Temperature Sensors - Thermocouples
(124)One group of commonly used sensors are temperature sensors. It is very often measured in industrial machines or during various processes that must take place at a specific temperature, hence temperature sensors are used in many industries, laboratories, control and measurement equipment, and R&D departments. Its measurement is useful or even necessary during all kinds of electronic works performed by DIY enthusiasts, e.g., when charging battery cells, soldering/ desoldering components with hot air on PCBs, when building systems where temperature is one of the controlled values. Also in 3D printers, which are becoming more and more common, a temperature sensor is essential to control the degree of heating of the print head or heat bed to ensure proper print quality.
Temperature sensorsare classified into subgroups. This is due to differences in structure and mode of operation. The most popular ones are NTC thermistors, PTC thermistors, resistance temperature sensors, and thermocouples.
Temperature sensors - thermocouple construction
Thermocouples are simple in design, reliable and durable, and are therefore one of the most commonly used temperature sensors. The systems that use them work by measuring the voltage (potential difference) generated in a closed eclectic circuit by a thermocouple consisting of two thermocouple wires. These wires are made of various conductors or semiconductors, selected in such a way that the voltage produced at their junction, the so-called thermoelectric voltage, increases proportionally with the measured temperature. This is the so-called Seebeck effect, whereby the so-called hot and cold junctions must have different temperatures. In practice, the reference temperature is measured with an independent sensor in the measuring system using an isothermal reference block.
Types and measuring ranges of thermocouples
Thermocouples, depending on the thermocouple materials, are divided into the following types: J, T, K, E, N, S, R, and B. These are standardized designations that indicate the type of thermocouple materials. Characteristics of particular thermocouple types, determining values of thermoelectric force for each of them, are described in PN-EN60584-1 standard: 2014-04. The most commonly used are type J (Fe-CuNi), K (NiCr-NiAl), E (NiCr-CuNi), T (Cu-CiNi), and N (NiCrSi-NiSi)thermocouples made of iron (Fe), copper (Cu), nickel (Ni), chromium (Cr), silicon (Si) or aluminium (Al), i.e., without the use of noble metals. The temperature range that can be measured with these sensors depends on the specific type of thermocouple. For T-type thermocouples, the maximum measurement temperature is only about 350° C, but for K and N-type thermocouples it is up to 1200°C. The lower end of the temperature range for the listed, most commonly used thermocouple is around -50°C, although some of them may be capable of measuring lower temperatures, as low as -200°C.
High-temperature thermocouples
The so-called high temperature thermocouples, made of precious metals, namely platinum (Pt) and rhodium (Rh), are used for measuring higher temperatures. These are type S (Pt10Rh-Pt), R (Pt13Rh-Pt), and B (Pt30Rh-Pt6Rh) thermocouples. They can operate and measure temperatures up to about 1600°C, with B-type thermocouples even up to 1800°C. Due to the use of noble metals, they are more expensive than the previously mentioned thermocouples type J, T, K, E, and N.
Using temperature sensors
A measuring system that uses a sensor in the form of a thermocouple to measure temperature normally consists of: a thermocouple, wires that connect the sensor to the measuring device, and the measuring device itself, e.g., in the form of a millivoltmeter or an advanced transmitter. Note that thermocouples are non-linear and their characteristics vary depending on the type. For this reason, digital linearisation, carried out by a microprocessor system, is most often used in measuring systems, as analogue linearisation is not cost-effective. Due to the very low voltage obtained from such a sensor, of the order of several microvolts per degree Celsius, signal amplifiers and various types of filters are used to make the measurement as accurate as possible and in accordance with the actual measured temperature
Choosing a temperature sensor
When deciding to purchase a thermocouple to measure the temperature in a particular device or process, there are several factors to consider. The first is of course the temperature range to be measured. It is extremely important that the range of the measured temperature value is within the measuring range of the selected sensor type. Otherwise, an attempt to measure with an improperly selected sensor can result in false or incomplete information about the processes taking place in the environment under test, or even lead to permanent damage to the sensor. Choosing a sensor with the right temperature range is also important for financial reasons. More expensive sensors, manufactured with precious metals, may have a wider measuring range and measure much higher temperatures, but these may never occur in the process whose temperature is being measured. In this case, buying a more expensive sensor is not justified and may prove to be an unnecessary additional expense.
Another aspect to consider is the length of the cable. It is worth noting here that, unlike other sensors, including some temperature sensors, thermocouples always have two wires. The sufficient wire length is required. If the wire is too short, it may not be possible to connect the sensor to the measuring device and it may not be possible to extend the cable. On the other hand, a wire that is too long can lead to greater signal interference, which can be caused by all sorts of ubiquitous electromagnetic interference, e.g., from the standard electrical network with an alternating current of 50Hz frequency and 230V voltage. You can also find thermocouples with connection head, i.e., without a cable provided by the manufacturer.
The sensor tips are protected by special heat-resistant Teflon or fibreglass insulation, thanks to which they are protected against damage caused by too high temperature and short circuits. Fiberglass is more resistant to high temperatures and only softens at around 740°C, while teflon softening point is around 260°C. For this reason, you should also predict the temperature conditions to which the thermocouple connection cable may be exposed and select the one with appropriate insulation.
The sensor tip and its external dimensions are also important. Usually, to protect the junction from physical damage or contamination, the thermocouple is placed in a metal, ceramic, or porcelain sheath having a cylinder shape. There are various forms of the measuring tip: flat, half-round, or conical with a point angle of 118°. Usually, the dimensions of such a sensor are determined by its diameter and length, but sensors with eyelet termination can also be found.
Of course, you must not forget to match the type of thermocouple to your measuring system or the measuring system to your thermocouple type. The manufacturer of systems designed to tracking temperature values using a thermocouple clearly defines which type of sensor is required. The user should follow these guidelines to ensure that the temperature reading is true and to avoid damaging any of the components used.
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