A temperature transmitter is used in a wide range of applications. Because of the wide variety of environments, sites, and supporting instruments, engineers, technicians, or maintenance personnel will encounter various problems during use. It is used in petroleum, chemical, chemical fiber, textile, rubber, building materials, electric power, metallurgy, medicine, food, and other industrial fields.
1. The principle of temperature transmitter
The temperature transmitter converts the thermocouple temperature measurement millivolt signal, RTD temperature resistance signal into 4~20 mA DC signal or 1~5V DC voltage signal for display instrument or temperature control system.
On-site temperature measurement process control is especially suitable for automatic measurement and control systems and can also be used in conjunction with instruments.
So, what is the role of temperature transmitters? How to deal with temperature transmitter failure?
2. The function of the temperature transmitter
2.1 Amplification function
Sensors and transmitters play a pivotal role in instrumentation and industrial automation. Unlike sensors, transmitters can convert non-electricity into measurable electricity and generally have a specific amplification effect.
2.1.1 Temperature probe function
An integrated temperature transmitter comprises a temperature measurement probe (TC or RTD sensor) and a two-wire solid-state electronic unit. It uses a solid-state module to mount the temperature probe directly within the junction box, thus forming an integrated transmitter. Integrated temperature transmitters are generally divided into RTD and TC types.
2.1.2 TC temperature transmitter
TC temperature transmitter is generally composed of circuit units such as reference source, cold-end compensation, amplification unit, linearization processing, V/I conversion, coupling break processing, reverse connection protection, current limit protection, etc. It is to amplify the TC generated thermal potential by cold-end compensation, then the linear circuit to eliminate the nonlinear possibility and temperature, and finally strengthen and convert it to 4 ~ 20 mA current output signal.
2.1.3 RTD temperature transmitter
RTD temperature transmitter comprises a reference unit, an R/V conversion unit, linear circuit, reverse connection protection, current limit protection, a V/I conversion unit, etc. The temperature measuring RTD signal is converted and amplified. The linear circuit compensates for the nonlinear between temperature and resistance and outputs a constant current signal of 4 to 20 mA linearly related to the measured temperature after the V/I conversion circuit.
2.2 The function in measuring temperature
The millivolt signal of the thermocouple and the resistance change signal of the RTD is converted into a unified current signal by the temperature transmitter and input to the display and recording instrument, which can be used as automatic temperature detection and input to the regulator and can also form an automatic adjustment system for automatic temperature adjustment. After conversion into the electronic computer, it can be used for temperature tour detection, computer control, etc.
2.3 The physical function of the temperature transmitter
We want to convert physical measurement signals or common electrical signals to standard electrical output or output devices in a communication protocol. A temperature-current transmitter converts the signal from a temperature sensor into a current signal connected to a secondary instrument to display the corresponding temperature.
According to the measurement method, we can classify them into two categories: contact and non-contact. In actual use, we usually use them together with some instruments, but there are also a lot of failure phenomena.
3. Temperature transmitter failure problems
When the measured medium temperature increases or decreases, the output of the transmitter does not change, most of this situation is the temperature transmitter sealing problems, may also be due to the temperature transmitter is not closed or in the welding time accidentally welded a small hole in the sensor, this situation generally requires replacement of the sensor housing to solve.
3.2 Output Signal
The output signal of the temperature transmitter is unstable, and this cause is the temperature source itself. The temperature source itself is an inconsistent temperature, and if the instrument display is unstable, it is the cause of the instrument’s anti-interference ability. It is not strong.
3.3 Output errors
The output error of the temperature transmitter is significant, and there are more reasons for this situation. It may be that the temperature transmitter we choose is not the right resistance wire leading to the wrong range, or it may be that the transmitter is not calibrated when it leaves the factory.
Temperature transmitter failure is rare. As long as the factory is for careful testing, these situations can be avoided, so the temperature transmitter in the factory must be tested. Customers can also find the transmitter manufacturer to ask for factory test reports for reference.
4. Notes on temperature transmitters
The power supply to the temperature transmitter must not have spikes. Otherwise, the transmitter will be easily damaged.
The calibration of the transmitter should be carried out after 5 minutes of power supply, and to pay attention to the ambient temperature at that time. Measurement of high temperature (> 100 ℃) between the sensor cavity and the junction box should be filled with material isolation to prevent the junction box temperature is too high to burn the transmitter.
In the case of serious interference using the sensor, the shell should be firmly grounded to avoid interference, power and signal output should be used Ф10 shielded cable transmission, crimp nuts should be tightened to ensure airtightness.
Only the RWB type temperature transmitter has 0 ~ 10mA output, for the three-wire system, in the range value of 5% or less, due to the triode-off characteristics caused by non-linearity. Therefore, the temperature transmitter should be calibrated once every six months. If the DWB cannot be corrected for linearity due to circuit limitations, it is best to select the range according to the instructions to ensure its linearity.
In the industrial automation process, the temperature is a significant controlled variable, and a good or lousy temperature control will directly affect the product quality. Therefore, our requirements for accuracy and scientificity of temperature measurement have been greatly improved.