I agree with the advice to try stirring and playing with different roast sizes.
I have a scale, and always weigh what I am roasting.

One other thing you can try is to tip you popper. I saw that somewhere, and found that I get better movement with larger roasts, if my popper is tilted in the beginning. Later, when the beans get lighter, and move more easily on their own, then you can level the popper.

As for modding your popper, you can try things that improve airflow. That should help.

Randy

Thank you all for your meaningful comments and suggestions. It was indeed the case that I needed to roast lesser amounts so that they will be roasted properly, which meant roasting "really" small quantities barely enough for a double . As said, it was not enough for the beans to circulate, rather, they had also to swirl vertically while circulating (beans on the top being continuously brought down by the stream of hot air directed downward)... This unfortunately meant very little quantities regardless of the maximum airflow available to the roaster and whether this airflow can lift larger quantities of green beans or not. That is, the design of the roasting chamber seemed to be the critical element in allowing someone to roast more reasonable batches (two doubles at least?).

To address this, I tried to cut a very cheap ($2.6) thermal thermos made of stainless steel to extract from it: (a) a rounded cap and (b) a hollow cylinder of the same diameter of the current roasting chamber. After sawing the thermos to extract both parts, I fitted them tightly (a) within (b) to have a new roasting chamber that can be fitted into the upper hole of the heating assembly using an o-ring gasket. To perforate this new, rounded-end stainless steel roasting chamber I used simple tools, which resulted in an ugly cuts that seem to do the job (sort of, still can benefit from some enhancements). The design of the new chamber is different from the original one in the aspect that once increased airflow is available, the quantity to be roasted is hopefully not limited by the design of the chamber (to an extent, of course). Below, you may find some photos for the current roaster and the new chamber added to it.


Higher neck, even more retarded .

The o-ring gasket I use to seal the interface between the heating assembly and the new roasting chamber.

Looking into the inside of the new roasting chamber.

The unclean perforation I did of the roasting chamber. I used a screw driver to direct the holes properly. I simply fit it in, and then tilt it in the right direction which secures a tunnel both on the inside and on the outside to blow the air in the desired, unified direction (counterclockwise).

This simple spring is intended to push the heating chamber away from sticking to the heaters. In the case of my rounded-end chamber, it gets very close to the heaters which means that you obtain more vertical space of the chamber immersed within the heating assembly. This gives you more height of the segment of the chamber that can be perforated, which is good I believe to allow you to obtain bigger batches.

Put into place, note that the end of the thermocouple is placed firmly at the centre of the bottom of the roasting chamber. I am unsure what type of measurement this positioning of the thermocouple would give but tend to think that it's the temperature of the "hot air" being blown rather than the temperature of the beans. It is reasonable to think that the end of the thermocouple is at the bottom of the beans and thus would be in touch with them, but I tend to think that it's more expressive of the temperature of the air being blown.

No comment.

Generally, I feel that I have moved to a type of roasting totally different from the one I am accustomed to. Now I can easily understand why people used to wonder about the long roasting times I've been having. I can barely now go beyond 13 or 15 minutes in my roasts, drying is very fast and so almost everything else.

I seem to prefer roasting environments in which the airflow is slower and the beans are being agitated mechanically. In the roasting environment imposed by the modified popper, however, both the speed of heat transfer and the speed of drying are tightly connected to the need to agitate beans (all achieved using the same mean: hot airflow). I am embarrassed to say that I used to obtain better (much... better?) results using ovens (with or without rotisseries). I know it's disappointing to say that, and maybe it's better that I experiment more before saying such a thing, but my feeling at the moment would still that I need mechanical agitation so that I can use lower fan speeds and longer roasting times. Doesn't know if it makes much sense to home roasters who have been always using poppers, but wouldn't mind enlightenment as to whether my feelings has a basis or not, that is, whether they were only due to my lack of experience in this new roasting environment.

Question one... what keeps the o-ring from melting?

Second more geeky question... For a given D of the roasting chamber what airflow is needed to create a full fluid bed for roasting? I can not see 9.8m/s * surface area * mass to keep it fully suspended.

This o-ring gasket I used is the same one for the washer screen of the La Pavoni Professional I own, which means it can take up heat (to an extent). Still, there is the possibility that it will not live long, in which case I will have to look for another sealing solution. I have asbestos sheets but they aren't bent easily, otherwise I could have used them to secure the gap. I will see.

I suppose D stands for depth, and that 9.8 is the same number I used to know as تسارع الجاذبية الأرضية in school. Don't seem to have many more clues as to how the question should be answered ...

Wait a second, are you saying that 9.8 multiplied by the surface area multiplied by the mass of the green coffee gives the amount of airflow needed to achieve a true fluid bed that carries along the thing to be roasted (in meters per second)? then what would the surface area in the equation be? the surface area of what exactly?

Anyway, I don't have much ways to increase the airflow except for trying to operate the motor using a higher voltage. I don't know how to measure airflow speed either.

