Learn more about Thermocouple Sensors
THE SEEBECK EFFECT In 1821 Thomas Seebeck discovered the continuous current flow in the thermoelectric circuit when two wires of dissimilar metals are joined at both ends and one of the ends is heated.
How does a thermocouple work?
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.
What is the response time of a thermocouple?
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. Be aware, however, that sometimes the
probe sheath cannot withstand the full temperature range of the
thermocouple type. Learn more about thermocouple response times.
Discussing the various Thermocouple Types
Various Temperature Ranges for various Temperature Measurements
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.
| Common Thermocouple Temperature Ranges | |||
|---|---|---|---|
| Calibration | Temperature | Standard | Special |
| J | 0° to 750°C (32° to 1382°F) | Greater of 2.2°C or 0.75% | Greater of 1.1°C or 0.4% |
| K | -200° to 1250°C (-328° to 2282°F) | Greater of 2.2°C or 0.75% | Greater of 1.1°C or 0.4% |
| E | -200° to 900°C (-328° to 1652°F) | Greater of 1.7°C or 0.5% | Greater of 1.0°C or 0.4% |
| T | -250° to 350°C (-418° to 662°F) | Greater of 1.0°C or 0.75% | Greater of 0.5°C or 0.4% |
- Determine the application where the thermocouple will be used
- Analyze the temperature ranges the thermocouple will be exposed to
- Consider any chemical resistance needed for the thermocouple or sheath material
- Evaluate the need of abrasion and vibration resistance
- List any installation requirements
Choosing a Thermocouple
Because a
thermocouple measures in wide temperature ranges and can be
relatively rugged, thermocouples are very often used in industry.
The following
- 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)
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.
Choosing the right type of style of Temperature Indicator
Various Temperature Ranges for various
Temperature Measurements
A beaded wire thermocouple is
the simplest form of thermocouple. It consists of two pieces of
thermocouple wire joined together with a welded bead. Because the
bead of the thermocouple is exposed, there are several
application limitations. The beaded wire thermocouple should not
be used with liquids that could corrode or oxidize the
thermocouple alloy. Metal surfaces can also be problematic. Often
metal surfaces, especially pipes are used to ground electrical
systems The indirect connection to an electrical system could
impact the thermocouple measurement. In general, beaded wire
thermocouples are a good choice for the measurement of gas
temperature. Since they can be made very small, they also provide
very fast response time.
Thermocouple Probe
A thermocouple probe
consists of thermocouple wire housed inside a metallic tube. The
wall of the tube is referred to as the sheath of the probe.
Common sheath materials include stainless steel and Inconel®.
Inconel supports higher temperature ranges than stainless steel,
however, stainless steel is often preferred because of its broad
chemical compatibility. For very high temperatures, other exotic
sheath materials are also available. View our line of high
temperature exotic thermocouple probes.
The tip of the thermocouple probe is available in three different styles. Grounded, ungrounded and exposed. With a grounded tip the thermocouple is in contact with the sheath wall. A grounded junction provides a fast response time but it is most susceptible to electrical ground loops. In ungrounded junctions, the thermocouple is separated from the sheath wall by a layer of insulation. The tip of the thermocouple protrudes outside the sheath wall with an exposed junction. Exposed junction thermocouples are best suited for air measurement.
Surface Probe
Measuring the temperature
of a solid surface is difficult for most types of temperature
sensors. In order to assure an accurate measurement, the entire
measurement area of the sensor must be in contact with the
surface. This is difficult when working with a rigid sensor and a
rigid surface. Since thermocouples are made of pliable metals,
the junction can be formed flat and thin to provide maximum
contact with a rigid solid surface. These thermocouples are an
excellent choice for surface measurement. The thermocouple can
even be built in a mechanism which rotates, making it suitable
for measuring the temperature of a moving surface. Type K is
ChrOMEGA™/AlOMEGA™.
Wireless Thermocouples
Bluetooth®
wireless transmitters that connect with smartphones or tablets to
log and monitor temperature measurements. These transmitters
measure different sensor inputs, including but not limited to
temperature, pH, RTD, relative humidity. The data transmission is
performed via Bluetooth wireless technology to a smart phone or
tablet with the app installed. The app will allow the smartphone
to pair and set up multiple transmitters.
Frequently Asked Questions
What are the accuracies and temperature ranges of
the various thermocouples?
You can find out more about
thermocouple accuracy and temperature ranges on this thermocouple
color code table. 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).
Should I use a grounded or ungrounded probe?
It depends on the instrumentation. If there is any chance that
there may be a reference to ground (common in controllers with
nonisolated inputs), then an ungrounded probe is required. If the
instrument is a handheld meter, then a grounded probe can almost
always be used.
Can I use any multimeter for measuring temperature
with thermocouples?
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.
How to choose between thermocouples, resistance
temperature detectors (RTD's), thermistors and infrared devices?
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.
Thermocouple Reference Tables
Thermocouples produce a voltage output that can be correlated to
the temperature that the thermocouple is measuring. The documents
in the table below 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.
Termopar
Thermocouples - French
Termopares
Termopares
Thermocouples
Termokobler
Thermocouples
Thermoelementen
Termocoppie
Thermocouples
Termopares
Thermocouples
Thermocouples
热电偶
Thermocouples
熱電対
열전대
Thermocouples
Thermocouples
Thermocouples
Thermocouples
Thermocouples