Basic knowledge of thermistor temperature sensor, thermocouple and RTD temperature sensor

Concept of temperature

From the perspective of physics, heat is a measure of the energy contained in the body due to the irregular movement of its molecules or atoms. Just as tennis balls have more energy with the increase of speed, the internal energy of objects or gases increases with the increase of temperature. Temperature is a variable, which together with other parameters such as mass and specific heat describes the energy content of the object.
The basic measure of temperature is Kelvin degrees. At 0 ° K (elvin), the molecules of each object are in a static state and no longer have any heat energy. Therefore, it is impossible to have a negative temperature because there can be no lower energy state.
In daily use, the Celsius scale (formerly called Celsius) is usually used. Its zero is placed at the freezing point of water, because this point is easily reproduced in practice. Now 0 ° C is by no means the lowest temperature everyone knows from experience. By extending Celsius to the lowest possible temperature at which all molecular motion stops, we reach a temperature of - 273.15 degrees.
Man has the ability to measure temperature within a limited range through his senses. However, he could not accurately reproduce the quantitative measurement results. The earliest form of quantitative temperature measurement was developed in Florence in the early 17th century, which depended on the expansion of alcohol. Zoom is based on the highest temperatures in summer and winter. One hundred years later, Swedish astronomers replaced it with a scale based on the melting point and boiling point of water. This gives the thermometer the opportunity to scale at any time and reproduce the reading later.

temperature measurement

Temperature measurement is important in many applications, such as building control, food processing, and the manufacture of steel and petrochemical products. These very different applications require temperature sensors with different physical structures and often different technologies.
In industrial and commercial applications, measurement points are usually far away from indication or control points. It is usually necessary to further process the measurement in the controller, recorder or computer. This type of application is not suitable for directly indicating thermometers because we know them from daily use, but we need equipment that converts temperature to another form (electrical signal). In order to provide such remote electrical signals, RTD temperature sensors are usually used. Thermistors and thermocouples.
RTD adopts the characteristic that metal resistance changes with temperature. They are positive temperature coefficient (PTC) sensors whose resistance increases with temperature. The main metals used are platinum and nickel. The most widely used sensors are 100 ohm or 1000 ohm RTD temperature sensors or platinum resistance thermometers.
RTD temperature sensors are the most accurate sensors in industrial applications and also provide the best long-term stability. The representative value of platinum resistance accuracy is+0.5% of the measured temperature. +0.05 ° C may change with aging after one year. The platinum resistance thermometer can cover the temperature range of - 200 to 800 ° C.

Change of resistance with temperature

The conductivity of a metal depends on the mobility of conducting electrons. If a voltage is applied to the end of the wire, the electrons move to the positive pole. Defects in the lattice interfere with this movement. They include exotic or missing lattice atoms, atoms at grain boundaries and lattice to lattice locations. Since these fault locations are temperature independent, they produce a constant resistance. As the temperature rises, the atoms in the metal lattice exhibit more and more oscillations around their stationary positions, thus hindering the movement of conduction electrons. Since this oscillation increases linearly with temperature, the increase in resistance caused by it directly depends on the first approximation of temperature.
The resistance material platinum has been widely accepted in industrial measurement. Its advantages include chemical stability, relative ease of manufacture, especially for wire manufacturing, the possibility of obtaining it in a high-purity form, and reproducible electrical properties. These characteristics make the platinum resistance sensor the most versatile interchangeable temperature sensor.

