Digital Real-time Processing of Thermocouple Cold-End Temperature

Computer RT Correction for Thermocouple Cold Junction Temperatur e

GUO Younan

(Department of Electrical Information Engineering, Jiangsu Teachers College of Technology, Changzhou 213001, China)

Key words: thermocouple; cold junction temperature compensation; digital sensor; real time correction

When measuring temperature, the measuring end is placed in the measured medium, and the cold end temperature is generally not 0℃, and its size varies with the ambient temperature. If the test temperature is directly obtained according to the graduation table when the cold end temperature is 0℃, a large error is bound to occur.
The traditional method is to use a compensation wire to extend the cold end to a constant temperature room away from the heat source. Measure the cold end temperature, and calculate the thermoelectric potential corresponding to the 0℃ cold end according to the thermocouple intermediate temperature law:

where eAB(T, T0) is the measured thermoelectric potential, and eAB(T0, 0) is calculated or obtained by looking up the table based on the cold end temperature T0. Based on this, eAB(T, 0) can be calculated, and thus the measured temperature T can be obtained.
The second method is to use the bridge compensation method, connect a compensation bridge to the cold end, and use an external compensation potential that can change with the cold end temperature to compensate for the test error caused by the change in cold end temperature. This method can be used in situations where the cold end temperature changes.
Since the thermoelectric characteristics of the compensation wire material or the temperature-sensitive element in the compensation bridge can only be approximated to a certain extent with the thermoelectric characteristics of the thermocouple, the compensation range and compensation accuracy cannot meet the requirements of high-precision measurement.
The development of microcomputer technology has made it possible to digitally process the cold-end temperature in real time. The author uses a microcontroller and an integrated temperature sensor to make real-time corrections to the cold-end temperature of the thermocouple, so that the temperature measurement accuracy of the thermocouple reaches ±0.5℃ in the full range. This article takes the K-type thermocouple as an example to introduce the design method of the system.

This design uses 89C51 as a microcontroller. The thermoelectric potential generated by the thermocouple is amplified by the precision operational amplifier 7650, and then sent to the four-and-a-half-bit dual-integral AD converter ICL7135 to convert it into a digital quantity and send it to 89C51. The cold end temperature test uses an integrated temperature sensor DS18B20, which is a digital sensor that uses a single bus protocol and is connected to the computer with one line. The hardware structure is shown in Figure 1.

(1) Measuring end temperature. 5-digit display, unit: °C; (2) Cold end temperature. 4-digit display, unit: °C; (3) Digital value obtained after AD conversion of thermoelectric potential eAB (T, T0). 5-digit display, unit: mV.
Use the display selection button to select the display. The indicator light indicates the currently displayed number. The default state displays the measuring end temperature. The working process of the computer is as follows: First, through AD7135, the thermoelectric potential data eAB (T, T0) generated by the thermocouple when the measuring end temperature T and the cold end temperature T?0 are collected. Secondly, the cold end temperature T0 is measured by DS18B20, and then the corresponding thermoelectric potential eAB? (T0, 0) is obtained by querying the graduation table through the software, and then according to the intermediate temperature law, it is calculated: Finally, the temperature T corresponding to eAB (T, 0) is obtained according to the graduation table.

ICL7135 is a commonly used 4-bit half-double-integral AD converter, and the output timing waveform is shown in Figure 2. When the R/H pin is "1" when ICL7135 is working, 7135 is in a continuous conversion state. Every 40002 clock cycles, an AD conversion is completed and the BCD code is output in 4-bit binary form (actually the result of the last conversion). At the same time, the bit synchronization selection signal of each bit is output: D5, D4, D3, D2, D1. The AD conversion result is output in a dynamic scanning mode. That is, when the selection signal D5 = "1", the BCD output is the ten thousandth digit. When D4 = "1", the BCD output is the thousandth digit..., and so on. The negative pulse generated by the digital selection signal STB can be used as the end signal of the AD conversion to send an interrupt request to the microcontroller. In the interrupt service program, first determine whether the highest bit selection signal D5 is valid. If not, wait. When D5 is valid, the 4-bit BCD code appearing on the data line at this time is read into the memory as the ten thousandth digit. Next, the thousands digit strobe pulse D4 is judged to be valid. If not, wait. If valid, read the data into the memory as the thousands digit, and so on. After all 5 digits are read, the interrupt returns.

The cold end temperature is measured using the DS18B20 integrated sensor from DALLAS, USA. This sensor has the following features:
(1) It uses a single bus protocol, that is, only one interface pin is needed for communication. (2) No external components are required. (3) It can be powered by the data line. The measurement range is from -55℃ to +125℃. (4) It outputs temperature data in 9-bit digital form. (5) The digital increment value is 0.5℃. (6) The conversion time is 100ms. (7) It has a user-definable temperature alarm setting.
The operation of DS18B20 is carried out through control commands. DS18B20 has its own instruction set. There are 6 control commands in total. Users can use these instructions to perform related read/write operations. The temperature value of DS18B20 has 9 bits and is expressed in 1/2℃ LSB form. DS18B20 has 9 bytes of data temporary storage memory, byte 0 and byte 1 store the measured temperature value. The low byte is in front and the high byte is in the back. Figure 3 is the flow chart of the DS18B20 temperature measurement program.