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Instruction Manual
760007-A
July 2003
Rosemount Analytical Inc. A Division of Emerson Process Management Operation 3-11
Model NGA2000 PMD
3-7 BAROMETRIC PRESSURE COMPENSATION
Although normally calibrated for readout in
percent oxygen, the PMD Analyzer Module
actually responds to oxygen partial pressure.
The partial pressure of the oxygen component
in a gas mixture is proportional to the total
pressure of the mixture. Thus readout is af-
fected by pressure variations.
For instance, assume that an instrument is
calibrated for correct readout with a standard
gas containing 5% oxygen, admitted at the
normal sea level atmospheric pressure of 14.7
psia (1013 hPa). If the operating pressure
now drops to one-half of the original value
(i.e., to 7.35 psia/506 hPa and the calibration
controls are left at the previously established
settings, the display reading for the standard
gas will drop to 2.5%.
It is therefore necessary to calibrate the in-
strument at the same pressure that will be
used during subsequent operation, and to
maintain this pressure during operation.
Alternatively, an optional Barometric Pressure
Compensation Board, typically used for sup-
pressed range applications, can perform sig-
nal corrections automatically.