Environmental Conditions for Calibration - Temperature and Humidity

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Ryan Wilde

howste said:
Reading this thread brought to mind a few questions:

1)If you calibrate all of your gages in a laboratory with a controlled environment, then use the gages in an uncontrolled environment, what kind of results do you get?

No environment should be uncontrolled. The gages must be calibrated in a controlled environment to give you a basis to measure/calculate errors in other environments, such as the shop floor, etc.

2) Does it make sense to let the standards "soak" and calibrate the equipment in the environment it will be used in?

It does not generally make sense to "soak" standards and calibrate in a non-standard environment due to different thermal expansion properties. Basically, what is the material(s) that comprise the item being calibrated, what is the material(s) that comprise the standard, and what is the difference between their thermal expansion? Example: Gage blocks can be made of tool steel, chromium carbide, tungsten carbide, or ceramic. Most tools, such as micrometers or calipers, are made of entirely different types of steel, such as stainless. You now have a difference in thermal expansion to deal with that will be very small at or near 20°C, but will be measureable at 30°C, to the point that at 10", it may mean the difference between in and out of tolerance.

3) Is it better to keep gages in the environment they will be used in, or in a controlled lab?

That's a tricky question. If I have to use a micrometer on product that is in a 35°C room, I would want my micrometer to be at 35°C rather than 20°C. The reason would be that the difference in thermal expansion would be much less at that point, and although it's not perfect, it is a more accurate measurement. If my micrometer was calibrated at or near 20°C, and I know it to be accurate, it is my best bet to not have false accept/rejects.

Ryan
 
G

Graeme

howste said:
1)If you calibrate all of your gages in a laboratory with a controlled environment, then use the gages in an uncontrolled environment, what kind of results do you get?
Every place I have worked, the rule has been that everything portable is sent to the cal lab for calibration. No exceptions. If it's not portable, then an in-place calibration is done.

I don't know how dimensional tools will react to "uncontrolled" environments -- I'm an electronics guy.

As for electronic equipment, operation in the "specified" temperature-humidity-altitude range is normally covered in the specification sheets. However the user may be expected to figure temperature coefficients ... it should be evaluated just in case the environment you actually use it in is not the same as the one the manufacturer thinks it should be used in.

For example: when I was working in a Naval Shipyard, I saw many cases where test equipment that had a "for laboratory use only" label from the manufacturer was in used out on the waterfront and on board ships that were under repair. (Note that when a ship is being repaired, the air conditioning or heating is often turned off!) But if the test procedure calls for using a network analyzer to check an installed radar feed and antenna, you have to take the test equipment to the ship because you can't take the radar mast to the lab.
 
A

AlbertPaglinawan

Ryan Wilde said:
I seem to remember that you had a Datron/Wavetek/Fluke/whatever 1281. The accuracy spec for that particular piece of equipment is 21 - 28°C, which may pose a problem for you with your present environmental specification.

Many labs, including ours, have separate rooms for dimensional and electrical calibrations, with entirely different specifications. The crux of the problem is that 23°C is standard laboratory temperature for electrical, and temperature does have measureable effect on high-accuracy equipment. Dimensional, on the other hand, has a standard laboratory temperature of 20°C, and variations from that temperature will have a measureable effect with very little temperature change.

You also may wish to worry about your humidity specification, as 60% RH on tool steel tends to cause rust.


Ryan

Hello,

Yes, we do have that equipment.

I thought that at lower temperatures, the measurements in electrical equipment are more stable. Am I wrong?

Thanks in advance.

cheers!
 
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Graeme

The weather outside is frightful ... I'm glad I work in a cal lab!

Lord Ituralde said:
Yes, we do have that equipment.
I thought that at lower temperatures, the measurements in electrical equipment are more stable. Am I wrong?
Thanks in advance.
In principle, electronic equipment can be designed to work over as wide a temperature range as you like. Test equipment that is intended to be used that way is designed for it -- for example, equipment that an airline would use to perform tests on an airplane. The equipment has to work as expected, whether it's in Moscow in the winter or Phoenix in the summer. However, that type of equipment does not have the same type of performance as a calibration laboratory standard. Generally the tradeoff is wider enviromental conditions and wider performance specifications, or laboratrory-grade performance and a very small range of allowable environmental conditions.

