Super stupid question - Vacuum in camera case and buoyancy

[Pyndle]

Hi,
As you can probably tell, I don't have a phd in physics

I use a seafrogs case with my sony A6500 and an 8inch dry dome port. There's a lot of air inside and I don't use a vacuum pump. Which means it needs a lot of extra weight. With my Z330s and all the arms and clamps (that are all negatively buoyant), I still need to add over 1kg of weight on the dome for it to be neutral in salt water.

I see a lot of divers with domes and floating arms, which suggests they have the opposite problem (their setup sinks) and I wonder how that is possible.

Is it because other brands are much heavier (metal cases) and despite the air inside they still float?
Or does removing the air inside with a vacuum pump has an impact (significant) on buoyancy?

Thanks

 

[scubadada]

Your camera floats because it weighs less than the water it displaces, positively buoyant.

Vacuum has nothing to do with it. At an extreme, it would remove weight, like an empty tank.

 

[PatW]

My rig with a dome is slightly buoyant and negative with a macro port. The dome has a much larger “air” volume, I routinely put a vacuum seal on my rig. Gas does not weigh much and the difference between a rig with a vacuum lock and with just normal air at standard temperature and pressure would be negligible.

 

[Pyndle]

Don't know why I didn't think of the analogy with a tank, that makes total sense, thanks!

I guess those people I've seen have very heavy cases. Mine is also super negative with a macro port. But with a dome, no way!

 

[dlofting]

At least you don't have to add flotation to your arms.

 

[Barmaglot]

I have the same setup as you (A6300, 10-18mm, SeaFrogs, 8-inch dome) and my rig goes negative because I have heavy strobes - Retra Pro with superchargers, so I use two float arms on the strobes, plus one more as a crossbar on top of the housing. This causes issues with trim, as the dome is still very floaty, so I have about a pound of auto wheel weights glued to the dome shade.
As already pointed out, vacuum has very little to do with it - it removes a few grams of air from the housing, that is all. I do, however, highly recommend getting a Vivid Leak Sentinel (not VPS-100 - it leaks underwater); it adds a LOT of safety. Over the five years I had my camera, it has saved me from a flood several times.

 

[Pyndle]

Hey Barmaglot

I actually have auto weight on my dome too thanks to your recommendation on another thread. I see, your strobes are exceptionally heavy (300g heavier than mine, per strobe)! But adding more weight to the dome and use float arms is not a bad idea, I do struggle with this dome the camera wants to flip all the time.

For the Vivid leak sentinel, I'm not too keen on getting one, they cost almost the price of my camera second hand, not really worth it. Might regret it who knows... I had 1 flood in 6 years, I forgot that I unscrewed the valve a little bit and went underwater with it. But I think if my camera floods again I will stop underwater photography, so I'll save money. Win win haha.

Btw you're not meant to go underwater with the VPS-100, I think (if I remember correctly) you used to go underwater with it anyways and it was fine - got a bad experience since then? Or am I misremembering?

 

[Barmaglot]

For the Vivid leak sentinel, I'm not too keen on getting one, they cost almost the price of my camera second hand, not really worth it.

It's not just the camera - although even there, the price difference is considerable; Leak Sentinel costs 230 euros (plus another 25 for pump), whereas a cursory search of ebay shows A6500 bodies going for around $500. In the event of a flood, you also lose the lens, and while 10-18mm f/4 has come down in price over the years (I paid $557 for mine, second hand, five years ago), it's still around $400, so you're pushing closer to a thousand, more than that if you're shooting macro with Zeiss 50mm or Sony 90mm. I also have a ~$500 UWTechnics trigger in my housing, but I don't imagine many people use one with a SeaFrogs. The bigger issue, however, is opportunity cost - imagine you're on a $5000 liveaboard trip to Raja Ampat or Solomons, or whatever, and your housing floods on the second day. That's a major oops if there can ever be one, and a vacuum system almost completely eliminates the chances thereof.

Btw you're not meant to go underwater with the VPS-100, I think (if I remember correctly) you used to go underwater with it anyways and it was fine - got a bad experience since then? Or am I misremembering?

You're not - I went through three of them over four years or so; after 70-80 dives their seals fail and water starts seeping in. Leak Sentinel, while it has its own quirks (it is VERY sensitive to pressure changes, and a slight compression exerted on the housing by water pressure is enough to trip its sensor, so I generally turn it off before splashing down - this keeps the vacuum, because unlike VPS-100, the on/off switch and vacuum release functions are separated) it has no issues with exposure to saltwater.

 

[Pyndle]

You make a good sales pitch haha. Lenses can be fine after a flood, when I had my leak it's not like the whole housing gets completely filled up with water, I ended my dive, even opened the case at the surface while swimming to empty the water in it, and my 16-50 survived. But yeah I get your point, it's a good insurance. The only thing is I feel like you can still have leakages with this (orings can snap), especially if it's off you have no alert if something goes wrong? (unless you kept the original water detector?)


Also, why do you use a UWTechnics trigger? I followed your IG you seem to be shooting at 1/160th, isn't the point of this being able to shoot at faster speeds?

 

[Barmaglot]

Lenses can be fine after a flood, when I had my leak it's not like the whole housing gets completely filled up with water, I ended my dive, even opened the case at the surface while swimming to empty the water in it, and my 16-50 survived

It can go either way. You were lucky that your leak was slow, but catastrophic floods where the housing fills up with water within seconds also happen.

The only thing is I feel like you can still have leakages with this (orings can snap), especially if it's off you have no alert if something goes wrong? (unless you kept the original water detector?)

The UWT trigger that I use slots in place of the original leak detector and has a leak detection function - same conductive strip along the bottom of the housing, and red LED plus buzzer if moisture gets on it.

Also, why do you use a UWTechnics trigger? I followed your IG you seem to be shooting at 1/160th, isn't the point of this being able to shoot at faster speeds?

I shoot HSS occasionally, but it's a situational tool. Most of the time, especially on wide angle, I'm considerably slower than 1/160. For macro, 1/160 is fine. The only situation where it gets limiting, really, is shooting into the sun.