Answering questions about gas detection
October 1st 2004 In a special piece for HSM Status Scientific Controls, designers and manufacturers of portable gas detectors, fixed gas detection systems and confined space communications answer some frequently asked questions concerning gas detection.
Q: Can infrared sensors be used for the detection of hydrogen gas?
A: No, hydrogen is infrared inactive.
Q: What is the recommended calibration period for gas detection equipment ?
A: It is normally recommended that equipment is calibrated using a certified concentration of a suitable gas every six months. Where instruments are exposed to prolonged high concentrations of gas this period may be reduced to between one and three months.
Q: What is the meaning of the term LEL ?
A: This is an abbreviation for Lower Explosive Limit.
Combustible gases are potentially explosive when mixed with air in particular concentrations, these concentrations vary from one gas type to another. For example, methane gas is potentially explosive for concentrations between 5 and 15% by volume with air under normal atmospheric conditions. The LEL for methane is therefore 5% by volume. The value of 15% is, for methane, the Upper Explosive Limit (UEL).
Q: What is the meaning of 25% LEL ?
A: A common method of expressing flammable gas concentrations is to express them as a percentage of the LEL value, in the case of methane, 25% LEL is equivalent to 1.25% by volume (i.e. 25%, or 14, of 5% by volume).
Alternatively, if the gas being measured were hydrogen, which has an LEL of 4% by volume, then 25% LEL would be equivalent to 1% by volume hydrogen (i.e. 25%, or 14, of 4% by volume).
Q: If Methane is potentially explosive in concentrations of between 5 and 15% volume with air, does this mean that a concentration of say 30% by volume is not dangerous ?
A: No, although this concentration is above the upper explosive limit, there is a danger of it becoming diluted with air to form an explosive concentration.
Q: For what reason is a charcoal cloth filter sometimes fitted in front of the flammable pellistor type sensor within an instrument ?
A:A charcoal cloth filter can reduce the risk of chemical "poisoning" of pellistors by substances such as silicones. However, care must be exercised in specifying a charcoal filter because this may prevent the sensor from responding to certain types of flammable gases. In general, these filters should only be used where there is a known 'target' hydrocarbon gas, such as Methane. Fitting a filter in this instance would be acceptable because it is known that the 'target' gas will through the charcoal filter and reach the sensor.
Questions About Portable Gas Detectors
Q: What colour rubber boots are available ?
A: The standard colours are black, yellow and green. If the colour is not specified when ordering then normally a black rubber boot is fitted to the instrument. Other colours are available to special order, subject to a minimum order quantity of 100.
Q: Can PGD1 style chargers be used to charge PGD2 instruments ?
A: No. The old style charger used with the PGD1 has three charging pins; the PGD2 chargers have only two pins.
Q: My previously fully charged Portable Gas Detector has been switched off and unused for 3 weeks but when I switch it on indicates LOW BATTERY.
A: The instrument continues to draw power from the battery even when switched off in order to maintain
the sensors in a state of readiness for immediate use.
It is recommended that the instrument is left in its charger when not in use. More articles from Status Scientific Controls Ltd.: | 
