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Instruction Manual
748213-S
April 2002
4-2 Theory Rosemount Analytical Inc. A Division of Emerson Process Management
Model 755R
4-2 VARIABLES INFLUENCING PARAMAG-
NETIC 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%.
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 ambi-
ent pressure, and is discharged to ambi-
ent pressure from the analyzer outlet.
However, in some applications, it is nec-
essary to insert an absolute back pres-
sure regulator into the exhaust line to
prevent the readout error that would oth-
erwise 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 ab-
solute pressure of less than 500 mm Hg
(66.7 kPa).