Buoyancy estimates for possibly common setups

[Yorkie88]

Hi everyone,

Just wondering if there's any resources for relatively common gear so I can get some kind of starting point for floats etc? Not sure if it's an exact science or individual to preferences and equipment.

 

I've got the Nauticam A7RV housing, and will have 2 backscatter MF-2's, 4 cheap 6in arms (might swap them for floaty arms), and the 90mm macro port (think it's called the 105).

 

I'm hoping someone out there might have a similar setup and might be able to give a pointer as to roughly how much buoyancy may be needed?

 

Or does it vary enough that you just have to do trial and error, or weigh it in a big bucket?

 

Thanks!

[Barmaglot]

Take a luggage scale, hook your assembled rig up to it, and submerge it in water (the rig, not the scale). The resulting figure will be the amount of buoyancy that you need to add. In saltwater it will be a little bit more buoyant than in fresh water, but the difference is negligible unless you take it somewhere like the Dead Sea.

[TimG]

As Barmaglot says: luggage scale. Works a treat.

[Phil Rudin]

I use the Sony A7R V with a verity of lenses, ports, port extensions, strobes and more. Rather than doing all Math I just attach one or two of the Marelux 1500ml Flexibouys to my housing and make adjustments in the amount of buoyancy on the fly. They can be moved for landscape and portrait, added around extensions behind heavy lenses like wet lenses and have a verity of other uses. They are $40.00US each, light weight, pack flat and take up very little space. Just remember that they will tend to hold water so need to be drained when you exit the water.

 

 

[Dave_Hicks]

You are supposed to dive with these balloons on your camera? It seems a bit like a cludge. It doesn't get in your way, entangled, or create drag?

[Chris Ross]

Just weigh it, the maths is not hard, the UW weight is equal to the required buoyancy.  Salt vs fresh doesn't really matter that much, it's a 3% difference, just make sure you are slightly negative. 

 

A great many accessories have their UW weight listed, for example  the MF-2  is 150 grams UW.    INON has a page with the UW weight of all their gear.  here :  http://www.inon.jp/underwater_weight/index.html

 

Housings generally don't have it listed as it varies with the attached port.  Once you have the weight of your rig work out a float combination that has a buoyancy slightly less than the the measured UW weight.

 

For example if your rig weighs 800 grams UW then you could add in two INON mega float arm S with 390 grams  buoyancy and your rig will then be 800 - 390x2 = 20 gram negative UW. 

 

Just be sure to err on the side of slightly negative, you don't want a positive housing.  Also it may be obvious - changing ports will change buoyancy a dome port is more buoyant , you can work out a float combo for each port/lens or just set it up to be OK with the dome and live with the macro setup being a bit more negative.

[Yorkie88]

Thanks everyone, I'll get the luggage scale out and go from there.

 

Those adjustable buoys look like they'd be great for someone who changes the rig often.

 

I was briefly looking at the adjustable arms from weefine but wasn't sure about them - they're relatively expensive and I'm assuming any kind of small leak would make them pretty useless for buoyancy. I might get some stix or similar floats that just attach to the arms, that way it can be adjusted if I change ports down the line, and if I'm settled with a specific setup later I could swap to the floating arms or something.

 

Thanks again!

[TimG]

Over the years I’ve found that a change of lens/port doesn’t usually make such a dramatic change of buoyancy that it’s worth endless shifting of floats, adjusting of buoyancy arms etc.

 

If you get reasonably neutrally buoyancy for a 60mm macro setup, I’ve found that to be about a reasonable average. Changing port and lens from that will increase or decrease buoyancy slightly. But rarely enough to be troublesome. 

[Chris Ross]

9 minutes ago, TimG said:

Over the years I’ve found that a change of lens/port doesn’t usually make such a dramatic change of buoyancy that it’s worth endless shifting of floats, adjusting of buoyancy arms etc.

 

If you get reasonably neutrally buoyancy for a 60mm macro setup, I’ve found that to be about a reasonable average. Changing port and lens from that will increase or decrease buoyancy slightly. But rarely enough to be troublesome. 

It depends, if you dial in really close on buoyancy then changing to a medium size dome can end up positive.  I know I've had it happen.  Two INON mega float arms S are just perfect when using a 60mm macro port.  I changed to a Zen 170mm dome and it was positive.   My little 100mm Zen port is fine with the bigger float arms.

[Barmaglot]

4 hours ago, Yorkie88 said:

I was briefly looking at the adjustable arms from weefine but wasn't sure about them - they're relatively expensive and I'm assuming any kind of small leak would make them pretty useless for buoyancy.

I got a pair of those about four years ago, and they've been trouble-free thus far. They're built quite solidly, and the package includes spare o-rings. On the downside, they're quite heavy (approximately 450g each) which is felt when packing for a trip, and in the time that's passed, the price increase considerably - I got mine for $99.80 apiece.

[TimG]

3 hours ago, Chris Ross said:

It depends, if you dial in really close on buoyancy then changing to a medium size dome can end up positive.  I know I've had it happen.  Two INON mega float arms S are just perfect when using a 60mm macro port.  I changed to a Zen 170mm dome and it was positive.   My little 100mm Zen port is fine with the bigger float arms.

 

Fair enough, Chris. I'm sure all system are not alike. It could be, too, that I'm just too lazy to make the changes.... 

[humu9679]

On 3/8/2025 at 7:31 AM, Yorkie88 said:

Hi everyone,

Just wondering if there's any resources for relatively common gear so I can get some kind of starting point for floats etc? Not sure if it's an exact science or individual to preferences and equipment.

 

 

As others have noted, a luggage scale and a large bucket is de rigueur. If you're interested in Stix floats on ULCS type arms, the "jumbo" floats are 181g, and the "large" floats are 96g each. They look cheap in photos, but they're durable and infinitely adjustable. And you might need buoyancy for the macro port if you add a flip diopter holder.

[Ar Splujer]

16 hours ago, Chris Ross said:

Just weigh it, the maths is not hard, the UW weight is equal to the required buoyancy. 

Sorry for the stupide question but... what is the technique used to weight your underwater setup underwater ? Thanks.

[ChipBPhoto]

On 3/8/2025 at 10:06 AM, Barmaglot said:

Take a luggage scale, hook your assembled rig up to it, and submerge it in water (the rig, not the scale). The resulting figure will be the amount of buoyancy that you need to add. In saltwater it will be a little bit more buoyant than in fresh water, but the difference is negligible unless you take it somewhere like the Dead Sea.

@Ar Splujer - Barmaglot explains it above.  Pls read the rest of the comments as there is other important info shared.