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Offlinesonoramo
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Registered: 02/27/19
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Pressure cooker temperature distribution with thermal images
    #27817522 - 06/13/22 12:08 PM (2 years, 7 months ago)

TL;DR: Keep the outside of your canner clean and shiny. It will work better.

While waiting for the my Presto 23-qt canner to be "done" with a round of grain jars, I got curious about its temperature distribution, and also how uniformly it heats the jars inside. That led to taking pictures with and without an aluminum foil wrapper (to see if that made any difference) and getting pictures with a thermal camera.

The canner is a Presto induction model and the cook top is an induction unit. Other than the induction base, it is the usual 23-quart canner.

First, without the foil wrapper. This is at least an hour into sterilization. Everything really should be in good thermal equilibrium.

   

Here's what I think is notable:

  • The burner is at about 1/2 power. This is enough to keep pressure at about 16 psi on the dial gauge. I could hear continual boiling inside. Presumably this is because vapor is condensing on the lid and sides, and falling back down into the water bath.
  • In the thermal image, you can really see my fingerprints and water spots on the side of the canner. This ought to be a motivation for me to clean it more carefully! A shiny canner is an efficient canner.
  • Even in its rather unclean state, the canner does a pretty good job of not emitting heat. The black handles are warm but still OK to grab with bare hands. Yet they radiate massively more heat than the much hotter canner itself.
  • This is an induction cook top. The glow around the bottom is due to much higher emissivity of the cook top glass than the canner. The cook top is warm but even cooler than the plastic handles.


OK, now to see how hard it would be to reduce thermal loss on the canner. In the above thermal image, there's a visible temperature gradient from bottom to top of the canner side. And that makes sense. It's cooling radiatively and by convection with ambient air. A heat shield should could make canning more power efficient, and maybe improve uniformity of temperature.


 

And what do I see?

  • The burner setting is dialed down significantly. Not half, but enough to make a difference.
  • Boiling is reduced immediately upon adding the foil.
  • The whole thing has a very nice Bobb-Fett look.
  • The foil reduces the radiated heat loss notably. Again, not half, but enough to make a difference.
  • The canner lid, handles, gauge and all the rest look about the same temperature, and the dial is still near 16 psi.


Incidental note: The burner delivers 3.7kW at full output. That is about 2.5x what a $70 induction single burner unit puts out. The $70 units are still a pretty good deal, and they easily maintain a canner at full pressure. They just take longer to heat it up there.

So, conclusions:

  • Clean the outside of the cooker. Get it mirror-shiny if I can, so I don't have fingerprints and mineral spots radiating power away.
  • Maybe use the foil wrap. It isn't that much extra work, and it does seem to improve thermal performance.
  • The Presta engineers who designed this canner probably did this already. They made sure the outside at least starts out shiny, and that is probably part of their design.

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InvisibleMurphSmurf
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Registered: 02/08/18
Posts: 192
Re: Pressure cooker temperature distribution with thermal images [Re: sonoramo]
    #27817620 - 06/13/22 01:12 PM (2 years, 7 months ago)

very cool

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