I
NSTALLATION
748213-P Rosemount Analytical June 1997
2-5
Model 755R Oxygen Analyzer
a check valve should be placed in the vent line if the analyzer is connected to a
manifold associated with a flare or other outlet that is not at atmospheric pressure. If
the detector is over-pressurized, damage will result.
The analyzer exhaust port is commonly vented directly to the atmosphere. Any
change in barometric pressure results in a directly proportional change in the indicated
percentage of oxygen.
Example:
Range, 0% to 5% O
2.
Barometric pressure change after calibration, 1%.
Instrument reading, 5% O
2
.
Readout error = 0.01 x 5% O
2
= 0.05% O
2
.
Fullscale span is 5% O
2.
Therefore, the 0.05% O
2
error is equal to 1% of fullscale.
Thus, if the exhaust is vented to the atmosphere, the pressure effect must be taken
into consideration. This may be accomplished in various ways, including manual
computation and computer correction of data.
2.3.4 O
PERATION AT
N
EGATIVE
G
AUGE
P
RESSURES
Operation at negative gauge pressures is not normally recommended, but may be
used in certain special applications. A suction pump is connected to the analyzer
exhaust port to draw sample into the inlet and through the analyzer. Such operation
necessitates special precautions to ensure accurate readout. First is the basic
consideration of supplying the standard gases to the analyzer at the same pressure
that will be used for the sample during subsequent operation. In addition, any leakage
in the sample handling system will result in decreased readout accuracy as compared
with operation at atmospheric pressure.
The minimum permissible operating pressure is 5 psig vacuum (34.5 kPa vacuum).
Operation of the analyzer below this limit may damage the detector, and will void the
warranty.
2.3.5 S
AMPLE
F
LOW
R
ATE
Operating limits for sample flow rate are as follows: minimum, 50 cc/min; maximum,
500 cc/min. A flow rate of less than 50 cc/min is too weak to sweep out the detector
and associated flow system efficiently. Incoming sample may mix with earlier sample,
causing an averaging or damping effect. Too rapid a flow will cause back pressure
that will affect the readout accuracy. The optimum flow rate is between 200 and 300
cc/min.
Deviation from the set flow should be held to within 10% or 20 cc/min, whichever is
