Good Morning
Cold and snowy here in Missouri. I'm catching up on some projects. I'm looking for input, ideas and any direction for this idea.
I have a 30 gal steam kettle that I'm recirculating hot water around in the jacket. For mashing some grain, I need to heat the water to 155F and hold it there for an hour or so. My boiler (an old stainless steel beer keg with 4500W heating element inside) provides the hot water. I'll have a DS18b20 probe in the steam kettle to provide a temp reading. I'm using an Arduino Uno and a 30A SSR with 220V. I'd like to run without any PID til I'm within 10% of the 150F. Then I'd like to switch to PID control to reach 150 and keep it there.
Several other temp probes measure room temp, return water temp to boiler and boiler water temp. I'm learning Arduino programming. Plan to use a 4X16 LCD to provide readout and to input settings.
In the past I've used a PSR-25, phase angle controller to control current to the boiler when I' distilling things. Orginally I wanted to used the Arduino to control this, but everyone seems to go straight to an SSR with some PWM.
The temperature in the cooker won't change very rapidly, so a full blown PID loop won't do you any good. You could use the "P" part and vary the PWM according to the difference between the probe and setpoint temperature, but I'll bet that in practice, the action won't be much different from the simplest possible controller, in pseudocode as follows:
loop {
temp=read_temp();
if (temp < lowsetpoint) heater_on();
if (temp > highsetpoint) heater_off();
}
The setpoints could differ by just a couple of degrees. This approach works for both the heating and holding phases.
jremington:
You could use the "P" part and vary the PWM according to the difference between the probe and setpoint temperature, but I'll bet that in practice, the action won't be much different from the simplest possible controller...
I think the biggest problem in that thinking is that you don't want a 20A heating element switching full on/off all the time. It's a fairly substantial amount of current draw for smaller buildings and would annoy other circuits. The ideal would be to have a second, smaller element that you could switch on/off to maintain temperature, but if you're sticking with one element then you should stick with a similar "dimmable" SSR.
