Why Use Thermocouples?
Still deciding on whether a thermocouple is the right temperature sensor for you or your organization? Below we've outlined what a thermocouple is and what technical advantages you can expect with specific sensor types. Specifically we've compared thermocouple sensors and thermistor sensors, so you can better understand why you might use thermocouples.
What is a Thermocouple?
There are several types of electronic temperature sensors. Each has
its technical advantages and disadvantages depending on the intended purpose or application of the sensor. A
common choice for commercial and professional instruments is the thermocouple. A thermocouple is made
of two wires of different alloys. They are welded together to form the “thermocouple”. This assembly of wires
produces a voltage that changes with temperature. That voltage can be measured, processed and displayed as a
temperature. Different alloys perform differently. Over decades, industry has settled on several specific
thermocouple “types” that each use a specific combination of specific alloys. The most common is the “type K”
thermocouple (See Thermoworks Type K Thermocouples),
which is made from the two metals, Chromel and Alumel. It is the most frequently used type in general industrial
use, science, food processing and food service.
In practice, and with the right probe and electronics technology, a thermocouple offers several advantages over other common sensors, such as the thermistor. Thermistors are found in many low-cost digital thermometers. They can be cheaply produced and the electronics needed to convert their signals to temperatures can be very inexpensively made. However, the low-cost variety of thermistors have some limitations that a thermocouple can overcome.
Temperature Range
Depending on the design and materials of a temperature probe that uses the Type K thermocouple, the temperature
range can be very wide. Some probes can measure as high as 2,200°F (See High-Temperature Type K
Thermocouples). Even with wire insulations that are intended for lower temperatures, such as PTFE, probe
ranges are often able to cover -58°F to 572°F. Many thermistor type sensors and thermometers have a narrower
range. Although some thermistors are now available with an upper limit of 572°F, their accuracy degrades
considerably over about 300°F. Also, a thermistor is more quickly, and irreversibly, damaged at temperatures
above its stated limit.
Speed
A fast response is important in many applications. The speed of a temperature sensor is directly affected by its
mass or size. The larger the sensor, or probe assembly, the slower the reading. A thermistor is made of a bead
of carbon with two wires attached. It is then coated with epoxy or glass. Though most thermistors are somewhat
large (fitting an 1/8” or larger probe tube) advanced technology has lead to thermistors that can be quite
small. However, a thermocouple can still be made smaller than even the tiniest thermistors. Afterall, a
thermocouple is made of two wires whereas a thermistor adds the bit of carbon and a coating. Using fine gauge
thermocouple wire, the sensing bead or weld can also fit inside very narrow diameter tubes, such as hypodermic
needles. Even in a slightly larger tube, say 1/16” diameter (that might fit an advanced thermistor) one can
locate a thermocouple bead further up into the pointed end of the probe than you could a thermistor. This
contributes to a thermocouple’s ability to equilibrate to your target temperature somewhat faster than a
thermistor.
Probe Design
Because a thermocouple is made from the junction of two different metals, the possibilities for various
mechanical designs are greater than for other sensors. Very fine gauge wire can be used for tiny sensor
assemblies. Flat wire is often used for surface thermocouples. Heavy gauge wire can be used for very
high-temperature probes or for extremely rugged assemblies. Thermocouple beads are also suitable for fast
response air or gas sensors. Other sensor types are generally more limited in design constraints.
Accuracy
In industry and science, thermocouples have not always been associated with high accuracy performance. Many
thermocouple manufacturers use wire and techniques that produce only accuracy levels that are similar to that of
low-cost thermistor products. However, ThermoWorks uses special grade thermocouple wire that renders an
interchangeable probe accuracy better than ±0.9°F between 32°F and 212°F. This is better than most thermistors
over the same range. Thermocouple accuracy can be further improved if the thermocouple probe assembly is
permanently attached to the electronic circuit and then calibrated together with the electronics in a “system
calibration”. This process removes the interchangeability error of the individual thermocouple and contributes
to a total measurement accuracy of just a few tenths of a degree. Advanced circuit designs make it possible for
ThermoWorks to offer accuracies better than ±0.5°F in some thermocouple products.
Cost
In a comparison of typical industrial temperature probe prices, thermocouples are generally
considered less expensive than some types of scientific or commercial sensors and probes. When compared to
mass-produced low-cost consumer product sensors, a thermocouple may be somewhat more expensive than some
thermistors. This is normally due to higher costs associated with more durable industrial grade probe materials.
Not all thermocouples are created equal. Some probe manufacturers substitute low-grade thermocouple wire in
their assemblies to quote lower prices. The result is lower accuracy and faster wire degradation even under
normal use.
Other Considerations
Thermocouples are sometimes avoided when total thermometer cost is very limited. The electronics required to
read a thermocouple are more sophisticated and more costly than required for a thermistor. Some manufacturers
cut corners here and deliver electronics that do not solve the accuracy challenges present with thermocouples.
Many thermocouple meters made in the Far East deliver measurements that are only accurate to a few degrees
instead of a few tenths. Specification claims should be read carefully.
Styles of Type K Thermocouples
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All ThermoWorks Temperature Probes
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