## Have you chosen the right parameters for the thermal resistance

The accuracy of the temperature measurement circuit needs not only the early selection of the thermal resistance, but also the subsequent hardware design and software algorithm optimization. For the platinum thermal resistance measurement, Hongde measurement and control provides the PT100 interface module tps02 with three wire interface, the high stability measurement circuit including the built-in excitation current source, 24 bit ADC, resistance temperature value linearization algorithm, 2500V electrical isolation, and connecting the platinum thermal resistance To read the temperature value through the IIC digital interface.

Platinum thermistor has good long-term stability and accuracy, and it is a common industrial temperature sensor. In recent years, the production process of thin film printing has reduced the amount of precious metal platinum, and the cost of platinum thermistor has been greatly reduced. When the platinum thermistor is used in combination with the subsequent circuit, we pay attention to the three basic parameters of its nominal resistance, temperature coefficient and accuracy level. We can decide the type selection of platinum thermistor. Understanding the temperature resistance conversion characteristics, measuring current and wiring mode can help us to introduce as little additional circuit error as possible and build a precise temperature measurement circuit.

1. Accuracy class

The accuracy grade and allowable error of platinum thermistor are specified in iec60751. Taking A-level platinum thermistor as an example, Zui large temperature error consists of two parts. The fixed error caused by the deviation of nominal resistance value at 0 ℃ is 0.15 ℃, plus the error introduced by the drift of temperature coefficient is 0.002 ×| t|. Where t is the actual temperature measurement range, when t does not exceed the application temperature range - 30 ~ + 300 ℃ in the accuracy class table, the platinum thermal resistance does not exceed the allowable error of the accuracy class.

When the measured temperature is 100 ℃, the total error of class a platinum thermistor is 0.15 + 0.002 × 100 = 0.35 ℃. In the selection, the nominal resistance, temperature coefficient standard, accuracy grade and application temperature range of platinum thermistor are the basis for our selection.

2. Nominal resistance

The nominal resistance is the resistance of the platinum thermistor at 0 ° C of the freezing point. Pt100zui with nominal resistance of 100 Ω is commonly used, and pt200, PT500 and PT1000 with nominal resistance of 200 Ω, 500 Ω and 1000 Ω are also available.

3. Measuring current

Almost all platinum thermistors are measured by DC current excitation. The measurement current will inevitably generate heat in the resistance and introduce self heating error. There are two parameters in the platinum thermal resistance Manual: measuring current and self heating coefficient. The typical measuring current I is 0.3 ~ 1mA, and the self heating coefficient s is about 0.015 ℃ / MW.

According to the self heating coefficient, the temperature error introduced by the measuring current can be calculated according to the following formula.

For example, given 1mA, when the resistance Zui of Pt100 is 400 Ω, the self heating temperature is about 0.01 ℃, in this case, the error can be almost ignored. When the self heating coefficient of platinum resistance is not affected, the measurement current is set to Zui first, the output voltage amplitude becomes smaller and the signal-to-noise ratio decreases when the current is too small. 1mA is a common measurement current value.

4. Wiring mode

The output lead way of platinum thermistor includes two-wire system, three wire system and four wire value, among which the error introduced by two-wire system lead resistance cannot be eliminated; four wire system has no lead resistance error, but there are many leads; three wire system is based on three leads under the same physical size condition, the lead resistance value is equal, and the lead error can be eliminated through calculation after the resistance value is measured twice. It is used to There are many ways to get Zui.

5. Temperature coefficient

The temperature coefficient TCR is the average resistance change of platinum thermistor per unit temperature between the freezing point and boiling point of water. Different organizations adopt different temperature coefficients as their standards. European iec60751 and Chinese GB / t30121 adopt temperature coefficients of 0.003851, American ASTM e1137 adopt temperature coefficients of 0.003902, and 0.003851 is currently the recognized industry standard in China and most countries.

The calculation process of temperature coefficient is as follows, taking PT100 as an example.

R100 = 138.51 Ω at boiling point of 100 ℃, R0 = 100 Ω at freezing point of 0 ℃. Divide 38.51 of the difference into the nominal resistance and then divide it into 100 ℃, and the result is the average temperature coefficient.

6. Temperature resistance conversion characteristics

The temperature resistance conversion relationship of platinum thermistor is described by the following formula, which can be divided into two cases: below 0 ℃ and above 0 ℃.

When t ≤ 0 ℃: RT = R0 • (1 + a · T + B · T ^ 2 + C • (T-100 ℃) • T3)

When t ≥ 0 ℃: RT = R0 • (1 + a · T + B · T ^ 2)

Among them, RT is the resistance value when the temperature is t, R0 is the resistance value when the temperature is 0 ℃; a, B and C are the three constants specified in iec60751.

The measured temperature T can be solved directly by substituting the resistance value RT into the formula, but it needs to solve the cubic equation, which is complex.

In order to simplify the calculation, the temperature resistance curve of Pt100 in the range of - 200 ~ + 850 ° C is output by the formula. The resistance of Pt100 changes in the range of 18 ~ 400 Ω, and there is a linear temperature resistance conversion relationship.

If the two ends of - 200 ° C and + 850 ° C are directly used for two-point linear calibration, try to simplify the calculation, temperature resistance value curve within the temperature range. At this time, the nonlinear error of Zui is larger than 16 Ω, and the error is larger.

It is a simple and accurate method to generate the temperature resistance table according to the formula, and then make a small range of linear interpolation in the look-up table. The temperature resistance value inquiry table with 1 ℃ interval is attached in iec60751