Multimeter Accuracy

[thstone]

If you actually want to test the accuracy, you'll need a verified voltage source like a DC power supply which also has a manufacturer's accuracy.

Set the DC power supply to 12.00 vdc and take several readings using your multimeter. Then work the numbers.

For example, let assume that the DC power supply is rated at +/- 2% or +/- 0.24 vdc at 12vdc (2%-3% error is typical of a $300 power supply). This means that the actual voltage of the power supply could be anywhere between 11.76 vdc and 12.24 vdc.

Now, add on the error in the multimeter. Let's assume that the multimeter is also accurate to 2%. This means that the meter could read as low as 11.52 vdc or as high as 12.48 vdc.

Thus, in the example above, the total uncertainty of the measurement is on the order of 1 vdc. This doesn't mean that affordable multimeters aren't useful, it just means that they aren't accurate enough for the measurements that you're tying to take.

Affordable instruments are still useful in a lot of practical applications like determining whether the battery voltage is 12 - 13 vdc or 8-9 vdc. Its just not going to tell you conclusively whether the voltage is exactly 12.50 vdc.

Of course, better instruments have lower errors. For example, if you spend $1,500 for a multimeter with a 0.25% error, you'd be able to read the 12.00 vdc to within a range of 11.97 - 12.03 vdc or an uncertainty of 0.06 vdc. This would be highly accurate.