Temperature is a critical parameter in industrial processes. Temperature transmitters are essential for both processing and temperature control. They can monitor temperature information for production safety and processing quality. Therefore, they play a vital role in temperature control.
This article primarily explains the working principles of temperature transmitters. Hopefully, this article will provide you with a general understanding of temperature transmitters.
What is a Temperature Transmitter?
A temperature transmitter converts temperature signals into a standard electrical signal. It is primarily used to measure and control temperature parameters in industrial processes. A current transmitter converts the measured main circuit AC into a constant current loop standard signal and continuously transmits it to a receiving device.
Temperature transmitters use thermocouples and RTDs as temperature measuring elements. The output signal from the measuring element is sent to the transmitter module. After processing through circuits including voltage stabilization and filtering, operational amplification, nonlinearity correction, V/I conversion, constant current, and reverse protection, the signal is converted into an electronic signal output. The signal is linearly related to temperature.
How does a Temperature Transmitter Work?
Temperature transmitters are mainly divided into thermocouple temperature transmitters and RTD temperature transmitters.
Thermocouple Temperature Transmitter
Principle of Operation
Thermocouples utilise the Seebeck thermoelectric effect, whereby two dissimilar metals form a closed circuit; a temperature difference between the two ends generates a faint thermoelectric potential.
The transmitter acquires this millivolt-level thermoelectric potential, performs cold-junction compensation, amplification and linearisation, and converts it into a standard 4–20 mA current output. The current value corresponds linearly to the measured temperature and is transmitted to a secondary control instrument.
Advantages
1. Wide temperature measurement range, covering temperatures from ultra-low to over 1,000 degrees, with superior adaptability to high-temperature operating conditions.
2. Compact probe size and low thermal mass result in fast temperature response, making it suitable for confined installation locations.
3. High-temperature resistance, shock and vibration resistance, and a robust structure make it suitable for high-temperature, vibrating environments such as boilers and heat treatment furnaces.
4. The low cost of the sensor itself means that large-scale deployment can effectively reduce the overall project cost.
Disadvantages
1. Measurement accuracy at low temperatures is relatively low; weak signals are prone to errors caused by line resistance and electromagnetic interference.
2. The thermoelectric potential is non-linear and relies on transmitter algorithms for correction; it is difficult to control deviations in the low-temperature range.
3. Cold-junction compensation is essential; significant fluctuations in ambient temperature can directly cause measurement drift.
4. Dedicated compensation leads are required; incorrect wiring or excessively long cables will increase measurement errors.
5. Measurement accuracy at room temperature is inferior to that of RTD (Resistance Temperature Detector) thermistors.
RTD Temperature Transmitter
Principle of Operation
Resistance temperature detectors (RTDs) measure temperature based on the property that the resistance of platinum increases in tandem with temperature; at 0°C, the resistance of a PT100 is 100 Ω.
The transmitter uses a Wheatstone bridge to measure changes in the RTD’s resistance; after compensating for lead resistance, it amplifies and conditions the signal, converting it into a linear 4–20 mA standard current for the stable transmission of temperature data over long distances.
Advantages
1. High measurement accuracy within the normal temperature range, with inherent good linearity and excellent measurement repeatability and stability.
2. No cold-junction compensation is required; changes in ambient temperature are unlikely to cause significant measurement drift, and commissioning and maintenance are straightforward.
3. Strong signal immunity to interference and a high signal-to-noise ratio, making it suitable for precision temperature control, clean piping systems and laboratory equipment.
4. The PT100 specification is universal, ensuring good compatibility with associated instruments and control systems, and facilitating the replacement of spare parts.
Disadvantages
1. The upper temperature limit is approximately 600°C; it cannot be used in ultra-high-temperature measurement scenarios such as smelting or high-temperature furnace chambers.
2. It has high thermal mass, resulting in response lag during sudden temperature changes; it is not suitable for rapid transient temperature measurement.
3. Lead resistance affects measurement accuracy; three-wire or four-wire configurations are preferred for long-distance wiring.
4. The internal platinum wire is prone to breakage due to severe vibration, resulting in a short service life in equipment subject to strong vibrations.
5. Prolonged exposure to high temperatures causes oxidation and drift in the platinum wire, reducing service life; procurement costs are higher than those of standard thermocouples.
