Technical Manual

XNX Universal Transmitter
Section 3 - Calibration
97
3.2.8 Zero and Span Calibration for MPD Sensors
Caution: Extended or frequent exposure to elevated concentrations
of combustible gases may affect sensor sensitivity. Verify sensor
performance by frequent calibration.
Caution: Before initial calibration allow the sensor to stabilize for 30 minutes
after applying power. When in zero and span calibration modes, the current
output from the sensor is inhibited (default 2mA) to avoid false alarms.
The Gas Calibration menu is for both zero and span calibrations.
This section describes how to calibrate MPD ammable sensors
tted to the transmitter. The calibration adjustments are made on
the transmitters display. Gassing is performed at the sensor,
which may be locally or remotely located.
The following equipment is required:
• Flow housing (P/N: 1226A0411)
• Test gas
• Regulator
Note: Zero gas and Span gas should be at about the same humidity levels to
avoid erroneous cell responses.
1. At the MPD, remove the weatherproof cap, if equipped.
2. Fit the ow housing onto the MPD.
Figure 214. Flow Housing
3. Reverse the cap removal procedure. Figure 215 shows the
ow housing accessory tted to the MPD.
Figure 215. MPD with Flow Housing
4. Connect the ow housing (using either gas pipe) to the
regulated cylinder containing a known concentration of the
target gas at approximately the sensor alarm point, e.g., 50%
LEL methane in air.
Warning: As some test gases are hazardous, exhaust the flow housing outlet
to a safe area.
5. Follow the procedure in Section 3.2.1 for both zero and
span calibrations.
6. Apply the target gas to the sensor. Pass the gas through
the ow housing at a rate of 300-375 ml/min for XNX EC
sensors or 500 ±200 ml/min for XNX mV sensors .