3.6 Enhancing the factory
calibration
When your XMO2 transmitter was calibrated at the factory,
the actual factory calibration data points were entered
into the XMO2 software. If requested on the original order,
calibration data points for expected field background gas
composition and/or measurement cell pressure variations
may also have been entered. To supplement this factory
calibration data, calibration data points generated in the
field for these parameters can be added into the XMO2
software.
The factory calibration can be further enhanced by
performing periodic recalibrations in the field. The XMO2
then uses the new calibration data to create offset and drift
curves that compensate the original factory calibration
data for variations that occur in the field.
When making a measurement, the XMO2 uses the Offset
Curve or Drift Curve, along with any background gas and/or
cell pressure compensation data, entered at the factory or
in the field, to update the factory calibration data.
To maintain the integrity of this process, the XMO2
should be recalibrated periodically. This is typically done
every 1-3 months with a single (offset) calibration gas,
depending on the application. The optimum recalibration
interval depends on such factors as %O
2
range, required
accuracy, components of the gas mixture, the cleanliness
of the sample gas, etc. In addition, the XMO2 should be
recalibrated with the two-gas (zero gas and span gas)
method at least once per year. Again, the optimum
calibration interval depends on the specific application.
Using the calibration procedures in this chapter, the XMO2
can be recalibrated for the same %O
2
range, background
gas mixture, and compensation signals used for the
factory calibration. However, if it has been some time
since the original factory calibration, or if you want to
calibrate the XMO2 for a different %O
2
range, gas mixture, or
compensation signal, contact the factory for instructions.
CAUTION!
The calibration procedures described in
this chapter require the use of specialized
apparatus and should be performed only by
properly trained service personnel, following
all applicable safety practices.
3.7 Required calibration materials
To perform a field calibration, the following materials are
required:
• Offset gas - for a one-gas %O
2
calibration
• Zero gas - for a two-gas %O
2
calibration and/or a 4-20
mA analog output calibration
• Span gas - for a two-gas %O
2
calibration and/or a 4-20
mA analog output calibration
Note: Suggestions for suitable calibration gases are listed on
the XMO2 Calibration Sheet provided with your unit.
Also, the accuracy of the calibration will only be as good as
the accuracy of the calibration gas(es) used.
• Panametrics XMO2 Calibration Sheet
• A sample system or individual components (e.g.,
flowmeter, needle valve, pressure gauge, etc.) for
introducing the calibration gas(es) to the XMO2
transmitter at the required pressure and flow rate. See
Chapter 2, Installation, for specific recommendations.
• A multimeter or ammeter (for a 4-20 mA analog output
calibration)
WARNING!
Avoid using explosive gas mixtures as your
XMO2 calibration gases.
3.8 Preparing for field calibration
To prepare the XMO2 for a field calibration, refer to Figure 10
on page 17 and perform the following preliminary steps:
1. Turn the power on and allow at least 30 minutes for the
XMO2 to reach temperature stability.
2. Loosen the set screw that locks the XMO2 cover in place,
and unscrew the cover.
IMPORTANT:
Remember to replace the cover after the field calibration
has been completed.
3. Refer to Figure 11 on page 17, and locate the following
items:
• Calibration pushbutton (Switch S3)
• Zero/span selector (Switch S1)
• Terminal block TB1
Note: If you plan to perform the field calibration at a
computer terminal via the XMO2 RS232 digital output, you
do not need to access the above items because you will
skip steps 1-2 above.
16
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