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Electrical Resistance Thermometer

Electrical Resistance Thermometer

weather familyThe electrical resistance thermometer used by the Observatory gives a resistance of 100.00 ohms at zero  degree Celsius and 115.54 ohms at 40 degree Celsius.  Throughout this range, the resistance experienced can be considered as a linear function of temperature.   In operation, there are 4 wires connected between the platinum resistance sensor at the remote site and the microprocessor-controlled unit.   Two wires are used to determine the resistance of the sensor.   The other two are used to compensate for the wire resistance.   One platinum resistance sensor is normally installed in the thermometer shed or Stevenson screen for routine measurement of ambient air temperature.

The microprocessor-controlled unit consists of a resistance to analogue interface, an analogue to digital converter, a microprocessor and an LED digital display.

The resistance to analogue interface utilizes a bridge circuit to convert the platinum resistance to an analogue signal of 0 to 10 volts when the temperature ranges between -10 degree Celsius and +40 degree Celsius.  The analogue to digital converter digitized the analogue signal to binary data.  The microprocessor is utilized as a system controller such that a reading is taken every second for display on LEDs.   The maximum and minimum temperatures registered are also recorded and displayed.

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