The diagram above illustrates how the 4-wire principle is used to eliminate lead and contact resistances as potential error sources. Errors normally caused by test lead and contact resistance are eliminated by use of 4-terminal configuration on all Valhalla Digital Ohmmeters. In many applications the contact resistance can exceed the value of the load by several orders of magnitude. Valhalla ohmmeters by-pass this potential error source by providing two terminals for a high input impedance voltage measurement. The result is a fast, accurate resistance measurement of the load, independent of the resistance in the current carrying leads. The internal current source inherently overcomes all series resistance (within compliance voltage limits) and delivers stable constant current. The high impedance DVM senses the voltage dropped across the load. There is no contact and lead resistance error created by the voltage measurement because there is essentially no current flow in the voltage sense leads. All ohmmeter probes, clips and cable accessories offered by Valhalla are 4-wire Kelvin configured.

For low resistance inductors (i.e., utility transformers) the ability of a micro-ohmmeter to perform a valid measurement requires considerable application insight and is dependent upon several parameters. The larger the test current (10 Amps is better than 1 Amp) the easier it is to resolve a low ohm value, however there is normally an increase in settling time with large inductors. Settling times can be decreased substantially if a large compliance voltage is used with the current source circuitry of the ohmmeter.

Valhalla offers a variety of four-wire Kelvin based lead sets. The most common lead set is the "option K" , which is a Gold Copper-Tellurium Kelvin Clip with 48 " cabling terminated with dual banana posts. There is also a wide selection of four-wire spring tipped microprobes, miniprobes, and surface probes to choose from. For hook-up to larger devices the "JAWS" terminated leads (opens to 2") are also available. Please refer to the back section of this catalog for a detailed listing.

When measuring 1 milliohm with 10 amps of test current the Valhalla 4300B senses a 10.000mV drop across the device. Competitive ohmmeters typically claiming equivalent low resistance ranges or resolution don't tell the whole story. For example an ohmmeter with only 100 milliamps of test current having a 1 millohm range would only sense 100.00µV drop across the same 1 milliohm device mentioned above. How does a 10.000 mV drop versus 100.00 µV drop impact the measurement? The lower level is one hundred times more susceptible to noise, and measurement errors. A 1-2µV thermal EMF offset, and 3µVolts of 60 Hz noise (i.e. fluorescent lights) are common. The result is a low current ohmmeter with several hundred counts of randomly changing readings and 5-10% error at best.