Regular operation of the transmitter’s three-valve manifold (and drain valve) during routine
instrument maintenance inevitably releases some liquid volume from the wet leg. Without a seal
pot, even a small loss of liquid in the wet leg may create a substantial loss in liquid column height within that tube, given the tube’s small diameter. With a seal pot, the (comparatively) large liquid volume held by the pot allows for some liquid loss through the transmitter’s manifold without substantially affecting the height of the liquid column within the wet leg.