Burner Bypass Switch; Checking The Alarms; Probe Or Sensor Calibration; Calibration Gas Check - Novatech 1635 User Manual

2.20 BURNER BYPASS SWITCH

Heated probes and sensors should have their heater supply interlocked if the 1231 probe is to be installed into a dryer /
oven which is directly heated with combustion flame, or if the probe heater is likely to be a source of ignition in the
appliance (See CAUTION 1 at the front of this manual). If the combustion appliance is not running, then power will
not be supplied to the heater. To commission an oxygen probe or sensor when the main burner is turned off, switch
power off the analyser, remove the probe from the dryer or oven or the dryer or oven pipe connection from the sensor.
Re-apply power to the analyser, press the burner bypass switch into the 'DOWN' or 'ON' position. This will apply
power to the probe or sensor heaters even when the plant is not running. The offset can now be set and calibration
checked with appropriate calibration check gases (typically 2% oxygen in nitrogen).
Ensure that the BURNER BYPASS switch and the power are turned off before the probe or sensor is re-installed. An
alarm will occur if the BURNER BYPASS switch is turned on (down) during normal operation.

2.21 CHECKING THE ALARMS

If any alarms are present the alarm LED will be lit, either flashing or steady. To interpret the alarms, press the alarm
button until all alarm functions have been displayed. Rectify the cause of each alarm until no further alarms appear on
the display. For details on the operation of the alarm button and the alarm functions refer to Section 3.

2.22 PROBE OR SENSOR CALIBRATION

The zirconia sensor provides an absolute measurement of oxygen partial pressure. There are no calibration adjustments,
apart from 'SENSOR OFFSET', for the probe or RGS-01 sensor. The sensor EMF is either correct or a replacement is
required. To check that the probe or sensor is functioning correctly, firstly check that the high impedance alarm is not
activated ('Sensor Fail' alarm). The actual impedance can be displayed on the lower line. It should be less than 9,000Ω
at 720°C.
Once it has been established that the impedance is normal, the offset may be set using the millivolt level marked on the
oxygen probe or sensor. Refer to Section 4.5. The probe offset can be tested on site. A small flow of air must be
admitted to both the 'REF' and 'CAL' ports when testing the probe or sensor offset. If the probe is in the process, the
air must fully purge the probe sensor without interference from the process gas sample.

2.23 CALIBRATION GAS CHECK

If the installation has a filter purge facility, set this up first. Refer to the previous paragraph. Press the 'CAL 1' button
while in 'SET-UP' mode to obtain a reasonable flow through the calibration check gas flow meter. If air is being used
as a calibration check gas, use the air from the regulator for filter purge. Then, when setting up a gas for calibration
checking, set the pressure from the calibration gas cylinder so that it is the same as the pressure set on the air regulator.
Then the setting on the rotameter / flow regulator will be the same as that for the air flow. The flow requirements vary
depending upon the length of the probe and whether or not the probe has a filter. Required flows normally range from 5
to 20 litres per minute. Too high a flow will cause cooling of the sensor, creating errors. If the flow is too low, gas
equilibrium will not be achieved and the reading will fall short.
Air is not the best gas for calibration checking on a zirconia sensor. The output of a zirconia sensor with reference air is
zero millivolts. It is better to choose a gas value which provides a reasonable output from the sensor and which is near
to the process oxygen level. A cylinder with 8% oxygen in nitrogen is a commonly used calibration gas. The
maximum pressure on the calibration check gas cylinder regulators is 100 kPa (15 psi).
August 2009
24 1635 Water Vapour Analyser