Posted by vahegan on 03/26/2014 11:05 AM
#14
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oldgearhead wrote:
As the roast progresses the beans double in size, and lose their moisture.
Therefore, they are lighter and have more surface area, they require much less air to remain 'fluid', so they 'fly' higher. As the beans become lighter your temperature probe is influenced more by the hot air, and less by the beans and the mounting plate.
OK, I now get what you mean
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However, your total roast times (11-15 minutes) appear perfect to me, and if you are happy with the taste of coffee, don't change anything.
Well, this is a tricky point. I've got several coffees and more are yet to arrive (I have to order the good ones from overseas, that takes some time). I am drinking it, and I like the results much better than stale store-bought coffee. But I don't know yet what they will max out, and I am sure it'll be possible to obtain much better results with what I have... I am afraid I am on a slow learning curve now
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Cooling - Are the cooling temperatures you reported from your roaster probe or the beans? I'm very surprised the 'popper' can cool that well with only a 20% increase in blower speed. But that's fine, don't bother with the colander
Yeah, I have just one probe in the bottom of the popper can, all my temperatures are from there. But when I remove the beans just after cooling, they are hardly warm to the touch - just about 40-42C (104-108F)
According to the sticker, my blower motor is rated at 24V DC. I measured the voltage in its original connection, and there was about 18V on it. That's what I supply to the motor presently, but its on a PWM control, and my power supply is capable of supplying up to 30V, so I can easily blow all the beans away should I wish to. When I speak about 20% increase of blower speed during cooling, I mean a 20% increase of PWM ratio, which translates into roughly 20% voltage increase on the motor, but it is hard to judge on how much the actual flow changes. Theoretically, for a brushed DC motor, the dependence of RPM on voltage is linear, so I expect that flow increase should also be around 20%.
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Ambient air - Electric heating units can only raise the temperature of the air supplied to them. Therefore, as long as your ambient air stays within range of your heater's capability you won't see much of an affect.But I know a couple of 'popper' users who rely either a cardboard box or a heat gun when it gets cold.
OK, I see your point. Yes, indeed, my friend who was using a 1040W popper reported that he was hardly getting there and baked a batch or two due to too low temps. My popper is 1200W, and this was sufficient even when the ambient was freezing cold (I am roasting at the balcony), although it seems a tad slow to get to the last stage. Well, I think I'll be able speed up this ramp, should I need it, by dropping the blower speed a little bit.
By the way, I inserted a meat probe in the chamber during the roast today, and I've got a reading some 10C (about 18F) higher than what my probe reading. I think this explains why I am getting too dark roasts with standard profiles. Will have to check the temperature difference also at the drying and Mayard phases, to find out whether it is a constant error or if it is proportional to measured temperature. to be able to make corrections. I think this difference is due to the cold junction compensation circuit in the thermocouple chip. There is no cold junction in the circuit, and the chip is compensating based on an on-the-chip temperature sensor. I placed the chip below the heater, in the hope that its case temperature will be equal to that of ambient air that is being sucked in by the blower. However, I am not sure what's the chip temperature during roast and this may be affecting my probe readings. The other reason may be a low-quality thermocouple... Anyway, I need to compensate for this error and therefore I shall find out how the error depends on the actual temperature range.