Calibration? Verification? Is this a calibration?

P

peschi

#1
I am taking analogue 0-1 dial (resolution .001") indicators putting them in a ridgid upright stand and setting zero. I put a total of 5 different gauge blocks over the full scale of the gauge. I am seeing that the needle doesn't always rest on exactly on the line, but, somewhere between lines. I am judging where the needle rests past or before the line and documenting the readings as accurately as possible. ie with a 1" gauge block, my results may be .9995. Is this a calibration? From what I read on the net, this does qualify a calibration.
Please help.
 
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#2
I am not a 17025 expert but I run a calibration system that satisfies TS16949.

Strictly speaking, it is calibration if you tweaked the guage so that the needle rested on the 1.000" line when measuring the 1" block. If you are not adjusting the guage, it is verification. For the purposes of meeting TS16949 Section 7.6 your practice is acceptable as long as you document that the .9995 reading is adequate to "provide evidence of conformity of product to determined requirements".
 

Jim Wynne

Staff member
Admin
#3
"Calibration" is the act of comparing a measurement device against a known and traceable standard--nothing more. Sometimes adjustment is necessary, sometimes not.
 
E

Ederie - 2007

#4
Yes it is calibration

Since you adjusted the zero to begin with, you are only checking the measuring scale inside the gauge.
Record the standards used and the actual readings (1.000 - .9995)
- make sure that you set tolerance limits(throw out or fix) for the gauges that are proportional to your part tolerances
 
#5
I split the hair only because ISO9001 uses both terms under Control of Measuring and Monitoring Devices and either one is acceptable. Calibration generally insinuates an adjustment or "setting" of the equipment (e.g. twiddling a pot until your DMM reads 1.000VDC with a 1VDC input). Although I could be wrong about the definition of calibration, I might be refering to adjustment. Although, Merriam-Webster seems to agree that Calibration or the state of being Calibrated would tend to support either definition. But you know how those British are with English.

Verification (don't make me quote ISO9000:2000) is a confirmation that the device is still correct where no adjustment is possible (e.g. checking your digital micrometer after you dropped it to make sure you didn't bend it).

This is fun Jim. I learned something.
 
Last edited:

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#6
peschi said:
I am taking analogue 0-1 dial (resolution .001") indicators putting them in a ridgid upright stand and setting zero. I put a total of 5 different gauge blocks over the full scale of the gauge. I am seeing that the needle doesn't always rest on exactly on the line, but, somewhere between lines. I am judging where the needle rests past or before the line and documenting the readings as accurately as possible. ie with a 1" gauge block, my results may be .9995. Is this a calibration? From what I read on the net, this does qualify a calibration.
Please help.
It could be.....once your zero is set and you run your comparisons then you have some measured difference. JSW05 is correct that easily qualifies on the surface as a calibration.

However, then it gets a bit more qualified. Calibration requires tracability. Traceability under the international definition requires two components: an unbroken chain of comparisons to SI through National or international standards; and stated uncertainty at each step.

Put another way, for it to truly be calibration you need to document the chain of comparisons and perform your uncertainty calculations. So how do you do that?

The unbroken chain is documented by taking the item number (or similar) of your gage blocks, the due date for their next calibration, and the certificate number from their last calibration (assumes an accredited calibration lab). If the calibration lab is not accredited you may or may not be able to trace the specific calibration through the cal lab to the next level, or more precisely the lab may not be able to provide evidence of the link.

Uncertainty is a bit more rigorous. Take into account half the least resolution of the indicator, the uncertainty of each gage block (divide by 2 to return it to standard uncertainty), the difference in temp from your shop/office to 20 degrees C (or stated temp on the gage block cert), thermal expansion caused by your handling, the flatness of the stand for the indicator (if known), these are all Type B contributors. Other than the uncertainty of the gage blocks one assumes a rectangular distribution so divide by square root of 3. Your readings are Type A and you should have 3 or more for each gage block value. Find standard deviation for each gage block value, divide by square root of n, where is the total number of readings. Take all the Type A and Type B numbers, square each one, add them together then take square root, this is standard uncertainty or about 67% confidence level. Multiply by two to achieve expanded uncertainty or about 95% confidence level.

