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Low Range Sensors
It is now possible to fit the madur flue gas analysers with special sensors for low ranges. This has been done successfully on the GA-40plus and would apply equally to the GA-40Tplus. The modern regulations in many countries call for such small levels of emissions that this has proved essential in certain cases. Naturally this will increase the absolute accuracy of the measurement, since the relative accuracy remains the same, but the range is reduced.
The specific possibilities are as follows:
Carbon monoxide CO
A range of 0 – 300 ppm with a resolution of 0.1 ppm
This range of CO will cover the carbon monoxide emissions from most modern equipment, assuming nothing else is to be measured. Carbon monoxide emissions have been slashed dramatically in the past decade, with limits of 100 ppm carbon monoxide being the rule rather than the exception. Calibration can be carried out using a concentration of CO most appropriate to the application, otherwise we would recommend about 250 ppm carbon monoxide. These levels of CO are also useful for measuring conditions inside of small, enclosed and occupied spaces, such as vehicles.
Nitric oxide NO
A range of 0 – 50 ppm with a resolution of 0.1 ppm
NO emissions in this range are now possible with some combustion systems. Whilst nitric oxide is still often produced in the area of 100 ppm, low range burners are now available that will only produce about 30 ppm NO. Nitric oxide is also to be found from other sources and this low range sensor makes it possible to measure these as well. As a matter of interest, nitric oxide is also used in the treatment of asthma, and NO is to be found in the heart muscle. Nitric oxide still counts as a pollutant, even with these low ranges, so all attempts should be made to measure NO, even if the nitric oxide level is held to be within perfectly acceptable boundaries.
Nitrogen dioxide NO2
A range of 0 – 20 ppm with a resolution of 0.1 ppm.
Nitrogen dioxide is heavily controlled nowadays and low range sensors may be needed in many cases where NO2 is to be measured. Nitrogen dioxide belongs to the particularly unpleasant pollutants but NO2 production will be reduced when NO emissions are also kept down. Nitrogen dioxide can also come from other processes, and the low range NO2 sensors make it possible to also measure these factors. Calibration can be carried out with a gas of around 10 – 15 ppm NO2, if available. Nitrogen dioxide is highly water soluble, so measuring these low ranges means strict control of the moisture content of any gas.
Sulphur dioxide SO2
A range of 0 – 100 ppm with a resolution of 0.1 ppm
With modern fuels the level of sulphur dioxide has sunk radically in most countries. This is a great advantage in most cases, since SO2 is a particularly unpleasant gas. A low range sensor for 100 ppm sulphur dioxide is adequate for most of these applications. Care must only be taken that the instrument is not used on coal fired power installations, which can have much higher levels of SO2 in the emissions. Sulphur dioxide is generally only present when combustion of a fuel containing sulphur has taken place, so SO2 is not to be expected under other circumstances. This can include the use of internal combustion engines, where the fuel may not be completely free of sulphur compounds. For this purpose a low range sensor for SO2 will be essential since the sulphur dioxide level should be minimal.
Hydrogen sulphide H2S
A range of 0 – 50 ppm with a resolution of 0.1 ppm
Hydrogen sulphide is a particularly nasty gas. The best known source of H2S is rotting eggs, and this is the typical odour of this gas. Hydrogen sulphide is also to be found in biogas as a result of fermentation. The H2S must be measured, even if the level is relatively low. Combustion of hydrogen sulphide will produce sulphur dioxide and H2S is a corrosive gas in its own right. Most applications such as sewerage measurement will require a low range sensor for hydrogen sulphide. It is important to remember that H2S is a combustible gas and should be treated as such.
Sensors with these ranges are all available from madur electronics, and can be combined if necessary in many cases with standard ranges for other gases. Care must just be taken to consider cross-sensitivity effects which may completely blanket the result from another sensor in these low ranges if a high level of interfering gas is present. The soluble gases (NO2, SO2, and H2S) must be considered carefully. Sensors for these low ranges will also require an appropriate gas preparation system capable of removing any moisture to preserve the gases for measurement.
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