Real pneumatic detection of bellows pressure, high-resolution pressure sensor
For Roland FR-3xb this info is on specs
New sensor for the detection of bellows pressure (high-resolution pressure sensor), Bellows resistance regulator: wheel with fine adjustment
They may have same sensor, but at the same time they may have different sensors because FR-3S and FR-3x(b) are probably different instruments. FR-3S is older something from beginnig era of their V-Acc. I did quick search and didn't find technicians manual for these instruments, manuals for technicians usually list these hidden modes and parameters.
I don't reccomend adjust these trimmer pot before it is really needed, these things just don't go easily out of calibration. These usually adjusted only once in factory when sensor is installed and output voltage calibration is needed. There may be dirt and dust in places where it shouldn't be, nicotine is also very sticky and nasty thing, I really enjoy cigars but not near my accordion