# Kelvin 4 Wire Resistance Measurement: 11 Important Facts

## 4 Wire Resistance Measurement

There are different methods to measure different types of resistance, where varies with the range of resistance. 4 wire resistance measurement method is a very accurate measurement method, which can measure very low resistance with high accuracy. It is used to avoid contact resistance or lead wire resistance problems in the circuit. Here every connection wire is called kelvin connection.

In 4 wire resistance measurement method, the four-wire connection is used where two-wire is used to deliver the supply current to the measuring component, and another two-wire is used to measure the voltage drop across the measuring element.

As we know, at constant temperature Ohm’s law define resistance ‘R’ as the ratio of voltage across the resistance to the current ‘I’ passing thru it, So with measuring the voltage drop across the measuring component with known current passing through it, the resistance of the measuring element can be calculated.

## Kelvin Bridge

The basic principle of the Kelvin 4 wire resistance measurement is based on Kelvin Bridge. Kelvin Bridge is a modified version of the Wheatstone bridge used to measure the very low resistance value, which ranges from 1 ohm to 0.00001 ohms. In this bridge, the effect of load resistance contact resistance and the resistance of the lead wires are taken into account.

## Kelvin Bridge Circuit:

$Y_b$ in the figure is the connecting lead wire Resistance.

Whenever the galvanometer is connected to point ‘a’, then the resistance of the connected lead is summed up to the resistance Rx and total impacts become $R_x + R_{ab} + R_{cb}$.

Whenever the meter is attached  to point ’c’ the resistance of the lead wires  summed up to $R_3 + R_{ab} + R_{cb}$.

And when the galvanometer is attached to the point ‘b’, which is between ‘a’ and ‘c’ point, in such a way that the ratio of lead resistance from ‘a’ to ‘b’ and ‘c’ to ‘b’ is the same as the ratio of $R_1$ to $R_2$.

Equation 1 : $R_1/R_2= \frac {R_{cb}} {R_{ab}}$

Now the overall equation of the circuit become

Equation 2 : $(R_x + R_{cb}) = \frac { R_1} {R_2} (R_3 + R_{ab} )$

From equation 1 and 2 after solving we get : $R_x =\frac{R_1} { R_2} R_3$

The final equation is the same as the balanced Wheatstone Bridge, which shows that connecting lead wire has been eliminated by connecting the galvanometer at point ‘b’. $Y_b$ is eliminated with the kelvin bridge.

## 4 Wire Resistance Measurement Method | 4 Wire Resistance Measurement Technique

When measuring low resistance, the connecting wires can cause an error in the result of measurement. If the error produced is higher than the tolerance, or if the accuracy of the measurement is required very high degree, then four-wire resistance measurement is used. Ideally, the wire does not have any internal resistance, but in practice, every wire has some internal resistance.

## 4 Wire Resistance Measurement Circuit:

In the 4 wire resistance measurement method, 4 wire connection is used where two-wire is used to deliver the measurement current to the measuring component, and another two-wire is used to measure the voltage drop across the measuring component.

In this 4 wire resistance measurement method fixed current generator is used. So if the resistance through the circuit varies, the fixed current generator will supply a constant current through the circuit.

The wire which is used in voltage measurement is connected straight to the legs of the resistance, which is to be measured, and the voltage metre is used in this method is of high impedance so that minimal current passes through it. With a small current through the wire, the overall voltage drop across the wire is negligible, which doesn’t affect the value of the measuring component voltage drop. This method eliminates the wire resistance, which is also called Kelvin or four-wire method. Hear special connecting clips are used, which is known as Kelvin clips.

## Kelvin Clip Circuit Connection:

Kelvin clips are also known as alligator or crocodile clips. Each half of the jaw of a Kelvin clip is insulated from one another; both Jaws of the Kelvin clip are electrically common to each other, which usually joint at the high point. The current delivering wire is connected to one jaw, and the voltage measuring wire is linked to the other jaw. Kelvin Clips are used when the accuracy of the measurement is required high.

## 4 Wire Resistance Measurement Application:

• Remote Sensing.
• Resistance thermometer detector.
• Induction hardening.

## Disadvantages of Kelvin 4 Wire Resistance Measurements:

• Expensive.
• Complicated circuit.
• Testing speed is very slow.
• The no. of test points is twice.
• Larger number of connection wires are required.

## 2 Wire and 4 wire Resistance Measurement

In the 2 wire resistance measurement, the total lead wire resistance adds to the measurement because the current through the whole circuit is the same. As the voltage drop through the wire and the measuring component can produce a measurement with error, It does not have a very accurate output for a small value of resistance when the measuring resistance is much larger than the wire resistance. Then the lead resistance can get negligible. If the length of the wire can be minimum as possible, then the measurement’s accuracy can be increased.

As we can see from the above figure, $RW_1$ and $RW_2$ are the lead wire resistance. This is because the Voltmeter measures the voltage drop across $R + RW_1 + RW_2$. 2 wire resistance measurement is a less accurate simple circuit structure, requiring fewer connecting wires.

## 3 Wire Resistance Measurement

3 wire resistance measurement, which is not accurate as 4 wire resistance measurement, is more accurate than two-wire resistance measurement. The complexity of the circuit is less than that of 4 wire resistance measurement.

