## General Metrology Forum

### ppm

ppm; meaning 'parts per million' is very similar to percentage (%), as you can think of percentage as 'parts per hundred'. Although ppm is used when referring to a smaller fraction than what would typically be expressed as a percentage.

1 ppm = 0.0001 %

To compute a specification given as a 'ppm' of range or reading you can use the following formula.

(Value ÷ 1 million) x ppm quantity = result in Value units

Example:
• Spec = 23 ppm of reading + 8 ppm of range
• (6.5 / 1,000,000) * 23 = 0.0001495 VDC
• range = '10 VDC'
• (10 / 1,000,000) * 8 = 0.00008 VDC
• Spec = 0.0001495 VDC + 0.00008 VDC = 0.0002295 VDC
Another way to express the example specification would be to use µV/V (micro-volt per volt) instead of ppm. It would read:

23 µV/V of reading + 8 µV/V of range

or going back to percentage

0.0023 % of reading + 0.0008 % of range.

I hope this helps anyone dealing with expressing small quantities.
For reference purposes I recommend the following documents: