Temperature sensors use thermocouples and RTDs as the core temperature measurement element. Temperature sensors are responsible for directly measuring temperature and converting it into electrical signals. The temperature transmitter is responsible for processing and converting these signals. The processed signals can be connected to control systems for long-distance transmission and control system integration.
In this post, we will focus on the difference between temperature transmitters and temperature sensors. We hope that this post will be helpful to you.
What is a Temperature Sensor?
A temperature sensor is a sensor that senses temperature and converts it into a usable output signal. Two common types of temperature sensors are thermocouples or RTDs. Once a temperature sensor senses a temperature change, the thermocouple or RTD will change accordingly. This change is then converted into a weak electrical signal.
Temperature Sensor Types
Temperature sensors are the core part of temperature measuring instruments and come in a wide variety. According to the measurement method, it can be divided into contact and non-contact. According to the sensor’s electronic component characteristics, Temperature sensors are divided into RTD and thermocouple.
An RTD temperature sensor utilizes the nature of an object’s resistance to change with temperature to measure temperature. The resistance value of the RTD shows a linear relationship with the temperature. The material of RTD is generally metal, such as copper, nickel, iron, and so on. It also has some platinum, gold, and other heavy metal materials.
RTD temperature sensor has the advantages of a wide measuring range, high precision, good stability, and strong anti-interference ability. But its response speed is slower, and the measurement error is larger. And need to compensate for the wire. In the selection of RTD temperature sensors, it is necessary to make comprehensive consideration according to the specific measurement needs and environmental conditions in order to achieve the best measurement results.
Thermocouple temperature sensors are the most commonly used temperature sensors for temperature measurement. Its main benefits are a wide temperature range and adaptability to various atmospheric conditions. It is also sturdy, inexpensive, requires no power supply, and is the cheapest. Thermocouple temperature sensors are not suitable for high-precision applications.
Temperature Sensor Working Principle
Thermocouple Temperature Sensors
The basic principle of thermocouple temperature measurement is that two conductors of different compositions form a closed loop. When there is a temperature gradient between the two ends of the loop, a current will flow through the loop. At this time there is an electric potential between the two ends – thermal electromotive force, which is known as the Seebeck effect.
Two homogeneous conductors of different compositions are used as thermal electrodes, with the higher temperature end being the working end and the lower temperature end being the free end. The free end is usually at some constant temperature. A thermocouple scale is made from the thermoelectric potential as a function of temperature.
When a third metal is connected to the thermocouple circuit, the thermal potential generated by the thermocouple will remain unchanged as long as the temperature of the two contacts of the material is the same. That is, it is not affected by the third metal access circuit.
Therefore, in the thermocouple temperature measurement, as long as there is access to the measuring instrument, after measuring the thermal potential, the temperature of the medium can be measured.
RTD Temperature Sensors
The principle of operation of an RTD temperature sensor is very simple.The sensor consists of a resistor made of metal wire or film. When a current is passed through the RTD sensor, the current heats up the wire, thus raising the temperature of the wire. Based on the relationship between resistance and temperature, temperature changes can be indirectly measured by measuring changes in resistance.
The operating principle of resistance temperature sensors relies on the linear relationship between wire resistance and temperature. According to Ohm’s law, there is a relationship between resistance and current, and voltage.
Ohm’s law formula: R=V/I,
where R is resistance, V is voltage and I is current.
And according to the effect of temperature on resistance, a linear relationship between temperature and resistance can be obtained.
R=R0(1+α(T-T0)),
where R0 is the resistance value at the reference temperature T0,
α is the thermal sensitivity coefficient,
T is the current temperature.
By measuring the change in resistance, the temperature change can be calculated. To improve the accuracy of the measurement, a bridge circuit or other correction method is often used to eliminate the non-linear effect of temperature on the resistance measurement.
What is a temperature transmitter?
Temperature transmitters use thermocouples and resistance as temperature measuring elements. The output signal from the temperature measuring element is transmitted to the transmitter module.
The signals are processed by voltage regulators, filters, operational amplifiers, non-linear correction, voltage/current conversion, constant current, and reverse protection circuits. And then it converted into electronic signals that are linearly related to temperature.
Temperature Transmitter Function
A temperature transmitter converts an ordinary electrical signal into a standard electrical signal or communication protocol.
The temperature transmitter converts the measurement main loop AC into a constant current loop standardized signal.
This signal is continuously transmitted to the receiving device, which displays the corresponding temperature value.
Read More about: How does a Temperature Transmitter Work?
Sino-Inst Featured Temperature Transmitter
Temperature Sensor vs Temperature Transmitter
1. Differences in core definitions and functions
A temperature sensor is a sensing element that directly detects temperature and outputs a raw, weak signal; it is solely responsible for acquiring the physical quantity of temperature and does not perform any signal processing.
A temperature transmitter is a secondary conversion device that receives the raw signal from the sensor, amplifies it, performs linear calibration and isolation conversion, and outputs a standardised industrial signal.
2. Differences in Output Signal Types
Temperature sensors output weak, non-standard signals: thermocouples output millivolt-level voltage signals, whilst resistance temperature detectors (RTDs, such as PT100 and Cu50) output resistance values that vary with temperature.
Temperature transmitters uniformly output industrial standard signals, commonly 4–20 mA current, 0–10 V voltage or RS485 digital signals, which suffer minimal signal loss during long-distance transmission.
3. Differences in transmission distance and immunity to interference
As the raw signals from sensors are weak, long-distance transmission is prone to interference from cable resistance and electromagnetic interference, limiting their use to short distances.
