From the PORSCHE Workshop manual:

Quick test of RDK

Whenever a wheel or tire is changed, it is essential to perform a quick test of the tire pressure control system.
If only one wheel was removed, the quick RDK test need only be performed at this wheel.
A short test drive for a distance of at least 20 meters, at a road speed of 5 and 4Uph and at the following tire pressures is sufficient:

1. Tire pressure 0.5 bar below nominal value:' front 2.0 bar rear 2.5 bar (2.0 bar for 17" summer tires) The relevant arrow (S) must flash on the instrument cluster display panel.

If an arrow is continuously illuminated instead of flashing, one pressure sensing switch for the wheel is faulty and must be replaced. (Check pressure sensing switch by using the 9288 system tester, see Page 44 - 311)

Do not drive the car at more than 50kph above this speed, the display always begins to flash.

2. Tire pressure 0.5 bar above nominal value:* front: 3.0 bar rear: 3.5 bar (3.0 bar for 17" summer tires) No fault display should occur. If it does, this indicates a system fault or a faulty pressure sensing switch (diagnosis from Page 44 301 on).

3. After the quick test, restore tire pressures to the nominal value*, or preferably 0.3 bar above the pressure sensing switches' switching points (Page 44-27). * Nominal tire pressures: front 2.5 bar / rear 3.0 bar (17" summer tires = 2.5 bar) at 20' C tire air temperature.

2.0 bar = 29.00lbs.
2.3 bar = 33.35lbs
2.5 bar = 36.25lbs.
3.0 bar = 43.51lbs
 

The front sensors are 2.5 bar and are tan colored, the rears are 3 bar and are black. In 1992 they switched to using the 2.5 bar sensors at all 4 corners. The tools for sensor removal are 00072192730 and 00072192740.

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By the way - I should point out that the tire pressure sensors in the 928 compensate for temperature. The pressure trigger values are for 20°C. If your temperature is lower, the sensors will trigger at a lower pressure, and (obviously) when the tires are warm, they will trigger at a higher pressure. If you adjust the tire pressure to 2.5 bar on a hot tire, the monitoring will start alerting.

Believe me, these sensors are very accurate. (I have not seen any other system with the same accuracy as the 928 have. But then again, I have not seen any other system with the same price tag as the 928 system
either...)

Johnny
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The sensor is temperature compensated, there is even more detail in the WSM, starting at page 44-21.
Especially interesting is page 44-27, the section "Accurate adjustment of tire pressures with the 9288 system tester", which describes how to really accurately check tire pressure.
There is a paragraph there, which says:
"If the wheels register different temperatures, for example because of exposure to the sun on one side of the car only, the switching point must be determined for ail wheels and the tire pressure adjusted as already described."

So to accurately adjust the tire pressure, using the RDK system, you must individually check each tire with the diagnostic tool, and adjust pressure given the sensor readings for that specific tire, since temperatures might differ between wheels, and pressure should accordingly be different. Using a normal pressure check tool will not give you the accuracy, nor temperature compensation, but the RDK system actually do.
Johnny Bilquist

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This is a 2.5 bar sensor:

This is a 3.3 bar sensor (see text on the back):