Two-wire and three-wire thermistor configurations

For resistors, no compensation wires are needed, but when transmitting resistor signals, a three-wire system is required strictly, as there is a bridge circuit in the instrument’s circuitry designed to eliminate line resistance; for rough measurements, a two-wire system can be used. With a temperature-sensitive integrated design, it is possible to transmit 4–20 MA signals using just two wires; however, a power supply wire is required as well, meaning a total of four wires are needed. For transmission via resistance and current, it is necessary for the current to be stable, and the environment must be good – there should be no moisture; otherwise, the circuits inside the unit will get damaged within a few days, and the cost of replacement will be high. So, it is recommended that you choose a three-wire system; the decision of whether to use three wires or two wires should be based on the actual circumstances. If you don’t understand, join my QQ group 44169652

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Thank you for the reply from the first floor. So it means that it’s better to choose a three-wire system, and it’s also cheaper, right? So what exactly is the concept behind compensation wires?

Compensation wires are only for thermocouples, got it?

Compensation wires are suitable for thermocouples. There are already many integrated temperature transmitters with a 2-wire configuration; a regular 2-core cable is sufficient. As for whether to choose a regular thermal resistor or an integrated temperature transmitter, the decision should be based on the actual circumstances. For example, by using a 4-wire system for the on-site thermal resistance and converting it to a 2-wire system through integrated temperature conversion, accuracy is improved while cable costs are reduced ； 4-20mA signals have much better long-distance transmission performance and interference resistance compared to resistance-based signals ； If your control room’s PLCs, DCS systems, etc. do not have cards capable of converting signals to appropriate resistance values, or if you want to reduce the number of different types of spare parts, an integrated temperature transmitter can also be used.

Some of the views upstairs appear to be misleading. Commonly used thermal resistors, whether Pt100 or Cu50, are generally connected to the corresponding input modules of secondary instruments, DCS, PLCs, etc., using a 3-wire or 4-wire configuration. The use of 3 or 4 wires is intended to improve measurement accuracy, as there is resistance in the wires. The integrated temperature sensor mentioned by the poster refers to a device in which the transmitter and the thermistor sensor are combined into one unit. Such an integrated temperature sensor typically outputs a 4–20mA signal over two wires; by \"2-wire signal,\" it means that these two wires serve both as power lines and as signal lines. For a more detailed explanation, you can read a relevant book. When the circuit length is large, it is advisable to use integrated solutions; the cost needs to be taken into consideration as a whole. The price per channel of the thermistor input modules for DCS or PLC systems is much higher than that for 20mA signals, and adding 1–2 more cable cores also incurs a significant cost.

1. Thermal resistors do not require compensation wires! Only thermocouples need them. 2. The choice between two-wire and three-wire systems depends on what the factory uses, namely whether there are corresponding secondary meters or I/O cards available.

When purchasing instruments, you need to consider what DCS modules you will use or what secondary instruments are required. 1. When used with modules or secondary instruments that accept 4-20mA input, a thermistor + temperature transmitter type must be selected. 2. If it is to be used with a thermistor module, then just buy a thermistor. The choice that suits the actual situation is the best choice. The resistance of the cable increases the value of the thermal resistor’s resistance; measuring the thermal resistor using a three-wire system is intended to eliminate the effects caused by the cable resistance, and this is achieved within the secondary meter. Thermocouples require compensation wires, and different thermocouples come with corresponding compensation wires that must be chosen correctly. Within the range of 0-100 degrees Celsius, the compensation wire can be considered to be made of the same material as the thermocouple wire; it serves to extend the thermocouple to the secondary instrument. Thermocouples also have cold-junction compensation, which is implemented in the secondary instrument in order to eliminate the effects of the ambient temperature there.

The main reason for choosing this is 1. the precision requirements for temperature control at your site； 2. Requirements for the wiring terminals of the display instrument ； 3. Distance between the site and the display terminal, cable cost ； 4. Level of concern regarding costs

Regarding whether to use a thermal resistor or a thermal resistor combined with a temperature sensor, I believe the following conditions should be taken into consideration: 1. The installation environment of the instrument on site; if the instrument is installed in a humid area or in close proximity to high-temperature equipment and other harsh conditions, it is better to choose a thermal resistor； If the instrument is installed in a location with favorable conditions, using a thermistor along with a temperature sensor is a good option. 2. Purchase price: If it is just a thermistor, the price is relatively low; the price is higher when it includes a thermistor and a temperature sensor. 3. Maintenance: Thermal resistors and temperature sensors, due to being installed on-site and being affected by the surrounding environment, are prone to malfunctions after long-term use. However, with thermal resistors, as long as the sealing is proper and the wiring is not loose, there are basically no problems, making maintenance simple.