Pt100 platinum resistor: temperature measurement principle and selection considerations for laboratory platinum resistors

Pt100 platinum resistor: There are usually many temperature measurement points in air-conditioning laboratories, and high-precision measurement points often use platinum resistors. We all know that resistance value is related to material, length, cross-sectional area, and of course temperature is also an indispensable factor. The thermometer that uses the resistance change brought about by temperature change to measure temperature is called a thermal platinum resistor. This thermometer has a simple structure, is easy to use, has stable performance, high measurement accuracy and sensitivity, and is convenient for remote transmission, automatic recording and centralized control of signals. It is usually suitable for temperature measurement between -200 and 500°.

Platinum resistors and thermocouples have similar names, but these two temperature measurement devices are essentially different: 1. As the temperature changes, the platinum resistor changes its own resistance value, while the thermocouple produces changes in induced voltage; 2. The thermal resistor is a single metal/semiconductor material, and the thermocouple is a bimetallic/semiconductor material; 3. Different measurement temperature ranges: In comparison, thermal resistors are suitable for low-temperature measurements, and thermocouples are suitable for high-temperature measurements; 4. Different connection methods: Thermocouples have positive and negative poles, and thermal resistors do not.

Platinum resistors have the following requirements for materials: 1. The temperature coefficient of resistance should be large. The temperature coefficient of resistance is defined as the relative change in resistance when the temperature rises by 1 Kelvin. The larger the temperature coefficient of resistance, the higher the sensitivity of the thermometer and the more accurate the measurement results. 2. High resistivity. The larger the resistivity, the smaller the volume of the thermal resistor, the smaller the heat capacity and thermal inertia, and the faster the response to temperature changes. 3. The chemical and physical properties are stable within the temperature measurement range. 4. The resistance value is approximately linear with the temperature.

Since the resistance of general platinum resistors is not large, the resistance change of platinum resistor leads and cables will have a great impact on the temperature measurement results. Generally, it is eliminated by wiring. The lead forms of the measuring element are two-wire, three-wire, and four-wire. The commonly used products when purchasing thermal resistors are three-wire and four-wire.

The two-wire system refers to a wire led out from both ends of the thermal resistor. This measurement method is simple, but since the resistance of the connecting wire cannot be eliminated, it will cause measurement errors and is often used in situations where short distances are not required. The two-wire wiring method is not suitable for thermal resistors with an accuracy level of platinum resistor A.

The three-wire system is to connect one lead at one end of the thermistor root and two leads at the other end. This method is usually used in conjunction with a bridge. Two wires are connected to the two bridge backs of the bridge, and the other wire is connected to the power supply of the bridge, eliminating the error of the wire resistance. This is a commonly used method in industry.

The four-wire system is to connect two wires at both ends of the root of the platinum resistor, two of which provide a constant current to the thermal resistor and convert the resistance into a voltage signal; the other two wires measure the voltage above the thermal resistor. Since the voltage measuring instrument has a high internal resistance and does not shunt, the measured voltage is completely the voltage at both ends of the platinum resistor. This method has high accuracy and is suitable for high-precision measurement.