Features and Capabilities 1-5
September 2007
Theory of Operation
(cont.)
Figure 1-3: Thermistor Bridge Circuit
As the background gases that comprise the balance of an oxygen-
containing gas mixture change, the magnetic and thermal properties
of the gas mixture also change. This affects the accuracy and response
of any paramagnetic oxygen analyzer. To compensate for such
variations, the XMO2 has a unique “bridge-within-a-bridge” design.
The oxygen measuring bridge circuit described on the previous page
is itself one arm of another compensation bridge circuit that maintains
the oxygen bridge at a constant temperature as background gas
composition changes. The electrical power change necessary to keep
the oxygen bridge at constant temperature is a function of the thermal
properties of the background gas. Therefore, this power fluctuation
provides a signal that is related to the thermal conductivity of the
background gas. That signal is then used to reduce the effects of the
background gas variation on the oxygen span point measurement.
In addition to maintaining a constant oxygen bridge temperature, the
XMO2 microprocessor compensates for any zero point shift in the
oxygen bridge circuit output caused by background gas changes.
Finally, the bridge circuit voltage is further adjusted for variations in
background gas composition and/or atmospheric pressure by internal,
microprocessor-based compensation algorithms. The compensated
signal is then amplified and converted to a 4-20 mA analog output
that is proportional to the concentration of oxygen in the gas mixture.
R1
W
1
W
2
R
2
W1, W2=Wind Generating (Cooled) Thermistors
R1, R2=Wind Receiving (Warmed) Thermistors
Zero Adjust
V
O2
V
comp
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