Model 755A

Instruction Manual

245364-V May 2002

4-2 VARIABLES INFLUENCING PARAMAGNETIC OXYGEN MEASUREMENTS

Variables that influence paramagnetic oxygen measurements include: operating pressure, Section 4-2a below; sample temperature, Section 4-2b below; interfering sample components, Section 4-2c (page 4-5); and vibration 4-2d (page 4-5).

a.Pressure Effects

Although normally calibrated for readout in percent oxygen, the Model 755A 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. However, the instrument incorporates electronic circuitry that provides automatic pressure compensation for pressure variations within ±3% of the target value, which may be set anywhere in the range of -2.7 to 3.3 psig ±3 psig (-18.6 to 22.8 kPa ±21 kPa).

WARNING

POSSIBLE EXPLOSION HAZARD

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

b.Temperature Effects

Magnetic susceptibilities and partial pressures of gases vary with temperature. In the Model 755A, temperature-induced readout error is avoided by control of temperatures in the following areas:

1.Interior of the analyzer case is maintained at 140°F (60°C) by an electronically controlled heater and associated fan.

2.Immediately downstream from the inlet port, prior to entry into the detector, the sample is preheated by

passing through a coil maintained at approximately the same temperature as the detector.

3.The detector is maintained at a controlled temperature of 150°F (66°C).

c.Interferents

Instrument response to most non-oxygen sample components is comparatively slight but is not, in all cases, negligible. Table 3-3 (page 3-11) lists the readings that would be obtained with various gases on an instrument previously calibrated with 100% oxygen, assuming that all gases are admitted at the same pressure.

During initial installation of an instrument for a given application, effects of the background gas should be calculated to determine if correction is required. See Section 3-5c, page 3-10.

d.Vibration Effects

Instrument Design

To minimize vibration effects, the detector/magnet assembly is contained in a shock-mounted compartment, Figure 1-2 (page 1-3).

Installation

Avoid excessive vibration. In making electrical connections, do not allow any cable to touch the shock-mounted detector/magnet assembly or the associated internal sample inlet and outlet tubing. This precaution ensures against possible transmission of mechanical vibration through the cable to the detector, which would cause noisy readout.

Electronic Response Time

If readout is noisy despite precautions mentioned above, obtain slower electronic response by counterclockwise adjustment of Response Time Adjust R30 (see Table 3-2, page 3-6, and Figure 3-2, page 3-7).

Rosemount Analytical Inc. A Division of Emerson Process Management

Theory

4-5