swimming pools + Linux

"Swimming pool control with Linux" puts together two technologies not normally associated with one another.
Richard Kinch, the inventor, lists the following reasons for using Linux to control his pool:

Reduced effort to manually check and treat chemistry. While Version 1 runs "open loop", having no sensors to use for "closed loop" operation, pool chemistry is predictable enough to be manageable in that form.

Saving money by running the circulation pump and chlorine feed only during ephemeral mid-daylight hours rather than a dumb timer's fixed, daily schedule. This requires the sophistication of PC software to know the present date and time, to compute daylight hours, and to sequence the interactions of various pumps. The equivalent function in a programmable astronomic timer costs about $700, such as this unit made by Intermatic. (Update: In 2004, Intermatic had available an wall-outlet version, the "SS8 Self-Adjusting Digital In-Wall Timer", specs here.) Solar cell daylight sensors cannot be used for this purpose because they cannot predict when the sun will set.

Heh. I can't think of any disadvantages to using shell scripts to control the feed of chlorine chemicals into a pool that you are using. What could possibly go wrong?
Of course, according to the inventor, "the chemical feed rates are limited and cannot rapidly overcontrol the chlorine or pH if they were to be 'stuck on' from hacking or system problems."

There's also a kind of hard-to-follow discussion about the chemistry of swimming pools. Apparently Kinch is very much against the use of "cyanurics" which are supposed to make chlorine solutions more managable, but which he claims make them much more ineffective.

There's some discussion of electronics as well. I'm surprised his original system didn't use optoisolators to isolate the parallel port of his computer from the outdoor cabling. I thought that was standard practice for systems like this. To be fair, he mentions that he would like to do this in the future.

I think it would be much better if the system were a closed-loop system, though. (A "closed loop" controller responds to changes in the thing it's controlling, rather than operating "blind.") A closed-loop system would know what the current condition of the pool was, and so could decide how much chlorine to add based on that, rather than just on a preset schedule. Also, it could alert the operator to potential dangerous conditions. I guess the main obstacle is how tremendously expensive automated pool sensors are.

There's a great potential for systems like this to control everyday things presently controlled by hand, like light bulbs. Combined with wireless technology, this could be pretty easy to install, and would save a lot of energy. I think I remember reading about a project related to this at Microsoft... I'm too lazy to dig up the link right now, though. I wonder if these systems will ever be widely accepted in households. Maybe if the price of energy goes up enough.


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