----- Original Message ----- From: "Graham North" To: Sent: Friday, September 29, 2000 12:24 PM Subject: [PIC]: DAMP! Startup problems? > Dear all, > > I have a few circuits that are kept in water proof boxes, but while in > storage (in not the most ideal place in the word) they seem to get damp. > My question is not how to prevent this, as I have sealed the boxes as well > as is humanly possible, and have put sachets of silica gel in the boxes > also. Does the silica gel absorb much of the moisture? Enough to change colour? Is the silica gel on the inside of the waterproof box? > My question is would a crystal or RC res circuit (in am currently using > cheap resonator) startup more reliabily in these conditions? > Has anyone come across this problem before? Yes, I had a similar problem with equipment at a transmitter site atop a small mountain. The equipment was only used occasionally and suffered from internal condensation. We solved the problem by keeping it powered and fitting a net-like arrangement of resistors in the lid to keep the kit very slightly warmer than ambient. If your equipment is in storage then keeping it powered might not be practical but the principle of keeping it slightly warmer than the ambient air temp still holds good. Maybe lay along the shelves a run of that resistance wire which horticulturalists use for soil heating, you don't need much heat to keep the condensation at bay IME. Another option is to seal the kit along with _freshly baked_ silica gel packs in either heat-sealed plastic or if that's not available you could try those zip-topped bags sold in supermarkets for storing stuff in freezers. (Do this in a warm dry environment.) Once it's completely sealed there's only the small amount of moisture in the bag and case to be absorbed, which fresh gel should be able to handle pretty simply. I did once have a handy table for silica gel showing how much was recommended for a certain volume at a given RH but I can't find it just now. > I know electronics should not be subjected to these environmments, but we > all live in the real world don't we! Lance is yer man for waterproofing really. I'm sure he'll have some other suggestions... . -- http://www.piclist.com hint: PICList Posts must start with ONE topic: "[PIC]:" PIC only "[EE]:" engineering "[OT]:" off topic "[AD]:" ad's