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??
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist