The principle of operation is as follows. A reel of paper tape impregnated with lead acetate is exposed to the gaseous process sample in the sample chamber. Hydrogen sulphide (H2S) in the process sample reacts with the lead acetate, after acidification, to form lead sulphide (PbS), a brown compound that stains the tape.
The higher the concentration of H2S the darker the stain. A focused LED and single photo detector system continuously measures the darkening stain and the analyser reports this as a 4-20mA output of the concentration of H2S.
Incremental measurements are made on the same spot until saturation occurs, when the tape is advanced to a fresh portion of tape. This avoids the need for frequent zero or span calibrations.
The benefits of this technology can be summarised as follows:
- As the measurement is specific to H2S, there will be no interference from other sulphur components
- The instrument has a wide range capability, measuring between 0-50ppb to 0-600ppm H2S
- The humidifier has a very low volume allowing response times as low as 10 seconds
- Stability of the instrument is improved with the electronics being temperature compensated
- Lower running costs as no instrument air or nitrogen is required
- The effects caused by ambient temperature swings are reduced with the use of a fibre-optic link between the housings
- Capital investment is minimised as up to four detection modules can be run from one controller
- Multi-streaming capability of up to 4 streams per analyser