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Although the iRoast is a great appliance for roasting green coffee beans, it does have a few limitations. A method for overcoming one of them is discussed on this page.
The temperature sensor on the iRoast is positioned in the inlet air stream to the roasting chamber. This gives no indication, however, as to the temperature inside the chamber.
Ideally you would want to know the temperature inside the coffee bean as it is roasting, but this is not easy to achieve. The next best thing is to measure the temperature inside the swirling bean mass, with the probe in contact with the beans.
A number of individuals on the newsgroups I monitor have fitted a common fiberglass covered bead-type probe to their roasters. This was also my initial approach, but I noticed over time that the sheath was eroding. I figured that at least some of those fiberglass particles would up on my beans, and since I brew without filters (via press pot, vac pot or swiss gold mesh cone) I have probably ingested some of them.
Concerns for my health led me to a different approach, which is described below.
Below is a picture of my roasting setup. It is not overly technical, as I have not opted to incorporate computer controls or automated data logging. I prefer to roast by sight and smell, with the temperature readout augmenting my senses by helping me keep track of the rate of temperature increase and assisting in deciding when to stop the roast.
Below is a close-up of the temperature probe inserted into the iRoast-1 chamber.
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The probe enters through the top of the Chaff Collector Assembly via a custom drilled hole, and is held in place by an alligator clip. The probe needs to be above the metal bottom of the roast chamber in order to keep from getting artificially low readings, and the clip keeps it from sliding down during roaster operation.
The picture of the probe, below, shows the bends I made to put it in the desired position.
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The bend near the tip places the measurement end slightly away from the glass wall of the roasting chamber. I did this in order to place the tip closer to the middle of the swirling bean mass. I found that leaving the tip straight would occasionally allow a bean to become trapped between the probe and the glass wall, leading to erroneous readings. I also found that bending the tip more sharply inward produced higher readings, as the probe was exposed to more of the rising stream of hot air emanating from the center of the bottom of the roast chamber. This is less than ideal since you are trying to get a reading of the outside temperature of the beans and not the air temperature inside the chamber. Note that it is not possible to eliminate the effect of the hot air on the reading, the goal here is to minimize it.
Beyond these little details, it is not really too important exactly where the tip is placed, only that it gets positioned in the same place for every roast. There can be large temperature variations within the roast chamber, and the goal is to get consistent readings from roast to roast. Through experience you will get to know what roast level a given temperature reading is indicating in your own setup.
Here is the only tricky part of this exercise. The hole I made in my chaff collector assembly is detailed in the picture below.
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I placed the hole mid-way between two of the locking lugs on the top cover of the chaff collector. I drilled the hole in the lower (metal) portion first, placing the hole right up against the vertical metal wall. I then assembled the chaff collector and carefully ran the drill bit up from the bottom side (through the first hole drilled) in order to mark the hole position on the underside of the top cover. I then disassembled the chaff collector and carefully finished drilling the hole in the top part using the mark from the drill bit as a guide.
My only advice here is to use a sharp drill bit and take your time. The plastic used for the chaff collector is very brittle and can crack if you are not careful. You can see evidence of the JB Weld repair I had to do on my unit after dropping it on the floor!
The final thing to do is to put a small dent in the side of the vent pipe adaptor that comes with the iRoast to provide clearance for the probe to enter the hole. I used a pair of needle nose pliers for this, carefully forming a v-grove at the proper location.
When I first started roasting I used sight and smell alone, but sometimes that was not enough. Certain beans would appear lighter or darker than their actual roast level, and variations in the light shining on the beans would sometimes impair my judgement. Sense of smell is also important, but is only a broad indicator for me. For instance, I can tell when the drying (grassy) stage is done, when first crack is coming on, and the short period of time just before the onset of second crack. Using this temperature probe setup I have now been able to refine the decision when to stop the roast to achieve any desired roast level, including the more elusive City+ and Full City levels.
If you have any questions or comments, please feel free to contact me at the email address shown below.
Thank you,
- Jeff Bensen
$Id: iRoast-1_Temperature_Probe_Mod.html 6 2007-01-05 09:51:34Z JBensen $