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In medical work this is mainly used to calculate the molar fraction of nitrogen in a gas mixture. It is used in some lung function tests.
A tiny quantity of the gas mixture to be sampled is drawn through a needle valve by a high vacuum pump to provide pressures of 1 to 4 mmHg and this gas mixture is ionized between two electrodes in an ionization chamber. The gases emit light which is detected by a phototube after passing though an optical filter to remove wavelengths outside the range of 310-480 nm. An amplifier and linearizing circuit is connected to the phototube to produce a figure for nitrogen concentration. Nitrogen concentration from 0 to 80% can be measured with considerable accuracy with response time of the order of 40 ms. Oxygen and carbon dioxide do not interfere with the accuracy. However, helium and argon can produce errors.
Content and Design Copyright 2000 Dr. Malcolm C Brown. See Title Page for more details