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

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nick1980

#1
:mg:
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.
 

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Marc

Hunkered Down for the Duration with a Mask on...
Staff member
Admin
#2
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?
 
C

Charmed

#3
Type B Uncertainty

Dear nick1980:

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

http://www.npl.co.uk/electromagnetic/dclf/tech-transfer/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/MeasurementUncertainCA34A.pdf

Charmed :)
 
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nick1980

#5
1 stand deviation=type-B uncertainty??

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. :eek:
 
N

nick1980

#6
definition of standard uncertainty.

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?)
 
C

Charmed

#7
Type B Uncertainty

nick1980 said:
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

Metrologist-Auditor
Staff member
Super Moderator
#8
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
 
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nick1980

#9
Simplify calculation!!

For most of my concern is that it's so trouble and time-consuming to calibrate one equipment with so complicated calculations, esp. type-B uncertainty.

I need to guess all the possibilities of type-B uncertainty. I never know it's correct or not. It's just my assumption.

If possible, I will assume type-B uncertainty as 1 standard deviation to simplify my calbration process.

Any experience can share?
 
N

nitram

#10
Distribution and 1 std.dev.

nick1980 said:
For most of my concern is that it's so trouble and time-consuming to calibrate one equipment with so complicated calculations, esp. type-B uncertainty.

I need to guess all the possibilities of type-B uncertainty. I never know it's correct or not. It's just my assumption.

If possible, I will assume type-B uncertainty as 1 standard deviation to simplify my calbration process.

Any experience can share?
Hi Nick,
I will try to help you in the best way I can.

One of the basic things in the GUM is that the square of all uncertainties should be added together at the 1 std dev. level. (You should not add an uncertainty on 1 std.dev with another that is expressed in e.g. 2.std.dev).

In order to do this you need to know the distribution and the coverage factor.
E.g. resolution is rectangular and the accuracy from a manufacturer might be normal distributed (or worst case=rectangular).

To get the 1-std.dev uncertainty (def. = standard uncertainty)for the resolution you divide the resolution with 2 (as the uncertainty is half the resolution) and the you divide it with the sqt.rot of 3 (due to the distribution).
To get the 1-std.dev uncertainty for the accuracy you need to know what coverage factor they used (2 =appr. 95% and 3 = appr. 99%).
The accuracy value from the manufacturer has to be divided with the coverage factor to get the 1 .std.dev. uncertainty.

In order to add all squared uncertainties you will also have to use the correct sensitivity coefficient.

In the end when you follow the GUM-method, you multiply the total uncertainty with the coverage factor 2 to get the expanded total uncertainty.

Did this give you any better understanding ? Even if it is not a in deept explanation.

Please also check the EA-10 documents on this link http://www.european-accreditation.org/documents.html where several examples are availiable.

Good luck, Martin :confused:
 
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