# Drawing tolerance vs. Measurement device

#### NRitacco

##### Registered
Hello,

I'm struggling with what I understand to be a measurement standard and what our QC department is saying.

I have a drawing that dimensions a length to ±0.25mm. My question is (again, based on what I think is the correct approach), what instrument I use to measure this length has to be capable / repeatable to 1/10th of the tolerance (10:1 ratio). Is that 10:1 compared with the .25mm or the .01 decimal place?

Any help would be appreciated.

Thank you

#### Jim Wynne

Leader
Admin
The 10:1 "rule" is just a guideline of sorts, and other information is needed in order to determine the device to use. In your case, the tolerance range is 0.50mm (~.020"), so using the 10:1 ratio your gage should resolve to 0.05mm (.002"). Other factors to consider include the risk(s) of error near the tolerance limits, and the thing being measured (is it 20 feet long or 2 inches, e.g.).

#### NRitacco

##### Registered
Ahh, OK, so it is based on the tolerance range, in this case 0.50mm. OK, I was thinking I needed to account for the hundredth decimal place when considering a measurement device.

That helps. Thank you for the clarification.

#### Mike S.

##### Happy to be Alive
Trusted Information Resource
4:1 is often acceptable in aerospace and defense vs. 10:1

#### Conrad Hoffman

##### Registered
As tolerances get tighter, 4:1 is often used because there's no practical measurement equipment that will get you 10:1.

#### Jim Wynne

Leader
Admin
As tolerances get tighter, 4:1 is often used because there's no practical measurement equipment that will get you 10:1.
I agree. I mentioned 10:1 because that was what the OP referred to specifically. 4:1 is fine in most cases.

#### Black Ships

##### Involved In Discussions
As said above, QC is referring to 1/10 of the measurement range. This is a practical rule, born out of a simple intepretation of the calculation of uncertainty of measruement. If there are more variables affecting this uncertainty, a 1/10 precision would assure that the contribution of the instrument appreciation to the total uncertainty is 1% (1/10 squared). As Jim said, 1/4 would contribute 6%. (1/4*1/4). Again, this is a simple explanation of a complex subject!