Duty: To measure the concentration of H₂S in a process gas stream.

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 reduces tape usage and 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
• A continuous analyser with response times as low as 10 seconds
• The instrument has a wide range capability, measuring between 0-50ppb to 0-100% H₂S and a sensitivity of 5ppb H₂S
• The humidifier has a very low volume allowing response times as low as 10 seconds
• Configuration of the system is via a touch screen which eliminates the need to open the enclosure.
• Single point calibration results in lower running costs
• A compact flash card allows for remote evaluation and diagnostics
• 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
• Suitable for gas group IIC environments.

With its wide-ranging ability to measure H₂S from % levels down to ppb levels, the HG400 range of H₂S process analysers can be utilised in many process industries, including:

• Natural Gas pipelines
• Reformer recycle gas
• Offshore gas production
• Fuel gas monitoring
• Sour gas treatment plants
• Gas processing
• Biogas