Page 33 - Instrumentation and Measurement
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Figure 2.30 Crankcase oil mist detector
2.6.2 Oxygen analyser
Gases can generally be classified as either diamagnetic or paramagnetic, the former seek the weakest
part of a magnetic field and the latter the strongest. Most of the common gases are diamagnetic but
oxygen is paramagnetic and use is made of this in the oxygen analyser shown in figure 2.31.
Two platinum wire resistances are heated by current from an ac bridge and the gas to be measured
enters the resistance chamber via a diffuser. One of the resistance wires is placed in a magnetic field
hence oxygen is drawn towards this resistance, thus convection currents are set up around this
resistance which is then cooled relatively to the other resistance. The bridge is then unbalanced; the
amount of unbalance is a measure of the oxygen content and this is displayed on the galvanometer.
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