Perfectly cromulent if you’re conducting your experiment in Canada.
I think you’re weighing the bucket, plus its contents, plus the entire column of air extending upwards. by displacing some of that air with a less dense gas, the weight of the bucket decreases by an infinitesimal amount.
As the helium rises and leaves the bucket, air will flow into the bucket to replace it. That downward-flowing gas has momentum and will press against the bottom of the bucket, slightly raising the weight.
After everything stabilizes and reaches equilibrium, the moving gas won’t be pressing on the bottom anymore, and the weight of the bucket will decrease, but will still be more than when the bucket was full of helium.
Your reasoning is wrong.
The Helium does not rise and create a vacuum.
It is a light gas, it has no anti-gravitational properties.
The helium is displaces by heavier gases.
Almost as though nature hates a vacuum.
In a vacuum the helium would stay put.
(More or less, not really)
The point I’m trying to make is that the Helium is not moving up on its own, it is pushed out.
Quicker than if it’s an unconstrained mass of helium, I would say.
This question is quite similar in concept to the thing about birds flying inside a truck, except this time it’s not a completely enclosed system, so the effects diminish rapidly (the bird/truck scenario would be even more similar to this if the truck had an open top; the the birds fly for a bit, having some effect, then leave, reducing their effect to zero).
One other way to think about this is to replace the air and helium with two other fluids, with a lesser difference in their densities - so maybe the bucket is on scales at the bottom of a swimming pool filled with salty water and the bucket is initially filled with fresh water.
Viscosity and inertia play bigger roles here, but the fresh water doesn’t just instantaneously float out of the bucket - it does float away and is displaced by salty water, over a period of time.
The ‘pouring a light thing into the bucket’ is the real confounding issue here because helium gas in air will ‘pour’ upwards and then if you have to squirt it down into the bucket, the squirting itself is exerting forces.
It’s probably better to just conduct the thought experiment with some magic in it; like if you use a Star Trek transporter to materialise two buckets on two sets of scales; one bucket is filled with air and the other is filled with helium, what happens next?
I think the answer is that the helium-filled bucket is lighter at first, but not for very long, and that this difference in weight is hard to measure properly because of the forces from sloshing and turbulence as air replaces the exiting helium.
You could pour water into the bucket. The bucket would be the still be the same weight. The bucket + water would not be.
I just love The Dope. Don’t you?