by JonS » Tue May 27, 2008 10:19 pm
Off the top of my head, I think you would aim to use a PID with multiple ramp/soak segments, and you would setup each segment with a ramp time and target temperature, to achieve a steady ramp up of temperature (the ramp part of each segment) as each segment progressed. I'm not sure you'd need to make use of the soak part.
The problem as I see it (and, with the benefit of using a Hottop programmable for some time now, by means of a comparison) is that this level of pre-determined time/temperature ramping can end up being somewhat counter-intuitive.
Let's say you want to steadily ramp up to 1st crack, then you want to keep it ticking over through first, and get a nice pause before 2nd crack, then drive it through second to the dump point. Time and target temperature are hard to adjust accurately, primarily because different bean moisture, different beans, humidity and ambient temperature will affect how long it takes the beans to reach a given state. I think that's where the author of that article is coming from. Sometimes, you want to progress to the next phase of the roast, when the beans reach a certain state, not necessarily after a given time.
This doesn't mean that you can't use a multi-segment ramp/soak PID to drive a roast, it's just that I think you'll get less control than you'd ideally like.
With a gas roaster, I would think you would have the benefit of lower thermal inertia and power in reserve for raising the temperature on demand. With an electric element, turning off the element because you've reached the target temperature is not ideal, because it will take a while to come back on when you decide you need to heat again. Consequently, I see a lot of people setting the hottop programmable up to basically under-achieve and never reach the target temperature for each segment, so that the element is always on to some degree. This is how I use mine, too.
One side issue is the difference between drum air temperature and bean mass temperature. The former is probably more easy to use as the control loop for applying heat, the latter gives a better indication of the progress of the roast. If you use the PID, you probably limit yourself to control via one or the other, but not both.
Like I said, off the top of my head (and I am pretty frazzled today). But, it very much depends on the open-loop behaviour of the roaster.
I can't really comment on the 200g part, to be honest, other than you'll end up with maybe 10-15% less than that post roast. That may suit you fine, as long as you are happy to make the time to roast. I enjoy roasting, but I have to make a concerted effort to roast 3 times a week in my hottop, which has a 250g batch.
Jon
Iberital L'Adri 1grp semi-auto | Macap MXKR Auto | Mazzer Super Jolly | Hottop Programmable
Isomac Zaffiro w/PID | Isomac Gran Macinino | Mazzer Mini
Scace Thermofilter
Custom-built grinder timer