Ha! I don't know why but the phrase "totally pimped out Behmor" tickles me for some reason... however, it occurs to me that this seems to be the right thread to pool our collective experiences/knowledge and come up with <Sunday! Sunday! Sunday! Monster truck rally announcer guy>
The Ultimate, ok, ONLY Guide To Pimping Your Behmor </Sunday! Sunday! Sunday! Monster truck rally announcer guy>.
Here, I'll kick it off with the hopes that others will jump in and share their observations, thoughts, ideas, etc...
Temperature MonitoringIt's easy enough to get a probe in the roast chamber to measure environment temperature. The real question here is: "What's the best location to place the probe?" It's tough to answer that question in a general way so let's make the simplifying and reasonable assumption that the measurements will be used as input to the heat control process (which may simply be a human hand turning a variac knob). This implies that a good probe location would be one that experiences the same or similar heat as the beans and will respond as quickly as possible to heat application changes.
The next question that comes up is: "In the airflow path or not?" It seems to be conventional wisdom that an environment probe should be located outside the airflow path in a drum roaster... although I'm not sure the Behmor qualifies as a "drum roaster" with respect to this particular guidance. The airflow path in the Behmor consists of air flowing from two inlets - the big flap in the back and the smaller opening above the top heat element - and out through the exhaust opening in the chamber ceiling when the draw fan is energized. If we want to stay out of the airflow path, that basically means placing the probe away from the inlet and exhaust openings.
A location that seems to meet both criteria is the space between the bottom heat element and the area of the drum where beans gather when it's rotating, about 1" away from the chamber wall (near the location of the stock temp sensor). The heat shield on the back wall comes in handy if you want to wrap a TC wire around the bottom rung a couple times and bend it such that the tip sticks out as close to the drum as possible without touching it (I have a picture of this somewhere, I'll post it later if I remember).
Getting a probe into the bean mass is a bit more tricky due to the completely enclosed drum. It can be done if you have the skills, however, as JohnMoody (
http://coffeegeek.com/forums/coffee/homeroast/333876) and farmroast have ably demonstrated.
Heat ControlThis is the part where we wrestle the Behmor's control and safety features to the ground, acknowledge that equipment and personnel damage may result from doing so and we agree to hold poor TahoeJoe faultless since we took it upon ourselves to hack his nice, well-mannered, consumer-friendly device. If, by adding environment/bean mass probes, we merely voided the warranty, controlling heat application in the Behmor requires
voiding the hell out of the warranty.
Actually, it's not quite that bad but there are aspects of the Behmor's control logic that need to be considered. We start with the assumption that heat is the only thing we want to control... I start with this seemingly basic assumption to distinguish the simpler heat control mod from the larger, more encompassing mod where we disable the stock control logic altogether and put
everything under manual control - the heater, the drum motor, the cooling fan, the draw fan, the main cooling fan, the afterburner heater, etc.
There are basically two approaches here and the one you select depends to some degree on how you plan to drive the heat control: variac and switched. A nice hybrid variation on this is the one mike points out a few posts back.
For variac control, it's best to split the heating elements from the Behmor's circuitry and power it exclusively through the variac. As others like drdna have pointed out, powering the digital control circuitry with a variac isn't particularly good for the gear nor beneficial in any way. Disconnecting the elements isn't difficult, as quick slide connectors are common on the Behmor's control board.
For switched control, you can either split the heating elements out and power them separately like we did for the variac or keep them in the existing circuit. In either case, we insert a toggle switch or relay in the supply circuit, allowing on/off control of the heating elements.
A description of the stock heat application logic is in order at this point so we can make an intelligent decision as to the degree of heat control we'd like to have (no pun intended). Internally, heat is switched on/off via a mechanical contact relay on the control board. On P1 (full power), the stock logic cycles the heat off at 400f and back on when the temp, as measured by the stock sensor in the chamber wall, drops to 360f. The draw fan seems to be activated at +7:30 and stays on for the remainder of the roast. It's been a while since I dealt with the stock logic and I don't have my [few] notes handy, so I can't recall if the 7:30 is a fixed time or a percentage of the total time. Also, IIRC, the other profiles simply use a lower cycle-off temp, while the cycle-on temp is 360f regardless of profile.
First, the simple approach. By leaving the heating elements in the existing control loop (note that this limits you to switched control), you effectively have
partial control over the heat application, or modulated control if you like. Let's say, for example, that you're perfectly happy with the default heat application pattern and all you *really* want is a way to kill the heat at certain times... say, to control a particular phase of the roast. If this is the case, partial control is all you need. Note that this approach precludes the ability to turn the heat on when the default logic has turned it off (technically that's not 100% accurate but it is in the simple approach).
Now, the more involved full control approach. If you're using a variac or you want to be able to profile or you have some other use case that makes partial control undesirable, the first thing you should do is liberate the Behmor from it's stock temp sensor. In addition to feeding the heat control loop, the control logic uses the sensor to detect an overheat condition (which, IIRC, is ~410f) and a too-hot-to-start condition (which, IIRC, is ~175f). It'd be great if you could simply unplug the molex connector for the temp sensor from the control board, but alas, the Behmor won't start if it doesn't detect the sensor. The easiest solution is to move the sensor from the chamber wall to outside the roaster so it always measures room temp, effectively disabling it. Taking the stock sensor out of the picture has done a couple things for us. First, it prevents the control logic from shutting down the roaster at an inopportune time. Second, it allows us to preheat the chamber before putting the beans in (something that's normally difficult to do given the too-hot-to-start check). And third (ok, a
few things), we can stick a switch/relay in the existing heat circuit and - unlike the partial control arrangement and so long as we always use P1 - have full control over the heat since P1 profile+room temp sensor=heat power is always on as far as the control logic is concerned.
Great, so now we have full control over the heat right? Eh, not completely. The draw fan is still being controlled by the stock logic, which means it will dutifully cycle on at seven and half minutes into the roast and *KERCHUNK* there goes that nice smooth heat curve you've been working on thanks to the sudden 20 degree drop in ET. Is that really such a big deal considering that the default heat application logic regularly causes 40 degree drops? I'd have to defer to someone with superior roast/flavor chemistry knowledge (virtually anyone here I would imagine) to answer that question with any kind of technical accuracy. However, I would have to assume that if you've gone through the trouble of placing the heater under manual control, not having manual control of the draw control as well would be irksome to say the least. Fortunately, splitting the draw fan off and putting it on a switch is trivial since it's an AC fan. Speed control is also an option here, although I've had limited success with the only fan speed switch that I had handy at the time that I was fiddling with it.
Ok, I think that covers the basics from my admittedly limited perspective. Hopefully there's something of value here as a starting point and others will add to it.
by popeye on Wed Nov 19, 2008 2:53 am 
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