Materials" (N. B. Hutcheon - correspondent)

M.!!.. March 1961

ELECTRICAL RESISTANCE METHOD OF MEASURING MOISTURE IN MATERIALS

The measurement of moisture in materials has for long been a problem of great importance because moisture is the prime factor in the process of deterioration and insta-bility in materials. The Division of Building Research has, in common with other Institutes, been aware of this and has explored the possibilities of improving the available moisture meters based on the electrical resistance method. No reports have yet been issued on this work but some of the implications

of it are now summarized.

This discussion relates to moisture measuring elements consisting of a system of electrodes encased in a small block of a porous body acting as the sorbing material. The method employing electrodes placed directly in the base material in which moisture is to be measured is now used extensively in the measurement of moisture content of wood.

The principle of operation of a practical moisture measuring element, such as that developed by Bouyoucos and Mick consisting of two tinned copper wire electrodes cast in a block of gypsum plaster, involves the measurement of the

electrical resistance between the two electrodes. The resistance is affected by the geometry of the system including the pore

sizes and pore size distribution in the plaster block as well as the amount of water present in these pores and the ion concentration in the water.

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-When the moisture of some porous base material in a structure is to be measured the moisture measuring 'element is placed in contact with it or embedded in it and when

equilibrium is reached the base material and the element are at the same moisture potential, or suction, but not at the same moisture content. This has serious implications in the calibration and use of these moisture meters, which are also discussed by Penner in DBR Technical Note No. 323 prepared for this Seminar. The complications resulting from hysteresis in the material in which moisture is to be measured are

unavoidable. But a choice is possible in the selection of a porous body for the meter. By suitable modifications,

hysteresis in the meter itself can be reduced and sensitivity and range adjusted within limits, thus improving the meter as a device for measurement of moisture potential, or suction.

A porous material like plaster has a characteristic curve of suction vs. moisture content. At moisture contents close to saturation there is very little change in moisture content for a large change in suction which results in low sensitivity in detecting changes of suction in this range by the resistance measurement. A more serious limitation involves the hysteresis between the wetting and the drying curves of suction vs. moisture content. It is possible to obtain two different values of moisture content at the same value of suction. Thus it is necessary to follow the history of the changes in moisture content in order that the correct value can be selected. This is not always possible when there is partial cycling from wetting to drying.

These limitations of the moisture measuring element are a function of the pore sizes and pore size distribution of the sorbing block. Thus the ultimate usefulness of such an element will depend upon the ability to produce a porous body of the desired pore geometry. The moisture content vs. suction curve should be regular and of reasonable slope in the range desired and should have a very small hysteresis between the wetting and the drying curves.

Some experience has been acquired in producing

plaster samples of different pore sizes and pore size distri-butions by compressing plaster of paris into a cake and then hydrating it at a controlled temperature in saturated vapour. The temperature of set, the pressure at which the cake is compressed, and the particle size of the plaster of paris are factors which influence the pore size distribution of the resulting plaster.

The ion concentration in the pore water seriously affects the electrical response of the measuring element.

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-Ideally, if there were no soluble salts present the element would have the highest sensitivity to changes in moisture

content and would ｾｶ･ a stable calibration curve. This, of

course, is impossible to achieve with normal porous materials

and therefore all that can be hoped for is a system in which

the ion concentration in the water does not change greatly.

Variations can be reduced in some measure if the system is one which will be saturated with respect to the more common ions

such as calcium and sulphate. Plaster has advantages in this

respect for use as a block material.

The moisture measuring elements developed to date have rather large electrode systems consisting of tinned copper

wire or monel screen. Experiments at DBR have shown that

platinized platinum electrodes provide improved sensitivity, tdthe extent that the size and spacing of the electrodes can

be reduced considerably. Reduction in size is in itself an

advantage but it also results in more rapid response of the system to moisture changes.

In summary, the resistance meter offers a relatively simple method for measuring moisture potential, or suction, in materials, but it has definite limitations as a moisture meter because of hysteresis in the material in which it is to be

bedded. The measurement of resistance is relatively simple

and a variety of meters are available for the job. These

meters can be improved through the development of more suitable porous materials for the meter block and by the use of platinum electrodes.