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Thread: Projected cooking time

  1. #1
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    Projected cooking time

    I'm doing my first cook with the Heatermeter and of course am very happy with it. But a question:

    The display says that my brisket will reach 205deg in 1hr 25 minutes. Neat! But how is this being calculated? Heat transfer is proportional to the temperature difference, so as the brisket temp gets closer to the pit temp, the temp rise rate will diminish. Is this being considered? Or is the current rise rate just being projected? Or something else?

    I know that the gadget can't understand about the stall, but a calculation based on temp differential would seem to be feasible and I think it would give me something longer than the hour and 25 minutes that I'm seeing as the projection.

    TIA

  2. #2
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    Although recently I did a cook where the estimated done time was spot on (prime rib), for the post part it is an unreliable prediction in my experience. I think the done time is estimated using the rate of change in meat temp once a trend is established, but as we know the rise in temp may not be linear over the entire cook, therefore the estimate is likely inaccurate. Particularly with something like a pork shoulder, where the temp usually plateau's when the fat cap hits the melt temp, the done time estimate is gonna be off. For something more like a lean meat it may be more accurate.
    I created the Roto Damper, RD3, HMv4.2 sliding back case, "air-burner" and the "ping-pong" valve in my quest for delicious "set and forget" BBQ.

  3. #3
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    Yeah. Makes sense. Actually my brisket has hit the stall and the time-to-done has skyrocketed. So apparently he is using the current rate of temp change to make the prediction. I have a commercial ham in there too just to get some more smoke and the time-to-done with that one looks to be about right. That one won't stall.

    FWIW, the brisket stall is apparently evaporative cooling rather than anything to do with rendering fat: https://amazingribs.com/more-techniq...stall-bane-all. I haven't done enough shoulders to see if they stall. I'm a pork rib fan but not much of a pulled pork fan.

  4. #4
    TVWBB Honor Circle Bryan Mayland's Avatar
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    Yeah the number there isn't great because it doesn't model the stall at all, it is just an extrapolation of the current temperature trend. The logic is simply: if there is more than 30 minutes of data in the past hour, run a least-squares fit against the past hour's data to make it fit y = a*x + b, and publish the slope 'a' as the "degrees per hour" statistic. Then the webui just subtracts current from target and divides by the DPH. So roughly it is just the number of degrees it has gone up in the past hour and determines the time it would take at that rate.

    It assumes the temperature will always continue at the same rate, which is wrong because it is constantly changing, but it doesn't know if we're on the curve before the stall (where the DPH will increase, then decrease) or after the stall (where the DPH will increase). It doesn't know if there is going to be a stall at all, because the stall can be shorter or non-existent depending on pit temp and meat or if the meat is foiled along the way. The stall temperature can also different depending on where you're probing. It needs a lot more logic and data added to it before it could effectively model through a stall curve and would need to have a set of conditions probably to apply the appropriate curve fit, linear, pre-stall ramp, no-stall ramp (for lower target temps or higher pit temps, or stall-integrated double ramp.

    I find it most useful for seeing where I am in the stall. If the DPH number is still dropping, we're going into the stall. Staying the same, stalled, BBQ is in the indeterminate future. If it is increasing, we're coming out of the stall and I know in about an hour I'll have a good estimate for when things will be done.
    I'm that HeaterMeter guy what ruins everybody's free time.

  5. #5
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    Quote Originally Posted by Bryan Mayland View Post
    ... I find it most useful for seeing where I am in the stall. If the DPH number is still dropping, we're going into the stall. Staying the same, stalled, BBQ is in the indeterminate future. If it is increasing, we're coming out of the stall and I know in about an hour I'll have a good estimate for when things will be done.
    Yes. Very nice. My DPH dropped from something like 11 to just above 5 in the stall, then recovered to above 10 when the stall ended.

    Using instantaneous DPH is conservative, too. After the stall, the actual time to the done point will be slightly underestimated. This might help keep the cook from being late to the party.

    Thanks for all your work.

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