COMMI-SSION OF THE EUROPEAN COMMUNITIES

·environment and quality of life

1976

A CALIBRATION DEVICE FOR LONG-PATH SENSORS OF ATMOSPHERIC POLLUTANTS

EUR 5472 e

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Copyright CECA, CEE, CEEA, Luxembourg 1976 ~.

Printed in the United Kingdom.

[cOMMISSION OF THE EUROPEAN COMMUNITIES

Jo±nt Nuclear Research Centre - Ispra Establishment

~, CALIBRATION DEVICE FOR LONG-PATH SENSORS OF

ATMOSPHERIC POLLUTANTS ,.---

eJ ol s. SANDRONI (~)~ E. BRAMBILLA (*), 'I

M. CERESOLI (**)

(*) Chemistry Dir. Euratom Joint Research Centre, Ispra

{**) Design and Fabrication Dir. Euratom Joint Research Centre, Ispra

Edited by the Dire~torate General

"Scientific and Technical Information and Information Management"

1976

ABSTRACT

The calibration of long-path optical sensors of atmospheric pollutants like the Barringer correlation spectrometer is obtained by a device in which the optical pathlength is varied. By using the device here described the calibration procedure requires about 6 min. for the full length expansion of the cell equivalent to concentration-pathlength variation of

1 : 250.

The device is simple and can be installed on an optical bench i.e. for simulation of polluted atmosphere.

CONTENTS

I The need of a calibration device 5

II The variable pathlength cell 6

III Detailed description 7

IV Mode of operation 8

Figures 10

5

I. THE NEED OF A CALIBRATION DEVICE

Within the framework of a research activity aiming at the application of optical methods to remote sensing of air pollution, some difficulties have been experienced in

calibrating instruments based on resonance absorption, and suitable for air quality measurements over a long pathlength. Instruments for this application are correla­tion spectrometers (i.e. Barringer {x) CospecTM)which uses a modulated Xe lamp for average concentration measurements of so2 at around 31 o nm and of No2 at around 440 run, and tunable laser analysers which use specific absorption of infrared light emitted by a diode laser or by other tunable laser device( 1 ).

The classical ways of calibrating these absorption analysers are: 1) by inserting two optical standards and extrapola­ting a linear relationship to a zero level or 2) by introducing some mixtures of gases at different concen­trations in a cell having a fixed length. Practically for a correct calibration following method 2), some difficulties are experienced, for example a) it is necessary to pre­pare and to store large volumes of gas mixtures at very low

concentration, b) the analysis of gas mixtures at a concen­tration of ppm or lower requires large volumes of gases or some sophisticated t~chniques and c) the wall effect in the cell and in the container may modify the mixture compo­

sition. For these reasons we studied a device which by varying the length could permit a calibration by using a gas mixture

only.

6

II. THE VARIABLE PATHLENGTH CELL

The device we realised and its schematic are shown in fig.1 and 2 respectively. Essentially, it consits of two cylin­drical teflon bellows cells (c1 , c2 ), having the left head fixed on the frame holder of the device and the right one moving on three bars; the cells are connected by silicone tubes in order to have a constant pressure. The cells are mounted one upon the other for use on an optical bench. A motor (M) operated by a button on the front of the holder moves by means of a driving orum (B) one of the right heads towards the fixed one, and the other in the opposite direc­tion. The PVC heads hold the optical windows (W); the left heads hold the optical windows by a teflon cylinder some em in length placed inside the bellows to allow a zero thickness in the cell when compressed. In one connec­ting tube, an inlet valve (!) for gas filling and a teflon fan (F) operated by a motor are inserted. To wash the gas circuit, an exit valve (V) is inserted in one left head. The mode of operation is quite simple: the circuit is flushed with the gas mixture to be used for several minutes; once (I) and (V) have been closed by simply pushing the buttons for (M) and (F) the cell length can be varied from about zero to the maximum value of b possible. The path­length b of one cell is given by a indicator on a meter scale on the front of the frame holder; in a more sophisti­cated way, the b values can be transferred to a recorder (R) via a potentiometer connected to (B). A two-pen recorder can display simultaneously the gas signal and b values. In our device the maximum pathlength is 50 em with a minimum of 0.2 corresponding to a concentration path lenqth (c.b) range of 1 : 250. By introducing i.e. the commercial gas mixture so2/N2 at 100 ppm (in vol.) we can get c • b values from 50 ppm-m down to 0.2 ppm-m. These data are equivalent over a 1000 m. optical path to concentrations from 50 to 0.2 ppb. The concentration range 1 : 250 fully covers the concentration range of most of the pollutants

7

in the atmosphere.

