U
NPACKING AND
I
NSTALLATION
2-10
April 2000 Rosemount Analytical 245364-U
Model 755A Oxygen Analyzer
CAUTION: HIGH PRESSURE SURGES
2.6.3 N
ORMAL
O
PERATION AT
P
OSITIVE
G
AUGE
P
RESSURES
Pressure at Sample Inlet (All Instruments) -
Normally, the sample is supplied to the
analyzer inlet at a positive gauge pressure in the range of 0 to 10 psig (0 to 69 kPa).
High pressure surges during admission of sample or standard gases can
damage the detector.
Sample Exhaust -
The analyzer exhaust is vented directly to the atmosphere through
an exhaust line with inner diameter sufficiently large as not to cause any back
pressure. Internal circuitry automatically corrects the oxygen readout to within
±
1% of
fullscale for atmospheric pressure variations within
±
3% of target value and within
±
2%
of fullscale for barometric pressure variations within
±
5% of target value. The target
value may be set anywhere within range of -2.7 to 3.3 psig
±
3 psig (-18.6 to 22.8 kPa
±
21 kPa).
2.6.4 O
PERATION AT
N
EGATIVE
G
AUGE
P
RESSURES
Operation at negative gauge pressures is not 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. There 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 will result in
decreased readout accuracy as compared with operation at atmospheric pressure.
The minimum permissible operating pressure is -1.9 psig (-13.1 kPa). Operation
below this limit may damage the detector and will void the warranty.
2.6.5 S
AMPLE
F
LOW
R
ATE
Operating limits for sample flow rate are the following: Minimum 50 cc/min.; maximum
500 cc/min. A flow rate of less than 50 cc/min. is too slow to sweep out the detector
and associated flow system efficiently resulting in a slow system response. Too rapid
a flow will cause a back pressure that will affect the reading. The optimum flow rate is
between 200 and 300 cc/min.
Deviation from the set flow should be held to within
±
1% or
±
2 cc/min, whichever is
smaller. If so, zero and span drift will be within the limits given on the specifications
page, provided that operating pressure remains constant.
Bypass Flow -
Preferably the analyzer should be installed near the sample source to
minimize transport time. Otherwise the time lag may be appreciable. For example,
assume that sample is supplied to the analyzer via a 100 foot (30.5 m) length of 1/4