Sino-Inst Featured Temperature Transmitter
Temperature Transmitter Input Signal
Temperature transmitter input signals come in two types: RTDs and thermocouples. The following is a detailed description:
RTD Input Signal
Based on the element structure, RTD input signals can be categorized as either wire-wound RTDs or thin-film RTDs. The following is a detailed introduction.
1. Wirewound RTD
This type of RTD has a small-diameter wire, most commonly platinum, wound into a coil placed within a ceramic/glass insulator. Extension wire is soldered to this platinum coil, which extends outside the insulator. Compared to thin-film sensors, this type of sensor is longer and more delicate. Wirewound RTDs offer good accuracy over a wider temperature range.
2. Thin-film RTD Element
Thin-film elements are made by depositing a very thin layer of resistive platinum metal onto a ceramic substrate. This film is then coated with epoxy or glass, which helps protect the deposited film and also serves as a strain relief for the external leads. This type of RTD performs better in vibration applications and for field temperature measurements.
Due to their versatility and cost-effectiveness, thin-film sensors are the most widely used sensor type. RTDs can also be differentiated based on the resistive element material. They are often referred to as Pt100, P1000, Ni120, Cu100, etc.
The letter indicates the element material, and the number indicates the resistance value at zero degrees Celsius. Therefore, a Pt100 element has a platinum resistance element and a resistance of 100Ω at 0°C. A Ni120 element has a nickel resistance element and a resistance of 120Ω at 0°C.
Thermocouple Input Signal
Thermocouple temperature transmitters (K, S, E, B, R, T, J, N, etc.) receive signals from field thermocouple sensors, process them, and convert them into a standard output signal. The signal is linearly proportional to temperature.
Common Thermocouples
1. Type K thermocouple: Made of nickel-chromium–nickel-silicon, with a temperature measurement range of –200 to 1,300 °C. It offers good value for money and excellent resistance to oxidation. As a general-purpose industrial model, it is suitable for routine temperature measurement in boilers and heat treatment processes.
2. Type S thermocouple: Made of platinum-rhodium 10–platinum, with a temperature range of 0–1,600 °C. It offers extremely high measurement accuracy and excellent stability, and is commonly used for high-temperature calibration and in precision kilns, though it is expensive.
3. Type B Thermocouple: Made of platinum-rhodium 30–platinum-rhodium 6, with a maximum temperature of 1,800 °C. It offers excellent high-temperature oxidation resistance and is suitable for ultra-high-temperature smelting applications; its low-temperature thermoelectric potential is extremely low, eliminating the need for cold-junction compensation.
4. Type E thermocouple: Made of nickel-chromium and copper-nickel, it has high sensitivity at low temperatures, with a measurement range of –200 to 800 °C. It is suitable for low-temperature and vacuum equipment, but is susceptible to corrosion by sulphurous gases.
5. Type J thermocouple: Iron–copper-nickel composition, with a temperature measurement range of –200 to 750 °C. It offers high sensitivity and is inexpensive, but cannot be used in high-temperature oxidising environments, as the iron wire is prone to rust.
6. Type T thermocouple: Copper–copper-nickel composition, with excellent low-temperature accuracy and a temperature measurement range of –200 to 350 °C. It is commonly used in low-temperature cold stores and for precision low-temperature measurements in laboratories.
Different thermocouples have different operating ambient temperatures. The following is a detailed introduction. For operating temperatures between 1300°C and 1600°C, B-type thermocouples are generally used when high accuracy is required.
- For operating temperatures between 1100°C and 1300°C, S-type and R-type thermocouples are generally used.
- For temperatures between 400°C and 1100°C, K-type and N-type thermocouples are generally used.
- For temperatures above 400°C, E-type and J-type thermocouples are generally used.
- For temperatures below 300°C and below negative temperatures, T-type thermocouples are generally used. At low temperatures, T-type thermocouples offer greater stability than other thermocouples.
Temperature Transmitter Output Signal
Temperature transmitters generally output the following signal types:
Analog signals are the most commonly used output signal types for temperature transmitters, and include the following:
Current signals are one of the most commonly used analog signals. And 4-20mA is the most common current output signal used in industrial measurements.
Voltage signals typically use a 0-5V or 0-10V voltage range. They are suitable for short-distance transmission and applications requiring high accuracy.
Resistance Signals
Resistance signals typically use platinum resistance sensors such as PT100 and PT1000. The output resistance value is proportional to the temperature. Resistance signals offer high accuracy, but relatively low interference immunity. They are suitable for short-distance transmission and applications requiring high precision.
