Rosemount 755, Instruction Manual

Page: 57 / 94

I
NSTRUMENT
T
HEORY
748183-H
5-5
Rosemount Analytical
December 1997
For convenience in servicing, the detector and the optical bench are self-aligning
assemblies that utilize slip-on sample connections and plug-in electrical connection.
Within the detector assembly, Figure 5-3B, the incoming preheated sample passes
through an integral 5-micron diffusion screen. It protects the test body by preventing
entry of particulate matter and/or entrained liquid mist. Additionally the screen isolates
the test body from flow effects, ensuring that instrument readout is relatively
independent of flow rate within the optimum range of 200 to 300 cc/min.
At the rear of the detector are an integral temperature sensor (RTI) and an integral
heater (HR2). Another heater (HR1) is attached to the magnet. Sensor RTI provides
the input signal to the detector temperature control section of the case circuit board
assembly, Section 5.3.3. This section controls application of electrical power to both
HR1 and HR2.
On the optical bench assembly, see Figure 5-3C, the source lamp and the photocell
plate are externally accessible, permitting convenient replacement.
5.2 VARIABLES INFLUENCING PARAMAGNETIC OXYGEN
MEASUREMENTS
Variables that influence paramagnetic oxygen measurements include: Operating
pressure, Section 5.2.1, sample temperature, Section 5.2.2; interfering sample
components, Section 5.2.3; and vibration, Section 5.2.4
5.2.1 P
RESSURE
E
FFECTS
Although normally calibrated for readout in percent oxygen, the Model 755 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 (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
controls are left at the previously established settings, the meter 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.
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
with most applications involving zero-suppressed ranges, and some applications of
zero-based ranges, 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 2.6.3.