> I'm looking for an easy-to-control water tank heater.

> The idea is that the 5 USGal (19L) tank goes in the
> freezer, where it is cooled to 1 deg C, or less, and
> then the heater starts to do its thing, bringing it up
> by about 1 deg C at a time until it reaches room
> temp, at which point we'll take it out of the freezer
> and go up from there to about 50 deg C.

> PS- Here are my "ideas for making":
> We have a heater already which is controlled by
> 10-turn pots.  I could (maybe) replace the pots
> with a digital pot, and use a PIC with a thermistor
> to determine temp stability.
> Use conventional "lamp dimming PIC" methods
> to vary the strength of an aquarium heater.

IF the existing arrangement meets your needs then it would be 
extremely easy to automate it. Thermal time constant of the 
tank/heater arrangement will be long compared to available switching 
periods. Even eg one cycle of mains is very short compared to the time 
required to change by 1 degree C. Using an off the shelf  "sold state 
relay" would allow logic level control of a heater with ease.

If you want to emulate existing pot settings then PWM control of the 
heater power would allow this. An eg 10 second cycle using whole mains 
cycles would allow 60 x 10 = 1 part in 600 variation. Probably a bit 
crude. If you can accept part cycle control of heaters then a far 
finer control would be available. Aspects such as zero crossing 
switching and inability to turn off until next zero crossing 
(depending on switching technology) complicate simple replacement.

Changing to DC heaters and a FET switch would allow infinitely 
variable control and almost trivially easy pot replacement.

I would imagine that mixing/stirring of the tank's water to ensure 
even temperature distribution would be essential if this was being 
used as a reference temperature for testing etc.

If you present system is "closed loop" with some means of measuring 
temperature and feeding back the result to the controlling mechanism 
rather than open loop then control is extremely easy, with the focus 
moving to control algorithms to hold the temperature near the target 
(eg PID or whatever).

A potentially superior alternative is to use Peltier devices for 
heating/cooling. These may be obtained in the 50w - 200w range at 
quite reasonable prices and allow the temperature to be driven in 
either direction around the target point rather than using only heat 
seepage in or out to move you in one direction. A Peltier (or array of 
Peltiers) driven from a bipolar output switch should allow control 
across the range without resort to a freezer. Using a freezer to "help 
it get going" and lots of insulation would be useful.




            RM 

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