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'[PIC] Condensation?'
2008\11\18@163454 by Carl Denk

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I have a 18F1320 PIC with a temperature sensor and 2 pressure sensors
mounted in a waterproof gasketed aluminum box. There are sealed
penetrations for weatherproof connectors, except the power from a wall
wart with the usual concentric connector on the bottom face of the box,
and a LED and temperature sensor sticking through a tight fitting rubber
grommets.The sensors are potted in epoxy coated with 3 part urethane
paint. All this is mounted in an insulated enclosure with several backup
heaters that maintains temperature above 40F (4.4C) minimum.

This is twice now, that the one pressure sensor (Motorola Freescale
MPX5700GP), after having the enclosure open for inspection for say 1/2
hour in near freezing temperatures, that the pressure reading was what I
 would think a maximum reading (110 PSI.). The sensor output is 0 - 5
VDC, and appears to me that I'm getting a short from the 5VDC power
supply for the sensor to the signal pin. The first time, I replaced the
sensor immediately, but today, I waited a couple of hours, doing other
chores, and when I checked, pressure reading was normal (30 PSI.). The
board is thru the hole with solder mask double sided. After soldering, I
took special attention to clean the board of flux. The board has no
additional coating.

I'm thinking it's condensation since the last time the aluminum box was
open, was much warmer temperatures and might have been light rain. What
would be appropriate actions- Coat most of the board with say a
catalyzed urethane clear paint (have stock of lots of it), let the
inside of the aluminum box open with cold low humidity ambient air (that
won't help if I open later in the summer though)? or??

2008\11\18@170840 by Jonathan Hallameyer

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The heaters are good, but 40F is probably below the dewpoint, so if it
is sealed well (RTV on the back the LED and power connector possibly?)
put a decent sized desicant package and seal it up in a dry
environment.

Though a conformal coating couldnt hurt either.

On Tue, Nov 18, 2008 at 4:34 PM, Carl Denk <spam_OUTcdenkTakeThisOuTspamalltel.net> wrote:
{Quote hidden}

> -

2008\11\18@174911 by Danny Miller

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Mounting the pressure sensors in epoxy may be a problem.  With any temp
change or even the curing action of the epoxy it creates stress on the case.

An absolute pressure sensor is a sealed reference chamber separated from
the outside port with a sealed diaphragm and very sensitive strain gauge
(gross oversimplification).  I suspect any mechnical stress which could
distort the case could put unexpected stress on the strain gauge which
would change the reading or even break something.

Epoxy usually does not expand or contract much as it cures.  There is
always some thermal expansion coefficient but even if it is small for
your epoxy note that the stress comes from *differences* in expansion
coefficients.  If a PCB expands by a few mils and the encapsulating
agent does not, then there is stress between them.

Danny

Carl Denk wrote:
{Quote hidden}

2008\11\18@182903 by Carl Denk

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Thanks for the replies:
I was ready to replace the pressure sensor (including 2 spares) with
ones that come with their own weatherproof case and pigtail wires to
eliminate the need for the epoxy encapsulation which is kind of minimal
cross section, just enough  to encapsulate everything, with a small hole
for atmospheric vent (it's a gauge pressure sensor) and a 1/4" brass
close nipple for connection to pressure source. The sensor case is
molded plastic, the coefficients of expansion may not be that different.
But the unit returned to normal operation in what might expect time to
evaporate any condensation. I don't discount the differential expansion,
and even that it could of healed itself (temporarily). Following that
line and replacing the sensors would not cure the problem of
condensation. The easiest fix and least costly would be to pot the
board. But what is interesting, it's only the one sensor that's an
issue. The board is only 2.5" square, I would more think that it would
be random between all the sensors, or just quit working. And that hasn't
happened.

Not over enthusiastic about desiccant (silica gel), though it would
basically work, and not that hard to do. Concern 5 years down the road,
someone opens the aluminum box, the desiccant should be refreshed, and I
don't think that's realistic.

Probably in the morning, I'll mix up a teaspoon of urethane and with an
artist's brush paint the board with clear paint.

Danny Miller wrote:
{Quote hidden}

2008\11\18@215339 by apptech

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Conformal coating is seldom a bad idea.

Dessicant may help.

I won't even mention that a controlled opening environment is a nice idea if
you can manage it :-). Maybe a portable glove box with controlled humidity?
Could be very low tech and cheap to implement. "Plastic" bag and air
cartridge or even a hand pump to initially inflate via a dessicant
cartridge.


  Russell


> I'm thinking it's condensation since the last time the aluminum box was
> open, was much warmer temperatures and might have been light rain. What
> would be appropriate actions- Coat most of the board with say a
> catalyzed urethane clear paint (have stock of lots of it), let the
> inside of the aluminum box open with cold low humidity ambient air (that
> won't help if I open later in the summer though)? or??

