Hall effect sensor direction detection using arduino
According to the data sheet (see below) the no-field PWM output is mid-range; north poles reduce it &amp; south increase.    

So if you had an arrangement whereby the float had north down &amp; south up, ie a vertical bar-magnet, this would presumably give a rise-before-0 or fall-before-0 at the float vanishes, or a fall-to-minimum an rise-to-maximum as the float centres? The bit in the middle could be interesting!    

From the datasheet:
"The presence of a south polarity magnetic field,
perpendicular to the branded surface of the package face,
increases the output duty cycle from its quiescent value toward
the maximum duty cycle limit. The amount of the output duty
cycle increase is proportional to the magnitude of the magnetic
field applied. Conversely, the application of a north polarity
field decreases the output duty cycle from its quiescent value."