A Thermocouple is a sensor used to measure temperature. Thermocouples consist of two wire legs made from different metals. The wires legs are welded together at one end, creating a junction. This junction is where the temperature is measured. When the junction experiences a change in temperature, a voltage is created. The voltage can then be interpreted using thermocouple reference tables to calculate the temperature. There are many types of thermocouples, each with its own unique characteristics in terms of temperature range, durability, vibration resistance, chemical resistance, and application compatibility. Type J, K, T, & E are “Base Metal” thermocouples, the most common types of thermocouples.Type R, S, and B thermocouples are “Noble Metal” thermocouples, which are used in high temperature applications (see thermocouple temperature ranges for details). Thermocouples are used in many industrial, scientific, and OEM applications. They can be found in nearly all industrial markets: Power Generation, Oil/Gas, Pharmaceutical, Bio Tech, Cement, Paper & Pulp, etc. Thermocouples are also used in everyday appliances like stoves, furnaces, and toasters. Thermocouples are typically selected because of their low cost, high temperature limits, wide temperature ranges, and durable nature.

1. THERMOCOUPLE

2. Thermocouples, RTDs, thermistors and infrared devices
Consider the characteristics and costs of the various sensors as well as the available
instrumentation.Thermocouples generally can measure temperatures over wide temperature
ranges, inexpensively, and are very rugged, but they are not as accurate or stable as RTDs 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 self heating. 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.

3. Thermocouple
● A sensor that measures temperature
● Consists of two different types of metals, joined together at one
end
● Works on the principle of seebeck effect
● When the junction of the two metals is heated or cooled, a
voltage is created that can be correlated back to the temperature

4. How it works
When two wires composed of dissimilar metals are joined at both
ends and one of the ends is heated, there is a continuous current
which flows in the thermoelectric circuit.
If this circuit is broken at the center, the net open circuit voltage
(the Seebeck voltage) is a function of the junction temperature and
the composition of the two metals. Which means that when the
junction of the two metals is heated or cooled a voltage is produced
that can be correlated back to the temperature.

6. How it looks like
Thermocouples are manufactured in a variety of styles, such as
thermocouple probes, thermocouple probes with connectors,
transition joint thermocouple probes, infrared thermocouples, bare
wire thermocouple or even just thermocouple wire

7. Response Time
A time constant has been defined as the time required by a sensor to
reach 63.2% of a step change in temperature under a specified set of
conditions. Five time constants are required for the sensor to
approach 100% of the step change value. An exposed junction
thermocouple offers the fastest response. Also, the smaller the
probe sheath diameter, the faster the response, but the maximum
temperature may be lower.

8. Thermocouple Types
● Thermocouples are available in different combinations of metals or calibrations.
The most common are the “Base Metal” thermocouples known as Types J, K, T,
E and N. There are also high temperature calibrations - also known as Noble
Metal thermocouples - Types R, S, C and GB
● Each calibration has a different temperature range and environment, although the
maximum temperature varies with the diameter of the wire used in the
thermocouple. Although thermocouple calibration dictates the temperature range,
the maximum range is also limited by the diameter of the thermocouple wire.
That is, a very thin thermocouple may not reach the full temperature range.

9. Accuracies and Temperature ranges
It is important to remember that both accuracy and range depend on
such things as the thermocouple alloys, the temperature being
measured, the construction of the sensor, the material of the sheath,
the media being measured, the state of the media (liquid, solid, or
gas) and the diameter of either the thermocouple wire (if it is
exposed) or the sheath diameter (if the thermocouple wire is not
exposed but is sheathed).

10. Thermocouple Reference Tables
Thermocouples produce a voltage output that can be correlated to
the temperature that the thermocouple is measuring. The documents
provide the thermoelectric voltage and corresponding temperature
for a given thermocouple type. Most of the documents also provide
the thermocouple temperature range, limits of error and
environmental considerations.

11. Why are Type K thermocouples so popular
Type K thermocouples are so popular because of their wide
temperature range and durability. The conductor materials used in
Type K thermocouples are more chemically inert than Type T
(copper) and Type J (Iron). While the output of Type K
thermocouples is slightly lower than Types T, J and E, it is higher
than its closest competitor (Type N) and has been in use longer

12. Color code
Calibration ANSI/ASTM E230 IEC 60584
Type K (+)/Red(-) (+)/White(-)
Type J (+)/Red(-) (+)/White(-)
Type T (+)/Red(-) (+)/White(-)
Type E (+)/Red(-) (+)/White(-)
Type N (+)/Red(-) (+)/White(-)
Type R (+)/Red(-) (+)/White(-)
Type S (+)/Red(-) (+)/White(-)
Type B (+)/Red(-) (+)/White(-)

13. How do I know which junction type to choose?
Sheathed thermocouple probes are available with one of three
junction types: grounded, ungrounded or exposed. At the tip of a
grounded junction probe, the thermocouple wires are physically
attached to the inside of the probe wall. This results in good heat
transfer from the outside, through the probe wall to the
thermocouple junction. In an ungrounded probe, the thermocouple
junction is detached from the probe wall. Response time is slower
than the grounded style, but the ungrounded offers electrical
isolation.

15. Thermocouple probes vs. Thermocouple wire?
Using a wire style sensor may be fine if the fluid does not attack the
insulation or conductor materials, if the fluid is at rest or nearly so,
and the temperature is within the capability of the materials. But say
that the fluid is corrosive, high temperature, under high pressure or
flowing through a pipe, then a probe style sensor, maybe even with
a thermowell, will be a better selection.

16. Vibration Damage
Choosing the right RTD or thermocouple for you application will
optimize performance and prevent sensor damage. Thermocouples
are a versatile and cost-effective means of temperature
measurement and offer the best protection against vibration. Wire-
wound RTDs offer superior accuracy and a wider measurement
range but are not as resilient. Thin film RTDs provide highly
accurate and consistent data and offer greater resistance to vibration
than wire-wound RTDs.

17. How to select a thermocouple type
● Determine the application where the thermocouple will be used
● Temperature range
● Chemical resistance of the thermocouple or sheath material
● Abrasion and vibration resistance
● Installation requirements (may need to be compatible with
existing equipment; existing holes may determine probe
diameter)

18. Thermocouple grade and Extension grade wire
Thermocouple grade wire is wire that is used to make the sensing point (or
probe part) of the thermocouple. Extension grade wire is only used to extend a
thermocouple signal from a probe back to the instrument reading the signal.
The extension grade wire typically will have a lower ambient temperature
limit in which the wire may be used. Namely, it may pass a signal representing
a higher temperature as received from the probe, but the wire physically may
not be exposed to higher temperatures. Thermocouple wire may be used as
extension wire, but extension grade wire may not be used in the sensing point
(or probe part) of the thermocouple.

19. Thank You