2. Thermistor
Thermistors are made of semiconductor materials, mostly with negative temperature coefficient, that is, the resistance decreases with the increase of temperature. Temperature change will cause large resistance change, so it is the most sensitive temperature sensor. However, the linearity of thermistor is very poor, and it is closely related to the production process. The manufacturer cannot give a standardized thermistor curve.
Thermistors are very small and respond quickly to temperature changes. However, thermistor needs current source, and its small size also makes it very sensitive to self heating error.
The thermistor measures the absolute temperature on the two lines, which has good accuracy, but it is more expensive than the thermocouple, and the measurable temperature range is also smaller than the thermocouple. The resistance value of a common thermistor at 25 ℃ is 5K Ω, and the resistance change of 200 Ω is caused by the temperature change of 1 ℃. Note that the lead resistance of 10 Ω only causes a negligible error of 0.05 ℃. It is well suited for current control applications that require fast and sensitive temperature measurements. Small size is advantageous for applications with space requirements, but care must be taken to prevent self heating errors.
Thermistors also have their own measurement skills. The thermistor has the advantage of small volume. It can be stable quickly and will not cause thermal load. However, it is also very weak. High current will cause self heating. Because thermistor is a resistive device, any current source will generate heat due to power. Power is equal to the product of the square of the current and the resistance. Therefore, use a small current source. If the thermistor is exposed to high heat, it will cause permanent damage.
Verification standard and technical index of temperature sensor:
1. Measurement accuracy: grade 0.01; Resolution 0.1uv and 0.1M Ω;
2. Parasitic potential of scanning switch: ≤ 0.4 μ V；
3. Temperature range: water tank: (room temperature + 5 ~ 95) ℃ oil tank: (95 ~ 300) ℃ low temperature constant temperature tank: (- 80 ~ 100) ℃ high temperature furnace: (300 ~ 1200) ℃;
4. temperature control stability: better than 0.01 ℃ / 10min (oil tank, water tank, low temperature constant temperature tank); 0.2 ℃ / min (tubular verification furnace);
5. Total uncertainty: for thermocouple verification, the measurement uncertainty is better than 0.7 ℃, and the repeatability error is less than 0.25 ℃; The measurement uncertainty of thermal resistance verification is better than 50mk, and the repeatability error is less than 10mk;
6. Verification quantity: 1-8 thermocouples can be checked at the same time, and 1-7 heating resistors in the same line can be checked at the same time;
7. Working power supply: AC220V ± 10%, 50Hz, with good protective grounding;
8. High temperature furnace power: About 2kW;
9. Thermostatic bath power: About 2kW;
10. Power of microcomputer measurement and control system: < 500.
Functions and features of temperature sensor verification device:
1. Verify K, e, J, N, B, s, R, t and other types of working thermocouples;
2. Verify PT100, pt10, cu50, cu100 and other working thermal resistance, glass liquid thermometer, pressure thermometer and bimetal thermometer;
3. Multi channel low potential automatic transfer switch, parasitic potential ≤ 0.4 μ V；
4. Control 1-4 high temperature furnaces;
5. Temperature field test: it can test the temperature field of verification furnace, oil tank, water tank and low-temperature constant temperature tank;
6. Wire system conversion: two wire system, three wire system and four wire system resistance verification can be carried out;
7. The software has the functions of comparison experiment, repeatability experiment, temperature field experiment and so on;
8. On the platform above Windows2000 / XP, all Chinese interface and standard Windows operating system are convenient and fast. Achievable:
1) Equipment self inspection and line inspection;
2) Display and save the temperature control curve on the screen ≤ 0.4 μ V；
3) Automatic collection of test data;
4) Automatically generate verification records that meet the requirements;
5) The verification results are saved automatically and cannot be changed manually;
6) Query various thermocouples, thermal resistance index tables and other help;
7) Intelligent management function for all historical verification data, temperature control curve query statistics and measurement of thermocouple and thermal resistance.
What is the main purpose?
Temperature is a physical quantity representing the degree of cold and heat of an object. It is a very important and common measurement parameter in the process of industrial and agricultural production. Temperature measurement and control plays a very important role in ensuring product quality, improving production efficiency, saving energy, production safety and promoting the development of national economy. Due to the universality of temperature measurement, the number of temperature sensors ranks first among all kinds of sensors, accounting for about 50%.
The temperature sensor indirectly measures the temperature by changing some characteristics of the object with the change of temperature. The characteristics of many materials and components change with the change of temperature, so there are quite a lot of materials that can be used as temperature sensors. The physical parameters of temperature sensor change with temperature: expansion, resistance, capacitance, electromotive force, magnetic properties, frequency, optical characteristics, thermal noise and so on. With the development of production, new temperature sensors will continue to emerge.
Because the temperature measurement range in industrial and agricultural production is very wide, from hundreds of degrees below zero to thousands of degrees above zero, and the temperature sensors made of various materials can only be used in a certain temperature range.
The contact mode between the temperature sensor and the measured medium is divided into two categories: contact type and non-contact type. The contact temperature sensor needs to keep thermal contact with the measured medium so that they can fully exchange heat to reach the same temperature. This kind of sensors mainly include resistance, thermocouple, PN junction temperature sensor and so on. The non-contact temperature sensor does not need to contact with the measured medium, but is transmitted to the temperature sensor through the thermal radiation or convection of the measured medium, so as to achieve the purpose of temperature measurement. This kind of sensor mainly includes infrared temperature sensor. The main feature of this temperature measurement method is that it can measure the temperature of moving substances (such as the bearing temperature of slow-moving trains and the temperature of rotating cement kilns) and objects with small heat capacity (such as the temperature distribution in integrated circuits).
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