There are reports that in long term use, a probe which is withdrawn and dries between reading sessions develops evaporated solids, salts, etc. According to Murphy's Law, any probe tip will eventually somehow pick up some deposits. In the H5, a probe can be removed, cleaned, and accurately replaced at any time in about a minute. Eaton's approach, in which a pointed probe approaches the surface from below, avoids waves and wavelets and makes it possible to obtain manual readings accurate to a micron or two fairly easily, but there is no readily apparent way to make readings automatically. Bubbles A very serious practical concern in the operation of any hydrostatic instrument is the necessity of knowing that there are no air bubbles in the liquid line.
Bubbles cause two sorts of problems.
One is a serious nuisance, but it still permits accurate work. The other prohibits accurate work entirely. If small bubbles which do not completely block the water tube are present, they affect the thermal common mode response because the air in the bubbles expands much more than water with a change in temperature. This causes more rapid variations in the surface level at each well, as if the thermal insulation of the water tube were defective or absent. If these small bubbles coalesce to completely block the tube with a single large bubble, surface tension forces cause the bubble to resist water pressure differences of up to a millimeter or two, giving a 'dead' and completely erroneous response.