How to Make B&G Analog Displays Moisture-Proof

Hello All;

Can anyone help me figure out how to make B&G analog displays moisture-proof?

I replaced the glass to face plate O rings with 1mm dia and the backing plate to glass O rings with 2mm diameter and liberally applied high-vacuum silica grease to them before reassembly. I then put a bead of silicone sealant around the perimeter of the backing plates before remounting them. They fogged up the next day!

Are the O rings I'm using too small in diameter, or is this the best I can expect from B&G?

Please bear ing mind that I want to be able to reopen them in the event they need servicing in the future.

Thanks

John

I am sorry but right now I cannot tell you anything on the thickness of the O-rings because I do not have them close to where I live but I can comment on the rest.
You probably did the right thing and the moisture that fogs the front glass comes from inside and not from the outside as you may guess.
You closed the display with air from your work ambient which probably had a high absolute content of moisture because it was warm and you were (fortunately!) breathing. When the display was placed outside, the colder temperature brought such moisture to a supersaturated condition, whence the fog.
Contrarily to common belief, usually colder air has less absolute moisture content which, in turn, will need even lower temperature to condense.
So what? Be sure to seal your display in an ambient with a low absolute content of moisture which may mean a pretty low temperature, for example.
Just a question: has your display a threaded cap on the back?
In this case you may, of course, lower its moisture content without opening again the front.

Daniel, 411-004

Daniel is right about the moisture content of air, but B&G used to put small bags with desiccant inside. These bags absorb moisture, and can be dried in an oven before put back in.

Lars

Daniel & Professor;

Thanks for your observations. Perhaps adding additional bags of silica gel is a good idea. I tool the precaustion of heating the entire display to about 50 deg C before sealing them to ensure they were dry inside. Today, I resealed the front and back O rings with silicone (my least favourite sealant). I'll let you know if anything improves.

Much obliged.

John 

Dear John,

before operating the B&G, be sure to get the screwed dryers you can find on the back of the instruments in the oven at a very high temperature (no less than 200 °) and wait untill they became pink, it may take 20/30 minutes, don't worry.

Fair wind

Matteo (38/067 Only You)

John,
I strongly discourage using silicone sealant on the O-rings: silicone grease is perfect and sufficient if the O-rings are of the right size.
First check that the O-rings are the right size; as I wrote before I cannot confirm the sizes although the numbers you cite sound rather small. You should also have a rubber seal for the threaded back cap.
To summarize all that has been written here, a good procedure to lower the moisture inside your instruments is the following.
1) assemble the display leaving the back cap open.
2) bake the silica gel bag till it becomes dry.
3) insert the silica gel in the display leaving the back cap open and put everything (display with silica gel in it and back cap) inside a refrigerator (possibly in a modern ventilated one) for a sufficiently long time till there is no apparent fog or frost on the display window.
4) quick as a lightning :-) open the fridge and seal the display with its back cap.
This procedure has the advantage of leaving slightly over pressurized almost-dry air inside the display which is very convenient. Your procedure left under pressurized air inside.
The best thing would be to have the inside display slightly over pressurized with pure dry air or nitrogen but it is more complicate.

Daniel, 411-004

Daniel;

Got it. Thanks.

John

Hi everyone.

Its really very nice to exchange recepies about our dear Swan mistresses!

On that one, may I confess that after different tries and failures we just leave the displays open, IE without the original dessication caps. So they can really breathe whatever air comes. No more condensation! 

Up to now it has been working fine for at least the last 15 years... I suppose it might no be OK for other climates than the Med.

Kind regards.

Philippe 41/022