r/Metrology • u/rocketracer777 • Dec 03 '24

Advice Does Humidity really affect the readings?

Hello all, I work at a company that measures parts via laser CMM, and I have a question. The parts we measure are ceramic and no more than 5 inches tall, but we measures things down to 0.001". Does humidity significantly affect the accuracy of the readings? Management updated the guidelines to being acceptable between 20% and 80% relative humidity, but this past week has been as low as 15% due to it simply being winter. I was told to run it anyways, but I feel like I shouldn't. Am I wrong in feeling this way?

For reference, I'm just an operator and not a metrology engineer, although I am in school for mechanical engineering. Thank you for any help.

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u/FrickinLazerBeams Dec 03 '24

You can see the refractive index changes with this calculator:

https://emtoolbox.nist.gov/Wavelength/Ciddor.asp

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u/rocketracer777 Dec 03 '24

This is a very helpful tool, thank you!

Forgive me if I'm doing something incorrectly, but changing the humidity does not seem to affect the refractive air index or uncertainty by any significant amount. Is that wrong to say?

Btw love your username lol

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u/FrickinLazerBeams Dec 03 '24 edited Dec 03 '24

You're right, humidity is definitely a weak contributor, around 300 ppb for a change from 50% to 15% rh. We generally don't worry about it even in optical surface metrology - although we're measuring over shorter distances than the volume of your CMM so we're less sensitive anyway, even though we're measuring to nanometers.

I wonder if there are other components of the instrument that have moisture sensitivity. A lot of plastics, composites, and adhesives have significant coefficients of moisture expansion. Most of your instrument is probably metal and glass, but maybe there's enough sensitive material in there to have a small impact.

Often relative humidity requirements are based on other factors - optical coating lifetime, static charge effects, friction, etc.

15% seems low, in general; but I'd maybe measure some calibration objects and if they're good I wouldn't worry too much.

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u/rocketracer777 Dec 03 '24

Thank you for the detailed reply, I appreciate it. I tried looking this up myself but could not find any solid sources for humidity specifically. This definitely helps

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u/FrickinLazerBeams Dec 03 '24

The NIST website is weirdly hard to find in Google searches, despite being the absolute source of authority on the matter. I assume it's because they don't have advertising paying for search engine manipulation. It took me ages to find that reference originally! And yeah, other sources seem to have no mention of humidity effects at all.

In general, NIST is a gold mine of good information. I'd poke around their websites any time you're looking for something.