I had assumed wireless comunications would not be possible underwater.
I thought all the extra um conductivity, and reflectence from the water surfaces (including pool edges) would muff every thing up. I know that remote control subs need to work at very low frequencies (around 20-30kHz)... and the is an asumption of no reflectance from big objects.
I had a small remote sub once, it worked in the sink, but not the pool. And it is dead now anyway(cheap China import).
A quick back of the napkin assuming my main problems involve signal atenuation from my (saltwater) conductive poolwater; and surface refraction from the pool top (not sides and bottom);
Attenuation of radio signals in water (dB/metre) = 0. 0173 sqrt( f * q)
where: f = radio frequency (hz)
q = water conductivity (M ohm per metre)
Ok I stick my multimeter in the pool and find 150 K resistance per cm.
that is 15 M per metre. This is less conductive than ocean water which is around 4 M pm in temperate oceans. So my mesurment is in the right ball park.
Now xbee gives a 900MHz and 2.4Ghz solution.
for 900MHz: 0.0173*sqrt(90000000*4) = 328.24 dB/m
for 2.4GHz: 0.0173*sqrt(90000000*4) = 1.6950e+003 dB/m
So the 900MHz is the least imposibly far fetched solution and my light is 70cm under water so . about 230db of attenuation... still not a good result.
Ok add to this signal loss by refraction at the water surface... (I call this rlaws)
going by the standard formula:
rlaws (dB) = -20 * log [(7.4586*10-6) * sqrt(f / q)]
where: f&q are as they were before:
Hey there is a surprise higher frequencies are atenuated much less by surface refraction. Also the salt reduces the refraction problem; Finally something going my way.
So I get 57dB atenuation due to reflectance at the surface.
Ok Im feeling all in all I need a lower frequency (but not to low to make the surface refraction a problem)... plug both formulas together and solve for minimum combined distortion and I get... a frigen headache... Its so bed time.
Anyway I figure the standard xbee will not cut it. But I know there are some "underwarer enabled" wifi devices around and I am not sure how they get over these hurdles.
I dont suppose anyone would care to test an xbee in a salt water pool for me (I dont have a xbee) to check my calculations... I saw a web page saying xbee could go underwater to a certain distance and I assume that is a fresh water scenario.