A SIMPLE DEVICE FOR TRACE GAS ANALYSES IN THE ATMOSPHERE

HAL Id: jpa-00223256

https://hal.archives-ouvertes.fr/jpa-00223256

Submitted on 1 Jan 1983

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

A SIMPLE DEVICE FOR TRACE GAS ANALYSES IN THE ATMOSPHERE

D. Sourlier, O. Oehler

To cite this version:

D. Sourlier, O. Oehler. A SIMPLE DEVICE FOR TRACE GAS ANALYSES IN THE ATMOSPHERE. Journal de Physique Colloques, 1983, 44 (C6), pp.C6-587-C6-591.

�10.1051/jphyscol:1983697�. �jpa-00223256�

A SIMPLE D E V I C E FOR T R A C E G A S ANALYSES IN T H E ATMOSPHERE

D. Sourlier* and 0. Oehl.er

Institute uf Applied Physics, Swiss Federal Institute of TeehnoZogy, CH-8093 Zurich, Switzerland

R6sum6

-

Abstract - A device, composed of a thermal light source and an acoustically decoupled photoacoustic non-resonant cell which allows the detection of atmospheric trace gases is described.

Introduction - The photoacoustic method allows the detection of gases at very low concentrations. By the use of infrared laser light sources it is possible to detect gases like methane, ethylene at concentrations down to the sub-ppb-range [1,2,31.

Very often a reliable detection of gases in the 1-100 ppm range is needed, since several gases, like CO, Nitric oxides, have their maximum allowable concentration values (MAC) L41 in this range. At these concentrations usually the direct optical method - the measurement of the optical extinction by the absorbing gas

-

used. Nevertheless the photoacoustic method often allows the detection of gases in a simpler way. If no extraordinary gas selectivity is required, the infrared laser may be replaced by a thermal light source together with an interference filter. If no high resolution is required it is even possible to modulate the light source thermally. This is feasible at low frequencies (below 20 Hz) only. Consequently the photoacoustic cell has to be operated at acoustical non-resonance; further care has to be taken with the acoustical decoupling of the detecting cell. Since thermal light sources are very weak in the infrared spectral range a highly efficient light collecting unit is needed. A description of such an optical device as well as a discussion of simple acoustical decoupling devices is given in this article.

The elliptical light collecting system - The use of an interference filter requires a fairly parallel light beam. The half cone angle should not exceed about 25 degrees [51. Further the beam has to be narrow. The fulfilment of this condition implies a large photoacoustic signal since a reduced cell volume can be applied. The covered el lipsoid-shaped half-mirror as given in Fig. 1 fulfills this condition satisfyingly.

Fig.1 Photoacoustic gas detection device G: filament, F: interference filter, C: gas cell, M: microphone, K: capillaries

'present address :

Triatex International AG, Heinrichstr. 217, CH-8005 Zurich, Switzerland

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983697

C6-588 JOURNAL DE PHYSIQUE

A plane mirror with a transmitting section at its center is attached near the cen- tralplane of the focal points. Therefore this plane mirror virtually-reglaces the second half of the ellipsoid. This construction uses the property of an ellipsoid to collimate the radiation of a point source which is positioned at a focal point.

The emitted light beam passes the source every two reflections while it approaches monotonously the main axis until it becomes decoupled through the central window of the plane mirror [61. An intense, converging beam is obtained. The disadvantage of this arrangement is the requirement for a partially transparent light source. A filament fulfills this condition satisfyingly. We used a gold plated ellipsoid- shaped half mirror of geometry A = B/2 = 40 mm and of a reflectivity of 71%. The reflectivity of the plane mirror was 95%. The filament was made out of a 0.05 mn Kanthal [71 wire. It was 0.7 mm long, had a diameter of 0.5 mm and its transparency was 65%. In Fig.2 the efficiency of the reflector - that means the ratio of decoup-

led light and the radiation produced by the source is given as a function of the diameter of the exiting window. Fig.3 represents the measured angular distribution of the light beam exiting through a 25 mm diameter hole in the plane mirror.

