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ING INSTRUMENT U T ~ L I S ~ N G PIEZOELECTRIC AND GAS-SENSITIVE POLYME

J. Watson*, R. Tamadoni", N McMurray** and G.S.V. Coles*

University of Wales, Swansea SA2 SPP, United Kingdom "Electrical & Electronic Engineering Dep'artment; **Materials Department

SUMMARY

The most usual form of instrument for the monitoring of CO, involves absorption bands in the infra-red spectrum, which in turn mandates the use of infra-red optics and sensors. The present instrument dispenses with the use of infra-red by employing visible light along with a gas-sensitive polymer which changes colour in the presence of CO, , this being a reversible process. The instrument itself also utilises a novel realisation of the well-known double-beam-in-time principle where fibre-optic light guides replace lens systems. The mechanical beam splitting is performed by a piezo-electric bimorph cantilever.

INTRODUCTION

In a classical double-beam-in-time system [1,2] a beam of light from a suitable source is mechanically split so that it sequentially passes through a sample and a reference cell. Both beams then impinge on a single photo-receptor so that an electrical waveform is generated such that the amplitude of each half-cycle corresponds to the optical transmittance of the sample and reference cells alternately. The ratio of these is then obtained by electronic means and this represents the transmittance of the sample cell as a fraction of that of the reference cell. When converted to a relative optical density this is proportional to the concentration of gas or liquid in the sample cell compared with that in the reference cell. By this means, changes in the intensity of the light source due to aging are negated, as are dirt accumulations in the optical components and also changes in the sensitivity of the sensor over time. This approach may be quantified as follows.

Let the light flux entering each cell be L, lumens, that leaving the sample cell be L, and that leaving the reference cell be L,. Then, the transmittance of the sample cell is Ts = (LJL,) and that of the reference cell is T, = (L,/L,). Hence, the relative transmittance is:

Optical density is given by D = log,,(l/T) and it is this quantity which is proportional to the concentration of a

T, = 'SmR =

light absorbing material in a cell. Hence, in the present case where the relative optical density D, is required,

and if the photodiode and its current-to-voltage- amplifier combination is linear, then,

where V, and V, are the output voltages representing the reference and sample beams respectively.

D, = "gIO('/Td) = log,O (LR/LS) rll

Drol = loglo ('R/V.S> [21

This is one example of a system suited to the monitoring of those gases, such as carbon dioxide, which are not easily addressed by other means such as solid-state or electrochemical sensors. However, previous instruments have utilised infra-red optical systems and (usually) motor-driven beam splitters. The present instrument replaces the infra-red components with their with visible light counterparts and the motor with a very simple piezo-electric vibrator Furthermore, all these constituent parts may reasonably be miniaturised for the production of a small, inexpensive instrument.

In the present case, the light source is a controlled filament lamp, but in principle may be a LED or a solid-state laser depending upon the absorption wavelengths of interest. This light (again in the present case) is injected into one end of an inexpensive plastic optical fibre, whilst the other end is attached to the free end of a piezo-electric bimorph cantilever. As is shown in Fig. 1, the emergent light is collected sequentially by the ends of two similar optical fibres.

Sampla/Refrrence

Light source

' I / Optical fibres

Fig. 1. The Basic System

776 TRANSDUCERS '95 * EUROSENSORS IX

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The other ends of these two fibres project their light components alternately through the sample and reference cells and eventually on to a single photo- receptor, which in this case is a silicon photo-diode.

THE PIEZOELECTRIC BIMORPH

The piezoelectric bimorph consists essentially of two flat plates of lead zirconite titanate (PZT) bonded together at their faces and poled so that when an electric field is applied to conductive layers covering their remaining faces, one plate lengthens whilst the other shortens, so producing a bending action. This bimorph is mounted as a cantilever so that the free end is able to vibrate under the influence of an applied alternating voltage. One end of a fibre optic light guide is fixed to this free end and the frequency of the supply is locked to the resonant frequency of the sub-system to produce maximal excursions (which for the present components is about 120 Hz).

THE FIBRE OPTIC LIGHT GUIDES

gas flow whilst the reference cell is gas-tight and is normally filled with clean air. Hence, the amount of light allowed to cross the sample cell is a function of the the gas concentration, whilst the the unaffected polymer in the reference cell provides a datum level.

The design of such a cuvette is crucial to the proper operation of the instrument and it should be such as to allow quick and easy insertion and removal of glass slides coated with the gas-sensitive polymer.

