Potentiometer FSD (VR1) on the main circuit board should be turned fully
anticlockwise. Apply a known calibration gas to the Pellistor and allow the reading to
stabilise for two minutes. Adjust VR3 on the Pellistor circuit board until the display
shows the correct gas value.
If additional gain is required then FSD (VR1) on the main circuit board can be used.
Pellistor span calibration (control unit without display)
The calibration is performed by using the coarse adjust potentiometer (VR3) on the
Pellistor circuit board and FSD (VR1) on the main circuit board.
Potentiometer FSD (VR1) on the main circuit board should be turned fully
anticlockwise. Apply a known calibration gas to the Pellistor and allow the reading to
stabilise for two minutes. Connect a DVM between TP1 and TP2. Adjust VR3 on the
Pellistor circuit board until the DVM shows the correct mV reading. Table 1 shows
the relationship between a mV value on the DVM and an analogue value.
If additional gain is required then FSD (VR1) on the main circuit board can be used.
Options board
The options board is connected to SK6 and SK9 on the main circuit board. A single
PCB pillar supports the top edge of the circuit board. The options board provides a
relay output for the TWA function, an RS232 output for the data/logger, and an
isolated 4..20mA current loop repeater.
The relay and its connections on the main board are distanced to permit connection to
either mains or 24Vdc potentials.
The RS232 conversion is done using the Maxim MAX232 chip. Physical RS232
output is provided by an RJ44SV connector on the ‘options board’ itself.
The galvanically isolated 4..20mA loop repeater uses an isolated 1W dc-dc converter
to power the ‘isolated-side’ circuits and provide the source current. Current may be
adjusted using VR1 on the options board.
General specification
Remote reset
Two terminals are provided for a remote reset switch. Under normal operation these
terminals are open circuit and require a momentary (0.5 seconds) closing to operate.
The remote reset switch also resets the TWA computation if the switch is held close
for approximately 10 seconds.
Repeat 4..20mA
See Options board
Issue 4 03/09/04
14 of 29
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