Auto water level?

I use an immersion heater I built myself (from Make's instructions) on whatever water tank will fit my project. I try to use water coolers whenever possible because I imagine they save me power.
Anyway, I usually cover the whole thing with cling wrap, I still need to check water level, but much less often.

How about an EarthBox watering kit?

http://store.earthbox.com/Automatic-Watering-System-Sensor/productinfo/1010105/

I recall reading someone used a layer of ping pong balls, which offered insulation and allowed the water to condense on them. Also easier to get into the bath to pull out/check pouches than with cling film.

Thanks for the different ideas, I remember reading about the ping pong balls. Then I thought about needing different sizes to fill in completely. well I don't want to dig out my old physics book to find that experiment.
Also my deep fryer is working well but is getting to be cramped and small so I am thinking of a new heater.
I have a block heater for a car, 1000 watts. Ordered a pump for circulating the water through the block heater and back into the cooking chamber. Probably will use a cooler for that version.
But back to the evaporation problem, I think I'll use the blue insulation cut into two or three blocks that cover the water surface to cut down the exposed area.

GeneK