Emerson NGA2000 PMD manual 35

Model NGA2000 PMD

Instruction Manual

760007-A July 2003

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 affected 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 instrument 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 suppressed range applications, can perform signal corrections automatically.

Rosemount Analytical Inc. A Division of Emerson Process Management Operation 3-11