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With my big 20 qt rice cooker, I never had a problem with just passive circulation.
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The myth of passive circulation during steady state has been debunked, see http://egullet.org/p1823461 !
Natural convection occurs only during ramping up at full power, in steady state with a power consumption of some 30-100W, there is no noticeable convection current.
And without circulation, surface heat transfer coefficient is significantly lower, see http://egullet.org/p1799312 .
Regarding bag position, see my article "Impact Of Bag Position On Temperature Uniformity".
Pedro - This is precious information - thanks!
My setup right now is a small rice cooker with no forced circulation, where I place one single small bag on a horizontal rack, with a SVM controller. Pretty basic, but I'm guessing also pretty common among home sous vide users.
Based on your experiments, in this setup, it seems that differences of temperature in the water are not significant (I always close the lid of the rice cooker). However, heat transfer is delayed because of lack of circulation.
I was wondering if I should start using an aquarium bubbler in my setup. However, it seems that simply cooking the ingredients at the higher time recommended would achieve results equivalent to a setup with circulation. Is this correct?
I was also wondering if it's worth making a rack to keep bags vertical in the rice cooker. My conclusion from looking at your tests is that it's not. If a covered bath with no circulation using bags in horizontal position only varies 0.1 - 0.2 C in temperature, I see no point in going through the trouble. The water temperature difference is not significant. Unless there are other implications of keeping bags horizontal that I'm not taking into account.
Would love for people to comment on this.
Edited by seabream, 30 January 2013 - 01:45 PM.