I was reading an earlier thread (Echo345) and a discussion was taking place on airships and vacuums.
I was wondering has anyone weighed a TV picture tube when it contains a vacuum then reweighed when it has lost its vacuum and noted a change in weight? If so what would be the percentage difference?
I know if I do get a tube I have to scrap I will give it a try!
I have two EF80's that have lost they're vacuum and loads that haven't. I don't have any scales that accurate but I can imagine that they would weigh more, because the air is now in the valve and has rwacted to whatever gas was in the valve to form the white residue.
you would need a very large light valve and a very very accurate balance.
"perfect" air, that is, air of typical composition, at exactly 20.00 degrees C, atmospheric pressure of 760.5 mm of mercury, and a relative humidity of 50% has a density of 1.2 mg/cm3.
so 1 litre of air has a mass of 1.2 grams.
A typical noval tube might be 40cc volume, but subtract space taken by all electrodes.
if you snip the nip and keep the broken bits of glass and weigh that, then fill with pure water at 20C and 1 Atmosphere and weigh all including the broken tip you can calculate the volume inside the valve by assuming 1g water = 1cc (it's in reality 0.9982071 gm / cm3 at 1Atm. and 20C)
If the answer was 15cc (no idea actually) then the extra weight of air, neglecting tiny amount of extra air reacting with getter is about 18mg or 0.018 gram (0.000018 kg).
I was thinking of something like a CRT rather than a valve. Weigh the tube in a box, then after knocking the neck off remeasuring the tube when air has entered. So if we say a tube holds a gallon of air when broken the weight (mass) would increase by around 4oz.?
Maybe about 5.4grams if it's nearly 4.5l (about 1gallon) (300x 18milligrams)
5.4g is about 0.2 ounces I think
But a CRT is too big/heavy for kitchen scale and I doubt a bathroom scale can see 1 or 2 gm. Maybe.
But yes a CRT has a noticeable amount of emptiness and if you have very "repeatable" scales you might see the difference. My bathroom scales are mechanical. They are not that degree of resolution.
As a rough estimate, 1 cubic metre of air weighs about 1.2kg.
Therefore, since 1 cubic metre is 1000 litres, 1 litre of air weighs about 1.2g.
So a CRT of 1 gallon (4.5 litre) capacity will weigh 5.4g heavier if the sealing pip is broken.
Alternatively, you could argue that when evacuated and sealed, the thing weighs 5.4g less than its true weight, due to atmospheric upthrust. Just think, if the glass was thin enough, and the electrodes etc were light enough, it would float up to the ceiling!
Sadly no-one has ever come up with a suitable material to make a vacuum Airship with. 
I'm glad Kalee20 we came up with about the same answer!
Hi Trev, So are they the same as lightbulb and CRTs in that sense? Is it better to have them on continuous use or the switch off switch on cycle?
