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BBQ Guru - Ramp function

EggSimon
EggSimon Posts: 422
edited November -1 in EggHead Forum
Hello to all,

I´ve got a question about my Digi Q II.

I have cooked a few low and slow cooks with the Digi Q and have always activated the ramp function.

Does anybody know, how is the algorithm behind that function ?
When starts the ramp function. How fast controls the Diqi Q the egg down ?
What I observated is, that the ramp function starts (led flashes), but the egg temp doesn´t really drop.

What´s your experience ?

Comments

  • Petunia
    Petunia Posts: 110
    Even at low and slow it is going to take the Egg a while to drop in temp. I personally disabled the ramp function and just monitor everything myself towards the end of the cook.
  • Mike in Abita
    Mike in Abita Posts: 3,302
    It's a PID loop. It is activated automatically once the temp reaches its' set point. I believe its 25 Deg below the temp you have set for the meat temp. The controller turns off (or pulses) the fan while monitoring the internal meat temp. Once the meat temp reaches the set point-25, the controller will lower the pit temp to keep from over shooting the done set point.

    Let's say you are looking for a an internal temp of 190 on a piece of meat, and your pit temp is set at 250. Once the meat reaches 165, the blower will slow down and the pit temp will start to drop. Remember once this PID tuning starts, the faster the meat temp rises the faster the controller will try to lower the pit temp. Once they both reach equilibrium the fan will only pulse to maintain the pit at 190. There are quie a few tutorials on the web that explain PID tuning in as much dtail as you need. This is by far the simplist information I have found.

    PID stands for Proportional, Integral, Derivative. Controllers are designed to eliminate the need for continuous operator attention. Cruise control in a car and a house thermostat are common examples of how controllers are used to automatically adjust some variable to hold the measurement (or process variable) at the set-point. The set-point is where you would like the measurement to be. Error is defined as the difference between set-point and measurement.

    (error) = (set-point) - (measurement) The variable being adjusted is called the manipulated variable which usually is equal to the output of the controller. The output of PID controllers will change in response to a change in measurement or set-point. Manufacturers of PID controllers use different names to identify the three modes. These equations show the relationships: .

    P Proportional Band = 100/gain
    I Integral = 1/reset (units of time)
    D Derivative = rate = pre-act (units of time)
    Depending on the manufacturer, integral or reset action is set in either time/repeat or repeat/time. One is just the reciprocal of the other. Note that manufacturers are not consistent and often use reset in units of time/repeat or integral in units of repeats/time. Derivative and rate are the same.
  • Jeffersonian
    Jeffersonian Posts: 4,244
    I'm pretty sure there's no PID tuning going on during the ramp, just a gradual adjustment of the pit setpoint. I think you're probably right about the 25-degree trigger point of the ramp function: Starting at 25 degrees below final meat temperature, the Guru starts dropping the pit setpoint 1/25 of the difference between the original pit setpoint and final meat temp for every degree the meat rises. Once the meat hits final temp, the pit and the meat should be at the same temp.
  • EggSimon
    EggSimon Posts: 422
    Thanks quys sor your replies so far. But, nevermind if it´s PID or not, I assume you are right with the starting 25 F below the set internal temp.

    but, is that really good ? Mike in Abita`s example with the low & slow cook @ 250 and meat setting @ 190. The pit begins to drop down, when the meat has 165°F internal. That´s the time, when the meat is in the stall (assuming talkin´ ´bout brisket or pork butt. Nothing worse than down dropping temp in the stall...

    So, I believe it makes sense to set the meat temp to at least 200 F.

    But - irrespective this effect, I observated no really dropping of the pit temp, after starting the ramp function.

    Perhaps it´s a general problem, to cool the egg down, like Petunia wrote. But all the facts brings me to the tendence, to disable the ramp in the future.
  • The Naked Whiz
    The Naked Whiz Posts: 7,777
    If the temp isn't dropping once ramp mode is in effect, that means you have too much natural airflow through the cooker for the temp to drop. You need to run with smaller vent opening at the top, or a smaller damper setting on the Guru's blower. If you have major leaks in your gasket, that can allow airflow also.
    The Naked Whiz
  • Jeffersonian
    Jeffersonian Posts: 4,244
    I'd agree, ES, you don't want to drop temp just as it's going into the plateau. Bumping final temp up should take care of that, but you still might think about just keeping an eye on it if you can. I'm not 100% sure that 25* figure is accurate...it was just a guess.

    UPDATE: I check the BBQ Guru site, and it is indeed a 25* band that the Ramp function works in:
    The Ramp Mode does NOT ramp the pit up in temperature. It ramps the pit temperature down proportionally as needed when your "meat done" temperature reaches within 25° of DONE, therefore your meat will NOT overcook. Consider this a "Cook and Hold" feature.

    UPDATE II: Oops, I spoke too soon. Apparently the DigiQ is different from the original Guru. From the manual:
    This mode is used for slow cooks so your food (meat) never over-cooks. This feature will gradually lower the pit temperature to the food set point temperature when the food is within 30° of being done.