Thermistors are made of some metal oxides, and their resistance decreases with temperature. Because the resistance characteristics decrease with the increase of temperature, they are called negative temperature coefficient (NTC) temperature sensors.
Due to the nature of the basic process, the number of conducting electrons increases exponentially with temperature; Therefore, this feature shows a strong rising form. This apparent nonlinearity is a disadvantage of NTC temperature sensor resistors and limits their available temperature range to around 100 ° C. They can, of course, be linearized by automated computers. However, the accuracy and linearity usually cannot meet the requirements of large measurement span. Their drift at alternate temperatures is also greater than that of RTD temperature sensors. Their use is limited to monitoring and indicating applications where the temperature does not exceed 200 ° C. In this simple application, they are actually preferable to more expensive thermocouples and RTD temperature sensors due to their low cost and relatively simple electronic circuits required.
Thermocouples produce the effect of voltage rising with temperature based on the junction between two different metals. Compared with resistance thermometers, their obvious advantage is that the upper temperature limit is higher, up to thousands of degrees Celsius. Their long-term stability is slightly poor (several degrees after one year), and their measurement accuracy is slightly poor (average+0.75% of the measurement range). They are often used in ovens, furnaces, flue gas measurements, and other areas where temperatures are above about 250 ° C.
Thermoelectric effect
When two metals are connected together, thermoelectric voltage will be generated due to different binding energies of electrons and metal ions. This voltage depends on the metal itself and also on the temperature. In order for the thermal voltage to generate current, the two metals must of course be connected together at the other end to form a closed circuit. In this way, a thermal voltage is generated at the second node. Thermoelectric effect was discovered by Seebeck in 1822. As early as 1828, Becquerel proposed to use platinum palladium thermocouple for temperature measurement.
If the temperature of the two nodes is the same, there is no current flow, because the partial voltage generated at the two points will cancel each other. As the temperature at the node is different, the voltage generated is also different, and there is current flow. Therefore, the thermocouple can only measure the temperature difference.
The measuring point is the node exposed to the measuring temperature. A reference junction is a junction at a known temperature. Since the known temperature is usually lower than the measured temperature, the reference end is usually called the cold end. In order to calculate the actual temperature of the measuring point, the cold end temperature must be known.
Older instruments used thermostatic control junction boxes to control the cold end temperature at a known value, such as 50C. Modern instruments use a thin film connected RTD temperature sensor at the cold end to determine its temperature and calculate the temperature at the measuring point.
The thermoelectric effect produces a very small voltage, only a few microvolts per degree Celsius. Therefore, thermocouples are usually not used in the range of - 30 to+50 ° C, because the difference between the reference junction temperature and the thermocouple temperature here is too small to generate a non interfering signal.
Thermal resistance wiring
In resistance thermometers, resistance varies with temperature. To evaluate the output signal, a constant current flows through it and the voltage drop across it is measured. This voltage drop follows Ohm's law, V=IR.
The measuring current should be as small as possible to avoid heating of the sensor. It can be considered that the measured current of 1 mA will not introduce any obvious error. This current produces a voltage drop of 0.1 V in Pt 100 at 0 ° C. This signal voltage must now be transmitted to the indication or evaluation point via the connecting cable with minimal change. There are four different types of connecting circuits:
2-wire circuit
The connection between the thermometer and the evaluation electronic equipment is made through a 2-core cable. Like any other electrical conductor, this cable has a resistance in series with a resistance thermometer. Therefore, the two resistances are added together, which is interpreted by the electronic equipment as a temperature rise. For longer distances, the line resistance may reach several ohms and produce a significant change in the measured value.
3-wire circuit
In order to minimize the influence of line resistance and its fluctuation with temperature, a three wire circuit is usually used. It includes connecting an additional wire to one contact of the RTD. This results in two measuring circuits, one of which is used as a reference. A 3-wire circuit can compensate for the amount of line resistance and temperature changes. However, all three conductors are required to have the same characteristics and be exposed to the same temperature. This usually applies to a sufficient degree, so 3-wire circuits are the most widely used method today. Line balancing is not required.
4-wire circuit
The best connection for resistance thermometers is a 4-wire circuit. Measurements are not dependent on line resistance nor on their temperature induced changes. Line balancing is not required. Supply the measuring current to the thermometer through the power connection. The voltage drop across the measuring resistance is picked up by the measuring line. If the input resistance of an electronic device is many times greater than the line resistance, the latter can be ignored. The voltage drop determined in this way is independent of the characteristics of the connecting line. This technology is usually only used for scientific instruments that require an accuracy of one percent.
2-wire transmitter
By using a 2-wire transmitter, the above 2-wire circuit problems can be avoided without using multi-core cables. The transmitter converts the sensor signal into a standardized current signal of 4 – 20 mA, which is proportional to the temperature. The transmitter's power supply also operates through the same two connections, using a 4 mA base current. The two-wire transmitter provides an additional advantage, that is, the amplification of the signal greatly reduces the impact of external interference. There are two ways to arrange the emitters. Since the distance of the non amplified signal should be as short as possible, the amplifier can be directly installed in the terminal of the thermometer. This optimal solution is sometimes impossible due to structural reasons or considering that the transmitter may be difficult to reach in case of failure. In this case, the rail mounted transmitter is installed in the control cabinet. The advantage of improving access is at the expense of the longer distance that the unamplified signal must travel.
Thermistor wiring
The resistance of a thermistor is usually several orders of magnitude greater than any lead resistance. Therefore, the influence of lead resistance on temperature reading can be ignored, and thermistors are almost always connected in a 2-wire configuration.
Thermocouple wiring
Unlike RTDs and thermistors, thermocouples have positive and negative electrodes, so the polarity must be noted. They can be directly connected to the local two-wire transmitter, and the copper wire can be returned to the receiving instrument. If the receiving instrument can accept thermocouple input directly, you must use the same thermocouple wire or thermocouple extension wire to return to the receiving instrument.
Select the right sensor for building automation
1. Platinum RTD is the most accurate and stable sensor for a long time. Their transaction costs are usually about $5 per point higher than thermistors. Some automation panels do not accept RTDs directly, for these panels they must be used with temperature transmitters
2. Thermistors are not as accurate or stable as RTDs, but they are easier to connect and have a lower cost. Almost all automatic panels accept them directly. Note that thermistors have many different base resistances and many different curves. You must specify the correct thermistor for the panel you want to use.
3. Thermocouples are widely used in industrial applications because they work reliably at very high temperatures and are cheaper than RTDs. They are rarely needed in building automation because most measured temperatures are below 100 degrees Celsius. However, they are often used with 2-wire transmitters for flue gas measurement.
4. Some consultants in the work assigned platinum RTDs because they improved accuracy and long-term stability.