Spare time experimentation...
Jan. 5th, 2013 07:48 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
alexpgpIt was time to break out the sous-vide equipment to make some beef, and after getting things started, I set up a camera with an intervalometer set to snap a photo every 60 seconds. The photo would show the temperature at the time the photo was taken and whether the state of the controller's output was "open" (the crockpot was off) or "closed" (the crockpot was on).
I did this to observe the behavior of the temperature, which I had previously noted continues to rise after it reaches the setpoint (the point at which the output opens and power to the crockpot is cut off) and continues to drop below the temperature at which the output closes to reapply power to the crockpot.
I started taking photographs early in the cooking process and just as the contact opened after bringing the water up to the setpoint temperature. The intervalometer continued to operate for about two and three-quarter hours, taking 223 photographs. The temperature and output state data captured in those photographs are shown in the graphic.
The red line represents the water bath temperature. The tall vertical blue lines show the times at which the controller output closed, applying power to the crockpot, and opened, and the time in between is hatched with shorter vertical blue lines.
The controller was set to open its output at a temperature of 57.0°C (134.6°F) and close it again after the measured temperature had dropped by 1.0°C (1.8°F). The minimum temperature over the recorded period was 55.7°C (132.3°F). The maximum temperature was 57.9°C (136.3°F).
Once the output closed (reenergizing the crockpot), it consistently took 10 minutes to get to the point where the output opened again. The average temperature over five complete on–off cycles of the output was 56.86°C (134.35°F). Curiously, the graphic seems to show that, after ten minutes of heating, the output opens at 56.5°C (133.7°F), and although the numeric data suggests that the contact opened before hitting the setpoint of 57.0°C (134.6°F), it never opened at 56.5°C (133.7°F).
Although it's not evident from the graphic, it gradually took longer and longer for the temperature to drop from its maximum value during a cycle to the point where the output closed again. The first such interval was 26 minutes long; the last, 35 minutes. I suspect this was the result of the interior of the meat coming up to temperature (i.e., the transfer of heat from the water bath to the meat slowed down as the meat heated up). In such case, with all other things remaining the same, it would take longer for heat to escape through the crockpot walls, etc., and so also longer for the temperature to drop.
The reason temperatures continued to rise after the contact powering the crockpot was opened is because of the heat stored in the heating element, which continues to be transferred to the interior of the crockpot even though no power is being applied. If you use an electric stove, the same phenomenon occurs once you turn off a burner, which will remain dangerously hot to the touch for some time afterward. Analogous reasoning explains why the temperature continues to drop by a few tenths of a degree Centigrade once the contact closes again—it takes a couple of minutes for the heat to build up in the element after being powered on.
Five complete output cycles took 210 minutes, during which the output powering the crockpot was closed for 51 minutes, or a shade less than one-quarter of the time.
The meat came out well, by the way. I had to exercise self-control and limit myself to one healthy slice!
I did this to observe the behavior of the temperature, which I had previously noted continues to rise after it reaches the setpoint (the point at which the output opens and power to the crockpot is cut off) and continues to drop below the temperature at which the output closes to reapply power to the crockpot.
I started taking photographs early in the cooking process and just as the contact opened after bringing the water up to the setpoint temperature. The intervalometer continued to operate for about two and three-quarter hours, taking 223 photographs. The temperature and output state data captured in those photographs are shown in the graphic.
The red line represents the water bath temperature. The tall vertical blue lines show the times at which the controller output closed, applying power to the crockpot, and opened, and the time in between is hatched with shorter vertical blue lines.
The controller was set to open its output at a temperature of 57.0°C (134.6°F) and close it again after the measured temperature had dropped by 1.0°C (1.8°F). The minimum temperature over the recorded period was 55.7°C (132.3°F). The maximum temperature was 57.9°C (136.3°F).
Once the output closed (reenergizing the crockpot), it consistently took 10 minutes to get to the point where the output opened again. The average temperature over five complete on–off cycles of the output was 56.86°C (134.35°F). Curiously, the graphic seems to show that, after ten minutes of heating, the output opens at 56.5°C (133.7°F), and although the numeric data suggests that the contact opened before hitting the setpoint of 57.0°C (134.6°F), it never opened at 56.5°C (133.7°F).
Although it's not evident from the graphic, it gradually took longer and longer for the temperature to drop from its maximum value during a cycle to the point where the output closed again. The first such interval was 26 minutes long; the last, 35 minutes. I suspect this was the result of the interior of the meat coming up to temperature (i.e., the transfer of heat from the water bath to the meat slowed down as the meat heated up). In such case, with all other things remaining the same, it would take longer for heat to escape through the crockpot walls, etc., and so also longer for the temperature to drop.
The reason temperatures continued to rise after the contact powering the crockpot was opened is because of the heat stored in the heating element, which continues to be transferred to the interior of the crockpot even though no power is being applied. If you use an electric stove, the same phenomenon occurs once you turn off a burner, which will remain dangerously hot to the touch for some time afterward. Analogous reasoning explains why the temperature continues to drop by a few tenths of a degree Centigrade once the contact closes again—it takes a couple of minutes for the heat to build up in the element after being powered on.
Five complete output cycles took 210 minutes, during which the output powering the crockpot was closed for 51 minutes, or a shade less than one-quarter of the time.
The meat came out well, by the way. I had to exercise self-control and limit myself to one healthy slice!
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no subject
Date: 2013-01-06 01:56 am (UTC)2) It would have been very interesting to also include the interior temperature of the meat.
I control my ale fermentation with a somewhat better controller via a probe held to the side of the fermentation bucket and covered by a piece of poly foam for insulation, the whole apparatus inside an insulated container (garbage can with a water heater blanket for insulation). I can control the probe temperature within less than 0.1F. Since the bucket contains 5 gallons of liquid, the thermal time constant is quite long, so the undershoot/overshoot is almost nil. My controller is a PWM controller, which also acts to minimize undershoot/overshoot. I'm not sure sous vide requires that kind of accuracy and control, though.
I suspect that if you had an insulated cover for the crock pot, at least some of the undershoot/overshoot would be eliminated. Again, I'm not sure that really matters from a reality point of view.
no subject
Date: 2013-01-06 03:30 pm (UTC)2) Dunno about "very" interesting (not sure there would be many surprises), but it would have been nice to be able to do that, I agree.
There are probably sous-vide applications where temperature is critical, but I'm pretty far from that point right now. For example, instead of using a vacuum sealer, I encapsulate my "payload" in a plain old Ziploc freezer bag that has had as much air removed as possible (by hand, while immersing the package in water as I secure the seam).
And although I'm pretty sure the seam is leaktight, I prefer to keep the top of the bag outside of the crockpot (sticking out from under the lid), and so I place a bath towel—folded in quarters—on top of the lid. The towel hangs down a bit on all sides of the crockpot, too, providing some additional insulation.
Besides my curiosity, the main point of the exercise was to see what kind of temperature range I can expect while stuff is cooking. Based on this run, I would appear I can expect a 4°F variation.
no subject
Date: 2013-01-06 05:51 pm (UTC)