1.10
1.6.1.2 Heaters and Temperature Sensor
The transducer unit is temperature controlled.
Heaters of type of construction 'N' (non-sparking) are fitted to the 1131 Transducer (Zone 2/Div
2) and of type 'd' (flameproof) construction to the 1132 and 1133 Transducer (Zone 1/Div 1).
A platinum resistance thermometer senses the internal temperature of the transducer and
transmits a signal to the amplifier board. Power to the heaters is switched through a solid state
relay located in the Control unit.
1.6.1.3 Pressure Compensation 01100997B
See Section 1.7.3 for a description of pressure compensation.
1.6.2 Control Unit
The Control unit has seven basic functional blocks:
1. Amplifier Board
2. Analogue to Digital Converter
3. Microprocessor Board
4. Isolated Current Output Board
5. Keypad
6. Display Board
7. Power Supply Unit
1.6.2.1 Amplifier Board 01100925B
The amplifier board accepts inputs from the platinum resistance thermometer, photo-cells and,
if fitted, the pressure transducer. The signal from the photo-cells is used to generate the
feedback current to the measuring cell and from this a voltage signal which is proportional to
oxygen content. The platinum resistance thermometer and the pressure transducer each form
one arm of a measuring bridge the outputs of which are amplified. The three analogue signals
are fed to the analogue to digital converter and hence to the microprocessor.
1.6.2.2 Analogue to Digital Converter 01100916A
The analogue to digital converter converts the three analogue signals from the amplifier board,
oxygen content, temperature and pressure, into a digital form acceptable for the
microprocessor board.
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