It was common practice to wire data acquisition thermocouples all the way back to the DCS using thermocouple extension wire. Over time the extension wire corrodes and becomes contaminated causing unacceptable measurement errors. When that occurs in facilities still using this wiring method, the choices are to replace in kind and perpetuate the problem or change to a different technology.
A common alternate technology is to mount multichannel temperature modules close to the thermocouples. There is a short thermocouple extension wire run between the thermocouple and the temperature module, but because it is short it is manageable over time. The output side of the temperature module uses copper wire back to the DCS and copper wire usually lasts the life of the installation. The output of our modules is typically RS-485 with a MODBUS RTU multidropped network, so we consume very little of the copper wire between the processing unit and the DCS.
The TCM Temperature Control Module is a 16-channel temperature multiplexer that accepts any sensors. The TCM is suitable for installation in Div 2/Zone 2 and IS hazardous locations and can tolerate ambient temperatures from -40 to +85°C. The TCM is a loop-powered, multipoint, HART temperature transmitter.
If MODBUS RTU is your preferred communication protocol, the HMC HART to MODBUS Converter converts the TCM HART output to MODBUS RTU. Each HMC can read two TCMs or 32 temperatures in ~2 seconds and deliver those readings to your DCS using RS-485.
This high density temperature multiplexer solution eliminates the constant degradation of thermocouple measurement accuracy caused by the thermocouple extension wire. Copper wire and an all digital measurement and delivery system assures the measurement is as accurate as the sensor.
- TCM is a 16-point HART temperature transmitter
- TCM is Div 2/Zone 2, IS certified, and can tolerate ambient temperatures from -40 to + 85°C
- HART to MODBUS conversion with the HMC