Monthly Archives: December 2010

Hello Blog readers – I recently received an e-mail asking me a question about Humidity testing, and I thought the question and the response might be relevant to you. Any comments would be appreciated.

Hi Steve,

I attended your MIL-STD-810G course in the spring.

I have a question on humidity testing.

I have an analysis to do on an electronic assembly and do not have the means to test it in a chamber.

The power supply is made up of a chassis with a cover. There is an o-ring seal between the cover and the chassis so I am not worried about that. In several places around the machined chassis we have counter sink screws that hold electrical components onto the chassis. These screws are not seal screws but they do have Loctite applied to them. In your experience would condensation go through the countersink into the unit? I have the same question about salt fog. All the hardware is Stainless steal so I am not worried about corrosion.

My response:

The question becomes – is the enclosure airtight, or not?

Keep in mind that the partial pressure of the water vapor will be higher then the dry air pressure, so EVEN if the internal and external pressure is the same, if you have high humidity air outside the enclosure, it may migrate in.

Then the question of temperature change comes in. Temperature cycling will cause a pressure differential. If the enclosure is not airtight, and there is a high moisture content in the external air, it will (again) migrate in.

Also – your O-ring seal – many o-ring seals only hold pressure in one direction. If your O-rings are designed to prevent air from coming in – then after it goes out, and there is a negative internal pressure, the air will try to get in around the screws.

I would definitely try to test.