o measured

- calculoled

Fig.2 ifficiency of the covered Fig.3 Angular distribution of the exiting ellipsoid-shaped half mirror 1 ight beam

Acoustical decoupling devices - The acoustical decoupling of the photoacoustSc cell is very important for two reasons, especially when it is operated at low frequencies:

First the equalization of the photoacoustically induced pressure with the surround- ing has to be prevented; secondly the incoupling of external sound into the photo- acoustic cell has to be reduced. In Table I the C02-pressure signal in air is given for a closed and partially closed photoacoustic cell of 50 mm length and a diameter of 5 mm. The excitation of the gas was produced by a mechanically modulated light source operated at an electrical power of 5.3 W (the optical power of the beam inci- denting the photoacoustic cell was approximatively (1.3 + 0.5).10-~-~v~.cm).

Table I shows the pressure equalization in a closed and incompletely closed photo- acoustic cell. The prevention of the mentioned pressure equalization can be achieved by a complete mechanical closure of the cell.

On the other hand, the use of mechanical valves is unsuitable since the absolute magnitudes of the pressure signals to be decoupled are very small; only a high acoustical attenuation is required. This condition can be fulfilled satisfyingly by gas-liquid-gas interfaces. Such a device makes use of the fact that gases and liquids have very different acoustical irnpedance,resulting in a high sound reflec- tion at the interface. In Fig.4 and 5 acoustical decoupling factors are given for two such devices.

Fig.4 Schematic and sound attenuation factors for the acoustical decoupling by oil-filled vessels

Fig.5 Schematic and sound attenuation factors for the acoustical decoupling by oil-filled capillaries

In Fig.4 the gas-liquid-gas interfaces are represented by small vessels of 15 mm diameter containing about 1 cm3 of oil each. The gas exchange is accomplished by

bubbling the sample gas through the liquid. In Fig.5 the fessels are replaced by oil-filled capillaries of 5 m length and an inner diameter.of 0.37 mm. A slight over-pressure produced by a small membrane pump drives the oil out of the capil- laries and allows an efficient gas exchange. After stopping the pump the liquid

C6-590 JOURNAL DE PHYSIQUE

r e t r e a t s i n t o t h e c a p i l l a r i e s b y t h e c a p i l l a r y force. Because of t h e h i g h p r e s s u r e f l u c t u a t i o n s on t h e gas exchange no gas c o n c e n t r a t i o n measurements a r e p o s s i b l e d u r i n g t h i s phase. A l t e r n a t i n g gas exchange and measuring procedures a r e necessary.

R e s u l t s - Fig.6 shows t h e t i m e e v o l u t i o n o f t h e CO2 c o n c e n t r a t i o n i n a c l o s e d room.

The measurements were accomplished b y a d e v i c e as g i v e n i n Fig.1. An e l e c t r e t m i c r o - phone [ 9 1 was used. Immediately b e f o r e t h e measurements which were taken a t a 10 sec t i m e c o n s t a n t t h e gas i n t h e p h o t o a c o u s t i c c e l l was changed. According t o t h e v e r y

low e l e c t r i c a l power o f

F i g . 6 t h e l i g h t source

( 9

( l i g h t power ,;1.10 ?.AV.W Scm), t h e s i g n a l - t o - n o i s e C02- concentrat~on ~n a 50m3 room occup~ed by 4 persons r a t i o i s low (S/N=16), b u t

neasureb ~ 8 t h a p h o t o o c ~ ~ s l t c ce:; the'mo~ty r n o a ~ l o t ~ d a t i t a l l o w s t h e save s u r -

a frequency 01 B HZ onci a t o n e l e c t r ~ c c l power OI 0 3 W v e i l l a n c e o f t h e C02 i n a c l o s e d room. By u s i n g a

MtCrophone

s ~ g n o l m e c h a n i c a l l y chopped l i g h t

source, o p e r a t e d a t an e l e c t r i c a l power o f 5.3 W

50 -. ( i n c i d e n t l i g h t power

1 . 3 - 1 0 - 6 ~ v . ~ . c m ) a save

LO - d e t e c t i o n (S/N=4) o f 1 -5ppm

C02 o r 23 ppm CO was found.

