Methods for improving the measurement accuracy of platinum resistance thermometers

The commonly used temperature measuring instruments in industry include glass liquid thermometers, thermocouple thermometers, thermal resistance thermometers, etc.
Glass liquid thermometers have advantages such as intuitiveness and simple structure, but they cannot automatically record, cannot be transmitted remotely, are fragile, and have low accuracy for engineering temperature measurement. The advantages of industrial thermocouples are low price, easy installation and maintenance, low thermal inertia, and convenient long-distance transmission. The disadvantage is that they require cold end temperature compensation and have low accuracy in industrial measurement. Platinum resistance thermometers in thermal resistance have good stability and repeatability, short thermal response time, and a wide working temperature range, and are widely used for temperature measurement in the medium temperature range (-200~650 ℃). Standard platinum resistance thermometers have high accuracy, but the stability assessment of materials during production is complex and the production cycle is long, which is expensive. At the same time, the precision requirements for measuring instruments are high, which is too costly for industrial measurement. According to the national verification regulations, the maximum allowable error of A-grade platinum resistors in OC is ± 0.15 ℃. Their price and usage conditions are easily accepted in industrial production. How to improve their accuracy without increasing costs, we have obtained from experiments.
This experiment tests the performance of a volumetric refrigeration compression condensing unit. According to the requirements of the national standard GBT "Volumetric Refrigeration Compression condensing Unit", the error of each measurement point is required to be controlled within the range of 0.1 ℃. Simply using a thermocouple thermometer or platinum resistance thermometer for temperature measurement cannot meet the accuracy requirements. This article proposes a comprehensive system calibration method, which still uses the traditional Pt100 platinum resistance temperature automatic detection system to test the entire measurement system including platinum resistance, signal lines, acquisition board, computer, etc. Linear regression is performed on each platinum resistance, and the regression data is input into the measurement program to correct each measured temperature data. This can improve the measurement accuracy of the entire measurement system and improve the overall performance Low price, economical and practical. Practical use has shown that the temperature detection system modified by this method not only meets the accuracy requirements, but also has very good stability and meets the deviation requirements of the test specifications.
2 Experimental System
This experiment is to test the performance of a volumetric refrigeration compression condensing unit, using an air-cooled method. The temperature points and maximum allowable values that need to be tested The error requirements are as follows:
(1) The inlet air temperature of the condenser of an air-cooled unit should be taken from the average reading of at least four temperature measuring instruments placed at positions that can indicate the average air temperature, or it can be measured using a sampling device. The maximum allowable error is required to not exceed ± 0.1 ℃ In this experiment, 5 temperature measurement points were arranged at the inlet of the condenser, labeled as 1, 2, 3, 4, and 5 respectively;
(2) The maximum allowable error of the refrigerant temperature before and after the subcooler should not exceed ± 0.1 ℃. Arrange temperature measurement points as 6,7;
(3) The maximum allowable error for the refrigerant temperature at the inlet of the compressor should not exceed ± 0.1 ℃. Set the temperature measurement points to 8;
(4) The maximum allowable error for environmental temperature should not exceed ± 01 ℃. Set the temperature measurement point to 9.
3 Solutions
The temperature measurement automatic detection system used in the experiment adopts the remote thermal resistance acquisition module RM411 and P100 platinum resistance of Zhongtai software. The RM411 remote thermal resistance module consists of a CPU, optoelectronic isolation, A/D conversion, channel conversion, three terminal current excitation, RS485 interface, site switch, and DC/DC. Using VB language to develop a collection program, the functions that can be implemented include: data collection and storage of various test parameters of the condensing unit, display of real-time data, generation of historical curves, etc.
Characteristics of automatic detection system:
(1) The Pt100 platinum resistor adopts a three wire connection method to reduce the impact of lead resistance; The signal line uses three core copper wire with shielded wire to reduce the interference of surrounding strong electricity on the data signal;
(2) Computer automatically detects and stores data in real-time, reducing human error;
(3) Inspect and verify the entire testing system of the experiment to eliminate secondary instrument errors and system errors. The entire system that has been verified in this way can be moved back to actual measurement, and the verification accuracy is the measurement accuracy.