You’re mixing and matching, Achernar. There are two sites linked in Zut’s post, and the more technical one states:
I don’t see anything about “55 cubic feet”; as far as I can see, Zut’s guess is pretty close.
Now, this is a standard gas cylinder, the size you see in laboratories and welding shops. The baby cylinders they sell at Party America are obviously much smaller, and less pressurized.
Hopefully, this link will put this one to rest.
Hopefully, this link will put this one to rest.
Yeah, I was mixin’ and matchin’ cites. I used the one you quoted because I wanted a cite for the pressure of helium bottles meant specifically for balloon-filling, and I used the other cite (with the larger size bottle) because he had done the calculations independantly.
Gas cylinders (even for balloon fillers, evidently) come in different sizes. For instance, these guys offer three sizes of tank for balloon-filling: 291 cu ft, 110 cu ft, and 30 cu ft, the first of which is apparently the same size as the one I was basing my calculations off. I suspect that the 291 cu ft size is the large, 4 ft tall type of gas cylinder, so 91 kg metal weight seems about right to me for that size.
Also, to quote the relevant part of scotth’s link:
As he says, that ought to put it to rest.
Mass is a constant; weight is not. Don’t confuse the two. A tank with helium in it is lighter than a tank that has air in it for the same reason that a balloon with helium in it is lighter than an empty balloon or a balloon with air in it.
But don’t consider the case closed until you’ve actually lifted and/or weighed a full helium tank and an empty one. Empiricism trumps theory.
This is just wrong; the balloon is lighter than air because the volume of the helium is preserved and it is only slightly pressurised inside the baloon (in the case of nonelastic mylar balloons it need not be under pressure at all).
In a tank, the helium is compressed; when you add mass to a system without changing the volume, the weight will increase. (this is pretty much the definition of density).
I’ve just fired off an email to the UK contact of http://www.boc.com :
I have attempted to frame the question in as neutral a fashion possible, although my email address is similar to my username here.
Here’s one theory on why a full tank might seem lighter. When pressure is increased, the air speed inside the tank will also increase. I believe this causes the tank to ring at a higher pitch when struck. We tend to associate high pitch sounds with lighter things, so one might get the impression that a full tank is lighter.
Um, I meant the sound speed inside the tank will increase.
By the way, does nobody have a can of balloon gas around? They are often sold in small cans that look like spray cans. It’d be trivial to weigh one, empty it and weigh it again. I’ll check a couple of stores on the way home tonight if I have time.
I was wondering about that myself, scr4; it may indeed be that full containers sound emptier that empty ones.
So is the helium in a rubber balloon. That’s why it explodes if pricked.
Hardly the same degree of compression as when it’s in the tank.
If you made a balloon that allowed you to compress helium to a greater density(total density including the substance of the balloon) that was greater than that of air, it would not float.
Or to put it another way; if you made a balloon that weighed (when empty) the same as an ordinary one, but was of a tougher material that required exactly twice the amount of helium to inflate it to the same size, do you think it would be lighter or heavier?
Here’s another snippet, the important parts being:
That the helium was rendered smaller in volume by cooling rather than compression is irrelevant; when you compress a substance, the density increases.
A given volume (the tank) of a more dense substance (compressed helium) is heavier than the same volume of a less dense substance (residual helium at 1atm pressure).
No one is confusing the two. And we’re talking about a full tank of helium, which is compressed to a pressure about 140 times that of normal atmospheric pressure with miniscule volume change. A balloon is pressurized at far less than one atmospheric pressure (gauge pressure), but has substantial volume change.
You’re not offering empirical evidence; you’re offering subjective anecdotes. How much heavier is an empty tank than a full one? What did you use to weigh it? What was the accuracy of the measurement device? What was the pressure in the tank? What was the construction of the tank? Additionally, why would your observation trump that from the cite scotth found, which directly contradicts what you say?
This case is closed. This experiment has been. The physical laws of bouyancy were determined doing experiments just like this.
I would without hestitation put my life at stake (I am not kidding or exagerating, I really would would not hesitate to do this) on the results of this experiment.
Any arguing to the contrary just indicates that you don’t understand the principals involved here.
Perceived weight on the surface of this planet is determined by two variables, the mass of the object and its displacement. With a balloon (which gets larger in displacement when filled) the displace increase faster than it mass when filled with helium, therefore its weight decreases as it is filled. The helium tank is a rigid structure and does not change displacement as it is filled. The only thing that happens as you fill it with helium is that its mass increases.
If you wish to understand this, study how a boat floats. The pricipals are exactly the same, but may be easier to grasp for the first time in that context.
The same principles govern why ice floats (water and ice are the same substance, but ice has greater volume/displacement for the same mass) and why hot air ballons rise (hot air occupies more volume for the same mass) and why an ordinary rock sinks in the ocean (greater mass than the same volume of water).