Temp is not temp, so how do we use it?

[Luca]

So, Scott Rao wrote about the importance of a straight line declining ROR, and it seems like many people are taking this as a rule that is their north star, at least to start off with.

Rao's ROR is based on drum roast readings, usually taken from a K-type thermocouple with a 3-6mm thick sheath, and often with a constant airflow reading, or airflow adjusted once or twice.

The kaffelogic has a bead thermocouple in the roast chamber, and most roast profiles have an airflow that declines progressively throughout it. Chris has already commented that if you get first crack at, say, temperature X on one profile, you're unlikely to see first crack at that temperature on another roast profile with different fan speed.

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?

Followup question. If you look around at other peoples' roast profiles, one of the things that can give you some idea of "doneness" is to take a look at how much higher than their first crack start temperature their end temp is. So, for a nordic style roast, they might finish 8.5C higher than their FCS temp, but for a more developed espresso roast, this might be higher ... 11 or 12 C, maybe. If the meaning of the temp reading changes depending on fan speed, how do we work backwards to figure this out? Is this problem solved by reaching a constant fan speed ahead of going into first crack?

Further followup question. How does the software calculate the ROR? Like does it use an average over a particular time?

[Damian]

Fan speed will effect how much heating power you need to maintain temperature, the roaster is tracking temperature, adjusting heat power to follow the temperature curve. changing fan or air flow isn’t changing ROR unless it means the roaster can’t track the curve as accurately.

One thing that is a little different to how Scott deals with air flow is, he increases air to remove colder air from FC and replace with hotter air faster, to help reduce a dip. But he also maintains heat or gas rather than continue to decrease it which helps offset the loss from faster air flow yet still maintain a declining ROR.
This is hard to simulate with this roaster.
I may play with some custom PID setting, try to hug the temp curve better and smooth out ROR, but probably not till after Easter.

[kaffelogic]

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.

[kaffelogic]

Followup question. If you look around at other peoples' roast profiles, one of the things that can give you some idea of "doneness" is to take a look at how much higher than their first crack start temperature their end temp is. So, for a nordic style roast, they might finish 8.5C higher than their FCS temp, but for a more developed espresso roast, this might be higher ... 11 or 12 C, maybe. If the meaning of the temp reading changes depending on fan speed, how do we work backwards to figure this out? Is this problem solved by reaching a constant fan speed ahead of going into first crack?

It is true that comparing two profiles that have different fan speeds is going to be difficult. In principle it is reasonably easy to model the temperature offset/air speed relationship. I expect that it is possible to develop a method for calculating true bean surface temperature given probe temperature and air speed and build this into Kaffelogic Studio. Until that time there is going to be a certain amount of intuition required when comparing profiles. If you are in a position to control the variables when comparing roasts, the best option is to use the same fan speed profile for both roasts so the changes are applied equally to both. Or, as you suggest, keeping a constant fan speed after first crack which would make your data more comparable with results from other roasters (provided they are not also making significant changes to air speed in their roast).

[kaffelogic]

Further followup question. How does the software calculate the ROR? Like does it use an average over a particular time?

The Nano 7 uses a combination of moving means and least squares regression to calculate a ROR on the fly for the roast control system. This value is also logged. Kaffelogic Studio provides an option to apply further smoothing using moving means of the logged data.

[kaffelogic]

Fan speed will effect how much heating power you need to maintain temperature, the roaster is tracking temperature, adjusting heat power to follow the temperature curve. changing fan or air flow isn’t changing ROR unless it means the roaster can’t track the curve as accurately.

One thing that is a little different to how Scott deals with air flow is, he increases air to remove colder air from FC and replace with hotter air faster, to help reduce a dip. But he also maintains heat or gas rather than continue to decrease it which helps offset the loss from faster air flow yet still maintain a declining ROR.
This is hard to simulate with this roaster.
I may play with some custom PID setting, try to hug the temp curve better and smooth out ROR, but probably not till after Easter.

If you make a step change to the fan speed then there are two factors to consider;
1. The change in air speed will cause a change in inlet air temperature. The control system will detect this and adjust accordingly, but it will take time. The roast will go off the profile curve and then swing back on again.
2. The change in air speed will cause a change in the offset between bean surface temperature and probe temperature. The control system will not detect this, and it could cause an undesired change in bean surface temperature.
So, as you say, difficult to simulate Scott's boost in air flow and simultaneous boost in heat. However, it might work if you ramp up the fan speed over say 20 secs, apply a +5 zone boost for the same time period, and kink the temperature profile curve up by a few degrees also over the same time period. This could be a use case for a 'power step' feature that enables you to increase power by a defined amount at a defined time?

[Damian]

This could be a use case for a 'power step' feature that enables you to increase power by a defined amount at a defined time?

I think a feature that allows us to set a zone/s that controls heater power would be worth exploring. Coming off power priority back to temperature priority might prove tricky.
But I would be interested in trying a profile that in stead of boost, I could hold the current heater power setting through FC and then step to a slightly lower power, revert back to temperature profiling or just ride out the roast at the set power and control heat with fan speed.

Or maybe even a full heater power priority controlled profile, like what traditional roasters do.

[Steve]

This could be a use case for a 'power step' feature that enables you to increase power by a defined amount at a defined time?

Or maybe even a full heater power priority controlled profile, like what traditional roasters do.

This would be really interesting to play with.

As mentioned chopping and changing between temp / power control too much would likely cause issues. But if i roast something via temp alone then after reviewing profile wish to create a large zone where can switch to power control or yes perhaps most of the roast is good on temp up until just before first crack then switch to power control for remainder of the roast?

[kaffelogic]

The Kaffelogic control system works by setting the fan to a pre-determined value, and adjusting the heat to keep the temperature on the line.

One alternative raised here is to set the heat to a pre-determined value, and adjust the fan speed to keep the temperature on the line. In the Nano 7 this will have the same effect as the current system. The reason is that the air is heated in the element chamber, from there it enters the roast chamber. So the energy makes it into the roast chamber by the same path either way. This is different from, for example, a drum roaster where energy enters through the chamber walls mostly, and not via the air inlet. So I believe there is nothing to be gained in the Nano by switching the control from heat to fan, and there is a risk of reducing the bean circulation if the fan speed were to drop significantly.

Another alternative is for the heat to be locked or held to prevent it changing for a portion of the roast. This would prevent the control system from dropping the heat if the roast went above the line, and from increasing the heat if the roast went below the line. I'm a bit doubtful that this would be advantageous, but it can be easily experimented with. In a zone (using engineering settings) make the zone multipliers both zero. This effectively turns off PID control and locks the power at the value it had at the start of the zone. Once the end of the zone is reached it will recommence adjusting power to turn towards and get back on to the line, provided that it has not got too far away from the line during the zone.

A third alternative is for the heat to be locked and then progressively increased or decreased by a pre-determined percent amount during the zone. This is easily implemented, but will require a firmware change. Let me know how valuable you think this would be.

[Damian]

Another alternative is for the heat to be locked or held to prevent it changing for a portion of the roast. This would prevent the control system from dropping the heat if the roast went above the line, and from increasing the heat if the roast went below the line. I'm a bit doubtful that this would be advantageous, but it can be easily experimented with. In a zone (using engineering settings) make the zone multipliers both zero. This effectively turns off PID control and locks the power at the value it had at the start of the zone. Once the end of the zone is reached it will recommence adjusting power to turn towards and get back on to the line, provided that it has not got too far away from the line during the zone.

This is good, I’ll have a play with it tomorrow.