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fritz said:
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Jim48 said:
I took off the caps, let them off for 30 minutes, and then turned on the ignition to check the readings. They were the same as with the metal caps and also the same as with the plastic caps on (not the original Porsche caps, but plastic is plastic).
I think fritz was correct about the metal caps interfering with the actual signal and not affecting the pressure reading.
Jim
So how was the coffee? 
Naturally pleased to hear that my intuition was right. So your initial concern was caused by assuming that your hand-held digital pressure gauge was correct and jumping to the conclusion that your TPMS display had to be wrong!

You should have more faith in German engineering.
fritz,
You're right about having more faith in German engineering.
I just re-read the owners manual description of the TPMS and realize that I had not read it carefully enough. For that, I apologize to the group.
From what I now understand, you should fill the tires to the proper level when cold (using your regular air gauge to monitor the pressure), and then get into your car, turn on the ignition without starting it, and use the computer control lever to enter the TPC function. Enter the "info pressure" section and this will tell you whether your air pressure is above or below the level recommended for your car, given the settings you have made (for 18 or 19 inch wheels, for summer or winter tires, for partial or full load).
The pressure display that can be accessed on the computer display below the digital speed readout, below the tachometer, is useful while actually driving. It will tell you the tire pressure as the tires warm up, but it is not to be used for setting pressure (page 110, U.S. version of the owners manual).
Now I just completed this process, and when I insert the key and turn on the ignition, after getting the oil level reading, my tire pressure reading says 30 lbs. for the front and 36 lbs. for the rear. That is 3 lbs. less than the recommended level to which I filled the tires. But when I turn on the computer TPC section, for the accurate reading when the tires are cold, I get "0" readings all around, meaning that my pressure setting is accurate.
It all sounds a bit confusing, particularly why the regular reading, that you can get without accessing the TPC info pressure section, wouldn't seem to be accurate, but I will take a guess. The basic info measure you can easily access only gives accurate readings when you are actually moving.
Anyway, I think this explains it, but I would like to get MMD's take on things, as he and I seem to have been having the same concerns with the monitoring system.
By the way, I would not go without the TPMS. The option price was only $590 in the U.S., and this is a very inexpensive way to prevent damage to the wheels and possibly tires.
Jim