How to determine Type-B uncertainty?What distribution?With Calibration beginner guide

[nick1980]

I don't know how to determine the type-B uncertainty because it has many possibilities such as past experience, calibration certificate.

In type-B uncertainty, the formula is varied by different distribution of measurement values (normal, rectangular, triangular, etc.). How do I know what distribution I should use? Does it assume all measurements rectangular distribution with formula 0.5*limit/sq. root of 3?

Also, I found a very good guide for beginner about calibration. I share it to you all.

[Marc]

Hopefully, someone can answer the thread starter's original question.

How do I know what distribution I should use? Does it assume all measurements rectangular distribution with formula 0.5*limit/sq. root of 3?

[Charmed]

Dear nick1980:

The following link takes you to a nice presentation by FLUKE on measurement uncertainties.

http://www.npl.co.uk/electromagnetic...sfer/fluke.pdf

See page 12, which I quote. Type B uncertainties are said to be 1 standard deviation estimates of the likely range of values the value may have (Often considered as systematic uncertainty). See also the following article by Dr. Stephen Phillips of NIST.

http://www.mel.nist.gov/proj/pdf/Mea...rtainCA34A.pdf

Charmed

[Atul Khandekar]

Nick,
Welcome to the Cove Forums!

I had asked a similar question in one of the threads here..It may help if you go through it:
http://elsmar.com/Forums/showthread.php?t=5689

I'm sure our calibration experts on board will chip in with more useful replies.

[nick1980]

Thanks Charmed!!

From your reference, 1 stand deviation=type-B uncertainty. Can you tell me why?

It's because I found many different formula to calculate type-B uncertainty for normal, rectangular, triangular or uniform distribution.

If type-B is really equal to 1 s.d., that's so great.
It makes the calculation easier a lot.

[nick1980]

I found a definition of it:

standard uncertainty = one standard deviation.
But in order to minimize the possibility of mistakes at a later stage of the evaluation, it is sometimes necessary to multiply values of type-A estimated standard deviations and type-B standard deviations by suitable sensitivity coefficient to bring them to the same units as the measurand or to take account of other factoes in the funcitonal relationship between input quantities and output quantity.

So what standard uncertainty equals to ? one s.d.? or typeA+typeB uncertainties? (type-A estimated standard deviations = type-A uncertainty??same meaning?)

[Charmed]

Originally Posted by nick1980

So what standard uncertainty equals to ? one s.d.? or typeA+typeB uncertainties? (type-A estimated standard deviations = type-A uncertainty??same meaning?)

Please read the article by Stephen Phillips which explains these things nicely, which I gave link to. I will check another source and get back to you.

Charmed

[Hershal]

Nick,

There are two answers going on here, and I will presume you are in a cal lab, not a test lab.

Standard uncertainty is one standard diviation, or approximately 67%. Expanded uncertainty is two standard diviations, or approximately 95%.

Type B uncertainty distribution model will vary depending on the influence. GENERALLY, the Type B is a normal distribution. GENERALLY, the Type A is rectangular, in that the true value can lie anywhere within the uncertainty range. These are rules of thumb, not hard and fast.

Type B can include the environmental concerns such as temp, RH, vibration, and dust. Recognize that the environmental influences might change. In some cases for example, temp and RH changes have no effect over a wide range. In the test lab world, that is common. Other effects include calibration uncertainty taken from the accredited calibration cert (plug and play number), time, viewing angles, technician proficiency, almost ANYTHING that can affect the measurement to a measurable degree.

Hope this helps.

Hershal