Saturday, July 25, 2020

Kitchen science: insulated cups

An impromptu science experiment this morning.  A few months ago we acquired some very nice insulated tumblers (initially from causebox and then more from here).  Like all such insulated items, the inner and outer walls are made from a comparatively lousy thermal conductor, in this case stainless steel.  (Steel is an alloy, and the disorder in its micro and nanoscale structure scatters electrons, making it have a lower electrical (and hence thermal) conductivity than pure metals.)  Ideally the walls only touch at the very top lip of the cup where they are joined, and the space between the walls has been evacuated to minimize heat conduction by any trapped gas in there.  When working well, so that heat transfer has to take place along the thin metal wall, the interior wall of the cup tends to sit very close to the temperature of whatever liquid is in there, and the exterior wall tends to sit at room temperature.

We accidentally dropped one of the cups this morning, making a dent near the base.  The question was, did this affect the thermal insulation of that cup?  To test this, we put four ice cubes and four ounces of water from our refrigerator into each cup and let them sit on the counter for 15 minutes.  Then we used an optical kitchen thermometer (with handy diode laser for pointing accuracy) to look at the exterior and interior wall temperatures.  (Apologies for the use of Fahrenheit units.)  Check this out.

The tumbler on the left is clearly doing a better job of keeping the outside warm and the inside cold.  If we then scrutinize the tumbler on the right we find the dent, which must be deep enough to bring the inner and outer walls barely into contact.

The bottom line:  Behold, science works.  Good insulated cups are pretty impressive engineering, but you really should be careful with them, because the layers really are close together and can be damaged.


Anonymous said...

Use a piece of dry ice on the dent while the cup sits in the hot sun to try and pop it back out as used on hail dented automobile roofs.

Anonymous said...

Can you be sure it's wall contact, and not a breach of the vacuum?

Douglas Natelson said...

Anon@4:08, I can’t be completely certain, but the dent itself is gentle and doesn’t have any cracking, and the actual sealed seam is a good 8 cm away from the dent.i also think that if the vacuum was breached the problem would be worse and ice water in the cup would lead to condensation on the outside.

Anonymous said...

If there was a leak in the vacuum seal, then immersing the whole object in a fish tank full of water would reveal it. You'd see bubbles.

Douglas Natelson said...

Follow-up: the cup doesn't seem to be getting worse, and the lack of condensation on the outside when there is cold contents says that the vacuum space is not completely vented.