I recently purchased 2 little voltmeters they look like the sort that would go in a control or instrument panel they are connected with just 2 wires which provide the operating supply ( they light up green and red) however the green one states it will work between 20and 500 volts and the red one between 60 and 480 volts. When they are both on the green one indicates normally around 241 volts the red one shows 235 volts why the discrepancy I know it's not much but makes you wonder if one of them is lying. Secondly I've noticed that the green one tracks voltage changes faster than the red one and that a few times the green one jumps down to 238 then up to 241 multiple times while the red one stays the same and I think can see a slight flicker in my filament lamps when this is happening incidentally both meters are connected to the same plug a 2 pin 5 amp one
This type of panel mount meter may either be intended to provide accurate measurement of the voltage, in which case the accuracy might be of the order of 0.1%, or provide an indication that voltage is present and about the right value, so the accuracy will be considerably lower.
There will almost certainly be a method of calibration to allow the indication to be more or less correct in the installation at the expected voltage. This may either be accessible by removing a cover, or be factory preset and sealed.
On the subject of Sat Nav speed measurement, I believe they measure the time taken to travel a particular distance. But the distance is the horizontal distance, so may be affected by hills and bends. On a hill the speedometer will measure the distance along the hypotenuse of the triangle (i.e. up the hill), but the Sat Nav will measure the horizontal distance, which will be shorter. In my experience, despite this the sat nav is almost always more accurate than the speedometer.
This type of panel mount meter may either be intended to provide accurate measurement of the voltage, in which case the accuracy might be of the order of 0.1%, or provide an indication that voltage is present and about the right value, so the accuracy will be considerably lower.
There will almost certainly be a method of calibration to allow the indication to be more or less correct in the installation at the expected voltage. This may either be accessible by removing a cover, or be factory preset and sealed.
On the subject of Sat Nav speed measurement, I believe they measure the time taken to travel a particular distance. But the distance is the horizontal distance, so may be affected by hills and bends. On a hill the speedometer will measure the distance along the hypotenuse of the triangle (i.e. up the hill), but the Sat Nav will measure the horizontal distance, which will be shorter. In my experience, despite this the sat nav is almost always more accurate than the speedometer.
