> Hi Ed, i've built many types of temp control
> systems over the years, and PID is probably real
> overkill and totally un-needed for a system like
> this. Most likely you will be using a standard type
> mains AC heater, which is essentially an on/off
> device. Add to that the huge thermal lag of your
> system, ie time from heater turns on to water temp
> actually rises, and you are going to get significant
> hysteresis from that anyway.
>
> You will get great results from an on/off system
> with high gain, you don't need any deliberate
> hysteresis because you have enough in the physical
> properties of the heater and water.
>
> Just detect the water temperature, using your sensor
> mounted very close to the heater and switch the heater
> when temp drops below the setpoint. You will probably
> get a few seconds thermal lag anyway, but if not try
> and switch the heater with at least 5 seconds on,
> if your period is too short you risk element failure
> from the continual thermal stresses. It should regulate
> within 0.1'C with no probs.
> -Roman

I agree with Roman.  A long time ago I built a water bath temperature
controller for photographic developing.  I used a fish tank pump to keep the
water circulating and an off the shelf immersion heater as the heating
element.  The kind sold to heat your coffee directly in the cup.  The ciruit
either turned a relay on or off every 64 power line cycles depending on
whether the temperature was above or below the set point.  No hysteresis,
just high open loop gain and a guarantee that the output never got changed
faster than every second.  I used two thermistors in series mounted on
opposite sides of the little tub, and a bridge with a pot to adjust the set
point.

The thing worked fabulously.  Once the set point was reached and it starting
controlling the temperature, I couldn't see any temperature variation on the
photographic thermometer.  Response was surprisingly quick when I dumped in
a liter of cold water or whatever.


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