Introduction of three temperature measurement methods for PT100 platinum thermal resistance

The three wiring methods of PT100 platinum resistance are different in principle: the two-wire system and the three-wire system are measured by the bridge method, and finally the relationship between the temperature value and the analog output value is given.There is no bridge in the four-wire, it is only sent with a constant current source, measured with a voltmeter, and finally given the measured resistance value, which is difficult and expensive to use.
Due to the small resistance value and high sensitivity of PT100, the resistance value of the lead wire cannot be ignored. The three-wire connection method can eliminate the measurement error caused by the lead wire resistance.

The cross-sectional area andThe lengths are the same (ie rl=r2=r3). The circuit for measuring platinum resistance is generally an unbalanced bridge. The platinum resistance (Rpt100) is used as a bridge arm resistance of the bridge, and a wire (rl) is connected to the bridge.At the power supply end, the other two (r2, r3) are respectively connected to the bridge arm where the platinum thermal resistance is located and the bridge arm adjacent to it, so that both bridge arms introduce lead resistors with the same resistance value, and the bridge is in a balanced state.Changes in lead resistance have no effect on the measurement results.
The result has nothing to do with the magnitude of the electromotive force, which avoids the errors caused by power supply changes, and the problem of excessive conductor voltage division.
The resistance and contact resistance of the connecting wires will have a great impact on the temperature measurement accuracy of the PT100 platinum resistance thermometer, and the platinum resistance three-wire or four-wire wiring method can effectively eliminate this effect.The two-wire system has poor measurement accuracy; the three-wire system has better accuracy; the four-wire system has high measurement accuracy and requires many wires.In contrast, the three-wire system used in this design is more suitable for integration and PT100 resistance acquisition.

We only need to output the voltage signal according to the bridge,The temperature state of PT100 can be known. When the resistance value of PT100 and the resistance value of Rx are not equal, the bridge outputs a differential pressure signal, which is very small.Since the output signal of the temperature sensor is generally very weak, a signal conditioning and conversion circuit is required to amplify or convert it into a form that is easy to transmit, process, record and display.Small changes in the measured signal quantity need to be converted into electrical signals.When amplifying a DC signal, the self-drift and unbalanced voltage of the operational amplifier cannot be ignored. After the operational amplifier is amplified, a voltage signal of the desired size can be output.
The resistance value of platinum resistance can be obtained by circuit calculation or multimeter measurement. When we know the resistance value of PT100, we can measure and calculate the temperature through the resistance value.
There are three common temperature measurement and calculation methods:
Temperature measurement calculation method 1:
When the exact temperature is not known, the temperature of PT100 thermal resistance increases by 1 ohm, the temperature will increase by 2.5℃ (used at low temperature).When the PT100 temperature sensor is at 0°C, the resistance value is 100, so the approximate temperature at this time = (PT100 resistance value -100)*2.5.
Temperature measurement calculation method 2:
The relationship between the resistance value and temperature of platinum resistance is in the range of 0～850℃: Rt=R0(1+At+Bt2);
In -200～0℃Within the range: Rt=R0[1+At+Bt2+C(t-100)3];
Rt represents the resistance of platinum resistance at t°C; R0 represents the resistance of platinum resistance at 0°CResistance value; A, B, C are constants, A=3.96847×10-3/℃; B=-5.847×10-7/℃; C=-4.22×10-12/℃;, and its temperature coefficient is about 3.9×10-3/℃.
Through the above formula, the temperature can be accurately solved according to the resistance value, but due to the large amount of calculation, this method is not recommended for this experiment.
Temperature calculation method 3:
The resistance temperature of PT100 has a good linear relationship with the temperature, which is more suitable for the temperature measurement of medium and low temperature.The resistance value of PT100 at different temperatures has a one-to-one corresponding measurement scale as shown in the figure below, which can intuitively display the corresponding relationship between different temperatures and the resistance value of PT100.
Check the corresponding resistance value through the PT100 indexing table to knowtemperature.In this experimental design, the temperature can be obtained accurately and efficiently by this method.