The Difference between Grounded or Ungrounded Thermocouples

Temperature control plays a crucial role in the world’s industrial process plants. These include the fabrication of steel, manufacturing food, petrol chemical production, power generation and many more industries. These developed process plants will produce the commodities that the majority of the population of the world will use and consume.

The thermocouple has become the most common device utilized, in the measurement and control of process plant temperature. But these devices do play an important role within the domestic heating systems in homes and offices.

Industrial Plant where Temperature
is crucial

Thermocouple Theory

The theory behind the thermocouple discovered by an Estonian, Thomas Seebeck in 1822 is that by applying a temperature gradient to a circuit of two dissimilar metals joined at one end, a thermoelectric effect takes place and an electrical potential at the open end of the circuit occurs.

In the circuit, the end joined together, referenced to as the hot junction and the other end, the open end referenced to as the cold junction. By connecting the cold end or cold junction of the circuit to an appropriate measuring device, then applying a temperature gradient the current can be calibrated and utilized in a temperature reading in a controlled environment.

Thermocouple Grounding Philosophy

In the grounded thermocouple the hot end is physically connected or bonded, to the sheath, well or pocket of the installation, but in the ungrounded thermocouple, the hot end is isolated from the sheath, well or pocket, normally by a mineral compound, there are advantages and disadvantages to both types of installation

Temperature Control is critical to
some processes

Grounded vs. Ungrounded

The grounded thermocouple loop has a faster response time to temperature change, but the disadvantages lay in that the hot end, grounded to the sheath, will exposes the circuit to transient earth loops and this causes noise on the circuit.

The ungrounded thermocouple response time to temperature change, has a slower response time than the grounded thermocouple. In this circuit the hot end will not have been directly coupled to the ground, so therefore regarded as isolated from the ground, by mineral insulation. The advantage being, that the loop will no longer have the same transient noise on the circuit, as the grounded thermocouple.

The Different Types of Thermocouples

Thermocouples consist of five main types, made of different types of metal, which depend on, what temperature ranges the thermocouples will measure, the types consist of Type T Copper Constantan for ranges 0 – 400 (*C), Type J Iron Constantan for ranges 0 – 850 (*C), Type E Chromel Constantan for ranges 0 – 700 (*C), Type K Chromel Alumel for ranges 0 - 1100 (*C) and Type N Nicrosil Nisil for ranges 0 - 1300 (*C).

For higher temperatures, Type B, R and S, made up of Platinum Rhodium are used, but these materials are a lot more expensive.

Amongst the group of temperature sensing devices, within the process industry the thermocouple rates very highly.

Heavy Industry is reliant on
Temperature Control

Resources

*  Thermocouple General Specification
   http://weedinstrument.com/product_groups/industrial/PDFs/TCGenspec.pdf

*  Thermocouple Manufacturer Technical Information
    http://www.peaksensors.co.uk/thermocoupledatasheets.html

References

*  National Plastics: How thermocouples work

    http://www.nphheaters.com/technical/thermo_letter.htm

*  Sensorland thermocouple

    http://www.nphheaters.com/technical/thermo_letter.htm

*  Omega introduction to the thermocouple

    http://www.omega.com/techref/themointro.html