Question from before that I did not answer: A dear member asked me whether I was using a fuse for safety sake, which I think is very important. Unfortunately I don't, and I don't ground the body of the heating assembly either as supposed . I should fix that shortly, but the matter was that I did not have a nice-looking fuse holder that allows replacing it externally, and I did not have the required 6A fuse (exact current drawn by both the heaters and the motor is theoretically 5.11A). I will do that soon when I buy these parts in case I did not give up my new roaster for a fancier and more satisfying roaster.

I would suggest to work within 70-100 gram batches and find a suitable stirring device and oven mitt to keep from burning your fingers. Stir the beans until they start to roll without assistance. I can't imagine a 25 gram or less batch size would work well at all. (I run 150 gram batch size, and need to stir early in the roast. The poppery 1 I use has a more powerful fan than your popper.)

Air roasting does happen fast and you need to stay on top of it or small mistakes may make bad results.
I'm not sure why you want to extend your roast longer than 13 minutes. This may be good for your other roast methods, but not generally so for an air roaster.

You have not mentioned any specifics about your 'roast profile'; the time and temperature of your roast. This is the key to a good roast and the reason you put in the controls and thermometer. Please take the advice to learn from those who have gone before. It will save you time, frustration, and coffee beans.
Jim Schulman uses a popper to roast and has been working on getting a good roast for quite a few years. Look in this current thread:
ET/BT rates of change

Study his roast time/temperature. Try to get your roaster to run somewhere in this range using the temperature and controls you have. This should get you in the range of something you can drink. Then you can start to adjust for your taste preference.

orwa wrote: ................

I seem to prefer roasting environments in which the airflow is slower and the beans are being agitated mechanically. In the roasting environment imposed by the modified popper, however, both the speed of heat transfer and the speed of drying are tightly connected to the need to agitate beans (all achieved using the same mean: hot airflow)................

I agree with you entirely on this point. My ideal roaster would provide mechanical bean agitation while using air/convection to deliver the heat to the beans and roasting vessel. This would allow separate control of heat transfer efficiency by air flow volume without the minimum bean agitation requirement. ....hence the stirring spoon will have to do for now...

Frost wrote:You have not mentioned any specifics about your 'roast profile'; the time and temperature of your roast. This is the key to a good roast and the reason you put in the controls and thermometer. Please take the advice to learn from those who have gone before. It will save you time, frustration, and coffee beans.
Jim Schulman uses a popper to roast and has been working on getting a good roast for quite a few years. Look in this current thread:
ET/BT rates of change

Previously I was unable to benefit from the great deal of information available on roasting due to the primitive methods I have been using to roast, which imposed a roasting environment drastically different from the one others are using, making most of the detailed advice irrelevant (where the "roasting profile" would count as such inclusive, detailed advice as how to conduct the roast exactly). Today this has changed, and so suddenly I can benefit from the great deal of advice available like you said.

I was having a feeling that my roaster isn't working properly, that is, that I can't ensure a uniform bean temperature for the batch sizes I am using and that I can't monitor the bean temperature well. It was more natural to me therefore to conquer these problems before starting to experiment with the roast.

One point to add: I may have also had problems with reading those discussions. They are conducted by people of high interest and parlance . I am not always lucky enough to fully understand what they are discussing .

Orwa, it seems you have the right stuff to get this sorted out and working. You don't have to understand all the discussion here I just refer to Jim's time/temperature graph for an example profile to approximate. As you have one temp probe, your choice of Bean Temp, or Environment Temp monitor will make this a bit more difficult but with some practice runs you can get by with just a bean temp probe. I suggest to put the probe tip very near the bottom vents to start. This may give you a better idea of the heater response to changes in power.

As you are also considering what makes a good roaster, I would point out one way to improve your popper (you no doubt are already aware of). Wrap an insulation blanket around the heater housing and roast chamber. That will get you a little closer to a uniform heated environment for the bean mass.
Good luck with your project.

After some experimenting, it seems that I need to finish the roast in relatively very short times to have a good, charcoal-free roast. This means a total roasting time as short as 3:30 - 4:30 minutes for the entire roast, starting at the moment I turn the heater on . If I do not do this, for example if I stretch the temperature ramp up to six minutes then I will inevitably have charcoal flavours. I tried to stay at a maximum temperature of only 400 degrees for some additional minutes after the 3-4 minutes elapse, but this doesn't help either in preventing charcoal flavours from developing. I usually break one of the roasted beans with my hands after roasting, and then observe the uniformity of colour of the fracture. When the roasting is good there is no charcoal smell that gets emitted the moment I break the bean and the colour of the side section is uniform. On the other hand, when the roasting time is a little bit longer, there is a clear charcoal smell that gets emitted and there is a clear black colour at the inside of the bean.

The reason could be that there is an abundance of airflow compared to the very little amount of coffee mass I am roasting. I did not succeed much in increasing the batch size with the new roasting chamber, however, due to the increased perforation of the new chamber the amount of airflow within the chamber increased. Could this be the reason why the drying and the roasting proceeds as quickly?

I don't understand why you can't turn down the heater power (dimmer switch) to slow the roast. If I turned the heat high enough to finish a roast in 4.5 minutes, it would be charcoal too. With your dimmer you should be able to slow the roast. Please explain, what am I missing?