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Good coating for condensation protection?

B

Bill Stock

Jan 1, 1970
0
I've got a temperature sensor circuit which is subject to severe
condensation. What's a good protective coating? I was thinking of filling
the enclosure with silicone caulk, but I imagine some spray coating might be
better?
 
R

Rich Webb

Jan 1, 1970
0
I've got a temperature sensor circuit which is subject to severe
condensation. What's a good protective coating? I was thinking of filling
the enclosure with silicone caulk, but I imagine some spray coating might be
better?

With my deep experience with silicone caulk (including leaky showers ;-)
I'd say that the problem with silicone is that it doesn't "wet" many
surfaces. If there's any motion or vibration, I'd be afraid that it
would develop a small gap at the surface interface and draw up water via
a capillary effect. I'd go with marine epoxy, myself.

One-off project or 100K/month commerial or ...?
 
R

R.Lewis

Jan 1, 1970
0
Bill Stock said:
I've got a temperature sensor circuit which is subject to severe
condensation. What's a good protective coating? I was thinking of filling
the enclosure with silicone caulk, but I imagine some spray coating might be
better?

How about using some conformal coating\lacquer made specifically for the
type of task you describe?
 
J

John Larkin

Jan 1, 1970
0
I've got a temperature sensor circuit which is subject to severe
condensation. What's a good protective coating? I was thinking of filling
the enclosure with silicone caulk, but I imagine some spray coating might be
better?

Most air-dry silicone caulk has acetic acid, which is bad news for
electronics.

Epoxy is good, or an acrylic casting compound. Use slow-set epoxy...
the fast stuff gets hot as it cures. The resin they use to fiberglass
boats is good and cheap. Mold-making (2-part) silicone is excellent.

For conformal coating, ordinary polyurethane varnish is pretty good.

John
 
B

Bill Stock

Jan 1, 1970
0
John Larkin said:
Most air-dry silicone caulk has acetic acid, which is bad news for
electronics.

Epoxy is good, or an acrylic casting compound. Use slow-set epoxy...
the fast stuff gets hot as it cures. The resin they use to fiberglass
boats is good and cheap. Mold-making (2-part) silicone is excellent.

For conformal coating, ordinary polyurethane varnish is pretty good.

John

Thanks John,

Epoxy seems to be the favourite. Although I imagine repairs would be a
bitch.

How do you find the drying time for a polyurethane dip? I think it would
take a while to harden? Do you favour any particular brand?
 
J

JeB

Jan 1, 1970
0
I've got a temperature sensor circuit which is subject to severe
condensation. What's a good protective coating? I was thinking of filling
the enclosure with silicone caulk, but I imagine some spray coating might be
better?

I'd imagine whatever you seal things up with is going to effect the
temperature response of the unit. Will that be a problem?
 
I would not rely on conformal coating on the circuit board. It is a
good vibration damper and protects against, at best, light levels of
dew formation. If you expect to have water condensation than I would
recommend epoxy. But it will depend on if your sensor operates mostly
in a narrow range of temperatures or across a broader range such as
below freezing to 90 deg. F.
 
B

Bill Stock

Jan 1, 1970
0
JeB said:
I'd imagine whatever you seal things up with is going to effect the
temperature response of the unit. Will that be a problem?

JeB,

The sensor is not on the board, it's attached to a short (30") cable. The
sensor actually measures water temperature, hence the serious condensation.
I used connector coating to waterproof the sensor, which seems to slow down
the temp response a bit, but not too bad.
 
D

D Akers

Jan 1, 1970
0
B. Stock wrote:
"I've got a temperature sensor circuit which is subject to severe
condensation. What's a good protective coating? I was thinking of
filling the enclosure with silicone caulk, but I imagine some spray
coating might be better?"
____________________________________
Re;
I've used a product called "Plasticoat" which is sold in many hardware
stores. I've dipped whole circuit boards in it. It's non-conductive,
water proof, acid proof, and tough, albeit probably not flame-proof.
It, and any other coating, will affect the response time constant of any
temperature sensor and increase any self-heating effect/error.
Additionally, you'll need to derate the power dissipation of any coated,
heat dissipating components. Repairs are made difficult, if not
impossible.

-Dan Akers
 
W

Watson A.Name - \Watt Sun, the Dark Remover\

Jan 1, 1970
0
Bill Stock said:
I've got a temperature sensor circuit which is subject to severe
condensation. What's a good protective coating? I was thinking of filling
the enclosure with silicone caulk, but I imagine some spray coating might be
better?

