Calibration Environment - Micrometers - NCSL RP 7 - Laboratory Design

[Marc]

From: "James R. Lloyd"
Subject: RE: Calibration Environment/Lemieux/Lloyd

Couple of questions first Kevin .... What do you mean by micrometer standards? I take it you mean the end user micrometers, etc... usually when I hear the term "standard" Im thinking of Calibration Laboratory level equipment such as gage blocks, parallel flats, and optical parallels used to calibrate outside micrometers .... As far as environmental effects on "floor" usage micrometers, it depends upon your resolution and accuracy requirements that you need on the floor. If I remember correctly expansion coefficients for steel are something like 1 micro inch (.000001) per degree F per inch, this is typically miniscule in most machining type environments. Considering that your typical micrometer is usually calibrated to an accuracy of +/- .001 inches, if your measurements are only in this range it would take some SERIOUS and I mean SERIOUS changes in temperature before the measurement accuracy of your floor micrometers would be affected.

The main concern for Micrometers on a floor is mishandling, and abuse that can affect the measurement accuracy rather than the environment.

For the calibration of Micrometers a bit more environmental control is required, not so much for the micrometer, but for the standards used to calibrate it. Referencing an USAF Metrology Procedure for micrometers .... There are three primary attributes that are calibrated ... measurement accuracy/linearity, flatness of the anvil and the spindle, and parallelism between the anvil and spindle. An acceptable environment to calibrate you typical +/- .001 0-1 inch micrometer would be even 75-79 degrees as the gage blocks would certainly not expand to a degree that would be even measurable ... but the larger the micrometer measurement the more likely that expansion WILL affect its calibration. Usually this threshold is around 4-5 inch micrometers, where you would need an environment controlled to about 71 +/- 4 degrees, and even this becomes excessive with micrometers that measure 10-11 inches where the optimum dimensional calibration environment of 68+/- 1 is required.

I hope I didn't inundate you with information, but metrology, and calibration programs are my specialty.

Best Regards
James Lloyd
QA Manager
Wesport Steel and Supply Inc.

One Comment, I hope all the other mics that are not calibrated are still
identified in your M&TE inventory and identified/labeled as "Not
Calibrated", regardless of use they are still M&TE and must be identified
and controlled.

-----Original Message-----

> From: Kevin Lemieux
Require information regarding consensus on what environmental parameters should calibration be carried out in? What do others do with their micrometer standards so they are protected as well as available for shop floor usage? We currently have calibration tractability on two sets of standards which are available for shop personnel usage. This eliminates the requirement to calibrate all personal standards. The question has came up on the environment in relation to temperature and location of the generally used standards. Help?
> Kevin - QA Manager

KEVIN,
YOU MUST HAVE A SEPERATE AND SEGRAGATED SET OF STANDARDS FOR CALIBRATION. YOU CAN NOT USE A SINGLE SET FOR CALIBRATION AND FOR SHOP USE. YOUR CALIBRATION STANDARDS MUST BE PROTECTED AN ISOLATED FORM THE REST OF YOUR STANDARDS. THEY ALSO SHOULD ONLY BE ACCESIBLE BY THE PERSON IN CHARGE OF YOUR CALIBRATION SYSTEM. I DON'T KNOW IF THAT ANSWERS YOUR QUESTION.

------------------

[Dawn]

After two hours of sitting in the QC department to be the same temperature as the calibration equipment, will the micrometer change when it is back out on the plant floor?
Supposing you adjust the micromter in QC, take it out on the floor and it gets warm again. Does this change the accuracy of the micrometer?

[Dawn]

Question # 2:
Why can't you use the same gauge blocks for calibration of micrometers that are used on the shop floor as long as they are certified and checked once a year.

[Marc]

Ummm, well,

After two hours of sitting in the QC department to be the same temperature as the calibration equipment, will the micrometer change when it is back out on the plant floor?
Supposing you adjust the micromter in QC, take it out on the floor and it gets warm again. Does this change the accuracy of the micrometer?

The issue here is sensitivity to temperature and differences between the standard and that being measured. Although some examples are self evident (e.g.: measures of length such as with a micrometers) such as coefficients of expansion, others (e.g.: electrical resistance properties) are not so easy to see and understand.

De Key:
One calibrates a micrometer in a 'lab' at a certain temperature because the standard is certified at that temperature (remember, every substance changes as temperature changes but different substances each temperature change differently with any given change). Bi-metal 'thermometers' are a good example of different coefficients of expansion.

The question always being asked is 'Well, we use it on the floor where it's hot so why not calibrate it there?' or something similar. Typically the items we measure and where we measure them (temperature, humidity, etc.) and with what with respect to required tolerances is not a big issue - BUT - it can be. This is in part the reason for QS9000's drive for Measurement Systems Analysis. It's not just calibration, it's the understanding of what you're doing. What I mean here is that all relative factors are considered from the design stage where tolerances and specs are determined, to use of the FMEA and thence the control plan from whence critical characteristics (as well as required Measurement and Test equipment) are determined, to the production environment. Like we have talked about in a couple of other forums, this is a holistic concept.

So - for question 1, the answer is: Typically it is not a problem (use of instrument on the shop floor after calibration in the lab) because differences in coefficients of expansion (or other temperature related effect{s}) with respect to the temperature difference (delta), the required 'sensitivity' (i.e.: precision, I think, is the word), and tolerances (to a small degree) are negligible.

Question # 2:
Why can't you use the same gauge blocks for calibration of micrometers that are used on the shop floor as long as they are certified and checked once a year.

You can if you want to. It depends upon how your system is designed, if you will. There is no requirement for a single standard to be kept in a lab for calibration. As long as you can show and explain your calibration cycle and show evidence that the yearly verification of calibration is sufficient and (if you will) effective, you may do this. Don't confuse reality with 'teachings'. We are taucht there is a master in the lab and then shopw floor 'standards' are calibrated against this 'golden' standard. The issue is frequency of use (wear) of mechanical/physical M&TE, possible damage on the floor and such factors. However, in reality not that many companies are all that big and complex to neccessitate a 'golden' standard.

[This message has been edited by Marc Smith (edited 12-31-98).]

[Marc]

Date: Wed, 13 Oct 1999 07:17:09 -0500
From: "SMITH, RON D. (JSC-EM)" jsc.nasa.gov
To: 'Greg Gogates'
Subject: RE: Environmental Conditions

There are a couple of documents we use as standards or guides for Calibration Laboratory environmental conditions.

NCSL Recommended Practice (RP) 7, "Laboratory Design" TO 00-20-14, "Air Force Metrology and Calibration Program"

Ron Smith
Johnson Space Center