Digital Signals
Digital signals have rapidly developed in recent years and include the following types of output signals. RS-485 and HART, a serial communication protocol that uses differential signal transmission. They are suitable for long-distance transmission and signal transmission in complex environments.
Differences between RTD and Thermocouples
1. Different measurement principles: Resistance temperature detectors rely on the characteristic that the resistance of a metal changes with temperature; thermocouples utilise the Seebeck effect, whereby a thermoelectric potential is generated by the temperature difference between two metals.
2. Different applicable temperature ranges: Resistance temperature detectors are mostly used in ambient and low-temperature scenarios ranging from –200 to 600 °C; thermocouples can measure temperatures from –200 to 1,800 °C and are suitable for various high-temperature operating conditions.
3. Different measurement accuracy: Resistance temperature detectors offer high accuracy and good linearity at ambient temperatures; thermocouples have poor accuracy at low temperatures, with non-linear signals requiring calibration and compensation.
4. Different response speeds: Resistance temperature detector elements have high thermal capacity, resulting in significant measurement lag; thermocouple probes are compact and offer faster temperature response.
5. Different wiring requirements: Resistance temperature detectors require three-wire or four-wire systems to compensate for lead resistance errors; thermocouples must be used with dedicated compensation leads and also require cold-junction compensation.
6. Differences in environmental tolerance: RTDs are not resistant to severe vibration and are prone to ageing and wire breakage at high temperatures; thermocouples have a robust structure and offer superior resistance to vibration and high temperatures.
7. Differences in operating costs: Conventional thermocouples are less expensive; high-precision platinum RTDs have a higher purchase cost.
For more information, please see：RTD vs Thermocouple: 8 Key Differences
How to Check a Temperature Transmitter?
We can verify the proper operation of a temperature transmitter by checking the following aspects:
- Check the power supply and wiring.
- Check the temperature sensor.
- Check the installation location and method.
- Check the signal wiring.
- Check other equipment.
How to Troubleshoot a Temperature Transmitter?
When a temperature transmitter exhibits the following faults, we can take appropriate measures to resolve the issue.
Low External Power Supply Voltage:
The temperature transmitter is drawing current internally but lacks external power. This may be due to a problem with the transmitter itself. This is usually caused by insufficient power to the transmitter.
External grounding or short circuit:
This may be caused by the temperature transmitter being ungrounded or improperly installed. In this case, it must be grounded.
Display error or no display:
If the meter shows no display at the output, it may be related to a problem with the signal source or output amplifier. If there is a problem with the input signal, contact the transmitter manufacturer.
Output indication is 0 or the output indication remains unchanged:
This is primarily due to a missing signal line within the measurement module. If this fault is detected, the test module must be replaced and the device must be reinstalled.
Sensor damage:
This is usually caused by damage to the sensor itself or internal wiring. The sensor must be replaced and the transmitter system reinstalled.
Power failure:
First, check whether the transmitter is powered. If it is not, try replacing the transmitter and resetting the power switch and transfer switch. Then, connect the meter to the load and enable the output port to restore normal function.
Ambient temperature is too low:
Low ambient temperature is often encountered at the temperature transmitter installation site. If this fault is detected, check the transmitter installation properly. Or it could be due to factors such as improper placement of the transmitter during installation.
Measurement accuracy is low:
If the test data is lower than the standard, first check whether the measurement precision meets the standard requirements. If the measurement accuracy exceeds the standard, consider replacing the measurement module or calibrating it before testing. If the measurement accuracy is lower than the standard, you can choose to replace the module or calibrate it.
The sensor is far away from the measured medium:
If the sensor is not in contact with the liquid and static electricity is generated during contact, the display may not be displayed. If the sensor and fluid are sealed (liquid seals can leak at high temperatures), reinstall the temperature transmitter to ensure normal measurement results.
How to test a 4 to 20 mA temperature transmitter with a multimeter?
The specific steps for measuring the two-wire 4-20 mA current of a transmitter are as follows:
Set the multimeter to current measurement mode. Connect the red test lead to the positive output terminal of the transmitter and the black test lead to the negative terminal. The value directly read on the meter is the measured current.
In short, temperature transmitters are common devices for measuring temperature. They are used in applications requiring temperature control.
Sino-Inst has over 20 years of experience in temperature measurement solutions. If you have any problems with temperature transmitters, please feel free to contact us. We are your ideal temperature measurement partner.
