DMM Accuracy Spec- How to Interpret? 1

[dnrepair]

How does one interpret the accuracy specification on a DMM.
In this case a 3 1/2 digit LCD display, 2000 count meter.

For example, under DC volts the following is shown for the 20 Volt Range:

+/- 0.8% of reading + 5 digits.

Range: 20.00V

Resolution: 10mV

Using a 10 VDC reading (as an example), I can see where +/-.8% of reading indicates that the meter can display anywhere between 9.92 -10.08VDC. How does one interpret the + 5 digits?

Thank you

 

[logbook]

The % of reading is a gain error. The % of FS (or just a number of digits) is the linearity error.

In modern terminology I should be saying "uncertainty" rather than "error"

You are nearly there with your 10V example. The meter might read 10.00V. ±0.8% is ±8 digits and then you have another ±5 digits. Result ±13 digits. The "digits" refer to the least significant digit position of course.

10.00V ± 0.13V.

 

[dnrepair]

Thanks. Is it correct then, in the 10V example, that the meter could display a reading anywhere from 9.87V to 10.13VDC. In effect an accuracy of 1.3%?

Or, in a different example, say the actual measured voltage is an automobile battery at 12.60VDC. The meter could display a reading anywhere from between 12.45 to 12.75VDC?

12.60V x .8% =.10 (12.6-.10) = 12.50
12.50-(5 digits)= 12.45

12.60V x .8% =.10 (12.6 +.10)= 12.70
12.70+(5 digits)= 12.75

Is this correct?

Thanks again

 

[logbook]

Yes. Top marks on your results.

This maths looks a bit dodgy though ...
12.60V x .8% =.10 (12.6-.10) = 12.50
Looks like a cut and paste error.

I would have gone
12.60 + 0.8% = 12.6*(1.008) = 12.70
12.60 - 0.8% = 12.6*(0.992) = 12.50

 

[catserveng]

Fluke has some really good application notes on DMM's, you can go to this link,

http://us.fluke.com/usen/support/ap...ukeProducts)&parent=APP_NOTES(FlukeProducts)#

and select the note for "Understanding DMM Specifications", does a great job explaining accuracy and uncertainty specifications.

We use a lot of these notes for technician training, as the info is pretty general in nature and most times applies to other makes of DMM's.

Hope that helps.

 

[dnrepair]

Thanks. Excellent explanation in 'Understanding DMM Specifications". I had searched on the Internet and could not find anything to the subject. This is great and what a great resource the applications notes are!