S
TARTUP AND
C
ALIBRATION
3-2
June 1997 Rosemount Analytical 748213-P
Model 755R Oxygen Analyzer
to perform span calibration on a range of higher sensitivity (i.e., more narrow span)
and then move the jumper to the desired operating range. For example, if the
operating range is to be 0 to 50% oxygen, span calibration may be performed on the 0
to 25% range to permit use of air as the span standard gas.
3.4.1 C
ALIBRATION WITH
Z
ERO AND
S
PAN
S
TANDARD
G
ASES
Note:
The same flow rate must be maintained for zero, span, and sample to avoid
measure error. The exhaust is vented to the atmosphere to avoid back
pressure. The following procedure is based on the standards in Table 3-2.
Performance specifications are based on recorder output.
S
ET
Z
ERO
C
ALIBRATION
P
OINT
Inject nitrogen zero standard gas through analyzer at suitable flow rate, preferably 250
cc/min. Allow gas to purge analyzer for a minimum of three minutes.
Adjust ZERO control so that the reading on the digital display or recorder is zero
S
ET
S
PAN
C
ALIBRATION
P
OINT
Inject span standard gas (see Table 3-1) through the analyzer at the same flow rate as
was used for zero standard gas. Allow gas to purge analyzer for a minimum of three
minutes.
Adjust SPAN control so that reading on display or recorder is appropriate to the span
standard gas.
3.5 COMPENSATION FOR COMPOSITION OF BACKGROUND GAS
Any gas having a composition other than 100% oxygen contains background gas.
The background gas comprises all non-oxygen constituents. Although instrument
response to most gases other than oxygen is comparatively slight, it is not in all cases
negligible. The contribution of these components to instrument response is a function
of the span and range used, and can be computed for each individual case.
If the zero and span standard gases contain the same background gas as the sample,
the routine standardization procedure automatically compensates for the background
components. Therefore, the zero and span standard gases would introduce no error.
If the background gas in the sample is different from that in the zero and/or span
standard gas(es), background effects must be taken into consideration to ensure
correct readout. During adjustment of the front panel ZERO and SPAN controls (see
Figure 1-1), the instrument is not set to indicate the true oxygen content of the zero
and span standard gases. It is set to indicate a slightly different value, relative to
background gas, calculated to provide correct readout during subsequent analysis of
the sample gas.
