Greetings,
Mr. Art. Not suggesting anything regarding the level of poop in the tank. I simply went below and "shot" expansion tank, transmission and isolation transformer. I got 3 different readings with the "read" button depressed so I ASSUMED the unit was capable of continuous readings. Another thing to consider is how much fuel is returned to your fuel tanks. A high flow return as in a DD may result in less or more accurate reading depending on where the warmed fuel is returned.
Mr. Art was asking about fluid level determination in various cavities. The IR unit, be it expensive or cheap has other uses and for $29 is worthy of investment. Heck, I have no gauges in any of my tanks. When the hull gets brown, one tank is full or when the engine quits, another. When I get thirsty, scotch.
Mr. SD. Try pouring ice water over the top of the tank when taking a reading. Cooling of the metal, if it is this material may give you enough of a differential to determine level.