Measurement Capability vs. Specification Tolerance

Mike S.

Happy to be Alive
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#1
A specification for a widget is 10 +/- 1 units and the specification also requires that "measurement capability meets or exceeds a 4:1 ratio between the product tolerance and the certified accuracy of the measurement device". Measuring device "A" has a certified accuracy of +/- 0.5 units.

Does measuring device "A" meet the specification?

Or does the measuring device need to have a certified accuracy of less than or equal to +/- 0.25 units?

Can you provide a published source that clearly demonstrates the correct technique?
 
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optomist1

A Sea of Statistics
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#2
Hi Mike,

It appears as though you are asking two questions:
1) What is the required accuracy of the measurment device. i.e accuracy is the difference between observed accuracy and the true accuracy or measurment as referenced to a master part or to a national bureau of standards.
2) What is the resolution of the measuring device - i.e. resolution is the smallest reading that an inspection instrument or porcess can reliably display.
You say a 4 to 1 ratio, I am only familair with a 10 to 1 ratio regarding the spec. so if the spec tolerance is 2 units the instrument must be able to reliably measure to 0.2 of a unit.

Have you performed a MSA on this instrument or set up?

Hope this helps..

Marty
 

Mike S.

Happy to be Alive
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#3
Op -- thanks for the reply, but you're making it more difficult that it has to be. All the info needed is there, the question is in the specifics of how "the 4:1 ratio between the product tolerance and the certified accuracy of the measurement device" is calculated.
 

Ninja

Looking for Reality
Staff member
Super Moderator
#4
'K, so I'm scratching my head thinking to myself..."What's the catch?"

The tolerance is 2 units.
The certified accuracy is 0.5 units.
2 divided by 0.5 equals 4

What am I missing?

Can I provide a published source...ummmm...my 5th grade math book.
And when I have thoughts like that with all the smart folks around here, I figure I'm missing something significant...:eek:

I waited all day to see if there was a post to show me what I was missing but it didn't come.
And I figure you are asking a serious question...So....what's the catch?

Is an auditor or customer or coworker insisting that the tolerance is really 1 instead of 2?
 

Mike S.

Happy to be Alive
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#5
It really does seem simple, doesn't it? But this is not intended as a trick -- just a "sanity check". Okay, here's the issue. I will try to ask the question without biasing the answer:

You could do this one of two ways. The spec. tolerance is 2 units wide (+/- 1). The measurement device certified accuracy is +/- 0.5 units. A 4:1 ratio is required.

You divide the total spec. tolerance (2 units) by 4 and get 0.5. Does that mean that (A) a measurement device that has a certified accuracy of +/- 0.5 is what you need as a minimum or (B) a measurement device that has a certified accuracy of +/- 0.25 is what you need as a minimum?

Does the question seem clear now?
 
N

NumberCruncher

#6
Hi Ninja

I don't agree with your calculation. You have divided a range by a semi range.
Spec = 1/-1 hence range = 1-(-1) = 2
Measurement instrument spec = +/-0.5 hence range = 0.5-(-0.5) = 1
Ratio = 2/1 = 2

It may help to draw a simple diagram. Mark three equidistant lines

11__________



10__________



9___________

Now measure the lines and get a reading of 10+/- 0.5


11__________

10.5.............

10__________

9.5...............

9___________

I think you can see that the instrument uncertainty takes up half of the specification.

Any other contributions from someone who knows for sure?

NC
 

Ninja

Looking for Reality
Staff member
Super Moderator
#7
Ahhh, that's what I was missing...the +/- symbol on the accuracy. Thanks NumberCruncher!
 

Mike S.

Happy to be Alive
Trusted Information Resource
#9
The 4:1 ratio for spec. tolerance vs. measurement accuracy is a fairly common requirement referenced in several standards, from old MIL-STD-45662 to current standards like SAE ARP 9013, although the exact wording varies slightly from place to place. The wording I used is from ARP 9013.

As you can see, it is a simple enough question on the surface...
 
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