Temperature Transmitters

In many cases, the temperature of a remote process must be monitored. Common temperature sensing devices such as thermocouples and RTD’s produce very small “signals.” These sensors can be connected to a two-wire transmitter that will amplify and condition the small signal. Once conditioned to a usable level, this signal can be transmitted through ordinary copper wire and used to drive other equipment such as meters, dataloggers, chart recorders, computers or controllers.

A temperature transmitters converts the thermocouple or RTD signal to a 4-20 mA output signal and is the ideal solution for many remote temperature measurement applications. 4-20ma transmitters have definite advantages over conventional temperature measuring devices, but must be selected with caution in order to avoid “ground loop” problems.

A temperature transmitter draws current from a remote dc power supply in proportion to its sensor input. The actual signal is transmitted as a change in the power supply current.
Specifically, a thermocouple input transmitter will draw 4 mA of current from a dc power supply when measuring the lowest temperature of the process. Then, as the temperature rises, the thermocouple transmitter will draw proportionally more current, until it reaches 20 mA. This 20 mA signal corresponds to the thermocouple’s highest sensed temperature. The transmitter’s internal signal-conditioning circuitry (powered by a portion of the 4-20 mA current) determines the temperature range that the output current signal will represent.
Physically, only two copper wires are necessary to connect the temperature transmitter output signal in a series circuit with the remote power supply and the process equipment. This is made possible since the signal and the power supply line are combined (one circuit serves a dual function).

Transmitters offer numerous advantages over the more traditional ways of measuring temperature.

1. ac power is not needed at the remote location to operate a twowire transmitter.

Since transmitters are powered by a low level 4-20 mA output current signal, no additional power has to be supplied at the remote location. In addition, the usual 24 Vdc signal necessary for operation is standard in plants that have large amounts of instrumentation.

2. Electrical noise and signal degradation are not a problem for two-wire transmitter users.

The transmitter's current output signal lends itself to a high immunity when it comes to ambient electrical noise. Any noise that does appear in the output current is usually eliminated by the common-mode rejection of the receiving device. In addition, the current output signal will not change (diminish) with distance as most voltage signals do.

3. Wire costs drop significantly when using transmitters.

Low voltage signals produced by thermocouples almost always require the use of shielded cable when they are sent any significant distance. Ambient electrical noise from arcing electrical relays, motors and ac power lines can raise havoc with these signals that are transmitted in an unshielded cable. In addition, expensive, heavy gage wire is often installed in applications that call for long cable runs (since it reduces errors from signal voltage drops caused by line resistance).

Ordinary copper wire can be used to connect all the pertinent equipment in a two-wire transmitter system. The 4-20 mA current output signal is relatively immune to ambient electrical noise and is not degraded by long distance transmission, even over a small diameter wire. Adding a temperature transmitter to a system eliminates the problem of having to provide long runs of costly wire and an extensive amount of shielding.

The head mount transmitter is a universal transmitter and has a HART communication protocol. We can program it remotely for the parameters like- temperature range, and type of sensor. Also, calibration of the temperature transmitter is possible from the remote station.

The salient features of the head mount temperature transmitter are as follows:-

  • It accepts all the temperature inputs from Pt-100 input and Types J/K/T/E/R/S/B/N Thermocouples.
  • We can configure the transmitter with PC software remotely.
  • Universal setting with HART protocol for various input signals pt-100, TC, etc.
  • The transmitter has two types of isolation - galvanic and optical isolation. As a result, the signal-to-noise ratio is more. It does not affect noise signals. And, the loop grounding problem is not there because of isolation.
  • It has non-volatile memory which can store the data permanently. Thus data logging is one of the best features of the head mount transmitter.
  • In case of an increase in temperature the transmitter measurement error increase.

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