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Geo_UW

How to get a neutral bouyancy without arms ?

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Posted (edited)

Hello everyone, 

 

I'm wondering how can I have a neutral bouyancy of my setup without arms. I'm using the vaccum system so it's very negative actually. This is a photo of my setup

117593901_631081091126557_4184507154240378879_n.thumb.jpg.40bc6973356c4539461539cc5ddae8fa.jpg

 

If you have any ideas I'll take it because it's start to be a bit heavy ... ! 

Thank's !  

Edited by Geo_UW

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Unless your name begins with ‘A’ and ends in ‘medes’, you may struggle.

But seriously, you need to add something with positive buoyancy. That might be in the form of ‘stix’ style foam or a bouyancy arm. You could attach an arm between the two posts rather than sticking out to the sides.


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Pardon the obvious question, but if you’re not using strobes or lights, why use a tray and handles?  Ditching those would significantly reduce the negative buoyancy, with little effect on stability.  

Otherwise, spend $30 on some Stix jumbo floats and experiment with zip ties on the bottom of the tray. 

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The vacuum system actually has no significant impact on buoyancy - this is set by the weight on land and the volume of water displaced by the housing.  If you ever plan to add strobes buoyancy arms will not be a regret purchase, I use a couple of my strobe arms attached with clamps between the two balls when I use my housing snorkelling .

Removing the tray and handles will approximately increase buoyancy by around 60% of the  weight of the components removed, based on being aluminium.   Probably little impact of removing the tray for photography, but I imagine video would be easier and more stable with the handles.

You could start with two clamps and a long arm running between the clamps to provide an attachment point for floats - or use float arms. You want the floats above the housing otherwise the housing will tend to twist upwards - with the float on top the heavier housing will hang underneath.  With float arms you need to weigh the housing underwater to know what size to buy - you do not wat to go positive, having the housing slightly negative is much more stable.

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I put a buoyancy cylinder (PVC from the hardware store) on a ball joint centered on a bar that runs above the housing from my two handles. The nice thing about the ball joint is you can adjust the tilt of the housing to match where you are shooting. Buoyancy arms are nice for making the rig neutral in a certain position, but when you go to shoot down or up you are fighting the buoyancy to do so. Photo of the rig below.

SYSTEM.jpg

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Posted (edited)

Alright thx to all of you guys. And yes, the reason why I'm using handles without arms it's because it's more stable for the video and I'm currently a videographer in a dive center.

Edited by Geo_UW

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On 8/21/2020 at 7:01 AM, troporobo said:

Pardon the obvious question, but if you’re not using strobes or lights, why use a tray and handles?  Ditching those would significantly reduce the negative buoyancy, with little effect on stability.  

Otherwise, spend $30 on some Stix jumbo floats and experiment with zip ties on the bottom of the tray. 

I'm in a similar situation with a compact, armless rig that is -600g negative without the wet lens and -900g negative with.

Trying to get the thing neutral with foam floats so it can be modular, but can't seem to find any dedicated foam floats in Thailand (tried Stix, Carbonarm...) only float arms -
Currently looking into deep fishing net buoyancy floats which should be relatively easy to source here and also EPP closed cell foam yoga things such as this:
https://shopee.co.th/SPA_2-in-1-EPP-Yoga-Column-Hollow-Foam-Roller-Muscles-Feet-Release-Tool-i.134960417.2246378037

Read that Stix was polyethylene closed cell foam but polypropylene closed cell foam seems easier to source in these parts...

The PVC buoyancy cylinder looks good as well.

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A great many foams will compress at depth and lose buoyancy - apparently Stix is quite resistant to this.   Just specifying the material is not going to solve this as the different grades available have different strengths and resistance to compression.   Float arms of course don't have this problem and most have published buoyancy figures, for example INON http://www.inon.jp/products/armsystem/arm.html 

For your rig , two mega float arm S at 390 gr each would be about right with 780 gr of buoyancy, I use them and I'm happy with them.

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I'm curious as to why you don't want to use arms unless it is just because of the bulk of them?  You can use Stix as mentioned to affix to parts of your tray.  Arms would get them in a position above the center of gravity though.  With floats affixed to the bottom you will potentially be fighting roll with the camera.  It is usually easier to manage tweaking the buoyancy if the lift is above the camera, think of hanging the camera from an air bubble verse balancing the camera on the bubble.  To keep things more compact , you can add two clamps and a single arm between the balls and add Stix floats to it to achieve the buoyancy you need.  I Use PVC sponge net floats https://www.memphisnet.net/category/floats-pvc which are dirt cheap, come in various sizes, can be cut and drilled to fine tune or fit onto arm section.  I have experienced no compression issues with these over repeated dives.  Maybe over time they will break down, but I have not noticed any degradation.  If they do I'll just replace with more as they are so cheap.  They do not look neat and polished though, but I'm ok with that, the fish don't care.

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