How do I Calibrate a volt meter?

I will start with the old saying, "A man with two watches never knows what time it is" and then reference the first line of the post. "I have a couple of cheap volt meters". Throw them away and buy one quality multi-meter.
Randy

Really, just how accurate do you need? I calibrated electronic instruments for decades. First you need a known good reference such as a meter that has been recently calibrated. Then you need a voltage source so you can see the meter under test and your reference at the same time.

The very simple way is to set the needle to be right on the mark for the voltage you have. It will then be accurate there, but not necessarily accurate in the rest of its range.

You can check at several places on the scale to get an idea just how accurate it can be overall. This may be only what you need.

Kayteg1 wrote:
Your house voltage early morning should be 120 +- 1 V

well, I wouldn't use that as a source because

1) it varies quite a bit from the 120V depending on your location
2) crest factor varies during the day which affects the RMS value of line voltage vs. peak value of the line voltage
3) your assuming the meter has a very accurate RMS converter
4) it is dependent on the line distribution to your location as well.
5) all that will tell anyway would be the error in that AC range. Doesn't tell squat about all the other ranges. And the AC ranges on the meter are probably the least accurate and most error prone in the design on inexpensive meters. you want to get the DC range(s) calibrated first and then live with the AC range(s) error(s)

I do have a two very accurate DVM's both with NIST traceable calibration. And have been involved in design and calibration of DVN's.

Solutions I can see are (1) comparing to a known calibrated source for the voltages you are interested in, (2) getting a quality meter.

But, for most uses that are needed for RV'ings needs, I'd say most any DVM will give you good enough data.

Trends and tendencies...

Inaccurate voltage / inaccurate resistance readings. I purchased a set of 5 ppm calibratiin resistors. The array is 1.0000 ohm through 20 M ohms. Not cheap (shiver).

There are multiple ways to calibrate a volt meter depending on the type (AC or DC) and what tools you have

The easiest is to connect in parallel with a meter that is known to be accurate.. (At least for the layman this is the easiest) This works for both AC and DC volt meters by the way.

Another involves a known voltage source. this works best with DC meters.. The problem is there are two different sources that are common.. and I forget the voltage of the most common.

Most common is a standard Carbon Zinc Dry cell battery AA AAA C or D all work well, alas, I no longer have access to the document that gave me the "Fresh cell" voltage.. This must be new, never used.. But the manual for my old VTVM (Vacuum Tube Volt Meter) RCA WV77E was the model as I recall.. Same one I used in college no less.

The proper method involves a variable voltage source, A mercury battery and a galvanometer.. Hou hook them up so the Galvanometer (Measures VERY tiny currents) is in series with the battery and variable source, adjust the variable source so no current flows.. Connect the volt meter to the voltage source (not the battery) and re-center the Galvanometer (Zero current).. THe meter shoudl display the battery's voltage.

Alas, that is Lab Grade Calibration and yes.. I've done that.

path1 wrote:
I have couple cheap volt meters. They don't give show the same numbers.

Multimeter or digital display?

In any event, it's not worth calibrating cheap meters. Get a better quality item. Accurate multimeter will cost at least $20. Displays - don't know, DROK has been reportedly accurate, about $12.

LCD display on my solar controller is same accurate as my multimeter, at least up to 0.1 V accuracy. But either one costs a bit more than $12.

Kayteg1 wrote:
Your house voltage early morning should be 120 +- 1 V

Where? I have an almost consistent 125

I tried various things like the 5 volt regulated voltage from a computer power supply and the 12.6 volts on a fully charged car battery 12 hours after charging and the 1.65 volts of a new Duracell AA battery.
Later I used lan inexpensive voltage reference from eBay
Ebay
I have the $8.47 one about 6th from the top and am confident it is accurate.
I have several cheap multimeters of a common type that happens to have a small voltage calibration potentiometer on its circuit board. I found that the meters are pretty stable over time periods of months but do vary tenths of a volt from a warm house to 20 below zero outside.

Since moving up to a UNI-T UT55 and a UNI T clampmeter (around $60 on eBay.ca) I haven't needed to calibrate - good thing because these meters have no calibration adjustment. I only need accuracy to the nearest tenth of a volt.

I use a regulated power supply. A battery will get you in the ball park, too. I have meters that I trust so comparing cheap meters to that measuring tool is what I do. I have a Fluke clamp meter that has a calibration function but is more a 0 balancing than a true calibration.

I've used the $3 Harbor Freight meters to set rail voltages and biasing in better audio amplifiers. I've found them to be fairly accurate. I usually double the measurement with a Fluke to verify accuracy but the HF meters are never much off. For me, it's a cheap solution for using multiple meters in that electronic setting.

Relying on utility 120V is a poor base for accuracy. My power at the house is usually on the high side, running from 123V to 125-6V.

Your house voltage early morning should be 120 +- 1 V