Transmitters perform signal conditioning, and standardised current signals offer strong immunity to interference, enabling cabling over distances of several hundred metres and adapting to complex electrical environments in industrial settings.
4. Distinct Differences in Power Supply Requirements
Passive temperature sensors, such as thermocouples and standard resistance temperature detectors (RTDs), require no external power supply, as they rely on temperature differentials to generate an electromotive force or changes in resistance.
Temperature transmitters, however, are active devices and must be connected to an external 24 V DC power supply to perform signal conversion and output.
5. Differences in Measurement Linearity and Compensation Capabilities
Sensors inherently exhibit non-linear errors; thermocouples also require cold-junction compensation, and when used alone, measurement accuracy can be significantly compromised.
Transmitters incorporate internal compensation circuits and linearisation algorithms that automatically correct the sensor’s inherent errors, with integrated cold-junction and resistance compensation, resulting in more stable measurement accuracy.
6. Field Application Scenarios and Integration Methods
Sensors are predominantly used in laboratories, for simple short-range temperature measurement, and in small embedded devices, often paired with dedicated display instruments.
Transmitters are predominantly used in industrial automation control systems and can interface directly with PLCs, DCSs and recorders, serving as standard components in complete factory temperature measurement systems.
7. Differences in Structural Integration
Sensors typically consist of a single probe element, featuring a simple structure and compact size; transmitters are available in split-type and integrated designs.
Integrated transmitters encapsulate the conversion circuitry within the probe’s terminal box, allowing for a single-unit installation without the need for additional conversion modules.
8. Differences in Cost and Usage Thresholds
Pure sensors have a low unit price, but require additional compensation leads and dedicated data acquisition modules for use; transmitters have a higher overall cost but can output a standard signal straight out of the box, eliminating the need for additional signal conversion components and making on-site installation and commissioning simpler.
FAQ
What is the difference between a temperature controller and a temperature sensor?
Temperature controllers use temperature sensors. A temperature controller is a device that controls temperature within a set range.
A temperature sensor is an electronic component that senses temperature. While the two are related, they are not the same device.
Temperature Sensors and Temperature Transmitters
1. Temperature sensors form the core of temperature measurement, including resistance temperature detectors (RTDs) and thermocouples, which directly sense the temperature of the medium and output a weak raw signal in the form of resistance or millivolts.
2. Temperature transmitters are used in conjunction with temperature sensors to amplify, linearly calibrate and isolate the raw temperature measurement signal.
3. Transmitters uniformly convert the signal into a standard 4–20 mA signal, facilitating long-distance transmission to control systems such as PLCs and digital displays.
4. Only when used in combination can stable, standardised industrial temperature acquisition and transmission be achieved.
What is the principle of a temperature transmitter?
The design principle of the temperature transmitter was earlier implemented through analog circuits. It includes RTD temperature transmitters and thermocouple temperature transmitters.
The RTD signal is a binary parabolic law. can be achieved by constant current plus compensating feedback at the output to achieve a linear output. Different input ranges require modification of the internal circuitry to adjust the amplification and compensation size.
The thermocouple signal can be seen as a folded signal around the ends of a straight line. Out of the accuracy range, it needs to be corrected by multiple folding line compensation to achieve an output within the accuracy range. For different input ranges, it is necessary to modify the amplification and adjust the multiple-fold compensation. The calculation and modification is more complicated, so the capacity is limited.
How do you use a temperature sensor?
Contact temperature sensors communicate with the manufacturer about the installation method when customizing. This is because proper installation ensures proper accuracy. For example, if the temperature of the surface of an object is not installed correctly, the temperature of the air near the surface of the object is often measured.
Contact temperature sensors should be strictly guaranteed to work within the allowable range. Prolonged work beyond the range caused skin accelerated aging, reducing the service life. It may even lead to chip damage. Out of range, some sensors will not collect data or data accuracy is poor.
Pay attention to the secondary instrumentation used to support the accuracy of the temperature sensor. Otherwise, the high-precision temperature sensor can not take advantage of the high precision. The temperature sensor should try to ensure that the wire has no connectors. Wire resistance can cause data deviation.
Temperature Sensor Precautions
1 Errors introduced by thermal inertia:
The thermal inertia of a thermocouple causes the meter display to lag behind the actual temperature change. This effect is particularly noticeable in fast measurements. Therefore, thermocouples should be used whenever possible. Thermocouples have thin heat electrodes and small-diameter protection tubes, which can even be removed if the temperature measurement environment permits.
2. Insulation degradation introduces errors:
Excessive buildup of dirt in areas such as thermocouple insulation, protection tubes and lead plates can lead to poor insulation between the thermocouple pole and the furnace wall. The situation is even worse at high temperatures, where not only is there a loss of thermopotential, but interference is also introduced. Such errors can be hundreds of degrees Celsius.
3. Improper installation errors:
Thermocouples and power lines should not share conduit. To avoid the introduction of interference errors. Thermocouples should not be installed in the sparse flow area of the measured medium. When measuring the temperature of a gas in a tube, the thermocouple must be installed against the flow direction and in full contact with the gas.
In summary, the temperature sensor is the core component of the temperature transmitter. Only the temperature sensor senses the temperature and then transmits the temperature to the temperature transmitter for data processing. In use, the temperature transmitter has self-diagnostics and automatic calibration, which makes it easy to maintain the accuracy.
Sino-Inst is a supplier of temperature measurement equipment from China. We have been engaged in temperature measurement for many years and have many successful cases around the world. If you have related measurement problems, please contact us. Our aim is customer first. Our technical experts can customize measurement solutions and answer related questions for you free of charge.