2008\11\19@091854 by Carl Denk

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Controlled opening not good idea. The Enclosure is 200 feet from house,
in the middle a mowed grass area. The only time needing to open the
aluminum box (which accessible once the enclosure is removed) is if
there is a problem with the PIC's board itself including the temperature
sensor. The pressure sensors, and wall wart can all be unplugged from
aluminum box mounted sockets. This could happen at any time (granted
probably rarely)  day or night, sun, rain, or snow. And then I probably
want to get things back together quickly and get back inside.

Right now my game plan is to coat the boards, less the DIP socket tops
with catalyzed clear urethane, make a little packet of silica gel, dry
it, and put things back together.

apptech wrote:
{Quote hidden}

2008\11\19@194702 by Jim Korman

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Carl Denk wrote:
> Controlled opening not good idea. The Enclosure is 200 feet from house,
> in the middle a mowed grass area. The only time needing to open the
> aluminum box (which accessible once the enclosure is removed) is if
> there is a problem with the PIC's board itself including the temperature
> sensor. The pressure sensors, and wall wart can all be unplugged from
> aluminum box mounted sockets. This could happen at any time (granted
> probably rarely)  day or night, sun, rain, or snow. And then I probably
> want to get things back together quickly and get back inside.
>
> Right now my game plan is to coat the boards, less the DIP socket tops
> with catalyzed clear urethane, make a little packet of silica gel, dry
> it, and put things back together.
>  
May I ask what pressure you are measuring?

Jim

2008\11\19@200226 by Carl Denk

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We have a natural gas well we drilled 31 years ago, it is just
sufficient to heat the house including  the garage, a small greenhouse,
hot water, kitchen, and clothes dryer. The PIC monitors the well head,
and regulated pressures and the enclosure temperature. The gas has a
water vapor dew point just below freezing, so it is important to know if
the heat fails. The well is 1000 feet deep, with 180 feet of steel
casing pipe, and the rest is bare hole. The PIC communicates via. fiber
optic to RS-485 to a PLC that manages a standby generator. Items
controlled with the PLC include changing the house heat from natural gas
to electric resistance heat if the well pressure is low which might
happen once or twice a year with having to pay for $30 electric. The
generator can run gasoline, natural gas or propane (no tank at this
time, but have tested) and switch under load automatically.

Jim Korman wrote:
{Quote hidden}

2008\11\25@145752 by Martin

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Carl Denk wrote:
> We have a natural gas well we drilled 31 years ago, it is just
> sufficient to heat the house including  the garage, a small greenhouse,
> hot water, kitchen, and clothes dryer. The PIC monitors the well head,
> and regulated pressures and the enclosure temperature. The gas has a
> water vapor dew point just below freezing, so it is important to know if
> the heat fails. The well is 1000 feet deep, with 180 feet of steel
> casing pipe, and the rest is bare hole. The PIC communicates via. fiber
> optic to RS-485 to a PLC that manages a standby generator. Items
> controlled with the PLC include changing the house heat from natural gas
> to electric resistance heat if the well pressure is low which might
> happen once or twice a year with having to pay for $30 electric. The
> generator can run gasoline, natural gas or propane (no tank at this
> time, but have tested) and switch under load automatically.

Carl, I think you should make a website with a description of your setup
+ pictures. I think I've asked you at least twice about it.

-
Martin

2008\11\25@151211 by Carl Denk

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I don't have a web site, and don't know how to set one up. it's not even
a low priority at this time. I am still trying to document the system so
if I am unavailable (70 years now), someone will be able to keep the
system reliable. The way things are going, I expect we will have rolling
blackouts that I may possibly see. I have had conversation with our
power supply, a rural electric coop, instead of just controlling my air
conditioning when peak shaving is needed, my generator will come on,
automatically transfer us off the grid. Sorry :(

Martin wrote:
{Quote hidden}

2008\11\25@173020 by apptech

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>I don't have a web site, and don't know how to set one up. it's not even
> a low priority at this time. I am still trying to document the system so
> if I am unavailable (70 years now), someone will be able to keep the
> system reliable.

Websites can be extremely easy to operate. There are many point and click
systems out there. And, if you have broadband then setting up an eg Apache
server (have somebody show you the basics or do it for you) then allows you
to simply copy files to a folder and they appear automagically "on the web".
Not pretty but from then on zero effort.

BUT if you want an 'almost good enough' system you can use a free photo site
like eg Fotki and publish images only.
You can set up a folder in a minute or two and then can email photos to it
as desired. They get added at the end.
Moving things around is also easy.
Not as flexible by far as a "real" website but very very very easy to run.

If you want a demo, email me offlist and I'll give you an email adress to
use and a weba ddrsss and any attached photo that you email to that adderess
will appear at the web address a few minutes after you send it.



  Russell

2008\11\25@175257 by Carl Denk

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As I said a web site is not a priority. If there are further details
that someone would like, I 'll try to either on the list or privately
provide. As for pictures, the well is only a piece of 8" steel pipe with
a blind flange bolted to the top, 16" off the ground. a 1.25" pipe
welded to the side with a couple of valves, pressure reducer and drip
legs (downward pointing short pipe with valve on bottom to catch any
liquid vapor). An insulated box encloses the piping. Almost enough not
to get out the camera. :)

apptech wrote:
{Quote hidden}

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