Digital panel meter measures resistance in all environments.

London Electronics Ltd are well known for their wide range of digital panel meters, and are often approached to give help and advice to engineers with unusual special applications.

In this case, they were contacted by a well known manufacturer of large electric motors, to produce a special portable digital panel meter they could give to their service teams worldwide. Part of their quality test involves the measurement of winding resistance.

However, because the winding is made of copper, which has a sensitive thermal coefficient of resistance, they cannot simply state that a windings resistance should be 5 Ohms. A measurement of the winding resistance on a winters day in the UK may show 4.000 Ohms, but in sunshine in the summer that same winding may read over 6.000 Ohms. That's around a 20% change in resistance, and a normal resistance meter would suggest that the winding has an excessive resistance.

London came up with a novel plan for a 'smart' resistance panel meter, which would give a referenced readout of 'Resistance at 25C'. It uses a precisely calibrated sample of the winding wire as a local reference. With this system, the reading for a good winding is constant at 5.000, regardless of the actual temperature. The meter reads the same value whether the winding is in Siberia in the middle of winter, at 3.9 Ohms or at the equator in direct sunlight, at 6.2 Ohms. The motor manufacturers can now issue clear guidelines to their teams, with well defined pass and fail readings.

This digital panel meter uses other techniques to further increase reliability and repeatability of measurements. A 4 wire connection means that the test lead resistance is excluded automatically from the reading, and the meter is designed to work with reliably highly inductive loads.

Other applications include the measurement of incandescent bulb filament resistance, transformer winding resistance, speaker coil winding resistance and so on.

The new meter is known as the INT-R-COMP and is now available from stock.