millivolt controller

[Junk Dr]

Greetings fellow techies - I have an old "pickle barrel" hot tub with a mechanically controlled Laars nat gas heater. I have tried to find a generic electronic controller for it but there seems to be only propriety units available. I am not an electronics genius but have sufficient tools, supplies and abilities to build one. What I need is a unit which has digital temp display, capable of two modes, i.e., temp maintenance ( not in use but holds temp to about 65 F ( tub is outside in Canada)) and in use ( maintains temp about 103 F with cut in and out @ +/- 1 F ) which would allow for adjustment of desired temp, up or down in a range of say 5 F. I would like to use supply voltage of 24V as my work provides me with components that use the same. It would also be nice to have the ability to link the unit to a second unit ( one inside the house and one at the tub ) either of which can be utilised. A design and component requirement list would be appreciated if anyone is up to the challenge. If anyone knows where such a device could be purchased that would also be helpful. :?:

 

[stevez]

I've noticed that a number of safety standards require a high temperature cut out and/or alarm to warn users that temperatures have reached or exceeded safe limits. I'd encourage you to add this to your design. "Too hot" might be a relative term and one could argue that a user would know when it's "too hot." Apparently that's not always the case as "too hot" relates to blood pressure and a number of other things you can't feel or sense.

Sounds like an interesting project. I wish I could be of more help. Sounds like excellent PIC project. In my readings on PICs (microcontrollers) I've found that some have worked out PID control programs. PID described a less than simple control scheme that might be work persuing for this application.

 

[jbeng]

If you're just looking to buy a controller, I'd probably look at something like this: **broken link removed**.
It's not too awfully expensive. You will, of course, need a temperature sensor (thermocouple or RTD) and some way to interface it with your gas valve system. And, like stevez said, an over temperature cutout/alarm may be a requirement (and obviously a very good idea).
JB