3 0 -. A p h o t o a c o u s t i c d e v i c e i s

I

Openlag of the I s e n s i t i v e t o e x t e r n a l

Oserrrg of t l p

w8ndow 10. !rnlnute w ~ n d o w for ! mfnute sound. T h i s i n f l u e n c e can be e s t i m a t e d b y u s i n g t y p i c a l n o i s e spectra, as g i v e n i n Table I 1 f o r a

6 ' 2 18 2 L 3 0 36 L2 L 8 5L 60 saw t y p e machine [81, t o -

time #a m l n u t ~ s g e t h e r w i t h t h e observed sound a t t e n u a t i o n f a c t o r s of t h e a c o u s t i c a l decoup-

l i n g elements ( F i g . 5 ) . Table I 1 Machine n o i s e d a t a (saw) and i t s i n f l u e n c e on t h e p h o t o a c o u s t i c

measurement

,

500 86

47.0 72.3 0.001 0.003 350 32

( 7 0 )

43.0 61.0 0.004 0.020 480 Octave band rnid f r e q u e n c i e s Wo(Hz)

Average sound p r e s s u r e l e v e l s P(d,AWl) measured i n dB a t d = 1 meter [ 8 1

Sound p r e s s u r e P(D,Aw2) c a l c u l a t e d i n dB a t D = 1.5 m, AW2 = 0.1 HZ Sound a t t e n u a t i o n f a c t o r s F o f t h e o i l f i l l e d c a p i l l a r i e s (dB)(Fig.5) Sound f r o m t h e e x t e r n a l source i n - co'upled i n t o t h e c e l l (Npin mPa) E l e c t r e l microphone [ 9 1 n o i s e N,

( e q u i v a l e n t i n mPa) S/N (S f r o m T a b l e 1)

11 (70)

47.6 58.0 0.006 0.029 240

16 ( 7 0 )

46.0 62.1 0.003 0.020 390

250 76

40.0 78.2 0.0003 0.005

620 63

67

37.0 68.1 0.0006 0.015

660 125

70

37.0 70.8 0.0004 0.009

650

distance D = 1.5 m and a reasonable detecting bandwidth of AN, = 0.1 Hz follow from:

AW 1

10.log10

nw,

The levels N of the sound incoupled into the photoacoustic cell in Pa are given by:

where the sound attenuation factor F was taken from Fig.5.

The electronic noise of the amplifier is much smaller than the noise of the electret microphone. It therefore has been neglected. Since the photoacoustical signals in a cell which is acoustically decoupled by oil-filled capillaries are very similar to those in a mechanically closed cell, the closed-cell values of Table I can be taken.

These values S correspond to a cell containing 450 ppm C02 in air. It was operated at an optical power of ^c 1 . 3 - 1 0 - ~ - ~ v - ~ - c m .

We thank Mr. M.Rosatzin for performing the acoustical attenuation measurements.

References

[21 L.G.Rosengren, Appl .Opt.

2

[ 3 ] E.Kritchman, S.Shtrikman and M.Slatkin, J.Opt.Soc.Am.

68

141 S.Moeschlin, Klinik und Therapie der Vergiftungen, G.Thieme Verlag, Stuttgart, 1972.

[51 Effects of the Variation of Angle of Incidence and Temperature on Infrared Filter Characteristics, Techn.Report, Mai 1967, OCLT Optical Coatings Ltd., Hillend Industrial Estate Dufermline, Fife, Scotland.

161 0.Oehler and D.Sourlier, Helv.Phys.Acta, in press.

[7] Kanthal DSD, Trade mark of High Temperature Resistent Alloy, Bulten-Kanthal AB, Kanthal Division, Hallslahammar, Sweden.

[8] Handbook of Noise and Vibration Control, 4th edition, Trade and Technical Press Ltd, Morden, Surrey, England, 1979.

[9] Electret Microphone Type CA-1832, Knowles Electronics Inc., Frank1 in Park, I11

.,

USA.