Four-terminal sensing

The four-wire measurement is used in the measurement of electrical resistance by a four -wire connection, as conduction and connection resistance can falsify the measurement. In the four-wire measuring arrangement flows through two of the lines, a known electrical current through the resistor. The voltage drop across the resistor voltage is high impedance picked up by two more lines and measured with a voltmeter; the resistance to be measured is calculated therefrom according to the Ohm's law.

Basics

If, in the four-wire measurement of the flow through the voltmeter is negligible, which is fulfilled in a good approximation by the very large, and ideally infinitely high internal resistance of the voltmeter, the following applies:

( no appreciable deviation through current branch ) and the voltage drop in the test leads is negligible

( no appreciable deviation through voltage division ), the resistance value is obtained from,

A power supply from a power source instead of voltage source is recommended because the current then is independent of and and thus has to be set or measured only once.

The line resistances are the resistances of the leads and the screw or connector.

Scope

• In industrial measurement devices over greater distances with copper lines outdoors, the temperature influence is to significantly; This is indistinguishable from changes in the measurement object when it is measured with two cables.
• Wherein the current measuring resistors, the contact resistance may be higher than the nominal value of the resistor; In addition, he is only estimated. A correction is not possible under these circumstances.

The four-wire measurement is primarily used in the measurement of low resistances when the parasitic resistances of leads and contact points are no longer negligible compared to the resistance to be measured, for example,

• At the temperature measurement by means of platinum resistance
• In Dehnungs-/Kraftmessung means of strain gauges,
• In the current measurement using a shunt or
• Generally at a low resistance measurement.

Measurement errors that may occur despite four-wire measurement, are caused primarily by thermal stresses due to temperature differences between the terminals. However, these can be avoided by harmonized contact point temperatures or by pairs of materials with low thermal stresses. As long as the resistors contain no appreciable reactance can also be measured with alternating current instead of direct current, which fall out the thermal voltage influences.

Three wire measurement

In industrial temperature measurement equipment of the measuring resistor and the other measuring circuit can be far away. Then one comes to savings of cabling costs with the three-wire connection. The current-carrying lines must have this same resistances ( single cable ); the contact resistances must be negligibly small.

For the three-wire measurement

The difference between two similar large stresses from measurements with two independent measuring devices is always uncertain. The difference can be recognized immediately but with a simple measuring circuit. With the usual non-overdriven and not congested operational amplifiers approximations,

• No appreciable voltage between the inputs and
• No appreciable currents in the inputs ( but with unobstructed input bias current )

Sets in.

With normally operated line 2, and with the same line resistances can be simplified to the

Is no longer included in.

Even the slightly detuned Wheatstone bridge works difference - making. If the two resistors in the circuit diagram are right the same size, creates a bridge transverse voltage

This voltage is independent of a measurement of a resistance variation from a fixed reference value. Since the voltage is not proportional to but to, changes can be seen particularly clearly.

Differentiation from the four -to-peak measurement

With the four -tip measurement ( also four-point measurement ), the resistivity of a layer is determined. It also uses separate wires for the measuring current and the voltage drop, but the contact points are spatially separated from one another by equal distances.

• Electrical Measurement