So I did some google math but my 24” intex has a circumference of 52”. That equals to 1.52 cubic feet. Helium has a lift of .069 lbs per square foot. This equals to 1.6 oz of lift in my beachball. I have to weight my ball deflated but I think it will be to heavy.
You can get .69 lbs of lift from a 48 inch beach ball. My math used my usual figure of 2/3 the rated size as the diameter, so 32 inches diameter. But if you were to heat stretch it you could get even more gain. Or just fill till its close to popping. The pressure isn't high at all, maybe 2psi really and after you've carried some argon bottles filled with hundreds of psi and can't tell the difference between full and empty I don't think the added pressure will add enough weight to change the measurement in a significant way
The stuff you buy in those balloon cylinders isn't pure helium, it's usually a lower grade with something else mixed in, so if you filled a beach ball to capacity it wouldn't actually be filled with 100% helium, so has less lifting power. If you go to a gas supplier and get welding helium for example, it's 100% helium.