Post by thread:[OT] 4-electrode conductivity meter circuit

I don't know of a "site", but the principles were pretty simple. Two terminals supplied a <high> current to the object under test D.U.T. The magnitude of the current was the controlled variable so contact resistances (and the induced voltages)didn't perturb that value. The other two terminals measured (with high impedance) the voltage resulting from the applied currents. Since this 'took' negligible current from the D.U.T. it did not steal current from that applied, and (more importantly) didn't add in its own (ohmic at least) voltage errors. I don't recall there being much 'magic' to all this, other than some attention to thermal (dissimilar metal thermocouple effects if not balanced) and in extreme cases chemical effects on the voltage probes. Probably the 'last word' on this is the "Review of Scientific Instruments" R.S.I. Journal at your local university, where sensor and measurement issues are carried out a few more decimal places than elswhere. Also N.I.S.T. pubs (formerly Nat. Bureau of Standards) which are in part online.

The 4 conductor resistance metering system is being used for long in the industry, mainly in RTD devices, and low value resistance. When measuring resistance, the meter needs to supply a small or large current, enough to generate a voltage drop on the resistance under measurement, enough to be read by the device voltmeter and indicate the representative resistance values. You know that wires and connections have their own resistance, so the current through the circuit would also generate voltage drops over those points too. If the voltmeter measures the voltage drop internally, it would be measuring not only the voltage generated over the resistance under measurement, but adding all the small voltages generated everywhere, so the real voltage measured is higher than the real one over the resistance under measurement. It means that the voltmeter would be indicating a wrong "relative" resistance. For example, suppose the meter supply a loop of 1 ma across the resistance under measurement. If the resistance value is 20 Ohms, the voltage generated across it would be 20 x 0.001 = 0.02 Volts, now, suppose the connections and probe wires offer a total of 3 Ohms, the same 1 ma current would generate a voltage on wires and connections equal to 3 x 0.001 = 0.003 Volts. Now, if the voltmeter measures the voltage right at the internal posts where the 1 ma goes out the instrument, it would be measuring all the external voltage, that would be equal to the sum of 0.02 plus 0.003 Volts, so 0.023 Volts. When the unit "calculates" the resistance based on Voltage/Current, it would indicate 0.023/0.001 = 23 Ohms, that is wrong (resistance under measurement is 20 Ohms). To avoid this problem, the 4 wires device use two wires to supply the current, and other 2 just to "measure" the voltage directly over the resistance under measurement. It means that the supply current wires can have several ohms and generate voltage on wires and connections, but the voltage generated over the resistance under measurement would be always the same, just 0.02 Volts. It works the same as if you have a voltmeter connected directly over the resistance. There are the possibility to use only 3 wires instead 4. It works this way: Suppose wires #1 and #2 supply the current, while the pair #1 and #3 is used to measure the voltage. The unit also measure the voltage on posts #1 and #2 (will be a higher voltage than #1 and #3), so it subtract one from another and this difference would be the voltage drop over the wire #2 and its connections. Considering that probably the wire #1 and its connections would have the same kind of resistance from wire #2, that difference (V#1#2 - V#1#3) is reduced from the voltage at #1 and #3 again (2 x V#1#3 - V#1#2) , so it goes to be very close to the real voltage over the resistance. Wagner. Richard Martin wrote: {Quote hidden}> > I don't know of a "site", but the principles were pretty simple. > Two terminals supplied a <high> current to the object under > test D.U.T. The magnitude of the current was the controlled variable > so contact resistances (and the induced voltages)didn't perturb that > value. The other two terminals measured (with high impedance) > the voltage resulting from the applied currents. Since this > 'took' negligible current from the D.U.T. it did not > steal current from that applied, and (more importantly) didn't > add in its own (ohmic at least) voltage errors. I don't recall > there being much 'magic' to all this, other than some attention > to thermal (dissimilar metal thermocouple effects if not > balanced) and in extreme cases chemical effects on the voltage > probes.

Hi Peter! Go to : http://www.keithley.com/ and try to order their free book 'Low Level Measurements' (Precision CD Current, Voltage and Resittance Measurements) I has some good explainations. Wagner Lipnharski wrote: ... > > When measuring resistance, the meter needs to supply a small or large > current, enough to generate a voltage drop on the resistance under > measurement, enough to be read by the device voltmeter and indicate the > representative resistance values. You know that wires and connections ... If you want to measure resistance, the above description is the correct one. For measuring resitivity (in Ohm x cm) of flat, homogeneous in thickness and arbitrary shaped materials such as semiconductors and foils the 'van der Pauw' Method can be used. (Also described in the book mentioned above) The dimension of the sample and the spacing ot the contacts are not important. It uses 4 probes on the edges of the sample, makes 8 measurments and calculates ro. Hope, this helps. Stefan.