Yesterday I actually did boil water.
I am missing a few pictures. The first thing to do was to take the 1/4-20 screw off the TC and to put it into an adaptor which threads into the top of the tank. The probe is now in direct contact with the airspace above the water. The lag between the installed TC and the faster-reading TC from the multimeter is now down to about 7 degrees. I set the temperature controller to auto-tune to a set point of 70 degrees C and did something else for an hour and a half. Then I moved the set point to 95C. I don't have the data with me but the controller overshoots by 5 or 10% or so during a cold start. After a short period (roughly 15 minutes total) the installed probe catches up with the reading from the multimeter. If the set point is then increased slowly, the two readings move pretty much in lock-step. This means that with a little tweaking of the PID coefficients (likely the D), the controller could probably be made to hit the set point with little or no overshoot. I think however, that it may just be easier to divide this into two problems. The first is heating up from a cold or warm start. The second is maintaining temperature at the set point. With a micro controller replacing the temperature controller it will be easy to set a safe initial set point, wait for the heat to disperse through the system so that readings become reliable, and then move the set point to its final value.
The good news is that the controller is already very good at keeping the temperature stable once it gets there.
The two pictures I did take yesterday :
I am missing a few pictures. The first thing to do was to take the 1/4-20 screw off the TC and to put it into an adaptor which threads into the top of the tank. The probe is now in direct contact with the airspace above the water. The lag between the installed TC and the faster-reading TC from the multimeter is now down to about 7 degrees. I set the temperature controller to auto-tune to a set point of 70 degrees C and did something else for an hour and a half. Then I moved the set point to 95C. I don't have the data with me but the controller overshoots by 5 or 10% or so during a cold start. After a short period (roughly 15 minutes total) the installed probe catches up with the reading from the multimeter. If the set point is then increased slowly, the two readings move pretty much in lock-step. This means that with a little tweaking of the PID coefficients (likely the D), the controller could probably be made to hit the set point with little or no overshoot. I think however, that it may just be easier to divide this into two problems. The first is heating up from a cold or warm start. The second is maintaining temperature at the set point. With a micro controller replacing the temperature controller it will be easy to set a safe initial set point, wait for the heat to disperse through the system so that readings become reliable, and then move the set point to its final value.
The good news is that the controller is already very good at keeping the temperature stable once it gets there.
The two pictures I did take yesterday :
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