PT100 temperature sensor uses 2 lines and 3 lines or 4 wire wiring diagrams - WeiLianFengRan Sensor Technology Co., LTD

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PT100 temperature sensor uses 2 lines and 3 lines or 4 wire wiring diagrams

2021-06-29

PT100 temperature sensor uses 2 lines and 3 lines or 4 wire wiring diagrams

The basic structure of the PT100 temperature is based on the precision resistance of the measurement point in the component, and is connected to the measurement instrument through a section of the cable. The cable may also include other components, such as interconnection and temperature transmitter. However, in all cases, there must be some electrical connection between the sensing element and the next component in the system.
Obviously, when using the PT100 RTD sensor to measure the temperature, we are only interested in the resistance of the sensor element. However, any component added to the measurement circuit, including the wires or cables used, also has its own resistance. If you don't consider it carefully, this may cause measuring errors.
PT100, PT1000, and NTC are the most commonly used measurement elements in resistance thermometers. Consider the problem of connection types in more detail here.

The resistance thermometer changes its resistance according to the temperature. This physical effect makes the temperature using the PT100 measuring process possible. The resistance is determined by the electronic device (such as the temperature transmitter) by using the constant current and the measurement of the voltage. According to the Om Law (R = U/i), under the constant current [i], the resistance [R] and the voltage [u] are proportional to each other. There are three possible ways to connect the PT100 to the transmitter: 2 lines, 3 lines, or 4 lines.

PT100 temperature sensor in the 2 -line connection

Although two -line connection can be used to connect to the PT100, you should understand that the measurement error will be introduced due to the resistance of the lead. This is because the measurement instrument will measure the total resistance of the measurement circuit, not just the total resistance of the sensing element itself.
If the resistance of the resistance of the temperature of each degree Celsius is only 0.3851Ω, the connection cable with a resistance of only 1Ω will also cause an error of about +2.6 ° C.
Having said that, the two -line connection is often used, and the process parameters create many sensors with this type of connection type. For machine manufacturers with very short cable lengths, it is usually used as a low cost solution. The two -line connection is suitable for the following special circumstances:
● Application for occasions with low accuracy requirements.
● The sensor cable is very short.
● The error is determined by testing and application of the offset of the instrument.
If your application needs do not belong to this, there is a standard method for compensation for the binding resistance.
When using the 2 -wire connection, the resistance of the cable will be added as measuring error. For copper cables with a cross -section of 0.22 mm 2, the following guidance value is applicable: 0.162 ω/m → PT100 is 0.42 ° C/m. For a version with a PT1000, the impact of the power cord (0.04 ° C/m) is 10 times less than the basic resistance. Compared to the basic resistance R25 (such as R25 = 10K) with the NTC measurement element, the lead resistance becomes not so important. Due to the inclined characteristic curve of NTC, the impact at higher temperatures does not properly increase.

The PT100 temperature sensor in the 3 -line connection

Add a third wire and connect to the side of the measurement element to help compensate the lead resistance. Each wire in the three wires used in the measurement circuit is the same in the conductor size and length, which is very important. This is because the measurement results are average. Only when all three wires have the same resistance can they provide good accuracy. The maximum length of the connection line depends on the compensation options of the conductor cross -section and evaluating electronic equipment (transmitters, displays, controllers, or process control systems).
The working principle of the 3 line connection specification is to measure the resistance value through the detector, and obtain the second resistance value by a pair of wires connected to the side of the detector. The resistance value of the isolated measurement component is obtained from the total amount of the resistance.
So far, the three -line connection is the most common type of wiring type used in the PT100 temperature measurement. Many instruments use this connection method, including temperature transmitters, temperature controllers, panel monitors and data recorders, and in many cases, if the 2 -line system is used, it will not run normally.
If you use the 4 -wire sensor and 3 wire, you can simply ignore the 4 lines and hang it.

PT100 temperature sensor in the 4 -line connection

In order to obtain the maximum accuracy, you should choose the four -line PT100 RTD specification. This measurement system is the only way to fully compensate all the binding resistors in the measurement system, even if each wire has different resistors.
The measurement system uses a pair of wires to carry the incentive current for measurement, and the second pair of wires are used to measure the resistance of the sensor by measuring voltage.
Because the 4 -line connection method can fully compensate all the lead resistance, we strongly recommend using this method when using high -specification PT100 (eg, 1/5 or 1/10 din tolerance). We believe that designated high -capacity differential temperature sensors with relevant costs, and then using inferior measurement systems is a wrong economic approach. The four -line connection is mainly used in laboratories and calibration applications and anywhere that requires the highest accuracy.
The 4 -wire connection completely eliminates the effects of the connection wire on the measurement result, because any possible asymmetric in the wire resistance of the connection wire is also compensated.

Alternative

Example: Measurement error at 150 ° C, 10 m in length of the cable, and the cross section of the conductor 0.22 mm2
Another possibility of significantly reduced the impact of the wiring is to increase the cross section of the conductor. When the cross section is 0.5 mm 2, the line resistance is only 0.036 Ω/m or 0.1 ° C/m. These two options (3/4 cables or increased cross -section) will cause higher wiring costs, which may bring problems, especially in market -sensitive markets such as machine manufacturing. As a compromise solution between cost and accuracy, for shorter cable lengths, the A -class 2 -wire connection PT1000 measurement element can be provided.

In summary, the highest measurement accuracy can be achieved only in the 4 -line connection.
Class A PT1000 measurement element also provides good measurement accuracy in the 2 -line connection. It is an economic alternative solution for 3 or 4 -line connections in the machine manufacturing.

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