What is a thermocouple sensor?
Thermocouple sensor is perhaps the most simple and robust electronic temperature sensing devices imaginable. They consist only of two wires made out of dissimilar metals, attached together in a junction, which is placed where the measurement is to be obtained. Thanks to the Seebeck effect—named after German physicist Thomas Seebeck—if there is a temperature differential between the junction of these two wires, and their other measured ends are some distance away, there will be a voltage differential between them.


This voltage differential can be correlated to the actual temperature at the junction, thanks to the Seebeck coefficient, which relates the voltage potential to the temperature. This is often expressed as microvolts per degrees Kelvin (μV/°K), which is equivalent to microvolts per degrees Celsius. If needed, that can be converted to Fahrenheit by dividing by 1.8. While this seems simple enough, several factors are at play here that make the correlation slightly less than straightforward.


Temperature measurement using a thermocouple


While thermocouples can be made from a wide variety of metal combinations, the Seebeck constant varies depending on the construction. This number also changes over the thermocouple usable temperature range, so even if you’re using a standard metal combination, you’ll either need a chart or a mathematical model to correlate the temperature value. You also need to know the temperature at the measurement point, since the voltage is due to the temperature difference, not the absolute temperature itself.


The good news is that standard thermocouple metallurgical combinations are well documented, so if you know the type of thermocouple and are able to measure the temperature at your meter, this can all be worked out. The even better news is that more than likely you don’t even have to do any of that. Electrical meters with thermocouple temperature sensing capabilities are able to automatically correlate this information when the proper probe is used, and can even measure the internal temperature to give an output in actual degrees—not just a temperature difference.


Type K thermocouples and more varieties


Standard thermocouple metal combinations are designated with a letter, such as “type M” or “type T,” and the most common general-purpose thermocouple is known as a “type K.” Type K thermocouples are made from alloys known as chromel and alumel, with a Seebeck coefficient generally pegged at 41μV/°C. This number, of course, varies over its useful measurement range of -200°C to +1350°C.


The type K’s measurement range isn’t unusual, and highlights this class of sensor’s versatility, temperature-wise. Since such a device is simply two wires coupled together, they can be made inexpensively and tend to be quite durable.


How does an RTD sensor work and what RTD types are there?


RTD stands for Resistance Temperature Detector, which refers to a type of temperature sensor. As the name suggests, it uses changes in resistance to detect temperature, but that in itself might not tell you how an RTD works. To know more about tenperature sensor types, you can read our article on temperature sensors


How does an RTD work?


When the temperature increases, the RTD’s resistance increases, and vice versa. The control system or transmitter constantly sends a current through the RTD sensor. When the temperature changes, the resistance on the current can increase or decrease. Here, the RTD detects this shift and thereby reports it.


Vendors build RTD sensors out of a variety of materials. Platinum, copper, and nickel show up the most often, with platinum as the fan favourite because it offers the best stability in a wide range of temps.


Also, we can choose RTDs with two, three, or four wires, but what does that mean? These wires create the compensation you need for the cable. Of course, the two-wire option doesn’t, so we should choose it when you need only an approximate value. The three-wire option is most common in many applications.


RTD sensor types


Many factors influence the properties of an RTD sensor. Various materials, such as platinum, copper, or nickel, can affect the range and linearity. Element types can have effects too, like thin-film elements or wire-wound elements. We can also have sensors with two, three, or four wires. The most common version is three, but each option has its pros and cons.


How Does an RTD Work?


"RTD" is an abbreviation for "Resistance Temperature Detector" An RTD is a type of temperature sensor which can be utilised in the manufacture of Variohms' temperature probe range.


They are available with different temperature / resistance values depending on the application requirement.


How Does an RTD Work?


