Help with camera buoyancy

[STITCH]

I'm in the process of building an underwater housing for my camcorder and I need help trying to get it neutrally buoyant. I have built it and I had to put 15lbs of lead in it to get it to sink so I could test it for leaks by tying it to a line and sinking it at 100 ish feet for 15 min. It passed the leak test and the cold test it rode in the fish box full of ice for 10 hours LOL (I wasn't planning on performing that test but hey it made it). Originally I had 10lbs of dive weights in it and it sat about 1inch out of the water so I grabbed 5lbs of specialized lead (fishing sinkers) and it sank nicely. This was done with out the camera inside. Now that I know it wont leak I need help finding the perfect weigh of ballast I need so I'm not fighting it from either floating away or dragging it around. I can figure out how much air it displaces by measuring the amount of water it holds then placing the camera in side (vacuumed sealed) and measure the difference. I just don't know how to find out how much "lift" it has so I can counter it. Please help

 

[bobf]

what are the exterior measurements?

 

[bobf]

Considering you've gone this far (I'm impressed!!!!!) you may prefer to calculate the displacement figure yourself.........if so......

First determine how much water the housing will displace. Using exterior measurements, height times width times length will give you the volume of the housing in cubic inches ( or centimeters if you choose.......).

Once you calculate the housing's volume, multiply it by the weight of water. If you plan on diving in salt water, use that figure. Salt water weighs 64 pounds per cubic foot or .037 pounds per cubic inch (fresh water weighs slightly less, @ 62 # per cu. ft).

cu foot equal 12"*12"*12" = 1728 cubic inches
64#/1728cu. in.=0.037#/cu. in.

Say your housing has measurements of:

height=8"
width=6"
depth=12"

8*6*12=576 cubic inches

576cu in times 0.037#/cu in = 21.31 pounds

Therefore a box that measures 8*6*12 will displace 576 cubic inches of salt water. The calculations above also point out that 576 cubic inches of salt water weighs about 21.3 pounds. Therefore to make the entire box neutrally buoyant, you will have to add enough additional weight to the interior of the housing so that the weight of the housing, camcorder, and the additional lead equal 21.3 pounds.

For instance, say the housing weighs 9 pounds dry and the camcorder weighs 2 pounds........then you would have to add about 10 more pounds to the interior of the box to make the rig neutral. Of course it would be most acceptable to add the additional weight to the exterior, but if you do, remember:

The mass you add to offset the buoyancy will also displace water. Therefore you will have to also take that volume into consideration (iow, a piece of lead that weighs 5 pounds on land weighs less in the water because the lead itself is displacing water.)

This is all based on Archimedes Principle.......that buoyancy is created by displacing water and that the weight of the water displaced will create an equal amount of lift.

If for instance an object displaces water that weighs 2 pounds, then 2 pounds of lift is created. And if that object weighs less than 2 pounds, it will float. And if the object weighs more than 2 pounds, it will sink. And finally, if the object weighs exactly 2 pounds, it will be neutrally buoyant.

Also take into consideration that any external lighting equipment used might be negatively buoyant and that can help offset the buoyancy created by the housing.

good luck with your project!!!

b

 

[STITCH]

bobf,
My housing is an irregular cylinder so it may be more of a pain in the arse to figure this out on paper. After I wrote this I thought "your best friend has a pool....add weight till it just starts to sink and it should be good enough for the ocean" Your post was great and you definitely made me think about how I need to add the weight to the inside due to the buoyancy of the ballast. Help me out with that one, lead is negatively buoyant right? I mean without getting into the whole metal ships float deal, I should be ok putting my ballast on the outside right?

 

[fppf]

For a good first guess, fill a bucket full of saltwater to the rim.
Put the bucket in a pan to catch water.
Force housing under the water in the bucket.
Weight the water that the camera overflowed out of the bucket into the pan.
Weight the housing with the camera in it.
Subtract the weight of the camera from the weight of the water, make up that amount with lead.

 

[pete340]

For a good first guess, fill a bucket full of saltwater to the rim.
Put the bucket in a pan to catch water.
Force housing under the water in the bucket.
Weight the water that the camera overflowed out of the bucket into the pan.
Weight the housing with the camera in it.
Subtract the weight of the camera from the weight of the water, make up that amount with lead.

Eureka!

 

[bobf]

For a good first guess, fill a bucket full of saltwater to the rim.
Put the bucket in a pan to catch water.
Force housing under the water in the bucket.
Weight the water that the camera overflowed out of the bucket into the pan.
Weight the housing with the camera in it.
Subtract the weight of the camera from the weight of the water, make up that amount with lead.

Simple yet effective..............well done! ..........or to now quote both Pete and Archimedes......Eureka!

btw, if instead in the example above you wish to use fresh water for convenience, consider this:

Salt water = 64 pounds per cubic foot
Fresh water = 62 pounds per cubic foot
64/62= 1.03

Therefore if you were to multiply the weight of the excess fresh water( the water expelled when immersing the housing and camera into the interior bucket) by 1.03, you could approximate the weight if you had actually used salt water.

For instance, let's say that 15 pounds of fresh water escaped the bucket when immersing your rig. Multiply 15 times 1.03 to determine the salt water weight of an equivalent volume of water.

.........lead is negatively buoyant right? I mean without getting into the whole metal ships float deal, I should be ok putting my ballast on the outside right?

Yes, of course. The specific gravity of lead is greater than water.

regards,
b

 

[STITCH]

I can’t say thanks enough guys! I will be taking this setup on its maiden voyage on the weekend of the 11th of next month to the Keys (Largo). Were diving the North Dry Rock reef and the Spiegel Grove with the possibility of a night dive mixed in. I will be putting the vids on you-tube and I will definitely post the links on here. Thanks again.