Your vehicle needs to sit undriven for a few hours, out of direct sunlight, to obtain the cold pressure reading. I suspect this is the issue the OP is having. Even with seasonal changes in temperature, you should be fine with checking your tires every two weeks.
Paying for Nitrogen is a waste of money for passenger tire use.
Here is a pretty decent article (even though there are number of minor mistakes and ommisions): POWERTANK - CO2 Air Systems
Here are a few additional points:
-Tthe air we breathe is approximately 78% nitrogen and 21% oxygen. (The article incorrectly states 17% oxygen).
- The effect of water vapour in tires is insignificant for passenger tire use.