Why does a integrated thermal resistance temperature sensor need to be connected in a three-wire or even four-wire configuration?
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As is well known, most temperature sensors can be selectively configured for two-wire, three-wire, or four-wire systems. Many years ago, they were separate units: the thermal resistor was located on-site, while the temperature-sensing element was far away from it, sometimes even installed indoors; as a result, the two-wire connection method did cause significant wiring errors. But these days, temperature sensors are widespread; resistive elements and temperature sensors are now readily available, and wire errors are minimal. So why is the three-wire connection method still used? Does this not make the internal structure of the temperature sensor more complex compared to the two-wire system? It increases the cost of the temperature sensor and introduces an additional wiring point that can cause problems, without any significant improvement in accuracy. I have a personal question; I hope everyone can help answer it. Thank you.
Are you referring to an integrated temperature transmitter? The output of an integrated temperature transmitter is a 4–20mA two-wire signal. From your explanation, it seems it’s not referring to an integrated temperature transmitter.
I'm referring to the wiring between the thermal resistor and the temperature transmitter; the 4-20mA signal output by the temperature transmitter is, of course, a two-wire signal.
Look at the input circuit of the temperature transmitter and you’ll understand.
Hello, do you have the wiring diagram for the temperature transmitter input? If two-wire operation is indeed feasible, then the input wiring for temperature changes can also be improved. Personally, I believe it is not the input wiring for temperature changes that determines the wiring configuration required for the thermistor, but rather a more appropriate wiring method for the thermistor that determines how the internal circuitry related to temperature changes should be improved.
It’s not because the thermal resistor and the temperature transmitter are close to each other; it’s a specification requirement for a 3-wire or 4-wire connection, and that’s why it appears in the instrument technician exam questions.
What did the original question say? The test bank for instrument technicians was compiled quite early on, and much of its content is outdated; it’s impossible for questions like the one I mentioned to appear in it Previously, the error was caused by the excessive length of the wires at both ends of the two-wire system; if the length of the wires is reduced to a few centimeters, the error becomes very small. So why go to the trouble of using a three-wire system?
According to the principle of thermistor temperature measurement, changes in the temperature being measured are detected through changes in the resistance value of the thermistor; therefore, changes in the resistance of the wires connecting to the thermistor can introduce errors in temperature measurement. Whether it is necessary to eliminate this effect depends on the application scenario, application requirements, and the required accuracy of temperature measurement; hence, there are various wiring methods available. However, the temperature measurement principle for two-wire, three-wire, and four-wire heating resistors is the same; the only difference lies in the wiring, which relates to whether the current circuit and the voltage measurement circuit are wired separately. It should be emphasized, however, that the three-wire and four-wire wiring methods must be used in conjunction with thermistor elements corresponding to those wiring systems in order to achieve true three-wire or four-wire wiring. The details are as follows: The two-wire connection method is mainly used for temperature measurement in short-distance applications. Since it does not take into account changes in the resistance of the wires, its measurement accuracy is low; therefore, it can only be used in situations where high precision in temperature measurement is not required. The three-wire wiring method is widely used in industrial production, mainly for remote temperature measurement, as it helps to reduce the impact of changes in the resistance of the connecting wires on the readings displayed by the instrument. In reality, however, the thermistors used in many factories have thermistor elements inside protective tubes that typically have only two leads; in other words, these thermistor elements operate on a two-wire system. Although three connection wires are used from the wiring box of the protective tube to the display instrument, this can still be considered a two-wire wiring method for thermistors, or it can be referred to as a two-wire wiring method for thermistors using three wires. http://www.yunrun.com.cn/News/UploadFiles_5183/201508/2015080119482890.png The four-wire wiring method is mainly used in laboratories, allowing for high-precision measurement results. When in use, a stable current is applied to the thermistor, and then the voltage (potential) across the thermistor is measured in order to improve the accuracy and sensitivity of the measurement. In industrial production, instruments, board modules, etc. connected to thermal resistors all have four terminal connections: I+, I-, V+, V-. Among them, I+ and I- are used to supply a constant current to the thermistor, while V+ and V- are used to monitor the voltage changes of the thermistor in order to determine temperature changes. Its wiring principle is shown in the figure. When the measuring instrument operates on the principle of potential difference measurement, although the wires have resistance, the voltage drop across them as current flows is not within the measurement range. The wires connecting the measuring instrument also have resistance, but no current flows through them; therefore, changes in the resistance of these four wires do not affect the measurement results. It is only necessary that the current from the constant current source remain stable.
In industrial production, thermal resistance temperature meters mostly use unbalanced bridges for measurement. The principle of its measurement circuit is shown in the figure below: http://www.yunrun.com.cn/News/UploadFiles_5183/201508/2015080119285183.png Since the resistance value of the copper wires connecting the thermistor to the bridge changes as the ambient temperature varies, if these connection wires are attached to only one of the bridge arms, the change in the resistance of those wires will combine with the change in the resistance of the thermistor RT, resulting in additional errors. Therefore, the three-wire wiring method is commonly used in industry, with wires 2 and 3 connected to the two arms of the bridge respectively; when the resistance of these wires changes, their effects can offset each other to reduce the impact on the instrument’s reading. However, the reduction in error is limited; for an unbalanced bridge, full compensation can only be achieved at the starting point of the instrument’s scale, while the additional error mentioned above is greatest at full scale. For unbalanced bridges, the additional temperature error caused by the power supply leads must also be taken into account. When current flows through wire 1 that connects the thermistor to the power supply, a certain voltage drop occurs. As the ambient temperature changes, the voltages in the upper and lower branches of the bridge also change, thereby introducing an additional temperature error in the instrument.
Personally, I think this is related to the internal circuit of the temperature sensor; however, a thermal resistor can be a two-wire type, and for three-wire or four-wire temperature sensors, the terminals can be shorted together.
The three-wire wiring method is widely used in industrial production, mainly for remote temperature measurement, as it helps to reduce the impact of changes in the resistance of the connecting wires on the readings displayed by the instrument. In reality, however, the thermistors used in many factories have thermistor elements inside protective tubes that typically have only two leads; in other words, these thermistor elements operate on a two-wire system. Although three connection wires are used from the wiring box of the protective tube to the display instrument, this can still be considered a two-wire wiring method for thermistors, or it can be referred to as a two-wire wiring method for thermistors using three wires.