The range of Ohmmeter and Microhmmeter proposed by Sefelec has been developed to offer a wide choice of solutions for the low resistance measurement.
In order to improve the quality of products they manufacture, many companies need to know the exact resistance value of a contact, a winding, a connector, etc…
Our range of microhmmeters is particulary suited for this type of test as well as for measurements of:
-resistance of cables;
-resistance of switching relays;
-resistance of transformer windings;
-resistance of transducers ;
-purity of precious metals;
-quality control of an electrical or electronic component.
Microhmmeter - MGR10 series
The heart of our microhmmeters and ohmmeters is the combination of two mains circuits:
-an extremely stable current source;
-a high impedance and very accurate voltage measurement.
The current source varies from 1µA to 10A depending on the model and the type of component test.
The resistance able to be measured by our microhmmeters goes from 0.1 µOhms to 30 kOhms.
The long term stability of measurement is directly dependant on the stability of reference semi-conductors and the quality of the resistance in the reference circuit of the microhmmeter.
Sefelec uses as reference elements, precision resistances which have gone through long aging cycles in order to guarantee their long term stability.
FOUR WIRE MEASUREMENT (METHODE KELVIN) ON MICROHMMETER MGR10
When measuring low values resistances, an important error source is coming from the contact resistance. Actually, in many applications, the contact resistance value can go beyond the value which has to be measured.
The method to cancel this error source, is to supply a high stability permanent current over 2 wire and to combine a voltage measurement on 2 other wire.
The voltmeter which is used to measure the voltage drop at the resistance terminals, must have a high impedance and a good measurement accuracy.
The diagram underneath shows the 4 wire measurement principle used in our microhmmeters.
Because of the high input impedance of the voltmeter, there is no error due to the leads resistance and the contacts in the voltage measurement circuit. Actually any current is flowing through these wires.
We are offering a wide range of 4 wire accessories (Kelvin type) to make easy the connection on the specimen to be measured.
AUTOMATIC COMPENSATION OF RESISTANCE VARIATIONS DUE TO TEMPERATURE
The resistance of elements such as copper or aluminium depends on the ambient temperature. A 10°C variation on these materials can lead to a 4% measurement error.
In order to compare measurements, the resistance value is expressed for a 20°C temperature.
To correct the value two techniques are used. The first one consists in measuring the ambient temperature and use a conversion table to correctly express the measurement. This method is long and mistakes can be made. The other method is automatic compensation by using an external temperature probe. The microhmmeter or the ohmmeter knowing what type of material is measured and having the temperature will automatically correct the value without manual intervention.