How to Determine if your Audi is Actually Short of Refrigerant

[mboucher70 at hotmail.com]

Short of taking a car to an AC shop, what is a good method to determine if 
its actually short of  refrigerant?

Having discussed the possibilities for refrigerant top-up and all but 
settled on Duracool from the nearest Wal-Mart, I thought I'd do a 
thermometer test.

The test was conducted on a cool day with temperature about 70F.  I started 
the car, set it to idle fast at about 1600rpm, turned the AC on max, and 
mounted an aquarium thermometer in front of the vent.  Somewhere around the 
2or 3 minutes mark, the vent temperature was below 50F.  After that, it 
continued a slow climb downwards, eventually bottoming out at 42F, after 
about 10minutes.

One thing that surprised me slightly was the frequency of the compressor 
cycling.  The compressor appeared to cycle regularly at about 15 seconds. 
I'm measuring the compressor cycling by the point at which the idle would 
jump from a steady idle of 1600rpm to a steady idle of 2000rpm during the 
test.

I was a bit surprised that the temperature got down as low as 42F.  I had 
assumed that the system was low on refrigerant based on a very sunny hot day 
last summer with 5 people in the car.  On that day, despite constant max/AC 
and max fan, the car just never got very cool.