231P Lake Shore Cryotronics Temperature Transmitter

The Lake Shore Cryotronics 230 Series Temperature Transmitters/Monitors feature sensor input fully isolated from power supply potential, support for various sensor types, 4-lead differential measurement, and output ranges from 4 to 20 mA or from 0 to 20 mA.

Once one of the 230 Series obtains temperature data, it transmits a current of 4 mA to 20 mA. The current output changes linearly with sensor temperature. Output scale depends on the selected temperature range. Several switch-selected ranges have been manufactured. Highest accuracy and sensitivity are achieved when the output is set for a narrow temperature band. A 0 mA to 20 mA output is also available to convert output to a voltage scaled from zero. A 500 Ω, ±0.02% output load resistor produces the maximum full-scale output of 10 V.

Circuitry for the Model 230 Series is powered by a single +5 VDC supply applied either from the front panel connector or the power pins on the VME bus connector. The outputs are isolated so several transmitters can be run off the same supply without interference.

The Model 231 operates with either silicon diode or gallium-aluminum-arsenide (GaAlAs) diode sensors.

Mechanical mounting is easy because the 230 Series is built on a standard size VME card. It fits directly into a single height (3U) VME card holder. The transmitter does not use the electrical bus format, only its physical shape and power supply.

Excited with a 10 µA current source from the Model 231, the sensors produce a voltage that depends on temperature. A microcontroller reads the voltage through an A/D converter and translates it into temperature using a temperature response curve. The Model 231 includes two standard curves for DT-470 and DT-670 diode sensors. It also supports a single CalCurve option for calibrated sensors (TG-120 diodes require a CalCurve).

The Model 231P uses a PT-100 Series platinum sensor. The Model 231P excites the sensor with a 500 µA current to produce a measurable signal. Either the standard platinum curve (IEC 751) or a CalCurve is used for temperature conversion.

The Model 234 operates with Cernox, carbon-glass, germanium, or other negative temperature coefficient (NTC) resistance temperature sensors. The Model 234 excites the sensor with a constant voltage of 10 mV or less to minimize the effects of sensor self-heating at low temperatures. The Model 234 includes a RJ11 serial interface.

The Model 234 employs an analog control circuit to maintain a constant voltage signal across the sensor. A series of reference resistors convert the resulting sensor current to a voltage. A microcontroller reads the voltage with an A/D converter, calculates sensor resistance, and converts the resistance to temperature by table interpolation (requires a CalCurve for temperature conversion). The sensor excitation voltage is reversed each reading to compensate for thermal voltages and offsets.