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| Typical Number in Hospital: 2 | Cost Bands: 3 | References: 9 |
This is a thermal conductivity detector for gases employing heated wires which are cooled by the gases to be identified. In general the thermal conductivity of a gas is inversely related to its molecular weight. Hydrogen and helium, for example, have conductivities approximately 6.5 times those of nitrogen and oxygen. The katharometer normally consists of two parallel tubes in which are mounted heating elements arranged in a bridge circuit so that resistance changes due to unequal cooling by the gases in each tube can be detected. One channel normally receives a reference gas (e.g. helium) and the gas mixture to be analysed is passed through the other.
Katharometers are used in lung function testing equipment such as the helium FRC analyser, and in the gas chromatograph. Response times are poor (e.g. 10 seconds to 1 minute) compared with a mass spectrometer, but the cost is low and accuracy can be high.
Content and Design Copyright 2000 Dr. Malcolm C Brown. See Title Page for more details