In my experience, it takes many,. many coats of spray to make a decent
thick coating on a PCB, and each coat has to dry, making it take a long
time. It's better to dip coat the part.
 
R

Rich Webb

Jan 1, 1970
0
In my experience, it takes many,. many coats of spray to make a decent
thick coating on a PCB, and each coat has to dry, making it take a long
time. It's better to dip coat the part.

Is that still effective if surface mount parts are used instead of dips?

<ducks head, runs for cover>
 
N

Nicholas O. Lindan

Jan 1, 1970
0
Bill Stock said:
I've got a temperature sensor circuit which is subject to severe
condensation. What's a good protective coating? I was thinking of filling
the enclosure with silicone caulk, but I imagine some spray coating might be
better?

Is it possible to have just the sensor in the condensing atmosphere?

If not, then you will need a small 'Hoffman Enclosure' with a NEMA (I
forget the #) rating.

If you use epoxy you may want to use marine epoxy, that's the stuff
they make boat hulls out of, available at boat shops. Also called
'West Epoxy'.
 
B

Bill Stock

Jan 1, 1970
0
Nicholas O. Lindan said:
Is it possible to have just the sensor in the condensing atmosphere?

Yes, but the non condensing atmosphere is outside, so then I've got
snow/ice/critters to worry about. The sensor measures the temperature of an
ornamental pond (coverered) and controls a heater to prevent fishsicles.
 
A

Art

Jan 1, 1970
0
Maybe Bee's Wax?? Seriously, melted Bee's Wax applied over the item, it
will not affect the ability to sense temperature changes as long as the
ambient does not exceed the melting temp of the wax. I presume the sensor is
immersed in the pond, water covered?? IMHO
 
B

Bill Stock

Jan 1, 1970
0
Art said:
Maybe Bee's Wax?? Seriously, melted Bee's Wax applied over the item, it
will not affect the ability to sense temperature changes as long as the
ambient does not exceed the melting temp of the wax. I presume the sensor
is immersed in the pond, water covered?? IMHO

Thanks Art,

I think someone else suggested wax too. Yes, the sensor (LM34) is at the end
of a cable, covered in "connector coating". Seems to be similar to
Plastidip. The sensor is in the water and the board/case is attached to the
bottom of the pond cover's escape hatch. Water temp is around 40°F and the
outside air (above cover) can be as low as -40°F. Although we haven't seen
that yet.

I removed the circuit (board/case/sesnor) this morning. The solder side of
the board, which rests against the case lid, was dripping in water. The
component side seems almost completely dry. Once I dried the circuit out
again, it started to work fine. On the bright side, all that water
emulsified the flux residue. Making my coating job much easier.

I bought some Acrylic spray for my coating, they didn't have the dip. I may
also put some Silicone in the case lid to be safe.
 
N

Nicholas O. Lindan

Jan 1, 1970
0
Bill Stock said:
[Fish pond heater for the winter ... exposed PCB
... dripping water ... etc. ]

There were completely submersible fish tank heaters - two
test-tubes IIRC, one with a heater and one with a fixed thermostat,
wired together and to a line cord. The test-tubes were sealed at
the top with some green gunk. Heating element was packed in sand
inside the test-tube.

Might this sort of construction be applicable?
 
B

Bill Stock

Jan 1, 1970
0
Nicholas O. Lindan said:
Bill Stock said:
[Fish pond heater for the winter ... exposed PCB
... dripping water ... etc. ]

PVB is inside a case. Now conformal coated.
There were completely submersible fish tank heaters - two
test-tubes IIRC, one with a heater and one with a fixed thermostat,
wired together and to a line cord. The test-tubes were sealed at
the top with some green gunk. Heating element was packed in sand
inside the test-tube.

Might this sort of construction be applicable?

I'm using a titanium aquarium heater to do the heating. But it's thermostat
starts at 68°F. My goal is to keep the bottom of the pond around 39°F (max
density of water). This should prevent too much convection and be much
cheaper to heat. These heaters aren't designed for prolonged use, they will
burn out quickly. Right now it only runs about 10 minutes per day, depending
on outside temp and snow coverage (insulation). The sensor is also hooked
into the house's automation system, so I can monitor any problems.
 
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