Controlled Environment - Environmental Requirements for calibrations

I have proposed performing internal calibrations of caliers and micrometers. My supervisor has said that we can not do this because ISO 17025, section 5.3 requires a controlled environement. I need some references to published calibration standards to convince him that the ambient temperature, if within say 70 +/- 5 degrees F. and if that temperature is recorded at the time of the calibration, that we will meet all requirements of ISO, Dod, Etc.

Temperature Standard

In ANSI/ASME B1.2-1983 Gages and Gaging for Unified Inch Screw Threads it states in paragraph 2.4.1 A temperature of 68 degrees F (20 C) is the standard temperature used internationally for linear measurements. --- For screw thread gaging, the acceptable tolerance on the standard temperature is ±2 degrees F.

This is the generally accepted standard for calibration labs worldwide. Hope this helps.

The typical accepted environmental parameters for dimensional calibration are:

20 C +/-1 C
30-50% RH

For more information, see NCSLI' RP-7 (Recommended Practice).

If you do internal cal, make sure your gauge blocks are sent to a laboratory accredited to ISO/IEC 17025 by an internationally recognized accrediting body to assure traceability, and that your procedures are documented and validated and personnel trainined, and that you have completed your measurement uncertainty calculations.

Traceability involves an unbroken chain of comparisons to SI through National or international standards, such as the standards maintained by NIST, and stated uncertainties at each step. This is according to the internationally accepted definitions in the VIM (Vocabulary of General and Specific Terms in Metrology).

Hope this helps.

Hershal

AlanJ_QA said:

I have proposed performing internal calibrations of caliers and micrometers. My supervisor has said that we can not do this because ISO 17025, section 5.3 requires a controlled environement. I need some references to published calibration standards to convince him that the ambient temperature, if within say 70 +/- 5 degrees F. and if that temperature is recorded at the time of the calibration, that we will meet all requirements of ISO, Dod, Etc.

Click to expand...

Welcome to The Cove! These tools have a resolution of thousandths? Ten-thousandths? Tight calibration lab environmental controls are usually called out for tight tolerances or to control conditions subject to regulations. Being ISO registered in itsself doesn't require such tight controls in many cases.

Are your working conditions far outside the 65-75 degree range you specify? (Don't forget humidity!) I used to work in a machine shop that reached 90 degrees and 80 percent humidity. Arguably it would be foolish to calibrate the tools at a strict 68 degrees and 50% humidity because the difference between lab and shop floor would invite error. Do you see what I mean? But we measured in thousandths as a rule.

Your need for ISO 17025 controls is based on what? Customer requirements, contracts and regulations might call out a need for strict calibration control, but as I understand the standards they don't get that prescriptive.

Here's an article on calibration that I hope will help. http://www.qualitydigest.com/feb97/gagecal.html

AlanJ_QA said:

I have proposed performing internal calibrations of caliers and micrometers. My supervisor has said that we can not do this because ISO 17025, section 5.3 requires a controlled environement. I need some references to published calibration standards to convince him that the ambient temperature, if within say 70 +/- 5 degrees F. and if that temperature is recorded at the time of the calibration, that we will meet all requirements of ISO, Dod, Etc.

Click to expand...

Many, if not most, companies I see calibrate (or verify) their basic hand gages like calipers and mics. If your tolerances are not especially narrow, the advice given above by Hershal and Jennifer should be adequate to meet your needs. If you work to very tight tolerances, you probably would not be using calipers in the first place.

I also wonder why ISO/IEC 17025:2005 is being quoted by your manager. ISO 17025 does not apply to your activities unless you're using that standard.

If you are not calibrating calipers/mics in house, you're certainly not using 17025.

The typical accepted environmental parameters for dimensional calibration are:

20 C +/-1 C
30-50% RH

Click to expand...

Hershal, where is this standardized? I see "generally accepted" and wonder on what that is based.

Daytime temperatures over here are about 27-30 degrees C and 32-36 degrees C outdoors. Calibration is usually done in air conditioned rooms of about 25 degrees C. Correction factors are then applied to cater for the difference between calibration room temperature and ideal temperature. If accuracy is important, the same correction table can be used to take care of the difference in ambient/shop temperature versus the temperature at which the instrument is calibrated.

Such tables are usually supplied with instruments for tropical climate.

atetsade said:

I also wonder why ISO/IEC 17025:2005 is being quoted by your manager. ISO 17025 does not apply to your activities unless you're using that standard.
Hershal, where is this standardized? I see "generally accepted" and wonder on what that is based.

Click to expand...

The standardization is documented in such locations as military requirements, NCSLI RP-7, and accepted international norms. It is certainly possible to calibrate outside the norms, but then the uncertainty values increase. Remember the gauge blocks are cal'd at 20 C, so if the calipers are cal'd at 25 C, then the 5 C difference is taken, divided by square root of three because it is a rectangular distribution, and included as one of the Type B contributions.

As for ANS/ISO/IEC 17025, for internal cal, it certainly does apply. ISO 9001 is woefully inadequate in the calibration requirements because that is not the intended purpose, where 17025 and ANSI/NCSL Z540-1-1994 are both intended to cover calibration activities. 17025 also covers testing activities, where Z540 does not.

Hope this helps.

Hershal

Lots of very good info in all of the above posts, to which I don't disagree with any (not to mention that my specialty is not dimensional - although I have been in the business nearly 30 years).

The temperature issue with calibration of calipers is more an uncertainty issue (as the coefficient of expansion for your gage blocks comes into play). A legitimate method if calibrating away from 20 Deg C (68 Deg F) is to apply coefficients. HOWEVER, if that temp is away from 20 Deg C, I am guessing temperature stability is not good either. Therefore, since expansion and contraction of the gage blocks takes some period of time (one of you dimensional experts can fill in better on this), and IF your temp fluctuates, there is an increase in the instability of the actual value of each block.

Therefore, in such an environment, you may expect fluctuations in the actual performance of your gage blocks, and fluctuations in your calibrated readings (away from the true nominal). Using 68 Deg F as a reference temp for calibrations was established to improve the uncertainty of the calibrations. You might consider TRYING to achieve as close to 68 Deg F as possible even if your spec allows the deviation. In a lab where I used to work, we had a looser spec, but the dimensional guy surveyed and found the coldest spot in the room. He does the dimensional work there.

I don't believe matching the operating environment of the calipers is necessarily beneficial UNLESS there is a very predictable operational environment. For example, if they are alway used at 85 Deg F, calibrating them at 85 Deg F (including applying the coefficient to account for gage block expansion -- a miust) may potentially provide better calibration results. Problem is, because the calipers and gage blocks were designed around the 68 Deg F context, you may find readings are always a little skewed.

Again, you experts chime in on the following.... Am I correct the coefficients on ceramic blocks are better for unstable environments? If so, you may want to consider a different type block.

Okay. I haven't posted in a while and wanted to chime in about something. Have a great weekend everyone, and I hope I added some value to the discussion.

Hi Jerry, welcome back.

Good advice.

Hershal