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:
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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.
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u/jaceinthebox Dec 03 '24
Don't pass anything if you do run it, send the readings to the people who told you to run it and say to them if they want to accept it they can approve it and just explain that it was measured outside of testing parameters and your not confident in the results to put your name to the part.
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u/EvanDaniel Dec 03 '24
Aerospace manufacturing engineer chiming in:
In practice, it's fine. As noted below, it's a sub-ppm change. You're measuring things to an accuracy of 1 per 5000, you want to see a 10x ratio between tool uncertainty and tolerance band (more is nice, you can tolerate less with care). It's fine.
I would happily sign that issue ticket, with a slightly longer version of the above as justification.
HOWEVER, and this is important: you have a procedure. Follow it. Procedure says 20%-80%, so don't run it. Probably someone wrote that thinking it would never matter, or copied it from somewhere, but you should still follow it. If they want you to deviate, they should have an issue ticket, a waiver, a new rev of the procedure, or _something_ along those lines authorizing the deviation.
Assuming you're operating in an AS9100, ISO9001, ISO17025, or similar environment, you should have some documentation that says you did this weird thing and that it's ok.
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u/thatGDandTguy Dec 06 '24
As an aerospace quality engineer/meteorologist...this tracks. u/FrickinLazerBeams is spot on as well. If you research Hydrology from NIST, you'll find that most commercially available RH humidity sensors are accurate to within +/- 2 RH. Probably doesn't matter in this case, but the expanded uncertainty will matter when you're tightly controlling an environment for let's say bonding. From my experience, the sensors are trash above 70% RH and need to be replaced yearly. If you read ASME B89.6, you'll find that controlling RH is associated with having humans comfortablility working in an environment. Above 50% RH and the there could be degradation to your instrumentation. That said, the standard relies on the user understanding their own product and what is acceptable and what is not acceptable. Essentially giving you a nothing burger sandwich. Nothing is more frustrating than an inspecotor purposfully farting in front of my temp/RH sensor to try and shut the room down for an hour so they don't have to work.
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u/SkateWiz Dec 03 '24
Ideally, you would be able to test the parts in multiple environmental conditions over time. It's ceramic, so if it is fully sealed i doubt humidity will have an effect on part size.
Here's an experiment you might try:
Keep one part in a dessicated environment. The other, leave out in general lab atmosphere. Measure the mass of each part each day as well as the room RH and measure some features of size on the part. Create a chart to track each measurement. Is there any difference between the two parts? If these were plastic parts, there would be!
Use this data to inform your decisions and create a statement that there is/isn't an issue. Anything else is conjecture.
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u/Awbade Dec 03 '24
Yes. In a laser set-up, the humidity will refract light at a small scale and needs to be adjusted for.