# What does PPM regarding measurement device accuracy mean?

L

#### lee01

I’ve been looking into some Laser Tracker measurement devices and have come across an accuracy reading of +/- 2.5ppm and +/- 10ppm? What does this mean and how do you calculate this in normal micron format?

Lee01

#### Jerry Eldred

##### Forum Moderator
Super Moderator
I suppose I may be missing a deeper implication on this one. But I'll start with the basic answer to see what I may not be understanding....

A ppm (part per million) is the same as 0.0001% percent. Add or subtract to a nominal quantity. I don't know the rest of how your tolerance spec is written on the instrument. But if you are measuring 1 inch, +/-2.5 ppm is 1 inch plus or minus 0.0000025 inches. If you are measuring 10 inches, +/- 2.5ppm is plus or minus 0.000025 inches. A plus or minus ppm would be computed similarly to a percentage (which is parts per hundred). Where a ppm is parts per million.

Did I interpret your question correctly?

#### Atul Khandekar

##### Quite Involved in Discussions
Does this, by any chance, mean 'positional accuracy'?
eg +/- 10 ppm would be accurate within +/- 10 µm/m ?

L

#### lee01

Okay, but if a product has an accuracy of +/- 10ppm, +/- 20ppm, +/- 40ppm? What is the accuracy of these machines?

Lee01

R

#### Ryan Wilde

lee01 said:

Okay, but if a product has an accuracy of +/- 10ppm, +/- 20ppm, +/- 40ppm? What is the accuracy of these machines?

Lee01
ppm = Parts Per Million

All this means is that you take whatever output the product is supplying (in the case of a source) or whatever measurement the product supplies (in the case of a measurement device) or whatever physical parameter the product has specified (in the case of an attribute gauge) and divide by 1 million to find the ppm accuracy.

Therefore, if my voltage source is supplying 120 V, then my accuracy would be:

ppm = nominal/1 million
ppm = 120 V/1000000
ppm = 0.000120 V
10 ppm = 0.0012 V (ppm * 10)
20 ppm = 0.0024 V (ppm * 20)
40 ppm = 0.0048 V (ppm * 40)

Therefore, my 120 V would be:

@10 ppm = 120 V ± 0.0012 V (or 119.9988 to 120.0012)
@20 ppm = 120 V ± 0.0024 V (or 119.9976 to 120.0024)
@40 ppm = 120 V ± 0.0048 V (or 119.9952 to 120.0048)

Hope this helps,

Ryan

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