Goal: measure linear position over 5 inches within 1/4 inch accuracy at top of range, in wet environment (hydraulic jack plate on outboard boat).
Readout/display: string of LEDs (from bottom to top of range).
Thinking of using these Hall Effect switches from Adafruit - US5881LUA. As the switches close it lights an LED in relative range. The bottom and mid range are not critical for accuracy, 1/2 inch increments or so would be fine. The top is important, 1/4 inch or less.
Anybody seen this type of "meter" around? I've never used Hall switches, so don't have a feeling for how sensitive magnetic proximity is. Any thoughts appreciated.
The US5881LUA specifically doesn't look like a great option; it's "low sensitivity" and will have more distance concerns.
Try http://bildr.org/2011/04/various-hall-effect-sensors/ for a little tutorial on these sensors. Notably the analog type (A1321 shown on their page) will give you a varying voltage output which you can analogRead() for a finer-grained distance measurement. You could use a long bar magnet and should be able to get a decent positioning that way.
Amazing Magnets will report the "surface gauss" (the magnetic strength directly on the surface of the magnet) in their product listings. I think that would be useful to you in getting a quantitivative idea of how strong a magnet is.