Resistance measurement; 2, 3 or 4 wire connection – How does it work and which to use?

What is 2 wire RTD?

A single wire connects each end of the RTD element in a two-wire RTD. It’s the simplest wire arrangement. The computed resistance will encompass all elements of the circuit, implying a higher margin of error. Because there are just two wires in the circuit, they are unable to compensate for any additional resistance imposed by other components.

Although systems can be calibrated to reduce inaccuracy, this is not always the most precise or best solution.

As a result, a two-wire RTD is frequently utilised in applications with short wires, high resistance sensors, or where precision is not critical.

2-Wire Resistance Measurement

The constant current approach is used to test a two-wire resistance arrangement. The total lead resistance (RLEAD) is included in the calculation with the two-wire approach for low resistance measurements, which is a major flaw. The voltage (VM) measured by the metre will not be exactly the same as the voltage (VR) immediately across the test resistance ® because the test current (I) causes a minor but significant voltage drop across the lead resistances.

Because typical lead resistances range from 10m to 1km, accurate two-wire resistance measurements are difficult to achieve when the resistance under test is less than 100, because the resistance of interest will be entirely swamped by the lead resistance.

Lead resistance will, in fact, be the most common source of inaccuracy. Using test leads with a combined resistance of 100m to perform a two-wire resistance measurement on a 500m resistor, for example, will result in a 20% measurement error in addition to the instrument’s error.

What is 3 wire RTD?

The three-wire architecture of an RTD is the most common, with one wire connected on one side and two-wire connections on the other. This compensates for the additional resistance induced in the circuit, resulting in a more accurate readout.

The resistance is computed by subtracting the resistance between wires 2 and 3 from the resistance between wires 1 and 2, yielding a precise measurement for the resistance element (Rb). Because this method implies that all cables measure the same resistance, they must be identical.

The most popular type of RTD is a three-wire RTD, which is accurate in most applications. However, if the cables have various resistances, there will be mistakes. As a result, a four-wire RTD is advised for a completely accurate solution.

3-Wire Resistance Measurement

In practice, using/installing four wires can be time-consuming and costly. The 4-wire connection can be simplified by using a 3-wire connection instead. It makes use of three wires, as you would have imagined.

Although a 3-wire connection is not as precise as a 4-wire connection, it is quite near if all three wires are comparable, and it is significantly superior to a faulty 2-wire connection. As a result, in many industrial applications, the 3-wire connection has become the standard.

In a 3-wire connection, the aim is to remove one of the wires and assume that the resistance of the remaining wires is similar.

The lower half of the diagram has only one wire in it. As a result, the lower connection resembles a 2-wire connection, while the upper connection resembles a 4-wire connection. The metre may correct for wire resistance in the upper half, as in a 4-wire connection. However, it lacks the ability to adjust for the wire (Rw3) resistance in the lower portion.

Because the resistance metre includes internal switching, it can measure just the resistance of the top loop (sum of Rw1+Rw2), then divide that number by two to get the average resistance of these two wires. The metre then thinks that the resistance of the third wire (Rw3) is the same as the resistance of Rw1 and Rw2.

Then it changes to a normal connection (as shown in the illustration) to measure the connected impedance R, and it incorporates the results of the resistance of the previously measured wire into the test result.

It’s important to remember that the 3-wire connection is accurate only if the resistance of all three wires and connections is the same. If the wire and connection resistances are different, the 3-wire connection will produce an incorrect measurement result. Depending on the resistance difference between the cables and connections, the error in 3 wire measurement might be either too high or too low.

In industrial applications, a 3-wire connection is frequently a suitable compromise; it is accurate enough while requiring one fewer wire than a perfect 4-wire measurement.

What is 4 wire RTD?

The 4 wire RTD is, without a doubt, the most sophisticated option; this is reflected in the price of this type of RTD. They’re mostly employed in scientific settings where high precision is required.

Because it can compensate for mistakes caused by wire resistance, this circuit is utilised for longer wire lengths between the measuring element and the measurement electronics.

With contrast, in a 3-wire circuit, all wires are believed to have the same resistance. The 4 wire RTD circuit assumes that each wire has its own resistance measurement. To obtain the most accurate reading of all RTDs, this compensates for the full wire resistance.

Two of the wires are attached to each end of the measurement element when utilising a 4-wire RTD (usually wires 1 and 4). A constant tiny measuring current is delivered to the measuring element, allowing it to be examined as a comparison variable at all times. Wires 2 and 3 can be used to measure the voltage drop at the measurement resistor. By employing the “fault current” flowing through the very high-resistance input, the voltage drop at the wire resistances can be ignored, and the measuring input can detect the real voltage drop at the measuring resistor practically 1 to 1.

4-Wire Resistance Measurement

For low resistance measurements, a separate methodology is employed to lessen the effect of test lead resistance due to the constraints of the two-wire method. Test engineers can utilise the four-wire (Kelvin) connection to measure DUTs with resistances equal to or less than 1k.

Voltage drop in the test leads is removed because the voltage is measured at the DUT (this voltage could be significant when measuring low-resistance devices).

In this design, one set of test leads drives the test current (I) via the test resistance ®, while the second set of test leads monitors the voltage (VM) across the DUT (sense leads).

Although the sense leads may carry a tiny current (normally less than 100pA), it is usually inconsequential and can be ignored for all practical purposes.

As a result, the voltage recorded by the metre (VM) is nearly identical to the voltage across the resistance (VR) ®. As a result, compared to the two-wire method, the resistance value can be established significantly more precisely.

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