Table of Contents
Advertisement
8.2.11 Burner Temp Override
Options: Disabled, or Enabled
Default:
Disabled
The mains power to the two probe heaters and purge/cal solenoids is switched through the
electromechanical 'burner' relay. In addition to being used to disable the probe heaters as part of the burner
interlock safety system, this relay can also be de-energised by the 1735 transmitter to protect a probe from
burning out in the event of a failure in the probe temperature controller.
In normal operation the burner interlock is de-energised if any heated probe exceeds 735°C (1355°F) as this
may indicate an electronic fault in the transmitter temperature controller or feedback loop. In certain
applications where heated probes are used in a process that exceeds 735°C, to use the purge/cal solenoids
at temperatures above 735°C this override option must be set to enabled.
WARNING: by enabling the burner temperature override, the operator acknowledges that in the unlikely
event of a probe heater solid-state relay short-circuit, the normal temperature protection will be disabled
resulting in the probe heater burning out.
8.2.12 Ref Pump Cycling
Options: Always On, or Cycle Above 30°C to 55°C in 5°C increments
Default:
Cycle Above 35°C
The internal reference air pump is used to move a small volume of atmospheric air to the reference side of
the oxygen sensor to allow for the continuous accurate measurement of oxygen concentration in the process
gas relative to ambient. If an oxygen probe is used without a reference air pump then the oxygen on the
reference side of the sensor will 'stagnate' and result in drift in the EMF output of the sensor.
The reference air pump is a small mechanical diaphragm pump located inside the transmitter case. As the
temperature inside the case heats up, the expected lifespan of the pump will reduce. In order to maximise
the life of the pump the transmitter can cycle the reference air pump to reduce its overall use.
Once the internal case temperature exceeds the pump cycle threshold, the reference air pump will run for 30
seconds on, then 30 seconds off. This is enough to prevent the atmospheric air on the reference side of the
sensor from stagnating.
8.2.13 Dry Reference Air
Options: Wet Air / Dry Ambient / Dry Air
Default:
Wet Air
The oxygen probe measures process gas oxygen concentration relative to the oxygen concentration of the
reference side of the sensor, which is normally exposed to atmospheric air. Atmospheric air will contain
20.95% oxygen by volume in dry air, however if there is any humidity in the air then the suspended water
vapour will displace the dry air reducing the oxygen content proportionally.. It is therefore normal for a probe
running with ambient air on both the reference and process side of the sensor to read oxygen concentration
slightly lower than 20.95%.
Some operators prefer to assume that atmospheric air is 'dry air', and therefore prefer to see an open air
reading of 20.95%. In this case, we have several options;
Always use the humidity compensated 'wet air' as the reference oxygen concentration
Wet Air
when calculating process oxygen. This is the default, and preferred option.
Dry Ambient Use humidity uncompensated 'dry air' as the reference oxygen concentration for
calculating process oxygen in the range 20.95 ± 1%, but use 'wet air' when the process oxygen deviation
is > ±1% from atmospheric oxygen. This allows for accurate readings at process level as well as seeing
20.95% in ambient air.
Always use humidity uncompensated 'dry air' as the reference oxygen concentration
Dry Air
when calculating process oxygen.
Technical Manual
48
August 2018
1732 Oxygen Transmitter
Advertisement
Table of Contents
