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I am currently performing an experiment in which it would be greatly beneficial to be able to accurately measure the mass of a hot object without waiting for it to cool. The current plan is simply to place the hot object in a crucible with a lid and attempt to measure the mass quickly, before the crucible has time to heat up. The thought is that the convection currents around the object won't matter as much if they are contained within the crucible.

I will be testing this method in the next few days for accuracy and reproducibility, but would appreciate any other ideas to mitigate the problems with weighing hot objects, or insights on how this idea could be better.

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    $\begingroup$ Some idea of how hot might be a good thing to add. Are we talking liquid lead, or liquid tungsten? (Or, if posting from Pluto, liquid nitrogen?) $\endgroup$
    – Jon Custer
    Commented Jun 21, 2016 at 20:24
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    $\begingroup$ Or are we talking rotavap water bath? $\endgroup$
    – Jan
    Commented Jun 21, 2016 at 20:44

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Depending on geometry, mass and temperature (hot is pretty vague), you might be able to use an inertial balance.

If the device used to hold the sample is rigid, you could calibrate its resonant frequency with and without the sample and with known weights.

See this high school lab and a video of the procedure.

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The issue here, is, even inside a sealed crucible, the mass fluctuations would still be an issue. The object won't go anywhere, true, but you'll get fluctuations up and down depending on how hot the object is (which, I assume, has an upper limit of about 3400 degrees Celsius). An issue with trying to take force exerted by convection is that you'll have to take into account the local pressure level, humidity, and air composition, as each of these will effect how much pressure the air will be exerting. For example, taking this measurement inside will mean the air will probably have more carbon dioxide in it, because there's people. Taking it on a humid day might increase how much convection affects whatever you're weighing. How much the air is pushing will definitely be different from the top of a mountain versus sea level.

TL;DR? Just wait for it to cool. It'll save you a lot of additional calculations and considerations, and ultimately be more accurate. If you want to do the measurement quickly, consider ways of cooling the object down without contaminating it. I would suggest placing it in a watertight container and dunk the whole thing in a bath of water, if that wouldn't cause stress fractures to form on whatever it is you're measuring.

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