Good day. My company has just set up a Calibration Lab and I am one of the people responsible for the calibration of the equipment, which consists mainly of calipers, micrometers, height gauges and other hand tools for a machine shop.

The humidity readings in the lab have hovered around 65 to 75% over the past few days so I haven't done any calibration. Our procedures state that the humidity must be a fixed rate between 35 and 55% RH, with a +- tolerance of 5% (taken from ISO 17025).

Although I know that the humidity is too high I would like to know the short term and long term effects of humidity on tooling, so I can explain to my manager why I can't do any calibration (and possibility get better climate control in the lab).

Any help would be much appreciated.

Our procedures state that the humidity must be a fixed rate between 35 and 55% RH, with a +- tolerance of 5% (taken from ISO 17025).

Although I know that the humidity is too high I would like to know the short term and long term effects of humidity on tooling.

With ferrous based materials, such as steel micrometers, unless they are made of stainless steel, the high humidity is likely to cause corrosion or oxidation. This also applies in the shop areas where they are used, not just in the calibration laboratory.

Preventative measures are to lower the humidity or to apply a rust preventative oil or mist. One problem with this is the oil film can affect the reading and may also leave a residue on the measured part.

For dimensional measurement devices made of metal, the temperature has a greater effect on the calibration than does the humidity. For electrical devices, a high humidity can lead to arcing in high voltage circuits, while too low a humidity can lead to static charge buildup on people in contact with the device. The use of electrostatic discharge grounding straps will drain off the static charge.

Wes R.

Also, if you're calibrating any good part or bad part masters, depending on what they're made of they could absorb moisture from the air, causing swelling and warpage.

Wes is correct, for long term, primarily the high RH accelerates oxidation (read: rust).....the best environment is one you mentioned that you have in your own internal standard, except I would drop it to an upper limit of 50%, based on recommendations from NCSLI contained in the RPs.

The short term effects are not as obvious. Temp has a more dramatic direct effect, but temp and RH do have a relationship.....sufficient RH can drive temp a degree or two, creating the reverse of a chill factor.

You might consider having your internal lab accredited, and in Canada the first folks to talk to is SCC/CLAS. That will also cement the requirement for control, which could help you.

Hope this helps.

Hershal

Thank you. We've only just started calibration so I doubt my work will want to get the lab accredited, but I can certainly bring it to their attention (We're just a small place with about 180 tools in total). Temp isn't a problem as I have a sufficent air conditioner, but I'm hoping to bring in the equipment to keep the RH at about 35%, +-5%.

Thank you. We've only just started calibration so I doubt my work will want to get the lab accredited, but I can certainly bring it to their attention (We're just a small place with about 180 tools in total). Temp isn't a problem as I have a sufficent air conditioner, but I'm hoping to bring in the equipment to keep the RH at about 35%, +-5%.
The air conditioner should be doing a good enough job of controlling humidity; it seems that if you have an air conditioner running and the RH is still in 65-75% range, there's something wrong with the air conditioner.

I don't know much about how to condition a room but I'm starting to think that the insulation they just installed on top of the room (it butts out from a wall) might be trapping the humidity inside.

They bought a 12,000 BTU air conditioner for an 8 X 8 ft room, so it's a little over-powered. Hopefully I can convince them to change that.

I've actually not done calibration before but I was apparantely the best choice since I'm the Incoming Inspector (I have no idea how top management got from point a to point b) but I don't think they realized the amount of work that would actually go into in-house Calibration. I kept repeating the Starrett phrase, 'When in doubt, send it out', but I guess no one heard me. :)