The temperature transmitter uses thermocouples and RTDs as the temperature measuring element, and the output signal from the temperature measuring element is sent to the transmitter module. It needs to go through voltage stabilization and filtering, operation amplification, non-linear correction, V/I conversion, constant current and reverse protection, and other circuits. That processing before the temperature transmitter can be converted into a 4-20mA current signal 0-5V/0-10V voltage signal with a linear relationship with temperature and RS485 digital signal output.
Temperature transmitters are used in petroleum, chemical, chemical fiber, textile, rubber, building materials, electric power, metallurgy, medicine, food, and other industrial fields to control on-site temperature measurement. Temperature transmitters are also particularly suitable for automatic measurement and control systems and can also be used in conjunction with instruments.
1. Features of temperature transmitters
- Power supply voltage: DC10V~32V.
- Output signal 4-20mA superimposed on HART□ protocol digital communication (two-wire system), HART communication does not affect the 4-20mA analog output.
- Temperature transmitters can be managed remotely through the hand controller and PC configuration and debugging software.
- It has an Internal Pt100 measurement of ambient temperature for thermocouple cold-end compensation.
- Cold end compensation accuracy: 0.5℃.
- Damping: 0-32 seconds adjustable.
- Working temperature environment: -40℃~+85℃ (LCD working temperature range: -20℃~+70℃).
- External dimensions: ¢44mm.
- Installation hole spacing: 33mm.
- It has an Anti-mechanical vibration performance: 10~60HZ, 0.21mm sine wave.
- Anti-radio frequency interference: IEC61000-4-3, 20V/M, 80~1000MHZ.
2. Advantages of temperature transmitters
What are the main advantages of temperature transmitters currently in industrial production, temperature transmitters have been widely used? Are their benefits shown in the technical aspects and make the preset more economical? If a thermocouple is used as deemed appropriate for measuring temperature in the field. If we use integrated temperature transmitters, we can eliminate the need for many reimbursement wires and use ordinary cables or shielded wires.
2.1 Survey range
The survey range and temperature output signal corresponding to the temperature range set by the user. When we chose the survey range, it should be larger than the actual application range, leaving a certain margin. The actual application of the temperature zone is generally only about 70 percent of the survey range.
2.2 Application conditions
The application conditions and background temperature requirements should depend on the specific location. It is essential to select the one with a wide application temperature zone for outdoor use in the north, while in the areas south of the Changjiang River, indoor requirements can be lower.
2.3 Survey temperature
When we survey higher temperatures, we should pay attention to the temperature at which the temperature transmitter plate can not exceed the specified range. Otherwise, the wall-mounted transmitter should be selected, or use the repayment wire to lead away from a little; power supply voltage changes, load resistance changes on the precision of the survey should be small, the error brought about by the background temperature should be small.
2.4 Temperature range
We often use 600 ℃, such as platinum resistance. The average temperature has reached its maximum limit of the use temperature. We need to consider the temperature overshoot, the use of components and their survival of the years, and other factors, so we should choose to use a K-type thermocouple to make the temperature range have a large margin.
2.5 Measuring range
The range of the group transmitter is set at 800℃, because the correctness of the survey is relative to the field, the larger the field, the error increases, so the range of the temperature transmitter should be left open and not too large, so as not to lose the precision of the survey.
2.6 Select dimensions
When we select the shape size, we should choose 500mm<800mm, and extend the difference between the total length and the depth of insertion from the standard 150mm to 300mm to reduce the impact of thermal radiation and thermal conductivity of the furnace body on the transmitter. When conditions permit, the transmitter plate should be made farther away from the high-temperature furnace body to reduce the impact of the background temperature on the precision of the survey.
3. Working principle of temperature transmitter
The temperature transmitter can convert physical measurement signals or common electrical signals into standard electrical outputs, and it is also capable of outputting devices using communication protocols. Current transmitters convert the AC of the primary circuit under test into a constant current loop standard signal, which is continuously delivered to the receiving device.
4. The role of temperature transmitter
A temperature transmitter transforms the signal from a temperature sensor into a current password connected to a secondary instrument, thus displaying the corresponding temperature. For example, the temperature sensor in the diagram is of type PT100, so the function of the temperature-current transmitter is to convert the resistance signal into a current signal, which is fed into the instrument and displays the temperature.
5. Inspection of temperature transmitter
Before we check, we can first short circuit Figure 1 circuit 2 and 1 terminals and observe whether the DCS display is the ambient temperature around the temperature transmitter. No display may be the temperature transmitter to the DCS line, or the temperature transmitter has a fault. If the ambient temperature is displayed, continue to check downwards, disconnect the thermocouple connected to terminals 2 and 1 of circuit 1 and the thermocouple connected to terminals 4 and 3 of circuit 2, and use the process calibrator to input a potential thermal signal to the temperature transmitter according to the thermocouple index number used. Observe whether the DCS displays the corresponding temperature to determine whether the temperature transmitter is standard.
6. Temperature measurement with temperature transmitters
6.1 Temperature measurement by RTD
An RTD (e.g., Pt100) is a temperature sensor that uses the principle that its resistance value changes with temperature to convert a temperature quantity into a resistance quantity.
There are three types of wiring between the RTD and the temperature transmitter: 2-wire, 3-wire, and 4-wire.
As in Figure 2, the transmitter applies an excitation current I to the RTD through wires L1 and L2 and measures the potentials V1 and V2.
As the resistance of the connecting wire RL1, RL2 can not be measured and is counted in the resistance value of the RTD so that the measurement results produce an additional error. For example, if the thermal resistivity of the Pt100 RTD at 100°C is 0.379Ω/°C, then if the resistance value of the wire is 2Ω, the measurement error caused will be 5.3°C.
Three-wire is the most common connection for practical applications. As in Figure 3, an additional wire is used to compensate for the measurement error caused by the resistance of the connecting wire. The three-wire system requires three wires of the same material, wire diameter, length, and the same operating temperature so that the resistance of the three wires is the same, i.e., RL1 = RL2 = RL3. The potential V1, V2, V3 is measured by applying an excitation current I to the RTD through wires L1, L2. wire L3 is connected to a high input impedance circuit, IL3 = 0.
This results in a three-wire connection that compensates for the measurement error caused by the resistance of the connecting wires.
A four-wire system is the ideal wiring method for RTD temperature measurement. The potential V3 and V4 are measured by applying an excitation current I to the RTD through wires L1 and L2. Wires L3 and L4 are connected to a high input impedance circuit, IL3 = 0 and IL4 = 0, so V3-V4 is equal to the voltage across the RTD.
It follows that the four-wire measurement method is unaffected by the resistance of the connecting wires.
6.2 Temperature measurement by TC
We solder two conductors of different materials, A and B, as in Figure 5. When the temperature at the connection point (the hot end) differs from the temperature at the other end of the conductor (the cold end, also known as the reference end), a thermal potential is generated at the cold end. A thermocouple is a temperature sensor that uses this phenomenon to convert a temperature quantity into an electrical potential abundance.
If the temperature at the cold end of the thermocouple is kept constant (e.g., 0°C), the output term potential and the temperature value at the hot end are in one-to-one correspondence. This is because the temperature transmitter measures the difference in potential at the output of the thermocouple and converts the difference in potential into temperature, thus enabling temperature measurement.
The temperature transmitter is a device that converts RTD, thermocouple, resistance, and millivolt signals into standard two-wire 4-20mA and transmits the call to the control room. It is generally used in industrial sites.