Back to the original topic, which is better, controlling the process by using the MT outlet temp or the HX tube temp ? Or both ?
So if you use the MT outlet temp, you are actually measuring the temp of the bed, which is actually what matters. That is where the enzyme reactions occur and that is where the temp needs to be "right". The time constant for controlling the bed temp will be very long.
If you use the HX (RIMS or HERMS) exit temp and control there, the mash liquid going back into the bed will be the "right" temp, but bed temp changes are going to take forever because you are relying on a very small delta T to warm the bed. Ie if the HX exit temp is 154F and you are doing a step mash from 144F, there is only 10F difference between the returning liquid and the bed itself. It will take forever to get the bed, where the actual reaction is occurring, up to temp. The time constant for controlling the HX outlet temp will be pretty short.
Then there is the issue of scorching with the RIMS system. If you measure and control at HX exit, the controller is going to put the element on full and scorching may result.
Then there is the issue of denaturing the enzymes. I tried to get at this point with this thread, but it never really got there.
https://www.homebrewtalk.com/f36/why-doesnt-mash-heating-denature-amylase-enzymes-518680/
Basically, what is the max temp that the mash liquid could be out of the HX such that it wouldn't be denatured ? I don't think denaturing happens instantaneously. I think its a time at various temperatures thing. At a decent recirculation rate, wort doesn't spend much time in the HX. Is it OK if it touches 160F ? 170F ? 180F ? Assuming of course that scorching isn't an issue.
HERMS has its own issue in that the HX water temp needs to be controlled as well. If you set it at the desired mash temp, a) the mash will be lower than the HX water and b) temperature changes are going to be SLOW. Fine for helping a mash keep its temp, but terrible for step changes. And if you set the HX water temp much higher than the step setpoint temp, the mash temp will keep rising after the setpoint is reached, unless you want to stop recirculation !
I think the best setup would be to control by the bed temp at the exit of the MLT, but have a secondary control loop that keeps the HX exit temp within bounds, to prevent denaturing. Unfortunately, none of the simple PID controllers control based on 2 variables, but one could serial chain the output of 2 controllers to achieve the same effect. It wouldn't be optimal, but it would work.
