Instruction Manual

748213-S April 2002

4-2 VARIABLES INFLUENCING PARAMAGNETIC OXYGEN MEASUREMENTS

Variables that influence paramagnetic oxygen measurements include: operating pressure (See Section 4-3a, page 4-4), sample tem- perature, interfering sample components, and vibration (See Section 2-1d, page 2-1).

a.Pressure Effects

Although normally calibrated for readout in percent oxygen, the Model 755R actu- ally responds to oxygen partial pressure. The partial pressure of the oxygen com- ponent 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, ad- mitted at the normal sea level atmos- pheric pressure of 14.7 psia (101.3 kPa). If the operating pressure now drops to one-half the original value (i.e., to 7.35 psia {50.65 kPa}) and the calibration con- trols are left at the previously established settings, the display reading for the stan- dard gas will drop to 2.5%.

Model 755R

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 op- eration.

Typically, the sample gas is supplied to the analyzer inlet at slightly above ambient pressure, and is discharged to ambient pressure from the analyzer outlet.

However, in some applications, it is necessary to insert an absolute back pressure regulator into the exhaust line to prevent the readout error that would otherwise result from fluctuations in exhaust pressure. The regulator must be mounted in a temperature-controlled housing (See Section 2-3c, page 2-3).

Operation at negative gauge pressure is not normally recommended, but is used in certain special applications (See Section 2-3d, page 2-4).

CAUTION

PRESSURE MINIMUM

Never subject the sensing unit to an absolute pressure of less than 500 mm Hg (66.7 kPa).

4-2 Theory

Rosemount Analytical Inc. A Division of Emerson Process Management