smith said:
I have a 2000 watt immersion heater that i'm putting in a 25 gallon water
tank. I want to keep the temp. at 200 deg. with a 5 deg. max differential
range. Could someone help me with type of controller i need.
thanks
Go here:
http://www.omega.com/guides/temperaturecontrollers.html
They have a selector guide to pick the controller. I would recommend also
getting a probe type thermocouple to measure the temperature.
I would recommend a solid state relay to run the heater. If you select "DC
pulse type" output, the controller will output 10V DC to feed into a
external solid state relay (SSR). You will also need to get a SSR with
enough current to handle the heater.
You want a PID controller, not a thermostat. PID controllers cycle the
heater fast with a variable duty cycle. The duty cycle is adjusted to
maintain the set temperature. This results in far finer temperature control
because a regular thermostat waits for the temperature to rise and fall.
There is a long delay, and thermostats will overshoot. A PID controller
cycles the heater on and off without a significant change in temperature.
You MUST UNDERSTAND that it will start cycling the heater on and off well
above and below the setpoint unlike a thermostat. This is to prevent
overshoot. Many people will look at it and say it's broken because it's
turning the heater on when it's too hot or it's turning the heater off when
it's too cool. In reality it's anticipating that the setpoint is about to
be reached and starting to dial up/down the heater. A thermostat will
dumbly hold the heater on right up to the setpoint, then of course it will
overshoot far more.
I have had good luck with the autotune feature of the Omron controllers in
similar applications. Is the immersion heater similar to a domestic water
heater element? They don't have too much lag. A element with a lot of
thermal mass can be more difficult to control.
