Many of the devices we sell on Tester.co.uk are autoranging products, but what exactly does this mean, and what do ghostly voltages have to do with it?

If you browse some of our products (for example, multimeters) you may spot that in the descriptions of technical specifications that that particular device is listed as either 'autoranging' or 'manual ranging'. The difference between these modes is simple - one does something automatically, and the other requires a user to manually do it themselves.

But how does this automatic ranging facility apply to testing equipment? It basically applies to the range, or series of ranges available on a particular testing device. If you're using an autoranging meter with multiple available ranges for instance, you may accidentally choose the wrong range for whatever it is you're testing, leading to a false reading. An autoranging device negates this as the meter will automatically scale itself to the relevant range, ensuring that the test that goes ahead is as accurate as that particular meter allows.

An auto-ranging facility still requires a measuring function to be selected. Imagine for example you're using a multimeter and you decide to use voltage mode. The autoranging multimeter will then decide which of the available voltage ranges is most applicable to the test being carried out. If you were using a manual ranging device you'd have to figure out this parameter yourself, and the change of error increases.

Despite autoranging sounding like it makes life easier, it isn't without its problems. If an autoranging mode is used on something such as a VAC measurement, it may be subject to displaying what's known as a ghost voltage. A ghost voltage is basically a reading before anything has been connected to the circuit, and this means that the change of a user interpreting this result as correct increases.

Meters generate these so called ghost voltages because under autoranging conditions the meter tries to search for a voltage, and if there's no circuit connected, it might suddenly accept a particular small mV voltage measurement as being correct, and thus displays a ghost voltage. Since the meter has switched to this sensitive mode, it's perfectly possible that a connected test lead will generate a ghost voltage simply because a electrical item is outputting electromagnetic interference. The meter will pick up on this electromagnetic output and interpret this as a voltage reading, hence why a ghost voltage might be displayed. Readings generated are generally either 108mV, 213mV or 455mV.

The effects of ghost voltages can generally be stopped by using a manual ranging mode for when ghost voltages are becoming a problem. This usually requires that the range be manually set to a higher setting such as 400V range or 4000V.