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Vacuum for Neutral Bouyancy

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Hi,

Did a search but couldn't really figure this out.

 

I was watching the Behind the Scenes for Highly Suspect's Lydia.

 

 

The ACs in here say that the vacuum helps with bouyancy for the housing. I don't really understand why since the housing is hard aluminum and since the weight of air inside is probably negligible. I thought it was purely leak protection.

 

I also read someone's comment on their their video talking also about balancing pressure, so I assume they knew what these guys were talking about.

 

Can anyone explain this to me?

Thanks!

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Makes no sense whatsover

Archimedes Principle: Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.

 

The vacuum created is around 200-300 mbar so the reduction in the number of air moles is only 20/30%.

 

So he is making it lighter say 25% of the weight inside. The weight of the air is only 20-25 grams as a starter so 20% less is 5 grams

 

The explanation is incredible too

 

thanks for checking it out Erik. the reality is sucking the air out of the housing is just one step to help balance. it does in fact close the seal but if there is air in there, no matter how little, the housing tends to bubble it up on either end and the camera will start to teeter. majoritatively however, the housing requires specifically placed weights to counter the teetering which took about an hour for us to set up.

 

The reason why the housing will be more buoyant in one part than another is because of the specific weight of the part. The port for example will be lighter if acrylic than the rest, for glass instead could be the opposite. So once you work out how much weight or float you need to make it neutral you need to put it in the right place so that there are no rotation. If there are areas that are very dense the specific weight will be higher than areas that have gaps. But vacuum does not change anything really.

 

What does vacuum have to do with this????

Edited by Interceptor121

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Indeed, makes no sense unless you need to get it perfect to within a few grams. Same for the rotation issue. Vacuum level can make a difference in theory. For instance if almost all the air inside is in a big dome port and little in the body, than increasing the vacuum will give more weight loss in the dome and thus causes the system to start tipping forward if it was perfectly balance without vacuum. I didn't look at the original post but it is hard to imagine a case where weight balance is so critical. Just moving a strobe a bit underwater will upset balance more.

 

Bart

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For instance if almost all the air inside is in a big dome port and little in the body, than increasing the vacuum will give more weight loss in the dome and thus causes the system to start tipping forward if it was perfectly balance without vacuum.

This makes absolutely no sense. Did I miss a smiley?

 

 

--

Sent from my Android phone

Typos are a feature, not a bug

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This makes absolutely no sense. Did I miss a smiley?

 

 

--

Sent from my Android phone

Typos are a feature, not a bug

Ultimately the housing will have a centre of gravity and when it goes up and down you can approximate the whole item to a single point. In absence of external forces the housing will not rotate around its centre of gravity. If you induce a force and make the item rotate on an axis this will continue to spin.

When people say the macro port makes the rig heavy in the front or the acrylic dome makes the rig light in the front is because the centre of gravity moves and now you have a torque on the axis between the handles of the rig if the overall system is negative or positive. When the rig is neutral it will stay in any position you leave it so if you are within grams this doesn't really matter much. In most cases though rigs are around 1 pound negative or less but not really 0.

So to have a perfectly balanced rig you need to ensure that the centre of gravity is on the axis between your two hands. Inducing an additional vacuum of 200-300 Mbar over a rig of more than 5 kg such as the gates housing in the clip will have an effect of few millimetre in determining the centre of gravity and therefore will be completely irrelevant

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These guys must add trim weights to their memory cards. Running with some round numbers 2.5 litres of nitrogen at 1 atm weighs around 3 grams. Pumping down to 0.75 atm lightens the housing by about 0.7 g, and is going to have no percievable effect on bouyancy.

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zzzzzzzzzzzzz..... some folks have got nothing better to do.

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Okay, I thought I was going crazy trying to figure out this one. When they explained it to me, I don't know why I thought it made sense when I read it (a bubble of air in a vacuum? it'd just dissipate)...

I worked/built the same exact Gates housing before, but didn't see anything about this supposed "vacuum balancing" in the operating manual. Unfortunately, mine was neither neutrally bouyant nor balanced properly so who am I to say.

 

Oh well, thanks for the replies and knowledge.

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as others have posted, they don't know what they're talking about. here's what the Seal Check system actually does.

 

Verify housing integrity before entering the water

 


Seal Check II represents the next generation in Gates legendary reliability by allowing confirmation of housing seals before entering the water – virtually eliminating water intrusion as a source of downtime and unexpected expense.

History reveals the top three flood causes are damaged o-rings, rushed setup and unfamiliarity with equipment. With Seal Check II there is peace of mind ahead of time. A precise digital readout provides a clear indication of housing integrity -- giving you the go / no go decision -- and aids with troubleshooting should the need arise. The electric pump quickly pulls a measurable vacuum, all contained in a small Pelican case. Seal Check II is also available with an optional self-install kit for non-Gates housings, too.

 

http://www.gateshousings.com/seal-check-ii/

Edited by sjspeck

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BTW, some one should invent a device to pump helium into the housing. Wouldn't that help with life the rig? :lol2:

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