Camera buoyancy

[npole]

Hi everyone, I need a little insight here... so I upgraded my rig that is now composed by a frame a mirror-less (with its housing), the frame (with carbon floating arms) and a couple of lights for a total weight of 5.2Kg.

I tested the buoyancy in fresh water and it's slightly positive: the camera housing floats at the 3/4 upper level of the dome, and two floating arms are like 2cm out of the water (the rest is submerged due to the lights weight).

Should I expect this to become less positive/neutral/negative after a certain deep? So I should leave it as is?

Unfortunately I cannot test the rig before a trip I'm having next week and I would like to resolve the issue (eventually changing the arms) before moving, rather than trying to find a sub-optimal solution on the place (ie: adding weights etc.).

Does anyone can give to me an idea of what I should expect? If there's no way that it could significantly change its buoyancy I'll try to find a solution before moving.

Thank you!

 

[Barmaglot]

It will be slightly more positive in salt water. Since you are using carbon float arms rather than foam, buoyancy will not change with depth - neither water nor any part of your housing will compress, your rig's mass remains constant, and since the water doesn't compress either, the depth will not have any effect on buoyancy.

If you want to get it to neutral, you will need to either switch to shorter/slimmer float arms, or add weights, such as car wheel balance weights. The latter method is more flexible, as you can add weight in smaller increments, and you can pick the location on your rig where the weights will result in the best trim.

 

[npole]

Thank you, considering the above... how much weight should I expect to add? I know that without testing it would be hard to figure... but we're in the order of 500gr. ? 1 kg ? ...I want to contain the weight but at the same time i don't want to trash two floating arms that I have paid a lot (!) to buy new ones.

 

[Barmaglot]

Try to estimate the volume, in cubic centimeters, of the parts that remain above the surface when your rig is floating free - this should get you in the ballpark for the amount of extra weight that you will need. It's pretty much impossible to generate a figure based on your description alone, without so much as a picture - you didn't specify the diameter of your dome or arms, nor which part of them remains above water - a photo would really help there.

 

[Barmaglot]

You could also take some small objects of a known weight and density - like lead shot, or steel ball bearings, or coins - tie a pouch to your housing, and keep adding those temporary weights until you achieve neutral buoyancy. Substract your weights' volume (if you're going to put your permanent weights inside the housing), add a couple percent of your rig's total mass to account for salt water and you should have the final number.

 

[npole]

Thanks guys, in the end it was 430gr. of copper (so little less if I'll use lead) difference, and even less if I manage to put inside the housing.. it is manageable after all, and I add something more to the rig they could be useful, so I didn't wasted money in floating arms.

 

[Akimbo]

Not sure how much this helps besides the method used. Sorry, the table got hosed up during one of the Scubabaord software updates. Click on the small white up-arrow on the following quote to see if there is anything useful to you.

I took some fairly accurate weight measurements in a trash can filled with salt water of different Nauticam components for the Olympus OM-D that may have some use to others.

I used a hanging 0-1 Kg electronic scale x 1 gram:

Amazon.com: American Weigh Scale Sr-1kg Gray Digital Hanging Scale, Gray, 1000g X 1 G: Health & Personal Care

My objective was to determine what size buoyancy strobe arms to purchase. I hope you find some of these measurements useful. Your mileage may vary...

 

[npole]

Tnx, unfortunately the infinite combinations of gears makes that table not much useful for me... however I think I solved the issue, in the worst case I'll just add more lead.

 

[Akimbo]

Thanks guys, in the end it was 430gr. of copper...

Be careful where you attach that copper. Aluminum corrodes scary-fast when in contact with copper in seawater. Ask any Aluminum workboat captain how nervous they get when electricians are onboard.

 

[npole]

I used copper (coins!) to get the weight... I will use lead! ...but tnx for the warning.