A tube containing a wire, which is electronically pulsed at fixed time intervals, is inserted into the entire height of the tank. A magnetic float travels along the tube as it rises and falls with changes in the liquid level. The interaction of the pulse with the float causes a stress wave to be induced in the wire. This wave propagates along the wire at a known velocity. A piezo-magnetic sensing element in the instrument converts the wave signals and measures the elapsed time between the start and return pulses. This is then converted into an analog (4-20 mA) output or a digital output that provides a very accurate (+-1 mm) measurement. The "pole and float" technique makes them suitable for light product applications (LPG, underground tanks, cryogenic storage) and small tanks.