In this method, the switch is used, so at first, the upper loop of the resistance is measured, the Voltmeter measures the voltage across $RW_1+ RW_2$, then divide the value by 2, which gives the average resistance of these two wires. $RW_3$ is assumed to be the same as the avg. of $RW_1$ and $RW_2$.

Then, switch the circuit to the regular connection, which measures the measuring component and the resistance of wire $RW_2 + RW_3$. The calculated value across $( R + RW_2+ RW_3 )$ then compared with the first measured value $\frac {(RW_1 +RW_2)} { 2}$, which is used to eliminate the lead resistance produced by the wire from the measured value.

3 wire resistance measurement connection can be very accurate if all the three wires connected are of the same resistance value $(RW_1 = RW_2 = RW_3)$. 3 wire resistance measurement is widely used in industrial applications, which offers good compromise; it is accurate and uses less wire than 4 wire resistance measurement.

## What is actual working of 2 wire 3 wire and 4 wire types of resistance temperature detector i e RTD ?

RTD stands for resistance temperature detector. It is known that the resistance of a metal changes with the temperature change, so by measuring the resistance with the temperature change, the temperature difference can be detected. They are some metals where the temperature Coefficient is positive, so with the increase in temperature, the electrical resistance of metal increases. RTD can use 2 wire, 3 wire or 4 wire method.

The error introduced by the lead can cause a significant error, so there are very few applications of 2 wire RTD, 2 wire RTD is used with short lead wire or where high accuracy is not needed. Three-wire RTD measurement circuit that minimises the effect of lead wire resistance as long as the connecting wires are of the same length. Some factors such as terminal corrosion or loose connection can still significantly differentiate the lead resistance.

Three-wire RTD is more accurate than two-wire RTD, whereas less accurate than 4 wire RTD, where three-wire RTD is commonly used in the industry relatively cheaper than that of four wires and has a more straightforward Circuit Design than that of a four-wire RTD. In 4-wire resistance measuring, RTD is where the lead wire resistance can be observed and separate from the sensor measurement 4-wire RTD is a true 4 wire resistance measuring Bridge 4-wire RTD is used where high accuracy is needed. Still, it is very expensive and complex in design.

## What are the disadvantages of the method of measuring resistance of a wire utilizing an ammeter and a voltmeter in a circuitry ?

Disadvantages depend upon the circuitry’s design, which will measure the resistance for two-wire resistance measurement accuracy is low and for four-wire resistance measurement accuracy is high. In contrast, the two-wire measurement circuit is very simple and cheap, whereas 4 wire resistance measurement is complex and expensive.

The disadvantage of measuring resistance using an ammeter and Voltmeter can be using meters that are not working correctly. The range of the measurement should be considered for the selection of meters, other disadvantage voltmeter and ammeter should be connected to the circuit in different branches. The Voltmeter should be connected parallel to the measuring load, where the ammeter should be connected in series with the branch where the current is to be measured.

## What is the resistance of an electric heater?

According to Joule heating or Ohm heating, heat is proportional to resistance. Joule heating is a process by which electric current passes through a conductor produces heat, so for an electric heater, there must be high resistance in the wire.

## What are the factors affecting the resistance?

• Temperature
• Length of the wire area
• Cross-section area the wire
• Nature of material

## Will a thick wire have more resistance than a thin wire Why ?

The thin wire usually have greater resistance than a thick wire because the thin wire has fewer electrons to carry the current and In comparison, the thick wire has more electrons to carry the current. In addition, the relation of resistance and area of cross section of a wire is reciprocally proportionate, because of this if cross section of a wire reduce, the value of wire’s resistance will be higher.

## How to increase the resistance of a wire ?

The increase in length of the wire or decrease in the area of the cross-section of a wire increases the resistance.

## What is the cross sectional area of a wire?

If we cut a wire vertically perpendicular to its length, then we get a circle face of the wire. The area of the circle face of the wire is known as the cross sectional area of the wire and this area of a wire does not depend upon the length of the wire, and it is generally uniform throughout the entire length of the wire.

## Why use a high impedance voltmeter ?

Ideal Voltmeter has an infinite impedance that does not consume any current from the circuit. Still, practically e infinite impedance is not possible. A high impedance voltmeter is used. The current that passes through the Voltmeter is very small, so it does not affect the overall circuit.

## Is temperature is directly proportional to resistance ?

The temperature is directly proportional to resistance for a metal conductor or the metal with a positive temperature coefficient.

## What are the effects of temperature on resistance?

The effect of temperature on resistance depends on the temp co-efficient of the resistance. This can be defined as the change in resistance per unit change in temperature,if co-efficient is positive, resistance will increase with the temperature rise and if Co-efficient is negative, resistance will decrease with the temperature rise.

## Can a wire have zero resistance?

Ideally, zero wire resistance is possible, but practically, no wire present has zero resistance.

## Why do we use three-wire RTD?

Three-wire RTD is most accurate when connecting lead wire resistance to three-wire RTD is cheaper than a four-wire RTD and has a less complicated Circuit Design than a four-wire RTD.

## What is the benefit of a four-wire resistance measurement?

Four wire resistance measurements can eliminate the lead wire resistance and have resistance measurement having the highest accuracy.