Abstract
The compact instrument is based on a light-emitting diode as radiation source and a photodiode as detector. A fiber optic cable is used for guiding the light to and from the membrane located at the tip. In contrast to earlier designs, the small sensor head is freely accessible to ambient atmospheres. The resulting effect of background light is removed by lock-in amplification using a single integrated circuit synchronous demodulator. Ratioing against a reference signal is carried out in order to eliminate instabilities of light-emitting diodes caused by temperature fluctuations or ageing. The system is demonstrated with membranes for the sensing of ammonia and carbon dioxide in the vapour phase.
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