An RTD probe consists of a resistance element and insulated copper wires. The most common number of wires is 2; however some RTDs have 3 or 4 wires. The resistive element is the temperature sensing element of the RTD. It is usually platinum because as a material it is highly stable over time, it has a wide temperature range, it offers an almost linear relationship between temperature and resistance and it has a chemical inertness. Nickle or copper are also other popular choices of material for the resistive element.


An RTD works by using a basic principle; as the temperature of a metal increases, so does the resistance to the flow of electricity. An electrical current is passed through the sensor, the resistance element is used to measure the resistance of the current being passed through it. As the temperature of the resistance element increases the electrical resistance also increases. The electrical resistance is measured in Ohms. The resistance value can then be converted into temperature based on the characteristics of the element. Typical response time for an RTD is between 0.5 and 5 seconds making them suitable to applications where an immediate response is not required.


Thermometers Information


Industrial thermometers are used for temperature measurement in many industrial applications. They are generally more durable than laboratory thermometers. The way an industrial thermometer works depends upon the type of temperature sensor it uses. Sensor options include bimetal, liquid or gas-filled, electronic, or infrared (IR).


Liquid or gas-filled thermometers use a gas or liquid as the temperature-sensing element. The liquid expands as the temperature rises in the tube and indicates the temperature. The basic design for this type of thermometer is a small-bore glass tube with a thin-walled glass bulb at the lower end. Typically, the liquid that fills is mercury or alcohol. As heat is transferred into the substance, it expands and pushes the column of liquid or gas higher up the capillary, which indicates the temperature.


What is a bimetal thermometer and how does a bimetal thermometer measure?


Bimetal thermometers are thermometers based on the functional principle that metals expand differently depending on the change in temperature. A bimetal thermometer always consists of two different metal strips that have a different thermal expansion coefficient. The two strips are joined together inseparably and thus form the bimetal strip. When the temperature changes, the different metals expand to different degrees, which leads to a mechanical deformation of the bimetal strip. This mechanical deformation can be detected in a rotary movement. The measuring system works in the form of a helical or spiral tube. This movement is transmitted to the pointer of the thermometer via the pointer shaft, which thus enables the temperature to be measured.?


In which applications are bimetal thermometers used??


Bimetal thermometers are used in many different industrial applications, e.g. in machine building, in heating technology and in air conditioning and refrigeration technology. The wide variety of models of WIKA bimetallic thermometers offer a large selection of thermometers in a wide range of applications.


The model 52 bimetallic thermometer, with its case and stem made of stainless steel, represents the entry-level model among the process thermometers, but is also used for demanding applications in machine building.


The model A48 bimetal thermometer has been specifically developed for use in air-conditioning and refrigeration technology to measure the temperature in air ducts, whereas the model A43 bimetal thermometer was developed as a standard version for simple temperature indication in heating technology. As you can see, WIKA offers a suitable thermometer for every application. Our specialists will be happy to advise you on your selection.


What are the advantages and disadvantages of bimetal thermometers??


Compared to gas-actuated thermometers, bimetal thermometers are much cheaper in terms of price. This price advantage is due to their simpler construction.


The scale ranges of bimetal thermometers are between -70 and +600 °C with accuracy classes 1 and 2 in accordance with EN 13190. For applications in which high vibrations occur, thermometers with liquid filling are used. For the filling, silicone oil is used, which damps the vibration and limits the maximum temperature to +250°C.


When the temperature changes, all types of dial thermometers react in accordance with the same principle; the expansion principle. The difference, however, is that with the bimetal thermometer the rotary movement of the bimetal can only be shown directly on the display. With the gas-actuated thermometer, on the other hand, the measured value can easily be transmitted via capillary due to its inert gas filling. Remote capillaries can even be used here to install gas-actuated thermometers up to 60 metres away from the measuring location. Expansion thermometers can also be installed via remote capillaries, but, instead of an inert gas, a filling medium is used for the measuring system, e.g. xylene or silicone oil.?


The final decision on which dial thermometer to use depends on a number of different factors. Our specialists will be happy to advise you in the selection of the right thermometer.