Of course the easy way is to have an accredited lab do the calibration and you only do verification.

Hope this helps.

Hershal
 

Jim Wynne

Staff member
Admin
#7
Icy Mountain said:
I split the hair only because ISO9001 uses both terms under Control of Measuring and Monitoring Devices and either one is acceptable.....
Some confusion is added by the fact that calibrations so often do result in adjustments, and those adjustments often have to be done by skilled people. But think of it this way: in any given shop, there might be lots of devices in use with calibration identification on them, and the assumption is made that those devices have been calibrated in accordance with requirements. But if it were found that one of them, during the last calibration, didn't need adjustment (it was OK as found), would anyone say that the device wasn't calibrated?
 
K

Kevin H

#8
In my last position as lab manager in a steel mill, we separated (at least in our minds) calibration from verification. I calibrated our calipers and micrometers quarterly, my operators verified them daily before use with a gauge block. If they would have failed, I would get them to calibrate and would have to analyze for potential nonconforming product.

We verified Rockwell hardness testers on a by turn/by scale use per the requirements of ASTM E18, which does differentiate between verification and calibration. We had corrective actions we could take if it didn't verify, such as examining the ball indenter for B scale hardness and replacing it. If we couldn't verify it and I or my technician couldn't verify it, we would have to have it serviced and calibrated by our vendor, Wilson Instruments.

We used a similar method for our tensile testing equipment, running 2 internally developed standard tensile specimens and plotting the results on an spc chart to determine whether the tensile tester was OK to use or not. Again, I or my technician had some steps we could take to clear the tester for use. If they failed, we would have to have it serviced and calibrated by our vendor, Tinius Olsen.
 

Marc

Hunkered Down for the Duration with a Mask on...
Staff member
Admin
#9
Icy Mountain said:
Calibration generally insinuates an adjustment or "setting" of the equipment...
I disagree.

JSW05 said:
"Calibration" is the act of comparing a measurement device against a known and traceable standard--nothing more. Sometimes adjustment is necessary, sometimes not.
I agree.

The Elsmar Wiki entry for Calibration:
Calibration
PLEASE help better define Calibration in the wiki if you can!

Also see these discussion threads here in the Elsmar Cove forums:
Calibration vs. Verification - A Good List of Definitions
Calibration vs. Verification - ANSI/NCSL/Z540 - Definitions of
Calibration vs. Verification - Definition of
Verification vs. Validation vs. Calibration - What is the difference?
 
C

crendfrey

#10
:bigwave:
JSW05 said:
"Calibration" is the act of comparing a measurement device against a known and traceable standard--nothing more. Sometimes adjustment is necessary, sometimes not.
:bigwave: greetings all. this is my first post. i do feel compelled to put my 2cents in here. please tell me what you think.

I work for a small calibration lab striving for 17025:2005. We calibrate scales. Our final audit is in 15 days. I can only ASSume the vocabulary will carry through to other measuring devices.

In my humble opinion,I feel there is a significant difference between verification, validation and calibration.

Verification is usually performed internally by a customer to verify the scale still remains within their determined tolerances, requireing nothing more than setting a known weight on the scale and seeing what the indicator reads.
If this reading is outside of their set tolerance, they call the lab for validation, calibration and or repair.

Validation requires a qualified technician using traceable weight standards (choosing the correct class of weights for the class of scale being examined)
performing several evaluations using proven methods and procedures, including environmental conditions, corner or quadrant, increasing and decreasing loads, repeatability, etc... and recording the results at each measurement.
If the results of these ("as found") measurements fall within the customer tolerance rangs, the scale may be validated for the stated accuracy.

However- Some scales may fall within the stated customer tolerance but in most scales there will still be some error found (using the correct weights and following the above procedure).

Calibration requires the same actions as validation and going the most important final step of correction of any error found. Adjustments are made to achieve as close to zero error at all steps of evaluation. this includes the recording of ("as left") corrected measurements.

Now-- this all feeds into measurement of uncertainty :bonk: which is a discussion for many many other days.

PS. How do i find spell check on this thing??
 
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