# Significant figures and uncertainty

1. Apr 8, 2012

### Miffymycat

Let’s say I have a class A 25ml pipet with an uncertainty of +/- 0.24ml.

If I want to express the volume of the pipet to the correct number of sig figs to say determine the sig figs in a final answer - what is it?

Is it 25 ie two, because that is the marking?

Or 25.00 ie four, because the uncertainty limits the range to 24.76 – 25.24, which are 4 sig fig values?

Or 25.0 ie three, being 0.24 rounded down to the nearest single sig fig?

Thanks!

2. Apr 9, 2012

### tiny-tim

Welcome to PF!

Hi Miffymycat! Welcome to PF!
Technically that's 2 significant figures.

(though I'd be tempted to call it 2.5 significant figures, since it obviously could be rounded to the nearest 1/2 ml)

I'd be inclined to call it ±1%, and wait and see what the final answer is …

if it's say 89.4, then make it 89 (2 sig figs) since that's ±1%

if it's say 11.4, then make it 11.4 (3 sig figs) since that's ±1%.

if it's say 33.4, then i dunno … judgment call!

What do other people think?

3. Apr 9, 2012

### Staff: Mentor

Re: Welcome to PF!

SF are faulty by design?