How Do I Test a DC Volt Meter?

[Andrew Shadow]

Question for the electricians/electronics people.

I have three multi-meters that all give me three different readings on DC voltage output in the 12 to 15 volt DC range. The difference between the three is almost 1 volt, with no two being close enough to matter. This is enough to not allow me to rely on the test results. I can always send them to a calibration lab for testing but I really don't think that it is worth the money. Two of them are fairly decent meters but they are by no means $1,000.00 Fluke meters. Is there something that I can use at home to reliably check their accuracy on DC voltage?

It is easy enough to verify the accuracy of the meters on AC voltage. The voltage from the utility company is very stable and varies very little and is a good reference point. When measuring that AC voltage all three meters are within a few Milli-volts of each other, so fairly trustworthy.

I did the internet search thing and the most common recommendation is to buy a new 1.5 V DC battery and use that as a baseline. The problem with that is that I know from past experience that there can be quite a lot of deviation from 1.5 V DC on new 1.5 V batteries. That variance in conjunction with the variance that I am seeing from the three meters means that I have no idea which one is correct. I was thinking about a cordless tool charger. I have one for an 18V cordless tool. I have never checked the voltage output of one of these chargers so I have no idea how close they normally are to their listed rating or how stable they are, so I wouldn't know which of the three meters is correct anyway. Would this be a reliable DC voltage source for comparison?
I can always compare with someone else's meter, but I won't know if theirs is correct either so not really much help. Any ideas if there is a way that I can confidently check the DC voltage reading on these meters myself?

 

[DannoXYZ]

Measure voltage drop across device of known value. Diode or zener. Or make voltage divider ladder with known steps.

For example:

1. measure native voltage output from power-tool + battery = 18,3v

2. run power through 8,9v zener in reverse bias and then small incandescent bulb

3. connect probes to input & output of zener, does meter read same as voltage-drop rating of zener? 8,9v regardless of input?

Repeat using different input voltages, bulb consumption and zener ratings to verify results.

 

[Uncle Phil]

Wouldn't the voltage across the poles of your ST1300 battery while is running be fairly consistent?
On a ST1100 it is usually 14.2 IIRC.
That should give you a 'standard' close to 14 if they are off by a volt.
Just a thought.

 

[Roger]

Fully charged, the bike battery should be 12.6V. I think this chart is accurate for a sealed lead acid battery......

 

[drrod]

Fully charged, the bike battery should be 12.6V. I think this chart is accurate for a sealed lead acid battery......

The problem with this (as it relates to the original question) is the state of the battery cannot be ascertained with certainty which would be needed if you are testing the DC voltage accuracy of the meter.

 

[Uncle Phil]

That's why I suggested testing the terminals with the bike running as I would think that should be pretty consistent.
But I've been wrong before!

 

[Al st1100]

test it on a car, many have dash volt meters. Or key off around 12.5 maybe a bit more but certainly not 13. Key on about 12 -11.8, running 14.5-.7. The one thats 1 volt difference is off. On a Fluke clean and tighten the leads.

 

[Chris09]

At less than half the cost of my cheapest Fluke Meter, my beat up dropped eight year old Amprobe AM-520 seems to be the best for DC Volts to two decimal places. Around $100.00

 

[Andrew Shadow]

Measure voltage drop across device of known value. Diode or zener. Or make voltage divider ladder with known steps.

I'll have to research that if a simpler solution doesn't come along because while I don't have difficulty with electricity I have never dabbled with electronics.

 

[Andrew Shadow]

The problem with this (as it relates to the original question) is the state of the battery cannot be ascertained with certainty which would be needed if you are testing the DC voltage accuracy of the meter.

Precisely my dilemma.

 

[Andrew Shadow]

Wouldn't the voltage across the poles of your ST1300 battery while is running be fairly consistent?
On a ST1100 it is usually 14.2 IIRC.
That should give you a 'standard' close to 14 if they are off by a volt.
Just a thought.

I would assume that it would be but I don't know what that reading should be. I know that I have checked it before but I don't remember what it was.
Anyone know with certainty what the voltage reading should be on an ST1300?

 

[Andrew Shadow]

test it on a car, many have dash volt meters.

None amongst anyone whom I know unfortunately.

Or key off around 12.5 maybe a bit more but certainly not 13. Key on about 12 -11.8, running 14.5-.7. The one thats 1 volt difference is off.

On older GM vehicles 14.2 V was the standard for healthy charging systems as a quick check. Stray only a couple of tenths of a volt and there was a problem. None of my meters give me 14.2. I get anywhere from 13.8 to 14.6. Any of those would be cause to run further charging system tests because they stray way to much from the 14.2 that should be present. The vehicles have current detection on the battery cables now and the BCM sends this data to the ECm which adjusts charging voltage to the minimum to maintain the operation of the vehicle and the battery while conserving fuel. Is the readings that I am seeing because of this or because the alternator is failing. I need to start with an accurate meter reading to find out.

 

[Obo]

If you had a bridge rectifier that converts AC to DC that would give you an accurate DC voltage, but it would be at 3 digits (120v DC)

You could try and measure the output from a USB charger device. They are usually quite accurate at 5V DC output.

 

[Larry Fine]

The only way to calibrate a voltmeter is to compare it to a calibrated meter on a known voltage.

 

[W0QNX]

My battery is 3 months old and the digital voltmeter slash USB was super expensive off of Amazon ($13) but my ST1300 usually says 12.9 most of the time sitting in the garage not running. It will drop down to 12.6 when I let it sit 10 days.

The best way to make 3 voltmeters to agree is attach all three to one power source and with the back of 2 removed turn the potentiometers so they 3 all agree. The only accurate way to calibrate is with a $80,000 Fluke 5522A machine which is calibrated annually to NIST.

If you're really worried grab a cheap 10v reference board off of evil bay like this TL431 board.

TL431 voltage reference board TL431 voltage source for calibrating multimeter | eBay

Materials for making resistor capacitor boards Yageo copper foot resistor;.

www.ebay.com

 

[Obo]

@Andrew Shadow what DC item are you looking to measure that you need to ensure it's that exact? Cheap meters are like cheap torque wrenches, there's often a certain percentage they can be out right from the factory.

 

[Al st1100]

None amongst anyone whom I know unfortunately.

On older GM vehicles 14.2 V was the standard for healthy charging systems as a quick check. Stray only a couple of tenths of a volt and there was a problem. None of my meters give me 14.2. I get anywhere from 13.8 to 14.6. Any of those would be cause to run further charging system tests because they stray way to much from the 14.2 that should be present. The vehicles have current detection on the battery cables now and the BCM sends this data to the ECm which adjusts charging voltage to the minimum to maintain the operation of the vehicle and the battery while conserving fuel. Is the readings that I am seeing because of this or because the alternator is failing. I need to start with an accurate meter reading to find out.

nothing wrong with 14.5. check for a/c voltage

 

[Andrew Shadow]

You could try and measure the output from a USB charger device. They are usually quite accurate at 5V DC output.

That was one of my thoughts as I have a couple hanging around like most people do, I just didn't know how close to 5 volts they usually are.

 

[Andrew Shadow]

The only way to calibrate a voltmeter is to compare it to a calibrated meter on a known voltage.

That is my opinion as well. Unfortunately I no longer work where there is a full calibration lab at my disposal.

 

[Andrew Shadow]

@Andrew Shadow what DC item are you looking to measure that you need to ensure it's that exact? Cheap meters are like cheap torque wrenches, there's often a certain percentage they can be out right from the factory.

Trying to figure out a possible vehicle charging issue. I don't even know if there is a charging issue. The first step was to check voltage. After getting what seemed to me like strange readings I can't tell if they are normal due to what I described in post #12 above or due to my meters being inaccurate.