Re: PID vs. temp stability question

Marshall touched on this, but I'd like to expand a little. The heat
transfer to the coffee and the temperature profile within the cake is
dependent on water temperature at the top of the cake, the dimensions
of the cake, the flow rate of water thru the cake, and time. Anyone
who believes that the temperature is constant is not thinking of the
temperature of the coffee during brewing.

In order to discuss temperature in a meaningful way, it's useful to
talk about water temperature at a point in the flow loop where coffee
cake issues do not alter the answer. The most relevant point is just
above the cake. It's not clear at all whether the temperature here
should be or should not be constant. None of us know and some of us
have been fooling around with this to a fair degree. What we can say
is that the starting point and the system behavior ought to be the same
time after time. If it ain't, then you're shooting in the dark and you
ain't gonna learn anything because too many things change from shot to
shot.

There are certainly methods available to the mechanical designer to
effect repeatable and variable temperature profiles. But prototyping
takes money and lots of time, as you know. Unfortunately the espresso
equipment industry is very traditional, insular and not very forward
thinking for the most part. Innovations seem to come from the outside.

So the answer is that the cake does not see constant temperatures at
all. There's no conceptual reason why constant temperature above the
cake is better or worse. We believe that reproducibility is key to
making advances in the art and that once the reproducibility is
improves sufficiently in all aspects of the process (I include green
coffee sourcing, roasting, grinding and distribution of the coffee in
the reproducibility business), then we can start to get meaningful
results on what temperature profiles above the cake work with what
coffees.

It's a fun hobby because science meets art here and to be really
stellar at it you have to work at your art.

-Greg




papenfussDIESPAM@xxxxxxxxxxxxxxxxxxxx wrote:
> Ken Fox <morceaudemerdeThisMerdeGoes@xxxxxxxxxxx> wrote:
> : This and related topics have been discussed here in very long threads over a
> : number of years. Every once in a while someone has a new idea on this topic
> : and that engenders another huge thread.
>
> : I suggest that you use the google groups archive function to read prior
> : threads. Use this link:
>
> : http://groups.google.com/group/alt.coffee?hl=en
>
> : with search terms like "temperature" "PID" "consistent temperature" "flat
> : temperature" and other terms that crop up in the threads that you pull up.
> : You will undoubtedly pull up more threads and posts than you can possibly
> : read over a weekend.
>
> I started doing that, but it quickly became apparent that the SNR is pretty
> low. I just figured I'd cut to the chase and ask the veterans of the group.... :)
>
> My current thought is that the time lag makes it impossible for a feedback
> system to control the temperature throughout the pull. A flatter profile is probably
> more likely with accurately instrumented temp surfing (if the heater power is correct
> for the water-in flow rate).
>
> -Cory
>
> --
>
> *************************************************************************
> * Cory Papenfuss *
> * Electrical Engineering candidate Ph.D. graduate student *
> * Virginia Polytechnic Institute and State University *
> *************************************************************************

.

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