Thermocouples can measure temperatures over wide ranges, inexpensively. They are very rugged, but they are not as accurate as RTDs and thermistors.

A thermocouple is basically two strips of wires of dissimilar metals. These metal wires are joined at one end and the voltage is measured at the other end. Changes in the temperature at the juncture induce a change in electromotive force (emf) at the other end. As the temperature goes up, this output emf of the thermocouple rises, though not necessarily linearly.

Here is a summary of some of the advantages and disadvantages of thermocouples:

Thermocouple Types

There are many different types of thermocouples. They are made of different types of wire and have very different properties, making one type better for a specific application than another. Below are descriptions of the types of thermocouples that United Electric Controls Co. makes.

Type J

The Iron - Constantan "J" curve thermocouple is the most widely used thermocouple; it is versatile and has a relatively low cost. It has a positive Iron wire and a negative Constantan wire and is recommended for reducing atmospheres. The operating range for this alloy combination is from 0º to 750ºC for the largest wire sizes. Smaller size wire should operate in correspondingly lower temperatures.

Type T

The Copper - Constantan "T" curve thermocouple, with a positive Copper wire and a negative Constantan wire, is recommended for use in mildly oxidizing and reducing atmospheres at temperatures from 0º to 350º C. They are suitable for applications where moisture is present. This alloy is recommended for low temperature work since the homogeneity of the component wires can be maintained better than with other base metal wires. Therefore, errors due to inhomogeneity of wires in zones of temperature gradients are greatly reduced.

Type K

The Chromel≥ - Alumel≥ "K" curve thermocouple is often used at high temperatures. It has a positive Chromel≥ wire and a negative Alumel≥ wire and is recommended for use in clean oxidizing atmospheres. The operating range for this alloy is from 0º to 1250º C for the largest wire sizes. Smaller size wire should operate in correspondingly lower temperatures.

Type E

The Chromel ≥ - Constantan "E" curve thermocouple has the highest emf of all standard thermocouples. It has a positive Chromel ≥ wire and a negative Constantan wire and is recommended for use in oxidizing, inert or dry reducing atmospheres or for short periods of time in a vacuum. These elements must be protected from sulfurous and marginally oxidizing atmospheres. Type E thermocouples can be used for temperatures from 0º to 900º C.

Thermocouple Insulation Types

United Electric Thermocouples are available in three versions:

GP - General Purpose thermocouples that are comprised of a pair of thermocouple wires inside a tube. These are used to measure temperatures of 260º C or less.

MI - For higher temperature applications, a Mineral Insulation can be added in the tube. For UE MI thermocouples compressed magnesium oxide is added inside the tube. These are used to measure temperatures of 260º C and higher.

BTC - Beaded thermocouples are mainly used in furnace applications.

RTD's are stable and have a fairly wide temperature range, but are not as rugged and inexpensive as thermocouples. Since they require the use of electric current to make measurements, RTD's are subject to inaccuracies from self-heating.

An RTD capitalizes on the fact that the electrical resistance of a material changes as its temperature changes. RTD's rely on the resistance change in a metal. The resistance will rise more or less linearly with temperature.

Traditionally, RTD's use a length of conductor (platinum, nickel iron or copper) wound around an insulator. Newer styles use a thin film of the conductor deposited on a ceramic substrate.

RTD's are used to measure temperatures from -196º to 482º C

Here is a summary of some of the advantages and disadvantages of RTDs:

Thermistors tend to be more accurate than RTD's and thermocouples, but they have a much more limited temperature range because of their marked non-linearity.

A Thermistor capitalizes on the fact that the electrical resistance of a material changes as its temperature changes. Thermistors rely on the resistance change in a ceramic semiconductor, with the resistance dropping non-linearly with a temperature rise.

Thermistors can be a low cost solution to temperature measurement. They tend to have large signal outputs and their small size permits fast response to temperature changes. UE Thermistors are used to measure temperatures from -45º to 260º C

Here is a summary of some of the advantages and disadvantages of thermistors:

The newest type of temperature sensor on the market is the integrated circuit (IC) temperature transducer. IC sensors can be designed to produce either voltage or current output and are extremely linear.

IC sensors are a very effective way to produce an analog voltage proportional to temperature. They have a limited temperature range and are used to measure temperatures from -45º to 150º C

Here is a summary of some of the advantages and disadvantages of IC Sensors: