Under water housings

4 replies [Last post]
tom hardwick
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Joined: Apr 8 1999

My camcorder and Sony Sports housing weigh in at a hefty 4,72kg (10,4 lbs).
This is partly because I've attached a big lump of steel to the base of the housing to give the combo a specific gravity (or is it density - I never remember) of 1,0.

The kit now gently sinks in fresh water and just floats in salt water - which is the way it should be if you happen to let go of the straps.
Without the added steel I found it nigh on impossible to film under water as it was like trying to manoeuver a helium balloon down there and all my effort was required to stop it rocketing to the surface.

An interesting side effect comes to light. Although in water the heavy combo is effectively weightless it is by no means devoid of mass, and the inertia of the kit under water is very apparent when you come to move forward or change direction. My physics master would've been proud of the
analogy.

tom.

Boogie Bear
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Joined: Jul 18 2001

You think 4.72Kg is heavy.
I once worked as a safety diver for a BBC underwater shoot. The cameras were Sony Betacams with umbillical cables. It took 2 butch camera assistants to pick up a housing once it was trimmed for neutral bouyancy - made a nice splash going in though ! I wasn't very happy with the underwater lighting though, it was stonking great HMI floods and you could hear them 'singing' all round the swimming pool - a Health & Safety nightmare!

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Alan McKeown
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Joined: May 9 2001

Tom,

I am not sure that your physics master would be happy!

1. The camcorder outfit has a Mass of 4.72 kg. (Not weight).

2. The Weight of the outfit is about 46.2 N (newtons) on Earth (well, at Teddington anyhow), when the outfit is not accelerating. Note that the weight force takes into account the centripetal force on the outfit due to the rotation of the Earth.

3. The Weight of the outfit is exactly the same in water as it is on land (46.2 N).

4. The only way to make your camcorder outfit weightless (weight = 0 N) is to allow it to accelerate freely in a gravitational field.

This can be managed (for a short time) in an aircraft flying in a parabolic path or for much longer periods by orbiting in a spacecraft.
One other possibility is to take your camera outfit to a region of space where the gravitational field is numerically zero (caused by equal but opposite gravitational fields, eg. between the Earth and the Moon).

Hope this is of help with your next project!

Alan.

Keitht
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Joined: Jan 8 2001

Don't you just hate a smart arse.

Regards Keith

tom hardwick
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Joined: Apr 8 1999

Thanks Alan - I enjoyed your analysis of the situation though feel my underwater kit is better underwater that free-falling in space. Anyway, I was using layman's terms (being wot I am a layman) and to me it feels weightless under water.

But thanks again - it all adds to the fun of learning.

tom.