THE ELECTRONICS

The output from the photodiode used in the present apparatus is shown in Fig. 3, where the differing amplitudes of the signals relevant to the sample and reference cells can be clearly seen. The intervening amplitude is not at zero level because, at the centre of the bimorph excursion, light will enter both receiving light guides concurrently. For this reason a sampling technique is used to determine the true amplitudes of the two signals.

Though most of the light from a source can easily be launched into the static end of the transmitting fibre optic light guide, the fraction of the light exiting the

very dependent upon the relevant geometry. This is

minimising the lateral gap between the transmitting and receiving light guide ends, (b) minimising the distance between the two receiving light guides and (c) making

e a - a 3 e oscillating end into the two receiving light guides is

shown in Fig. 2 and can obviously be maximised by (a)

0 .- : L

c n

I

* t

the diameters of the two receiving light guides larger than that of the transmitting light guide. However, Fig. 3 Waveform from the photo-diode.

_ _ _ these geometrical factors do influence the shape of the electrical waveform produced by the photo-diode. Following Fig. 4, the signal from the photodiode passes

through a signal conditioning amplifier to a sample-and- hold circuit synchronised from the bimorph drive

Transmitting oscillator. This effectively generates a square wave light guide. representing the peak values of the signals arriving from

each half of the cuvette. These are then digitised in preparation for the ratioing and conversion to log'arithmic which is carried out by the microprocessor. The final readout is displayed alpha-numerically and may be in the form of the relative optical density.

. T O ! Receiving

light guides. Bimorph.

Fig. 2 The light guide geometry.

The sample and reference cells are two parts of a transparent cuvette into which can be inserted a glass slide coated with the gas-sensitive polymer [3,4] which, in the present version, undergoes a colour change from blue to orange when exposed to carbon dioxide and does so in a consistent and reversible manner. This cuvette is designed so that the sample cell is open to a

To bimorph

Fig. 4 Electronics block diagram

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434 - D 8

ER

The gas-sensitive material consists of a plasticised lymer in which are dissolved organosoluble

quartenary alkyl ammonium salts of acid-base indicator 61. Such materials may be dissolved in volatile solvents and cast from solution to form thin (10- transparent films, the optical absorbances or

fluorescent intensities of which are strongly influenced by the conccntration of carbon dioxidc. Thcsc films arc completely insoluble in water, contain no residual volatile conlponents and consequently function over wide ranges of lemperalure and relative humidity. The

explained in terms of libria involving the quartenary salts of indicator acids, water molecules these salts and carbonic acid Using

prepare sensors which (sub-second) response,

long-term chemical and physical stability (storage lives exceeding a year) and the ability to survive biological sterilisation by y-irradiation.

n dio~de-sensitive material utilised ent may be regarded as a specific class of thin optical film sensors

range of gases, both acidic ore, films incorporating

organosolub~e salts of the ruthenium (11) tris-bimdyl cation have been shown to act as reversible non- consumptive oxygen sensors operating via a lum~ne~ence quenching mechanism.

C N

As a preliminary test, the sample cell was replaced by a sequence of neutral density filters and the ratio of the peak ampli~des of the series of sample signal to that of a constant reference signal was converted to optical density form. Fig. 4 indicates that correspondence between these output optical densities and those of the filter densities is promising and further work on both calibration and ~ a l ~ t i o ~ of the gas-sensitive polymer

with gas-sensitive s may be for visible

ces costs. It is recognised that these two i ~ o v a t ~ o n s are not inextricably

interwoven and that the gas-sensive film technique is also potentially useful as nt in various other forms of gas monitoring i

3 T ‘*:I 1.5

0.5 I t 0

AC

The authors wish to thank Hoechst Ceramtec UK Ltd. for providing the selection of piezoelectric bimo which made this project possible.

CE

[ I ] J. Ingle & S.R. Crouch, ‘ Chap. 3, sec3 &: 4; Chap. 1988). [2] C.N. Banwell, Spectroscopy’, Ed. 3. 1983).

ctrochemical Analysis’, sec. 6 (Rentice- Hall,

.N. Murray, ‘Carbon Dioxidc W091/05252, (1991).

[4] A. Mills 62 H.N. Murray, U.S. Pat. App. 07/853753, (1992). [SI H.N. McMurray, ‘Novel thin film optical sensors for the detection of carbon dioxide’, J. Mat. Chem. 2(4), (1992), 401-406. (61 A. Mills, W n g Chan & H.N. Murray, ‘Equili~rium studies on colourimetric plastic film sensors for carbon dioxide’, Anal. Chem. 64, (1992) 1383-1389.

778 TRANSDUCERS ‘95 * EUROSENSORS IX

The 8th International Conference on Solid-state Sensors and Actuators, and Eurosensors IX Stockholm, Sweden, June 25-29, 1995