kaffelogic wrote: ↑Tue 10 Mar, 2020 12:23 pm
Luca wrote: ↑Mon 09 Mar, 2020 2:01 pm
If the absolute temperature reading doesn't really mean the same thing at different fan speeds, then what is the relevance of the ROR if the fan speed is changing? Isn't the one thing that we can basically conclude that, so long as the fan speed is changing, a straight line linearly declining ROR is the one thing that
cannot produce the results that Rao writes about?
There is a relationship between fan speed and temperature probe offset from true bean surface temperature. The faster the fan, the bigger the offset. The existence of this offset should not be a cause for concern - after all there is also an offset between bean surface temperature and bean core temperature. The probe offset is not just influenced by air speed, it is also changing as the relationship between air inlet temperature and bean surface temperature changes. We have to manage with the offsets, and without certain knowledge of exactly how big they are at any point in time. Rao writes about steadily declining ROR. If the temperature profile curve exhibits steadily declining ROR, then a changing air speed will alter that ROR, but it will retain its steadily declining nature provided the air speed is also steadily declining.
A steadily declining fan speed will almost always be compatible with a steadily declining ROR. On the other hand, a steadily increasing fan speed could potentially shift the offset in such a way as to cancel a steadily declining temperature curve causing it to go flat. For this reason, the rule of thumb is to use a steadily declining fan speed.
Chris, the bold part makes no sense to me. Let me try and explain the issue again, because maybe I'm not being clear.
I think what you're saying is that the machine will adjust the heater temp with whatever airflow is present such that the bead probe reads the target temperature. If the air speed is higher, then it might need a higher temperature. This means that the air temperature at the inlet to the roast chamber will be higher. Since the bead probe is on the exit side of the roast chamber, in this circumstance the bulk of the green coffee will have been exposed to hotter air and will thus reach first and second cracks at lower temperature readings. (Or maybe I got the relationship between airflow and temperature reading the wrong way around and it's the opposite, but that's still fine for the purpose of this discussion - the point is that the beans in the roast chamber are actually experiencing different temperatures at different fan speeds at the same temperature reading.)
The ROR reading is based on the temperature reading. But if the fan speed is different, then the temperature reading actually means different things. So if I'm correct that there's a lower inlet temp with a lower fan speed, and you lower the fan speed after first crack and end up with a perfectly linear ROR decline, in fact the ending temperature would have been even lower, had the fan speed stayed the same. Hence, the straight line ROR is in fact dipping if you adjust for fan speed. So isn't the position actually something like if you are aiming for a straight line decreasing ROR as per Rao's book, then the kaffelogic graph only in fact gives you this over the portion over which the fan speed is constant?
The extent to which the straight line fan speed adjusted ROR dips, I don't know, since this is information that neither the machine, nor the software, provides to the user. Note that Ikawa ended up providing both an inlet and an exit temp on their pro machine. Maybe you don't want to do that to keep costs down - and having a machine that can deliver similar (or perhaps better) results to the Ikawa Pro at 1/4 of the price is admirable, so don't let me sound too critical of this! Since the machine seems to have a lot of power and a lot of flexibility, it seems to me like there's a very good chance that we should be able to get excellent results out of it, once we figure out how to use it. But is there some way to work backwards to work out what the air temp is when going into the inlets in the roast chamber? On most drum roasters, you get some reading of an environment temperature that's hotter than the bean temp, often the maximum environment temp. The BT inexorably asymptotes towards it. Not having this information feels really disorienting ... though I guess that not being able to make meaningful changes on the fly, nor see the roasting beans is also disorientating.
There's a further issue, which is that the fan speed on a regular drum roaster is calculated from BT information from a 3mm-6mm probe, which will have its own buffering, lag and smoothing by virtue of the physical properties of the sheath. So I guess what I should be asking is how the temp readings displayed by the software compare with what one would get from such a probe (and, again, with how the derivative is calculated in artisan, cropster, etc). Again, all of this is just maths, so presumably there's some way for the data to be transformed to be similar, if that's necessary or desirable. At the moment, I can't make head nor tail as to how similar it is or isnt. For example, do the numbers on a drum roaster lag more? Would one need to shift the kaffelogic ROR graph 15s to the right to make it more equivalent to a drum roaster?
Now, I do have to add to this that, as Damian has observed, it may be that the fan speed, damper settings and convective heat transfer are changing in drum roasts, but if they are, there are a number of reasons why they are likely to be less dramatic; there tend to be only a few changes, there's a mix of conductive and convective heat transfer, and the maximum amount of airflow is probably waaaaay lower than a fluid bed roaster to start off with.
Finally, I guess I'd like to know, are we making this all too difficult for ourselves? Do we actually need a fan curve at all? Why can't we start high enough to agitate the green and just leave it there? What do we lose by having it that high? I guess you might say that the inlet temp would be too high, and might cause charring. Or you might point out that the beans might be hurled around ridiculously towards the end. However, I'd observe that we have neither inlet temp/MET data, nor any sensible way to observe the agitation of the hot beans during the roast, so I think these are pretty legit questions for users to ask. We are literally roasting blind on these issues. I understand you're working on a transparent roasting chamber replacement, and that will hopefully help with at least one of these issues.
I don't want to sound overly critical. I do think that this roaster has tremendous potential, but I also find it difficult to use. Some of these difficulties are just the inherent difficulty of roasting coffee.