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Effect of vacuum on bouyancy of rigid-bodied containers

Discussion in 'Food and Drink' started by zwiefel, Jul 23, 2015.

  1. zwiefel

    zwiefel Rest in peace brother

    I have been learning about and intermittently experimenting with Sous Vide cooking for about 3 years now. I'm still trying to find the boundaries of the technique for my style of cooking. One of the keys to a successful experience with it, is to have a good vacuum sealer for your product. This ensures that there is no air trapped in your bag, which would make your bags float, reducing the evenness of heat transfer into the food, as well as creating an insulating layer on one or more sides of the food.

    However, with some softer foods, pulling a strong vacuum will cause the food to become misshapen or even damaged. So, I was thinking today, what if I could use a rigid-bodied container, such as a mason jar? Experiment time!

    I took two 1-pint mason jars, filled them with an equal amount of water, and pulled a -9Bar vacuum on one, and no vacuum on the other. Afterwards, I placed them both in a 2gal cambro and filmed the process. I was surprised to discover that I couldn't see any difference between the two!

    After some contemplation about the physics involved, I think I've figured out what's going on. Whether something floats or sinks is a function of the density of the item (as compared to the density of the water it displaces). With a standard plastic bag, pulling a vacuum keeps the weight the same while reducing the volume, thus increasing density significantly. With a rigid-bodied container, it has the opposite effect! It reduces weight (the weight of the air that is pumped out) while keeping the volume the same, DECREASING density.

    Once again, reality is under no obligation to be intuitive.

  2. Lucretia

    Lucretia Founding Member

    You had to bring up a fluid dynamics issue, didn't you? I hate fluid dynamics.

    Archimede's principle states that a body immersed in fluid is buoyed up by a force equal to the weight of the water displaced by the body. If the body is heavier than the water displaced, it will sink. If it is lighter, it will float. I'd bet a nickel that the difference in the weight of your two jars is negligible if any. That's why they behave the same.

    Did you mean millibars for your vacuum measurement?
  3. zwiefel

    zwiefel Rest in peace brother

    I looked at the dial more closely...it's -.9Bars, or -26" Hg. I hadn't noticed the . before....
  4. Lucretia

    Lucretia Founding Member

    OK, with making some assumptions (treating the air as nitrogen, guestimating some dimensions on your jars, correct density for nitrogen found on internet, correct values entered into calculator before coffee, etc)...before you pulled a vacuum, the air contributed about 0.01 oz to the weight of your jar. After the vacuum was pulled, it only contributed about 0.001 oz. So you lost a whopping 9/1000 of an ounce in weight by pulling the vacuum. Your jar with the water probably weighs a good 5 or 6 ounces at least--you're looking at a fraction of a percent weight change. Not going to affect the buoyancy very much at all.
  5. I'm going to say that this prob won't work as you want it to. What we know to be temperature is just a measure of the average kinetic energy of particles; vacuum = no particles -> no heat transfer. Assuming a perfect vacuum, the only transfer of heat would occur where the food is touching the sides of the container unless you plan on soaking the food in some sort of liquid medium (oil, water, etc).
  6. Right, the way I understand the mechanism within souse vide is that the air is evacuated from the bag to ensure the only heat transfer medium between the food and water is the bag. The less insulation the better.

    You could always partially freeze the food before placing it in a vacuum?
  7. zwiefel

    zwiefel Rest in peace brother

    This was pretty much what I observed. the vacuumed jar seemed to float slightly higher, but it's not like I had a method to measure it and there are any number of non-uniformities that could be assigned responsibility for such a small difference anyway.

    I actually hadn't considered this....but....where there's a vacuum, by definition, there wouldn't be anything I'd want to transfer heat to. From a more practical perspective, this reinforces my idea to completely fill the jar with oil. but would still have a buoyancy issue. On another forum, someone mentioned using bricks/pavers to hold floaty things below the surface of the water.

    That's the standard approaches, I think....I kinda just having fun here, but also thinking through what kinds of consequences other approaches might have...obviously, this would make confits very easy :)

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