First, the difference: 1. Although they are contact type temperature measuring instruments, but their temperature range is different, thermocouples are used in higher temperature environments, such as platinum rhodium 30---platinum é“‘6 (type B) measurement range is 300 degrees ~ ~ 1600 degrees, short-term measurement of 1800 degrees. S-type measurement of 20 ~ ~ 1300 (short-term 1600), K-type measurement of 50 ~ ~ 1000, (short-term 1200). XK type of 50 ~ ~ 600 (800) E type is 40~~800(900). There are J type, T type, etc. These meters are generally used for higher temperatures above 500 degrees, because they have small output thermoelectric potential in the middle and low temperature areas. The table can be seen), when the potential is small, the anti-interference measures and secondary tables and requirements are very high, otherwise the measurement is not accurate, and, in the lower temperature region, the change of the cold junction temperature and the change of the ambient temperature The relative error is very prominent and it is not easy to get full compensation. At this time, at low and medium temperatures, the thermal resistance range is generally 200~~500, and even lower temperatures can be measured (such as the low temperature of about 1K can be measured with carbon resistance). Now the normal use of platinum thermal resistance Pt100, (also Pt50, 100 and 50 represent the resistance of the thermal resistance at 0 degrees, expressed in BA1, BA2 in the old indexing number, BA1 is 46 ohms at 0 degrees, and copper is also used in industry. Resistance, indexing numbers are CU50 and CU100, but the temperature range is small, between 50~~150. In some special occasions, there are indium resistors, manganese resistors, etc.)
2. The basic principle of thermocouple measurement of temperature is the thermoelectric effect. The secondary meter is a voltmeter or an electronic potentiometer for improving accuracy. The resistor works based on the characteristics of the resistance of the conductor and the semiconductor as a function of temperature. The secondary table is an unbalanced bridge.
3. According to the thermocouple temperature measurement principle, the measured temperature is a single-valued function relationship with the thermoelectric potential only when the cold junction temperature is constant. In actual use, a thermoelectric characteristic is similar to that of the corresponding thermocouple. Cheap connecting wires (also called compensation wires), so that the cold end of the thermocouple is extended to a relatively constant temperature (preferably 0 degrees), such as copper--constantan as a compensating wire to extend nickel-chromium-nickel Silicon thermal resistance. Therefore, the thermocouple to the secondary meter extension line is two. The thermal resistance and the secondary meter are connected by copper wires. In order to reduce the measurement error caused by environmental changes, the three-wire connection method is generally adopted. There are two wires that connect the RTD in series to the adjacent two bridge arms, and the other wire is the power source. In use, the sum of the resistance value of each wire and the adjustment resistor is guaranteed to be 5 ohms (± 0.01).
On-the-spot judgment at work.
1. Thermocouple. Thermocouple has positive and negative poles, and the compensation wire also has positive and negative points. Firstly, the connection is ensured. The configuration is correct. In operation, there are common short circuits, open circuits, poor contact (multiple gauges can be judged) and deterioration (according to Surface color to identify). When checking, make the thermocouple separate from the secondary meter. I judge the method in practice for your reference: short the compensation line on the secondary meter with the tool, the table indicates the room temperature (if not, Table is bad), then short the thermocouple terminal block, the table indicates the ambient temperature of the thermocouple (not, the compensation line is faulty), and then use the multimeter mv file to roughly estimate the thermoelectric potential of the thermocouple (if normal, please check the process).
2. Thermal resistance. No more than short circuit, and open circuit. Can be judged with a multimeter. In operation, suspect short circuit, just remove the wire end of the resistor end, look at the display instrument, such as to the maximum, short circuit of the thermal resistance. Short circuit. To ensure normal connection and configuration, the table value shows low or unstable, the protection tube is likely to enter the water. The display is the largest, the thermal resistance is broken. The display is minimum and short circuit.
Generally, a thermal resistor having a temperature of 300 degrees or less and a thermocouple having a temperature of 300 degrees or more are used.
As the temperature changes, the resistance of the thermal resistance changes, and the thermoelectric potential of the thermocouple changes.
Thermal resistors currently use copper thermal resistance and platinum thermal resistance. According to different thermal resistance values ​​at 0 degrees, they are divided into different index numbers, such as PT100, PT1000, CU50, etc., PT100 is taken as an example, PT stands for platinum, and 100 represents The resistance of the thermal resistance at 0 degrees is 100 ohms.
Thermocouples currently have K, B, S and other indexing numbers, which represent different materials for different temperature ranges. For example: K type is nickel chromium-nickel silicon material, generally measuring 0-800 degrees, type B is platinum iridium 30-platinum iridium 6, generally measuring 800-1600 degrees.
In practical applications, the thermal resistance generally uses a three-core copper wire for removing the influence of the resistance value of the wire. The thermocouple uses a two-core dedicated compensation wire for removing the influence of the thermocouple on-site temperature.
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