Neutral buoyancy and DSMB

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Mr.S

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Location
Hertfordshire, England
# of dives
200 - 499
This might sound like a pretty basic/stupid question, but one that's been puzzling me.

I've just weighed my reel and SMB, and they weigh 2Kg. It's a heavy reel and SMB with a crack bottle.

I believe I am perfectly weighted / neutrally buoyant for my dive, for example, diving in fresh water with a dry suit and 5 kg of lead, split between my BCD and weight belt.

Now here's the question.....

If the reel/SMB are 2kg, I guess that I could have been taking that into account when putting the correct amount of weight to make me perfectly weighted / neutrally buoyant.
(1) Does this mean that if I wasn't diving with the reel/SMB I would need to add an extra 2Kg to my weight belt or BCD?
(2) What happens if I deploy them - would [my body] become positively buoyant given the reel/SMB are in my hand?
(3) Or does it mean I have been overweighted for most of my dive [by 2kg] and only truly properly weighted when I deploy the SMB, since when I have done I have not felt positively buoyant?

The reason I ask is that I now plan to dive with a lighter reel / SMB (Mares all-in-one, at only 300g) so just trying to work out if/how I need to adjust my weight.

Thanks for your help.
 
1) Assuming you just put everything on some scales to come up with the 2 kg; remember that under water the reel & dsmb will have some buoyancy themselves, so the actual weight underwater will be (a lot) less. It is very unlikely that you would need to add 2 kg if you are diving without reel.

2) worst case, yes, but as said above, the actual total difference will be a lot less than 2 kg. In reality it is likely you can just compensate by adjusting your breathing a bit.

3) best tip is to do a proper weight check without all "extra's"; weigh yourself for your suit, bc, regs, cilinder, fins, boots, hood, gloves and use that as your baseline weight. All other stuff (lights, reel, camera etc) can be thrown away in an emergency, but you always need to be able to control your depth.
 
If you are referring to the the weight as weighed by a scale on land that number may not be right. It is the weight in water that matters. The reel will displace a certain volume of water. If you weight that volume of water it will weigh more than or less than the 2Kg weight of the real. That difference is your positive or negative buoyancy number. If they weigh the same its neutral. You can grab a luggage scale and attach it them lower it into a tub of water deep enough to see the weight in water.
 
If you are perfectly neutrally buoyant with your DSMB and reel then if you dive without them you would theoretically be positively buoyant by the 2kg they weigh and would need to add 2kg of ballast/lead to make yourself neutrally buoyant.

If you deploy them, and all you are left with on your person is the reel then your buoyancy will theoretically change only by the amount the DSMB with the crack bottle weighs...you would theoretically be positively buoyant.

The reality is that there are factors that affect buoyancy that are not taken into consideration by what I will call straight stick calculations. For instance the density of the water changes with temperature so while you may be perfectly neutral today, you may not be tomorrow if the temp changes. Also your lung volume has an impact on your buoyancy at while it is relatively the same day to day it is not necessarily exactly the same. Then there is the issue of being irregularly shaped and its impact on buoyancy...as you suggest holding the reel off to the side may impact the "moment of mass" of your combined body and gear. Add in the fact that buoyancy will change as you descend due to the compression of your exposure protection along with the increased weight of the water column above you and the fact that a dsmb is typically deployed from a depth below the surface....the idea that you will neutrally buoyant throughout the dive based on a calculation of mass is theoretically possible but just not practical.

Don't overthink this. Your goal is to be able to establish neutral buoyancy at your safety stop with a relatively empty tank (50bar/500psi +/-) if your DSMB and crack bottle have significant weight then calculate that in...or do a dive where you can conduct your safety stop along the bottom and stash some weight there along with having some easily removable weight so you can play around with how much you actually need to be neutral with that relatively empty tank at safety stop depth...in reality and all practicality it is all a ballpark figure/swag.

-Z
 
If you are perfectly neutrally buoyant with your DSMB and reel then if you dive without them you would theoretically be positively buoyant by the 2kg they weigh and would need to add 2kg of ballast/lead to make yourself neutrally buoyant.

If you deploy them, and all you are left with on your person is the reel then your buoyancy will theoretically change only by the amount the DSMB with the crack bottle weighs...you would theoretically be positively buoyant.

The reality is that there are factors that affect buoyancy that are not taken into consideration by what I will call straight stick calculations. For instance the density of the water changes with temperature so while you may be perfectly neutral today, you may not be tomorrow if the temp changes. Also your lung volume has an impact on your buoyancy at while it is relatively the same day to day it is not necessarily exactly the same. Then there is the issue of being irregularly shaped and its impact on buoyancy...as you suggest holding the reel off to the side may impact the "moment of mass" of your combined body and gear. Add in the fact that buoyancy will change as you descend due to the compression of your exposure protection along with the increased weight of the water column above you and the fact that a dsmb is typically deployed from a depth below the surface....the idea that you will neutrally buoyant throughout the dive based on a calculation of mass is theoretically possible but just not practical.

Don't overthink this. Your goal is to be able to establish neutral buoyancy at your safety stop with a relatively empty tank (50bar/500psi +/-) if your DSMB and crack bottle have significant weight then calculate that in...or do a dive where you can conduct your safety stop along the bottom and stash some weight there along with having some easily removable weight so you can play around with how much you actually need to be neutral with that relatively empty tank at safety stop depth...in reality and all practicality it is all a ballpark figure/swag.

-Z

I don't think this is correct.
 
I don't think this is correct.

It is assuming the 2kg quoted weight is the "in-water" weight of the DSMB w/crack bottle and reel.

Why do you think it is not correct? your in-water mass is affected by your ballast whether that is lead or your regulator, a torch, etc. Removing an item that contributes to your overall in-water mass will have an impact on your buoyancy so if you are initially neutral, removing gear that contributes to your in-water mass, you would then become positively buoyant....you would need to add back that same amount of mass to become neutrally buoyant again.

Lung volume at any given moment not withstanding.

-Z
 
It sounds like I made the basic mistake of weighing the reel/SMB on land without taking into account what happens in the water (physics stuff I think) and, therefore, the actual affect it will have on buoyancy under water?
 
It is assuming the 2kg quoted weight is the "in-water" weight of the DSMB w/crack bottle and reel.

Why do you think it is not correct? your in-water mass is affected by your ballast whether that is lead or your regulator, a torch, etc. Removing an item that contributes to your overall in-water mass will have an impact on your buoyancy so if you are initially neutral, removing gear that contributes to your in-water mass, you would then become positively buoyant....you would need to add back that same amount of mass to become neutrally buoyant again.

Lung volume at any given moment not withstanding.

-Z

Sorry you are still not quite correct. All matter has mass. If you remove something from your gear which is "floaty" your mass will go down and you will sink more.
 
It sounds like I made the basic mistake of weighing the reel/SMB on land without taking into account what happens in the water (physics stuff I think) and, therefore, the actual affect it will have on buoyancy under water?

Yup, seems to be the case. Like suggested above, if you really want to measure and calculate it, use some luggage scales and test the equipment. for all practical purposes, don't be too concerned for something that is rather big and weighs 2 kg, you will be able to compensate.
It is the little stuff that weighs a lot (like lead blocks) that really impact your buoyancy.
 
Sorry you are still not quite correct. All matter has mass. If you remove something from your gear which is "floaty" your mass will go down and you will sink more.

Um yes....but his dsmb with crack bottle and reel is negatively buoyant....so removing it reduces his mass therefore making him more positively buoyant....to regain a state of neutral buoyancy the OP would need to add ballast to make up the difference created by removing negatively buoyant gear.

-Z
 

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