Thermocouples FAQ

The magnitude of the thermoelectric voltage depends on the closed (sensing) end as well as the open (measuring) end of the particular thermocouple alloy leads. Temperature sensing instruments that use thermocouples take into account the temperature of the measuring end to determine the temperature at the sensing end. Most millivoltmeters do not have this capability, nor do they have the ability to do non-linear scaling to convert a millivoltage measurement to a temperature value. It is possible to use lookup tables to correct a particular millivoltage reading and calculate the temperature being sensed. However, the correction value needs to be continuously recalculated, as it is generally not constant over time. Small changes in temperature at the measuring instrument and the sensing end will change the correction value.

You have to consider the characteristics and costs of the various sensors as well as the available instrumentation. In addition, Thermocouples generally can measure temperatures over wide temperature ranges, inexpensively, and are very rugged, but they are not as accurate or stable as RTD's and thermistors. 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. Thermistors tend to be more accurate than RTD's or thermocouples, but they have a much more limited temperature range. They are also subject to selfheating. Infrared Sensors can be used to measure temperatures higher than any of the other devices and do so without direct contact with the surfaces being measured. However, they are generally not as accurate and are sensitive to surface radiation efficiency (or more precisely, surface emissivity). Using fiber optic cables, they can measure surfaces that are not within a direct line of sight.

To select the optimal temperature measurement device for your application, you must first have an understanding of the differences between Thermocouple, RTDs and Thermistors.

The surface being measured must fill the field of view, and the surface emissivity must be taken into account.

For a specific instrument, check its specifications to see if there are any limits to the input impedance. However as a rule of thumb, limit the resistance to 100 Ohms resistance maximum, and this depends on the gage of the wire; the larger the diameter, the less resistance/foot, the longer the run can be. However, if the environment is electrically noisy, then a transmitter may be required which transmits a 4-20 mA signal that can be run longer distances and is more resistant to noise.

There are equations called “Inverse Functions” that can be used to calculate temperatures from thermocouple millivolt outputs. These equations are included in the standards ANSI/ASTM E230 and IEC 60584, and each type of thermocouple has its own set of coefficients.

These coefficients are quite long, so care must be used when entering them into a program. It should be noted that these equations are based in degrees Celsius so any conversion to Fahrenheit should only be done after the calculation is made.

Yes and no.

Some thermocouples can be hundreds of feet long and work very well, others can be relatively short and be problematic. Thermocouples produce outputs in the millivolt range. These signals can easily be affected by electro-magnetic interference from radios, high voltage devices and electric motors among others. In these cases, the thermocouple must be protected from the interference. Shielding on any non-metallic portions of the thermocouple (the extension wires) is one way of eliminating the noise. Another is to use a transmitter that converts the millivolt signal to one that is not affected by these conditions.

Each type of thermocouple has a different output value, but all are in the millivolt range per degree. For example, here are the millivolts of each thermocouple type at 250C and the mV per degree for each type at 250C:

Thermocouple Type	Output at 250C	Output Millivolts per degree
Type T	12.013 mV	0.055 mV/C
Type J	13.555 mV	0.056 mV/C
Type K	10.153 mV	0.041 mV/C
Type E	17.181 mV	0.076 mV/C
Type N	7.597 mV	0.034 mV/C
Type R	1.923 mV	0.010 mV/C
Type S	1.874 mV	0.008 mV/C
Type B	0.291 mV	0.003 mV/C
Type C	3.963 mV	0.019 mV/C

Output tables can be found on this webpage

Thermocouple sensors consist of the wires often called thermoelements, insulation, sheath, end seal and a means of connection (extension wires, connectors, etc.). The wires are connected together on one end to form the “measuring” or “hot” junction.

The sensor must be connected to a read-out device that compensates for the reference temperature. If a thermocouple readout is used then it will contain the needed circuitry. If a millivolt meter is used, then either a properly prepared ice bath or some other cold junction compensation is needed.

The output of a thermocouple depends on the type of thermocouple it is. The normal thermocouple categories include Types J, K, T, E and N which are called “Base Metal” thermocouples, Types R, S and B which are called the “Noble Metal” thermocouples, and Types C and D which are called the “Refractory Metal” thermocouples.

The output vs. temperature characteristics for each type are defined in two standards, ANSI/ASTM E230 and IEC 60584. Copies of the output tables can also be found here.

The thermocouple output or temperature tables are often referred to as “Thermocouple Curves”. Thermocouple output is not linear over temperature, this is the reason that the thermocouple equations contain a number of rather long coefficients in order to properly define them.

Yes and no. The thermocouple is one part of a circuit that is needed in order to measure temperature. The total circuit includes:

• The thermocouple sensor.
• A thermocouple instrument such as a panel meter, controller, data logger or other device that is designed to measure the thermocouple signal and convert it to a temperature
• Any extension wire or connectors necessary in order to connect the sensor to the instrument.
• Any other accessories that are needed for mounting or protecting the sensor, wiring or interconnections.

The most commonly used accessories for thermocouples are associated with installing the sensor or connecting the sensor to an instrument. For mounting, compression fittings, feedthroughs, flanges and thermowells are most common. For connecting to an instrument, connectors, extension wire, spade lugs, connection heads with terminal boards, transmitters or signal conditioners, and in the case of surface sensors sometimes epoxies or ceramic cements.