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Characteristics and Classification of Temperature Sensors

The temperature sensor is composed of a temperature sensitive element and a detection circuit. Temperature sensors can be roughly divided into two categories: contact type and non-contact type from the perspective of use. The former is to allow the temperature sensor to directly contact the object to be measured to be sensitive to changes in the temperature of the object to be measured. The latter is to keep the temperature sensor at a certain distance from the object to be measured and detect the infrared rays emitted from the object to be measured, thereby achieving the purpose of temperature measurement.

Traditional thermocouples, thermal resistors, thermistors and semiconductor temperature sensors convert the temperature value through a certain interface circuit and then output an analog voltage or current signal. Measurement control can be performed using these voltage or current signals. By integrating the analog temperature sensor with the digital conversion interface circuit, it becomes a digital temperature sensor with digital output capability. With the rapid development of semiconductor technology, semiconductor temperature sensors and corresponding conversion circuits, interface circuits and various other functional circuits are gradually integrated to form a powerful, accurate and cheap digital temperature sensor.
Digital temperature sensor probe with M4 thread
Temperature sensor characteristics

A temperature sensor is a device that detects temperature. It is widely used in industrial and agricultural production, scientific research and daily life. It has many types and is developing rapidly. Temperature sensors are generally divided into two categories: contact and non-contact. The so-called contact type means that the sensor directly contacts the object to be measured for temperature measurement. This is the basic form of temperature measurement. The non-contact method measures the infrared rays emitted by the thermal radiation of the object to measure the temperature of the object. Telemetry can be performed, which is not possible with contact methods. Contact temperature sensors include thermocouples, thermistors, platinum resistors, etc., which use the thermal electromotive force or resistance that they generate to change with temperature to measure the temperature of an object. It is widely used in household appliances, automobiles, ships, control equipment, industrial measurement, communication equipment, etc. In addition, there are some newly developed sensors, such as semiconductor integrated sensors that use the current/voltage characteristics of semiconductor PN junctions to change with temperature. There are use of optical fiber propagation characteristics that change with temperature or semiconductors.

Fiber optic sensor whose light transmission changes with temperature. There are sensors that use elastic surface waves and the oscillation frequency of the vibrator to change with temperature. There are NQR sensors that utilize the oscillation frequency of nuclear quadruple resonance to change with temperature. There are magnetic temperature sensors that utilize sudden changes in magnetism near the Curie temperature, and sensors that utilize liquid crystal or paint color changes with temperature, etc. The non-contact method measures the temperature of an object by detecting infrared rays in a light sensor. There are quantum sensors that use semiconductors to absorb light and move electrons, and thermal sensors that absorb light and cause temperature changes. Non-contact sensors are widely used in contact temperature sensors, radiation thermometers, alarm devices, visitor notification devices, fire alarms, automatic doors, gas analyzers, spectrophotometers, resource detection, etc. The company specializes in the production of various temperature sensor series: thermocouples, thermal resistors, bimetal thermometers, temperature transmitter series, etc. Customers are welcome to come and choose.

1. Application principle of temperature sensor thermocouple
Temperature sensor thermocouple is one of the most commonly used temperature detection components in industry. The advantages are:
①High measurement accuracy. Because the temperature sensor thermocouple is in direct contact with the measured object, it is not affected by the intermediate medium.
②Wide measuring range. Commonly used temperature sensors, thermocouples, can be continuously measured from -50 to +1600°C. Some special temperature sensor thermocouples can measure as low as -269℃ (such as gold, iron, nickel and chromium) and as high as +2800℃ (such as tungsten-rhenium).
③Simple structure and easy to use. Temperature sensor thermocouples are usually composed of two different metal wires and are not limited by size or opening. There is a protective sleeve outside, which is very convenient to use.

1. Basic principles of temperature sensor thermocouple temperature measurement
Two conductors or semiconductors A and B of different materials are welded together to form a closed circuit. When there is a temperature difference between the two attachment points 1 and 2 of conductors A and B, an electromotive force is generated between the two, thus forming a large current in the loop. This phenomenon is called the thermoelectric effect. Temperature sensor thermocouples take advantage of this effect to work.

2. Types and structure of temperature sensor thermocouples
(1) Types of temperature sensor thermocouples
Commonly used temperature sensor thermocouples can be divided into two categories: standard temperature sensor thermocouples and non-standard temperature sensor thermocouples. The so-called standard temperature sensor thermocouple refers to a temperature sensor thermocouple whose national standards stipulate the relationship between thermoelectric potential and temperature, allowable errors, and have a unified standard graduation table. It has matching display instruments available. Non-standardized temperature sensor thermocouples are inferior to standardized temperature sensor thermocouples in terms of range or order of magnitude. Generally, there is no unified graduation table, and it is mainly used for measurement in some special occasions. Standardized temperature sensor thermocouple In my country, starting from January 1, 1988, temperature sensor thermocouples and temperature sensor thermal resistors are all produced in accordance with IEC international standards. And designated seven types of standardized temperature sensor thermocouples S, B, E, K, R, J, and T as my country's unified design temperature sensor thermocouples.
Serial port DB9 temperature sensor
(2) Structural form of temperature sensor thermocouple
In order to ensure that the temperature sensor thermocouple works reliably and stably, its structural requirements are as follows:
① The two hot electrodes that make up the temperature sensor thermocouple must be welded firmly;

② The two hot electrodes should be well insulated from each other to prevent short circuit;

③ The connection between the compensation wire and the free end of the temperature sensor thermocouple must be convenient and reliable;

④ The protective sleeve should be able to ensure that the hot electrode is fully isolated from harmful media.