The device operates at atmo?pheric pressure but a somewhat lower pressure can be used. It is quite simple, easy to realize and can be installed on an optical bench, i.e. for simulation of polluted atmosphere.

III. DETAILED DESCRIPTION (fio.2) -·~·-~-~-

Cells ( c1 , c2) They consists of teflon bellows (fig .4), 600 mm. length, external diameter 120 mm•, internal diameter 80 mm. with 45 coils. The bellows are made by direct melting. The minimum length one can obtain is 120 mm. At one side of the cell a teflon cylinder 113 mm length, 78 mm diameter is inserted. At the internal end of the teflon cylinder the optical windows (w) are welded by Araldite type Ay 101. For operation in the ultraviolet/visible spectral region, which is the case of Barringer Cospec analyser, Tetrasil A

discs 78 mm diameter, 6 mm thickness have been used. In the infrared region KRS-5 windows could be used.

Fan (F) It is a mixin~!- teflon fan moved by an electric motor Crouset type 82160, 3000 rpm. The gas tightness is assured by Viton type a-ring-s resistent to most of corrosive gases.

C<?EE~Etin9" tu.E.~2__(T) These are silicone tubes IDv/55 type, 14 mm external diameter.

Motor (11) It is a Isotermic Swiss speed reducer with a reduction ratio 405 : 1 with 5,9 rpm. The motor moves the screw cams (B) by gears.

Ba:~~?_.(B) Both the screw cams and t1'1_P fixed bars are in stainless steel. The sc:r·ew cams hl.ove on ball bearings and their position is regulated by two microswitches •

. Po!~!l-_!3:-omete£_ (P) It is a Helipot type potentiometer with an accuracy of 0.25% a.nd a resistence of 0.5 K ~-!..

Recorder (R) It is a two pen recorder 10 mV full scale

v1e use a Philips type 8221 PN recorder.

8

Va.lves(V and three ways valve Y) are teflon or glass valves. The device holder is in alurainium AG-3: the front panel has two buttons for (M) and (F) and a meter for the direct reading of the· lower cell length. The bellows are moved by the motor (M)throu two gears; when one cell reaches

the maximum optical length of 500·mm, the other reaches

the minimum optical length of 2 mm (obviously these distan­ces refer to the internal walls of the windows). The speed of pathlength variation is 82 ram/min., the full expansion being obtained in about 6 minutes. The length of the lower

cell can be read (a) directly on the meter scale or (b) on the recorder simultaneously vrith the gas signal. So one can directly verify the relationship between path­length and absorption.

IV. HODE OF OPERATION

The mode of operation of this dP.vice is quite simple. First the .flushing oB the system is obtained in the following manner; the va.lve (I) is connected to a gas supply and (V) is opened• The gas circulates alternatively via (F) and by (c1 ) and (c2), and then on closing (I) and (V) the fan (F) is started for some minutes, after which the gas is removed by (v). This operation is repeated several times until a stable instrumental response is obtained. Once filled the cells and closed (I) and (V), the variation of the pathlength is obtained by pushing the button for (N) on the front panel. In about 6 minutes the pathlength varies from 2 to 500 mm continously and the length b is

recorded togeth~r with the gas signal~ fJ·. 4)

9

ACKNOWLEDGMENTS

we are indebted to Mr. G. Giovanelli and G. Cesari,Istituto di Fisica dell 1Atmosfera del C.N.R. Bologna for the helpful discussions in the realisation of the device.

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