For the last few days the analysis of my temperature reading showed it as being high compared to nearby stations.  Sigh, here we go again.  I’d been watching the temperature readings along with neighboring stations on Weather Underground and had come to suspect my readings.  So I started doing some investigating to find whether I had a problem or not.

First I opened up the FARS to have a look inside and make sure everything was OK and still in place.  I didn’t find any problems there.  My humidity sensor has a thermometer in it that isn’t as accurate as my main temperature sensor but could be used for comparison.  It agreed with my temperature sensor, so I was able to rule out a sensor malfunction.  I have another digital thermometer located in another part of my yard.  It was reading several degrees lower than the weather station.  So I knew I had some kind of problem inside the FARS housing.

Since I had some problems in the past with a voltage regulator getting slightly warm on my temperature board, I removed the jumper that supplied power to that regulator.  It is only used to supply external power for the 1-Wire sensors and isn’t strictly necessary since the sensor can run using parasitic power.  I waited and watched the temperature readings, but they didn’t change.  So now I need to put the jumper back on the board.  Oh well, no rush.  It seems to be reading fine using passive power for now.

I thought I might still have a problem with the fan in my FARS generating some heat.  I tried adding a potentiometer outside of the housing to slow the fan down, but that didn’t seem to help.  I had also tried just disconnecting the fan altogether, but I wasn’t certain that was the problem or not.  In testing i had done in the past it had appeared to be working the same regardless of whether the fan was plugged in or not.  After giving it some more thought, I decided to turn off the fan again and observe the behavior.  .If I was right about the fan causing some heating, the expected behavior would be a spike in temperature immediately after shutting off the fan due to the heat generated by the motor not being evacuated from the housing by the fan.  Then as the fan cooled off I should see the temperature fall to match the neighboring weather stations and my digital thermometer.  This wasn’t what I had observed the last time I tried, but this time the fan really looked like the most likely culprit.

I unplugged the fan and left for work.  Later in the day I checked the graphs from my station and saw a spike in temperature just after I unplugged the fan, then a gradual fall in temperature.  Eventually the temperature dropped to where the daytime temperature from my station more or less agreed with the neighboring stations. The nighttime temperature was significantly lower than most of the neighboring stations except for one.  But it also agreed with my spare digital thermometer.

Looks like my fan heating theory was correct.  I’ve left the fan disconnected for now.  This means I will probably get some solar heating in the morning when the sun shines on the housing.  I can live with that for now.  And I plan to order another fan in the next few days for installation next weekend.  Tine to hit the catalogs and research fans.