Hydrogen Sulfide H₂S

Hydrogen sulfide (H₂S) is a strong-smelling compound often associated with industrial processes, such as sewage treatment or waste disposal. H₂S is not listed among the six criteria health pollutants cited by the US EPA.

However, legislation controlling the levels of H₂S in ambient air is defined in most countries and many jurisdictions have ambient exposure standards for H₂S.

The hydrogren sulfide module uses a patented automatic baseline correction (ABC) design to measure H₂S in ambient air. It combines an advanced electrochemical H₂S sensor with a sample/zero cycling operation to deliver near reference performance in ambient H₂S monitoring.

The sensor used in the H₂S module is an electrochemical sensor. The sensor generates an electrical current in nA which is proportional to the concentration of gas which comes in to contact with the sensor.

The module uses two different flow paths, one of which passes through a proprietary scrubber to remove H₂S and produce a baseline measurement. The flow rate is determined by a flow control orifice on the exhaust side of the sensor and measured at the inlet to the sensor module (expected to be approximately 60 mL min^-1).

The sensor current is measured during both the baseline and sample measurement states and these values are used to calculate the H₂S concentration using a proprietary algorithm approximately once per minute.

The H₂S module performance specifications are given in the table below.

Just like a regulatory H₂S analyzer using UV fluorescence, the H₂S module can be field calibrated using standard calibration equipment and reference gases. This ensures the module calibration is fully traceable to NIST primary standards.

The H₂S module factory calibration is achieved using a zero-air source, certified calibration gas standards and a certified gas dilution calibrator. The Aeroqual AirCal 1000 or 8000 systems can be used to calibrate the H₂S module in the field.

Calibration frequency will be dependent on the user’s data quality objectives and Quality Assurance Project Plan (QAPP), but based on the low rate of field drift the recommended calibration frequency is 1-3 months.

To demonstrate module performance, a six-point gas test, at known H₂S concentrations, was carried out using the equipment outlined above. The module shows good linearly (R² = 0.99) over the 0–1600 ppb range studied.

The Aeroqual H₂S module is specific towards H₂S but other gases in the environment may also cause a response from the H₂S sensor. These do not indicate a fault. These effects are part of the module behavior and should be considered when examining data.

The H₂S module response to other ambient air pollutants is documented in the table below.

The non-catalytic chemical scrubber has an expected life of 2 to 3 years. The internal solenoid has an expected life of 2 to 3 years. The GSE sensor has an average expected life of 1 to 2 years in climates with moderate humidity and particle loading.

The Aeroqual hydrogen sulfide sensor head uses a gas sensitive electrochemical (GSE) sensor. The electrochemical gas sensor in the sensor head generates an electrical current in nA, which is proportional to the concentration of gas that comes in contact with the sensor. The sensor current is converted into H₂S concentration.

The table below contains the hydrogen sulfide sensor head specifications. These can be used on our Ranger, S Series and Series 900 products. The 0-10 ppm sensor head is designed for outdoor environmental air monitoring, while the 0-100 ppm sensor head is designed for industrial health and safety.

Although the GSE sensor in the H₂S sensor head is designed to measure H₂S, it does show cross-interferences to other gases. The table below shows the response in ppm of the H₂S sensor head to different gases.