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
The biggest difference between a temperature sensor and a temperature transmitter is the output signal. The output signal of a temperature sensor may be a resistance signal, a digital signal, and so on. The temperature transmitter outputs current 4-20mA, voltage 0-5V, 0- 10V, or RS-485, which are signals processed by the transmitter module.
In addition, temperature sensors and temperature transmitters have different structures. Temperature sensors are usually encapsulated by a single component. A temperature transmitter consists of the temperature sensor probe and the transmitter module.
Temperature transmitters and temperature sensors have different applications.
A temperature sensor directly measures the ambient temperature or the temperature of a specific object. It mainly plays a vital role in home automation, medical devices, environmental monitoring, industrial process control, and other fields.
A temperature transmitter processes and converts the signals from temperature sensors, facilitating data organization at a later stage. It can also provide additional signal processing and diagnostic functions. The temperature transmitter can be supplied with a supplemental cable for self-diagnostic functions.
Temperature transmitters are mainly used in applications where remote monitoring and control of temperature is required. Examples of scenarios include petrochemical, power, metallurgical and other industries that require process control systems.
In these applications, the temperature transmitter converts the weak or non-standard signal output from the sensor into a stable standard signal. The signal is transmitted via cable to a controller or display away from the site.
There is no difference in accuracy between temperature sensors and temperature transmitters. They can both be customized with levels such as high accuracy and normal accuracy. There is also no difference in the temperature measurement range. They can measure both high and low temperatures.
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.
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.
