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Why Use Thermocouples?

ThermocouplesThere 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 with 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, 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. 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.

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.

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.

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.

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