U
NPACKING AND
I
NSTALLATION
2-6
October 1999 Rosemount Analytical 748250-L
Model 880A Non-Dispersive Infrared Analyzer
2.9 SAMPLE FLOW RATE
Recommended sample flow rate is 1 to 2 SCFH (500 to 1000 cc/min). A subnormal
flow rate will not affect readings but may result in an undesirable time lag. However,
an excessive flow rate can result in cell pressurization and reading error.
Assume that two cell volumes are required to flush any cell. Approximate flushing time
for the Model 880A's 12-inch cell at atmospheric sampling pressure (i.e., the outlet of
the cell venting to atmosphere) is approximately 12 seconds. Flushing time decreases
with cell volume (see Table 2-1). Flushing time is inversely proportional to flow rate.
The primary effect of flow rate, other than flushing time, is cell pressure. Due to
restrictions in exit flow configuration, an increasing flow rate increases sample
pressure in the cell.
In all cases, the effect of pressure on readout is eliminated if the same flow rate is
used for the measured sample as well as for the zero gas and span gas.
Note that at higher flow rates the nonlinearity of the calibration curve increases,
because of increase in sample cell pressure. Therefore, if higher flow rates are
required, the calibration curve should be redrawn at the higher rate.
At flows up to 2 CFH (1 L/min), gaseous sample temperatures are equilibrated to
instrument temperature regardless of stream temperature. At extremely high flow
rates, this may not be true, but no such effect has been noted up to 18 CFH (9 L/min).
CELL LENGTH
MM INCH
CELL VOLUME IN CC
WITHOUT INLET
TUBE
TOTAL VOLUME IN CC
CELL WITH INLET
TUBE
TIME FOR 2
VOLUMES
AT 2 SCFH (1 L/MIN)
AT 750 MM HG
3
0.118
0.85
12
2 sec.
4
0.157
1.14
12
2 sec.
8
0.315
2.28
13
2 sec.
16
0.630
3.56
16
2 sec.
32
1.25
9.12
20
2 sec.
64
2.52
18.24
25
3 sec.
128
4.03
35.48
44
3 sec.
232
9.13
65.12
73
6 sec.
343
13.50
97.76
105
13 sec.
381
15.00
108.60
116
14 sec.
T
ABLE
2-1. P
URGING
T
IME AT
A
TMOSPHERIC
S
AMPLE
P
RESSURE
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