Principle and construction of thermal resistance
Principle and construction of thermal resistance
Thermal resistance is the most commonly used temperature detector in medium and low temperature region. Its main characteristics are high measurement accuracy and stable performance. Platinum thermal resistance measurement accuracy is the highest, it is not only widely used in industrial temperature measurement, and is made into a standard reference instrument.
1. Thermal resistance temperature measurement principle and materials
Thermal-resistance thermometry is based on the fact that the resistance of a metal conductor increases with increasing temperature. Most of the thermal resistors are made of pure metal materials. Platinum and copper are most commonly used at present. In addition, thermal resistors have begun to be made of Dian, nickel, manganese, rhodium and other materials.
2. Structure of thermal resistance
(1) proficient thermal resistance
According to the temperature measurement principle of thermal resistance, the change of the temperature measured is directly measured by the change of the resistance value of the thermal resistance, so the change of the leading wire of the thermal resistance body and other wire resistance will affect the temperature measurement. In order to eliminate the influence of lead resistance, three-wire system or four-wire system is generally adopted.
(Two-wire system: two wires and transmission power transmission signal, that is, the sensor output load and power supply is connected in series, the power supply is introduced from the outside, and the load is connected in series to drive the load. Three-wire sensor: Three-wire sensor is the positive end of the power supply and the positive end of the signal output are separated, but they share a COM end. Four-wire system: two power lines, two signal lines. Power and signal work separately.
(2) armored thermal resistance
Armored thermal resistance is a solid body composed of temperature sensing elements (resistance body), leads, insulating materials, stainless steel sleeve and its outer diameter is generally φ 2~φ 8 mm. Compared with the ordinary thermal resistance, it has the following advantages: ① small volume, no air gap, thermal inertia, small measurement lag; (2) good mechanical properties, vibration resistance, impact resistance; ③ Can bend, easy to install ④ long service life.
(3) end face thermal resistance
The end-face thermal resistance temperature sensing element is wound by special resistance wire and is close to the end face of the thermometer. Compared with the general axial thermal resistance, it can reflect the actual temperature of the measured end face more correctly and quickly, and is suitable for measuring the end-face temperature of the bearing bush and other parts.
(4) flameproof thermal resistance
Flameproof thermal resistance through the special structure of the junction box, the shell of the explosive mixed gas due to spark or arc and other effects of the explosion is confined to the junction box, production site will not lead the explosion. The flameproof thermal resistance can be used to measure the temperature in Bla~B3c class area with explosion danger.
3. Composition of thermal resistance temperature measuring system
Thermal resistance temperature measuring system is generally composed of thermal resistance, connecting wire and display instrument. The following two points must be noted:
① The thermal resistance and the display meter must be consistent
② In order to eliminate the influence of the change of the resistance of the connecting wire, the three-wire connection method must be adopted
Thermal resistance, as the name implies, the resistance value of its resistance is changed with the temperature change, for example, with relatively linear platinum wire, copper wire resistance. Industrial thermal resistance is generally Pt100,Pt10,Pt1000, Cu50,Cu100, platinum thermal resistance temperature measurement range is generally -200-800 degrees Celsius, copper thermal resistance is -40 to 140 degrees Celsius.
Such as the heat resistance made of platinum wire, its indexing known as Pt100. That is, its resistance is 100 ohms at 0 ° C, 18.52 ohms at minus 200 ° C, 175.86 ohms at 200 ° C and 375.70 ohms at 800 ° C.
For example, the thermal resistance made of copper wire, the indexing number Cu50. Its resistance is 50 ohms at 0 ° C and 71.400 ohms at 100 ° C.
The thermal resistance formula is Rt=Ro (1+A*t+B*t*t); Rt=Ro[1+A*t+B*t*t+C (t-100) *t*t*t], where t stands for Celsius temperature,Ro is the resistance at zero Celsius, and A, B, and C are all specified coefficients. For Pt100,Ro is equal to 100.
Scale definition: represents the temperature range, and represents the specific temperature of each scale of the thermocouple or thermal resistance output voltage of how many volts or millivolts.