Equipment like your Wavetek 1281 - and, in general, most "laboratory" electronic test and measurement equipment - is designed for best operation at a stable temperature in a narrow range. For example, look at the headings and footnotes of the 1281 specification sheet:
  • The preferred temperature range is 23 ± 1 °C. This is for the 24-hour or the 12-month specification.
  • 23 ± 5 °C is acceptable provided the Selfcal routine is performed any time the temperature changes more than 1 °C from what it was at the time of the last Selfcal.
  • It "can" be used from 13 °C to 33 °C provided the Selfcal is performed first at 23 ± 1 °C, and the Temperature Coefficient values are added to the measurement uncertainty. (Note that this also implies that you can't turn it off between the 23 °C Selfcal and the time is is used in the "other" temperature.)
As you can see, this piece of equipment really expects a stable temperature (variation less than ± 1 °C); and a temperature close to the standard 23 °C. This is also an example of why it is beneficial to continuously monitor and record the laboratory temperature.

Would it be correct for me to assume that all of your calibration facility is in the same place -- that is, you don't have separate rooms with individual air-conditioning systems? If separate facilities are not possible you have to compromise, which might be what the specification you originally quoted was trying to do. The compromise means tolerating greater uncertainty from the 1281 (because it is too cool) and dimensional offsets from your gage blocks (because they are too warm).

As Ryan said as well, the relative humidity you mentioned (50 ± 10 %RH) is really high for a dimensional calibration lab, and I would be uncomfortable with that in an electronic lab as well. Moisture causes corrosion, creates leakage paths for voltage and current, and promotes mildew and fungus growth. A "do not exceed" limit of 45-50% would be better if your environmental control system can do it. On the other end, it should not go below 20% or you get problems with static electricity, which can damage electronic equipment. I don't know what the climate is like in Arad Doman (I prefer Manticore, myself) but often in the winter it is necessary to add a lot of moisture to keep the humidity above 20%.

Does this help, or have I simply added confusion?
 
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Mario Alberto83

Re: The weather outside is frightful ... I'm glad I work in a cal lab!

Hello everybody, hope somebody sees this post on this old and useful thread.

We will be audited on 1st Quarter next year and we are reviewing our QMS.

I am working with my metrologist who is an experienced guy in these matters. He says that we need some special SW in order to record the actual values for humidity and temperature periodically (every 1, 5, etc. minutes, depending on SW configration).

We currently have a calibration report format where it is required to record the temperature the calibration was performed at.

I agree that it would be better to have evidence on these conditions fluctuations every time; however for audit pusposes I believe the actual readings recording would be enough (as long as readings meet established conditions).

I believe we are covered at this point in accordance with standard requirements, I would consider more stringent controls per customers requirements in future.

Please let me know your feedback.

Regards
 
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dv8shane

Are there a standard set of environmental conditions, e.g., temperature and humidity, that must be met during the calibration of standard measuring instruments [calipers, micrometers, etc.? A prospective supplier of calibration services is offering to perform such calibrations on site [to save transportation time and $] but I am concerned that the proposed location does not meet the required [?]controls. :confused:
If they are 17025 compliant they should issue the certificate with expanded uncertainties including the environmental effects, all scopes of accreditation to 17025 show if there are any differences in the uncertainty of the measurement when performed on site are documented
 
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Camilekz

Ryan, I got a question, in what standars are these values based? Thanks for the information
 

Hershal

Metrologist-Auditor
Trusted Information Resource
Re: The weather outside is frightful ... I'm glad I work in a cal lab!

Hello everybody, hope somebody sees this post on this old and useful thread.

We will be audited on 1st Quarter next year and we are reviewing our QMS.

I am working with my metrologist who is an experienced guy in these matters. He says that we need some special SW in order to record the actual values for humidity and temperature periodically (every 1, 5, etc. minutes, depending on SW configration).

We currently have a calibration report format where it is required to record the temperature the calibration was performed at.

I agree that it would be better to have evidence on these conditions fluctuations every time; however for audit pusposes I believe the actual readings recording would be enough (as long as readings meet established conditions).

I believe we are covered at this point in accordance with standard requirements, I would consider more stringent controls per customers requirements in future.

Please let me know your feedback.

Regards

Mario,

You are both correct and here is why. For an assessment, the actual at time of calibration is - generally - acceptable. However, the long term stability of the environment is something that an astute assessor will look at. That is where a datalogger comes in handy, which appears to be what your metrologist suggested.

One thing you should do is talk with the calibration program manager at your selected AB. Since you are in Mexico, I suspect it will be ema. Get the AB's advice and position, then you have a better idea how to prepare.

Hope this helps.
 
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kgott

Are there a standard set of environmental conditions, e.g., temperature and humidity, that must be met during the calibration of standard measuring instruments [calipers, micrometers, etc.? A prospective supplier of calibration services is offering to perform such calibrations on site [to save transportation time and $] but I am concerned that the proposed location does not meet the required [?]controls. :confused:

Calibrate these instruments in the same environments in which they are used.

You may consider it prudent to calibrate them in summer and again in winter to account for seasonal differences in temperature. I have found this usually takes care of concerns.
 
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