Environmental Monitoring

Fixed gas detection systems are usually deployed in tunnels and mines and usually comprise of sensors for measuring many types of gases, both toxic and flammable. Typical gases being measured are as follows –

• Oxygen (O2)
• Methane (CH4)
• CO2 (Carbon Dioxide)
• CO (Carbon Monoxide)
• H2S (Hydrogen Sulphide)
• NO2 (Nitrogen Dioxide)

Other gases can be added and on some recent projects MTI Tech have supplied and integrated Benzene detectors for specific contaminated ground conditions.

As well as ‘live’ data from the sensors being recorded, it is important that STEL (Short Term Exposure Limit) and the TWA (Time Weighted Average) data is recorded and stored. The STEL and TWA datapoints are common for toxic gases.

STEL

The STEL is a limit value above which exposure to a chemical substance should not occur and usually relates to a 15 minute reference period. The aim of a STEL is to prevent adverse health effects and other unwanted effects due to peak exposure that may not be controlled by the application of an 8 hour TWA limit.

TWA

The TWA for the exposure to a chemical can be used when both the chemical concentration and time for exposure varies over time. It is thus used as the average exposure to a contaminant to which workers may be exposed without adverse effect over a period such as in an 8-hour day or 40-hour week (an average work shift). They are usually expressed in units of ppm (volume/volume) or mg/m3.