3. Temperature Compensation of Cold Junction of Temperature Sensor Thermocouple

Since the materials of temperature sensor thermocouples are generally relatively expensive (especially when precious metals are used), the distance between the temperature measurement point and the instrument is very long. In order to save thermocouple materials and reduce costs, compensation wires are usually used to extend the cold end (free end) of the temperature sensor thermocouple to the control room where the temperature is relatively stable, and connect it to the instrument terminals. It must be pointed out that the temperature sensor thermocouple compensation wire only serves to extend the hot electrode so that the cold end of the temperature sensor thermocouple moves to the instrument terminals in the control room. It itself cannot eliminate the influence of cold end temperature changes on temperature measurement and does not have a compensation effect. Therefore, other correction methods need to be used to compensate for the impact on temperature measurement when the cold end temperature t0≠0℃.

When using the temperature sensor thermocouple compensation wire, you must pay attention to the matching model, the polarity cannot be wrong, and the temperature of the compensation wire and the temperature sensor thermocouple connection end cannot exceed 100°C.

2. Application principle of temperature sensor thermal resistance
Temperature sensor thermal resistance is the most commonly used temperature detector in medium and low temperature areas. Its main features are high measurement accuracy and stable performance. Among them, the measurement accuracy of platinum thermal resistance is the highest. It is not only widely used in industrial temperature measurement, but also made into a standard reference instrument.

  1. Temperature sensor thermal resistance temperature measurement principle and materials

Temperature sensor thermal resistance temperature measurement is based on the characteristic that the resistance value of a metal conductor increases with the increase of temperature to measure temperature. Temperature sensor thermal resistors are mostly made of pure metal materials. Currently the most widely used are platinum and copper. In addition, materials such as titanium, nickel, manganese and rhodium are now being used to manufacture temperature sensor thermal resistors. For example, Omega's PT100 temperature sensor contains a 100-ohm platinum resistance temperature probe.

  2. Structure of temperature sensor thermal resistor

(1) Proficient temperature sensor thermal resistor: commonly used industrial temperature sensor thermal resistor temperature sensing element (resistor body). It can be seen from the temperature measurement principle of the temperature sensor thermal resistor that the change in the measured temperature is measured directly through the change in the resistance value of the temperature sensor thermal resistor. Therefore, changes in the resistance of various wires such as the lead wire of the temperature sensor thermal resistor will affect the temperature measurement. In order to eliminate the influence of lead resistance, a three-wire system or a four-wire system is generally used.

(2) Armored temperature sensor thermal resistor Armored temperature sensor thermal resistor is a solid body composed of temperature sensing element (resistor body), lead wire, insulating material, and stainless steel casing. Its outer diameter is generally φ2~φ8mm, and the minimum can reach φmm.

Compared with ordinary temperature sensor thermal resistors, it has the following advantages:
① Small size, no air gap inside, small measurement lag due to thermal inertia;
②Good mechanical properties, vibration resistance and impact resistance;
③Can be bent for easy installation
④Long service life.

(3) End face temperature sensor thermal resistance: The end face temperature sensor thermal resistance temperature sensing element is wound by specially treated resistance wire and is tightly attached to the end face of the thermometer. Compared with the general axial temperature sensor thermal resistor, it can reflect the actual temperature of the measured end face more accurately and quickly, and is suitable for measuring the end face temperature of bearing bushes and other parts.

(4) Explosion-proof temperature sensor thermal resistor: The explosion-proof temperature sensor thermal resistor passes through a specially structured junction box. The explosion of the explosive mixed gas inside the shell caused by sparks or arcs is limited to the junction box, and no super explosion will occur at the production site. Explosion-proof temperature sensor thermal resistance can be used for temperature measurement in places with explosion hazards in Bla~B3c level areas.

3. Composition of temperature sensor thermal resistance temperature measurement system
Temperature sensor thermal resistance temperature measurement system generally consists of temperature sensor thermal resistance, connecting wires and display instruments. The following two points must be noted:
①The graduation number of the temperature sensor thermal resistance and the display instrument must be consistent
② In order to eliminate the influence of changes in resistance of the connecting wires, a three-wire connection method must be used.

Main types of temperature sensors:
There are four main types of temperature sensors: thermocouples, thermistors, resistance temperature detectors (RTDs), and IC temperature sensors (see table below). IC temperature sensors include two types: analog output and digital output.

Thermocouples are widely used because they are very robust and not expensive. There are many types of thermocouples, and they cover a very wide temperature range, from -200°C to 2000°C. Their characteristics are: low sensitivity, low stability, medium accuracy, slow response, easy aging and drift at high temperatures, and nonlinearity. Additionally, thermocouples require an external reference terminal.

RTD has extremely high accuracy and moderate linearity. They are extremely stable and available in many configurations. But their maximum operating temperature can only reach about 400°C. They also have a large TC, are expensive (4 to 10 times more expensive than thermocouples), and require an external reference source.

Analog output IC temperature sensors have high linearity (digital output can be generated if paired with an analog-to-digital converter or ADC), low cost, high accuracy (about 1 [[%]]), small size and high resolution. Their disadvantages are that they have a limited temperature range (-55°C ~ +150°C) and require an external reference source

Digital output IC temperature sensors have a built-in reference source, and their response speed is also quite slow (on the order of 100 ms). Although they inherently generate heat, automatic shutdown and single-conversion modes can be used to minimize self-heating by setting the IC to a low-power state before measurements are required.

Compared with thermistors, RTDs and thermocouple sensors, IC temperature sensors are highly linear, have low system cost, integrate complex functions, can provide a digital output, and can perform temperature measurements over a fairly useful range.
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