Rosemount 755, Инструкция по эксплуатации

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M
ODEL
755 O
XYGEN
A
NALYZER
2-14
748183-H Rosemount Analytical
November 1997
S
AMPLE
E
XHAUST
With positive sample pressure, the proper choice of arrangement for sample exhaust
depends principally on whether the analyzer has zero-based or zero-suppressed
ranges. as explained below.
S
AMPLE
E
XHAUST
A
RRANGEMENTS FOR
Z
ERO
-B
ASED
R
ANGES
With zero-based ranges, the analyzer exhaust port is commonly vented directly to the
atmosphere, and any change in barometric pressure results in a directly proportional
change in the indicated percentage of oxygen.
EXAMPLE
Range, 0% to 5% O2
Barometric pressure change after calibration, 1%
Instrument reading, 5% O2
Readout error = 0.01 x 5% O2 = 0.05% O2
Fullscale span is 5% O2, therefore the 0.05% O2 error is equal to 1% 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: manual computation,
computer correction of data. etc.
2.6.4 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. 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
in the sample-handing system will result in decreased readout accuracy as compared
with operation at atmospheric pressure.
The minimum permissible operating pressure is 660 mm Hg absolute (88.1 kPa
absolute). 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 as follows: 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, it will therefore allow the incoming sample to
mix with earlier sample, causing an averaging or damping effect. 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.