Thermal resistance digital thermometers use the nature of the resistance of conductors or semiconductors to vary with temperature to measure temperature. They are widely used in industrial production to measure temperatures in the range of (-100 to 500) ° C. The main feature is the accuracy of temperature measurement. High for easy automatic measurement. Because the thermoelectric potential generated by the thermocouple in the low temperature range is small, the measurement instrument is required strictly, and it is very suitable to use a resistance thermometer to measure the low temperature. According to the structural form, the thermal resistance thermometer can be divided into ordinary industrial type, armored type and special type.
The common industrial thermal resistances commonly used are:
1. Platinum thermal resistance: It is widely used to measure the temperature in the range of (-200 ～ 850) ℃. In a few cases, the low temperature can be measured to 1K, and the high temperature can be measured to 1000 ° C. Its physical and chemical properties are stable and reproducible, but it is expensive. Platinum thermal resistance is approximately linearly related to temperature. Its division number is mainly Pt10 and pt100.
2. Copper thermal resistance: widely used to measure the temperature in the range of (-50 ～ 150) ℃. Its advantages are easy to obtain high-purity copper wire, cheap price, good interchangeability, but easy oxidation. Copper thermal resistance has a linear relationship with temperature. Its division number is mainly Cu50 and Cu100.
Armored thermal resistance is developed on the basis of armored thermocouples. It is made of a combination of thermal resistance, insulation materials and metal sleeves. Its characteristics are that the external dimensions can be made small (the minimum diameter can reach 20 Millimeters), so the reaction speed is fast, has good mechanical properties, vibration and shock resistance, has good flexibility, and is not easily eroded by harmful media.
Before using the thermal resistance, you must check its good ring. The simple inspection method is to extract the thermal resistance from the protective tube and measure the resistance with a multimeter. If the multimeter reads "0" or the multimeter reads less than R0, the thermal resistance is short-circuited, and the short-circuit must be found to repair it; if the multimeter reads "∞", the thermal resistance is open and cannot be used; The resistance is higher than R0, indicating that the thermal resistance is normal.
When the resistance value of the thermal resistor is incorrect, the resistance wire should be increased or decreased from the intersection of the lower ends, and should not be adjusted from other places. After the adjustment is completed, the resistance wires should be arranged neatly, and they should not be touched, and they should still be wrapped as they are. The repaired thermal resistor must be qualified before being used.
When installing a thermal resistor, its insertion depth should not be less than 8 to 10 times the outer diameter of the thermal resistor protection tube, so as to increase the heated part of the thermal resistor as much as possible. Install the thermal resistor as vertically as possible to prevent bending and deformation at high temperatures. In order to reduce the error caused by radiant heat and heat conduction during the use of thermal resistance, the surface of the protective sleeve and the temperature of the measured medium should be as close as possible to reduce the black coefficient of the thermal resistance protective sleeve.
When the temperature is measured with a secondary meter that matches the thermal resistance, the thermal resistor is placed at the site of the measured temperature, and the secondary meter is placed in the operating room. If an unbalanced bridge is used for measurement, then the wires connecting the thermal resistance are distributed on one arm of the bridge circuit. Because there is generally a long distance between the thermal resistance and the meter, the resistance of the two connecting wires with temperature changes will be added to the arm of the unbalanced bridge together with the change of the thermal resistance value to make the measurement Larger errors occur. In order to reduce this error, generally when the temperature measuring thermal resistance is connected to the instrument, a three-wire connection method is used (Figure 1), that is, three wires are led out from the thermal resistance, and the two wires connected to the thermal resistance are exactly adjacent to each other. Within two bridge arms. When the ambient temperature changes and changes the resistance of the wires, the effects of the wires exactly cancel each other, so that the unbalanced voltage output by the bridge will not change due to it. The change of the other wire resistance R1 has only a slight influence on the bridge voltage, but it is within the accuracy range.