Rtd, Thermocouples

Thermocouples are the most commonly used temperature sensors. Thermocouples are self-powered, (require no excitation current), can operate over a wide and high temperature range, and have fast response times.

Two dissimilar metal wires are joined together forming an electrical junction. The temperature difference of the two dissimilar conductors produces a temperature dependent voltage as a result of the thermo electric effect, (Seebeck effect). The voltage is measured and used to calculate the temperature.

There are different hot junction types: insulated hot junction (SO), grounded hot junction (SP), one hot Junction for 2 TC insulated from the sheath (SOA), hot junctions insulated from each other and from the sheath (SOB).

Thermocouples have a built in electronic cold junction compensation to correct the missing thermoelectric voltage due to the fact that the thermocouple cold end at the instrument is not at 0°C.

There are several types of thermocouples made from different materials for different temperature ranges and different sensitivities. The most commonly used is the K type. Here a list of the common types of thermocouples:

Industrial Temperature Measuring Devices describe probes with

• one (or more) temperature sensor elements (usually RTD or Thermocouple)
• a connection head (head sensors) or plugs, surface magnets/clips (cable sensors for machinery or surface measurement)
• a protection tube or Thermowell

Sensor elements (inserts) are available as replaceable or non-replaceable types. They include the sensing wires, insulation materials and a shaft. Mineral insulated inserts are bendable and protect against vibrations, ceramic types are used for high temperatures.

Head sensors include the sensing elements which is connected to connection heads or head mounting transmitters (A). They can be fitted to the pipe or to extension tubes (Thermowells) by flange, threaded or with a clamp on (for hygienic aplications).

Cable sensors are used for

• movable or replaceable parts of machines and devices, (compressor, turbine,…)
• outer surface measurement of devices and machines, especially their bodies, blocks

The inserts are connected to a head (A), exposed wire ends (B), with sleeves (C), plugs (D) and/or compensation cables (E, F).

An RTD is a temperature sensor to measure temperature using the principle that the resistance of a metal changes with increasing or decreasing temperature. They required an electrical current (exitation current) which is transmitted through a piece of metal, called the RTD element or resistor. RTDs have a near linear response to temperature changes, they are stable and accurate and give repeatable responses.

Most RTD elements consist of a length of fine coiled wire (most commonly Platinum, Nickel or Copper)
wrapped around a ceramic or glass core. The element is usually quite fragile, so it is often placed inside a sheathed probe to protect it.

RTDs are available in 2, 3 and 4 wire configuration. (2 wire for short lead length,  3 wire (most commonly used) adds a line for the exication current to cancel the wire resistance effect, 4 wire config for highest accuracy)

The PT100 probe (el. resistance of 100.00 Ohm at 0°C) is most commonly used in industrial applications. PT500 (500 Ohm at 0°C) and PT1000 (1000 Ohm at 0°C) probes are available.