What Is A Temperature Transmitter?

Temperature transmitters receive a standard temperature signal from a sensor or thermocouple. These signals are typically based on resistance or voltage. The temperature transmitter converts this to a standard industrial signal such as 4-20mA or 0-10V. 

The transmitter can help to amplify the signal for longer transmission distances. It can also help to filter noise and scale/adjust the signal to give an accurate reading.

One of the other benefits of converting the sensor output is that standard analogue signals such as 4-20mA or 0-10V are compatible with a greater range of industrial equipment. This includes PLCs, SCADA and Data Acquisition Systems.

Where Are Temperature Transmitters Used?

Temperature transmitters are used where a standard signal direct from a sensor can degrade. For example, long cable runs in industrial environments prone to electrical noise.

They are also used where the signal type requires conversion to a different format.

Head mounted transmitters are often fitted to bulk tanks containing liquids or inline pipework. They are generally located in the field. Rail mounted transmitters are generally found inside a main or remote control panel containing other equipment.

How To Install A Temperature Transmitter

Mechanical installation depends on the product type. Generally there are two variations, head mount or rail mount. Selection depends on the process requirement.

Head mounted transmitters fit inside a terminal head which acts as junction box. This contains the wiring from the probe itself, which normally slides into a thermowell. A head mounted transmitter forms part of the overall assembly as per the below image. The terminal cover can be unscrewed to access the transmitter wiring connections. The thermowell and probe are situated behind the transmitter.

 Figure 1: Novus TxBlock-USB

There are many advantages to this arrangement. Particularly from a maintenance perspective where ease of access can save time and money when parts require attention.

Novus TxBlock-USB is an excellent example of a temperature transmitter designed to be fitted into a terminal head. It's compact design ensures maximum compatibility. The built in USB Micro port allows fast commissioning with Smart Phones or Laptop/PC.

Rail mounted temperature transmitters are designed to be located inside control panels and electrical enclosures. This allows conversion where perhaps the cable lengths involved are shorter or existing wiring is in place. In this instance a product such as TxRail-USB is a good solution. This is an ultra slim DIN rail mounted device. On account of its width many devices can be located within a small physical footprint. They are often used to convert an entire PLC card of temperature signals to standard analogue signals. Reducing the dependency on specialised PLC input cards capable of directly reading the sensor.

Figure 2: Novus TxRail-USB

Electrical connections on both types of transmitters are normally limited to:

1. Sensor Signal Input - direct wiring from the sensor itself

2. Analogue Signal Output - typically 2 cable cores 4-20mA or 0-10V, often wired as a current loop

3. Power Supply In - depending on product, some transmitters have dedicated terminals requiring a separate power supply

For loop powered transmitters such as the TxBlock-USB typical wiring is shown below. Note the 24VDC supply forms part of the current loop. The indicated load on the diagram is the device reading the converted temperature signal. It could be a PLC Input, Digital Indicator, Datalogger etc.

The important point to emphasise is that only 2 cable cores are required between the transmitter and the measurement device receiving the signal. The 24VDC can be sourced from the same location as the measurement device. It can often be derived from an existing PSU within the same electrical cabinet.

Isolated Vs. Non-Isolated Temperature Transmitter

An isolated transmitter contains an electrical barrier which can help eliminate interference and protect against voltage spikes.

An example of an isolated head mount temperature transmitter is the TxIsoPack.

An example of an isolated DIN rail mount temperature transmitter is the TxIsoRail.