Views: 1 Author: Site Editor Publish Time: 2023-05-17 Origin: Site
A temperature transmitter is a device that converts the signal produced by a temperature sensor into a standard instrument signal that represents the temperature of the process variable being measured and controlled. It is mainly used for the measurement and control of temperature parameters in industrial processes. The temperature transmitter uses thermocouples and thermal resistors as temperature measuring elements, and the output signal from the temperature measuring elements is sent to the transmitter module, after voltage stabilization filtering, operational amplification, nonlinear correction, V/I conversion, constant current and reverse After protection and other circuit processing, it is converted into a 4-20mA current signal 0-5V/0-10V voltage signal that is linearly related to the temperature, and the RS485 digital signal is output.
1. Amplification of temperature transmitter
Sensors and transmitters play a pivotal role in instrumentation and industrial automation. Unlike sensors, transmitters generally have a certain amplification effect in addition to converting non-electricity into measurable electricity.
Thermocouple temperature transmitters are generally composed of circuit units such as reference source, cold junction compensation, amplification unit, linearization processing, V/I conversion, burnout processing, reverse connection protection, and current limiting protection. It amplifies the thermoelectric potential generated by the thermocouple through cold junction compensation, and then uses a linear circuit to eliminate the nonlinear error between the thermoelectric potential and temperature, and finally amplifies and converts it into a 4-20 mA current output signal.
The thermal resistance temperature transmitter is composed of a reference unit, R/V conversion unit, linear circuit, reverse connection protection, current limiting protection, V/I conversion unit, etc. After the temperature measuring thermal resistance signal is converted and amplified, the linear circuit compensates the nonlinear relationship between temperature and resistance, and outputs a 4-20 mA constant current signal that is linear with the measured temperature after passing through the V/I conversion circuit. .
The integrated temperature transmitter is generally composed of a temperature measuring probe (thermocouple or thermal resistance sensor) and a two-wire solid-state electronic unit. The temperature measuring probe is directly installed in the junction box in the form of a solid module, thus forming an integrated transmitter. Integrated temperature transmitters are generally divided into two types: thermal resistance and thermocouple.
2. The role of temperature transmitter in temperature measurement
The millivolt signal of the thermocouple and the resistance change signal of the thermal resistance are converted into a unified current signal by the temperature transmitter, and input to the display and recording instrument, which can be used as an automatic detection of temperature and input to the regulator. An automatic adjustment system is formed to automatically adjust the temperature. After conversion and input into the electronic computer, it can carry out temperature circuit detection, computer control and so on.
According to the measurement method, it can be divided into two categories: contact type and non-contact type. In actual use, it is usually used with some instruments, but there are also many failures.
First, the output of the transmitter does not change when the temperature of the measured medium rises or falls. This situation is mostly caused by the sealing of the temperature transmitter, which may be due to the fact that the temperature transmitter is not sealed well or is not careful during welding. There is a small hole welded in the sensor, which generally needs to be solved by replacing the sensor housing.
Second, the output signal is unstable. The reason for this is the temperature source itself. The temperature source itself is an unstable temperature. If the instrument shows instability, it is because the anti-interference ability of the instrument is not strong.
Third, the output error of the transmitter is large. There are many reasons for this situation. It may be that the resistance wire of the selected temperature transmitter is wrong, resulting in a wrong range, or it may be that the transmitter was not calibrated when it left the factory. It is rare for a temperature transmitter to fail, as long as it is carefully tested when it leaves the factory, these situations can be avoided. Therefore, the temperature transmitter must be inspected when it leaves the factory, and customers can also ask the transmitter manufacturer for a factory inspection report for reference.