# Can't calibrate an analogue voltmeter: Is this normal?

I've just obtained a ALCRON ACROMETER 60 DC 30 V / 3 A panelmeter and have some problems calibrating it.

I plan to use it in a home-made bench power supply and the supply's range is 2.75 V to 35 V. The meter has a zero centering screw and using it I managed to set the meter to precisely display the voltage (compared to my other two digital multimeters, which show the voltage to be within 30 mV from each other). This works fine until I set the power supply voltage to around 28 V. At that point, as the panelmeter nears the edge of its range, it starts to show lower voltage. Once the meter reaches end of its scale at 30 V, my multimeters show voltage to be 31 V. If I set it to correctly show 30 V, it shows incorrect value at lower voltages.

Since this is my fist time working with analogue voltmeters, my question is: Is such behavior normal and expected?

The most obvious answer would be to check the manufacturer's web site for datasheets but I was unable to find the site. The only interesting thing written on the box (if my German is good) says that the meter is pre-wired for two 6 V lightbulbs.

• Native German speaker here who can help with your manual. Just post what is not clear and I will try to help. – zebonaut Jun 10 '11 at 19:32
• @zebonaut The inscription says Beleuchtbares Drehspul-Einbaumeßinstrument mit Spiegelskala and Dieses Einbaumeßinstrument ist beleuchtbar und entsprechend vorverdrahtet. Duch einfaches Einshieben von zwei 6V-Birchen wird das Instrument mit Flutlicht ausgestattet. Der passende Beleuchtungssatz ist als Sonderzubehör erhältlich.. I think I got the translation right. Unfortunately, that's the whole manual. – AndrejaKo Jun 10 '11 at 20:41
• You are right. Other than mentioning that the instrument has a mirrored scale, the text is really just about the optional installation of a kit with two little bulbs. The word Flutlicht is funny. I would expect such a thing in a football stadium, but not in a panel meter. – zebonaut Jun 10 '11 at 20:47

Could be an issue with flat vs. upright mounting.

• When the needle is too far towards the center at values close to the ends of the scale, the needle of an instrument intended for an upright mounting position might lack the gravity that puts it down just enough to yield a good reading.

• When the needle is too far away from the center at values close to the ends of the scale, the needle of an instrument intended for a flat mounting position might be pulled down too far by the gravity that should not act in the direction of the needle's rotation.

Usually, little symbols indicate the specified mounting position, e.g. an upside-down T means you are to use the meter upright, and a table-like symbol similar to $\sqcap$ means the instrument must be used in a horizontal position.

Examples may be found in this link. (Sorry German only, but you will certainly understand vertikal and horizontal.)

• That was it. The meter had the upside down T on it. Looks like I didn't put it in correct position when testing it. When it is completely upright, it shows correct voltage. – AndrejaKo Jun 10 '11 at 21:26

The zero centering screw is usually a mechanical adjustment for the meter movement. This is to make sure it is showing 0 when the input is 0. This screw should only be set at a known zero input, like when the leads are shorted in voltage mode or open in current mode.

For simple mechanical meters, that may be all you get. The gain is set by the meter movement and fixed resistors. For a powered meter that contains some additional electronics, there may be additional adjustments. In that case, the mechanical zero should be adjusted with everything off. There is usually another knob to set the zero of the circuit. In some cases you may get a gain adjustment, but often not. Gain is usually set with fixed resistors and the like and givin you a means to adjust it would make it less reliable.