What causes the error of thermal resistance thermometer?

Thermal resistance thermometers are used to measure a certain range of temperatures in industrial production. They are widely used because of their high temperature measurement accuracy and convenience for automatic measurement. Since the thermoelectric potential generated by thermocouples in the low temperature range is small, strict requirements are placed on the measuring instrument, and thermal resistance thermometers are very suitable for measuring low temperatures.
(1) Heat transfer error. It is an error caused by insufficient contact with the object being measured and failure to reach thermal equilibrium during temperature measurement. When using, you should pay more attention to the relevant instructions.
(2) Graduation error. The standardized thermal resistance graduation table is generated by statistical analysis. However, the specific thermal resistance used will vary due to materials and manufacturing processes, which will form a graduation error, such as the error introduced by any discrepancy with the nominal resistance value.
(3) Self-heating error. This is an additional error caused by the rise in current when the current flows through the thermal resistance during the measurement process. It is related to the current size and the heat transfer medium. The thermal resistance used in my country's industry limits the current to no more than 6mA, so that the temperature error can be limited to within 0.1℃.
(4) Errors in the measuring circuit and display instrument. It is determined by the accuracy level of the display instrument itself and the line resistance. For example, when using a Cu50 type copper resistor to measure temperature, the resistance of the copper wire is 50 under specified conditions, and the instrument indicates that the measured temperature is 4070. If the ambient temperature changes by 10°C at this time, the two-wire connection wire will bring an error of about 2°C to the measured value, and the three-wire connection will bring an error of 0.1°C. In addition, changes in the limit of the lead resistance and the connecting wire will also cause errors.
(5) Other errors. This refers to errors other than the above errors caused by poor shielding insulation, insufficient insertion depth, and thermal resistor degradation.
When installing the Pt100 thermal resistor, its insertion depth should not be less than 8 to 10 times the outer diameter of the thermal resistor protection tube, and the heated part of the Pt100 thermal resistor should be extended as much as possible. The Pt100 thermal resistor should be installed vertically as much as possible to prevent bending and deformation at high temperatures. In order to reduce the errors caused by radiant heat and heat conduction during the use of Pt100 thermal resistors, the surface temperature of the protective sleeve should be as close as possible to the temperature of the measured medium, and the black coefficient of the Pt100 thermal resistor protective sleeve should be reduced.
How many types of thermal resistors are there?
Thermal resistors can be divided into platinum thermal resistors with PT10, PT100, PT500, PT1000, and copper thermal resistors with CU50 and CU100. Among them, the commonly used platinum resistor temperature measurement range is -200~850℃, and the copper resistor temperature measurement range is -50~150℃.
Thermal resistor error
Thermal resistor allowable error (℃) = ± (0.15+0.002 X|t|) t---actual temperature
For example: t=0℃ Thermal resistor allowable error value = ±0.15℃
t=100℃ Thermal resistor allowable error value = ± (0.15+0.002X100) = ±0.35℃