How much does my camera rig weigh underwater?
Tired of having a negative rig that is hard to control underwater? There are many good reasons for having a neutral/slightly negative rig. If like myself you do not have an endless supply of money to keep buying equipment to try and get the right balance or little access to a pool you need to find out your starting point.
By understanding the exact weight of your camera rig underwater you can then work out what best to do next.
Measuring the weight of your camera rig in fresh water
Bath deep enough to submerge rig (If the rig is too large you will need to do in segments)
Board/Plank to span the bath
Step 1: Place the Board/Plank across the bath and place the scales on top
Step 2: Carefully place your full camera rig under the board
Step 3: Place your Wooden Spoon/Stick on the scales and zero the scales
Step 4: Tie one piece of string to each side of your rig and tie the other to the Wooden Spoon/Stick. Place the stick on the scales. The string at this point does not need to be tight
Step 5: Keep looping the string around the Wooden Spoon/Stick until the camera is lifted from the bottom of the bath evenly. Make sure that all of the rig will be under the maximum fill line of the bath. Your scales will now measure the actual weight of your rig, record this down for later to convert to salt water buoyancy.
Step 6: Start to fill the bath and watch the scales start dropping in weight as it becomes submerged. Check that the rig is fully submerged and not touching the bottom at all. Make sure you move the camera around under the water to clear any air pockets. This scales will now be reading the underwater weight of your camera rig in fresh water (Not 100% as tap water is slightly denser than fresh water but good enough!)
Converting your camera rig weight to salt water buoyancy
You could just pour lots of salt into the bath from above and repeat but that will cost a lot. Alternatively to convert the underwater weight to salt water buoyancy we need to ultimately know the weight of your rig and its mass. We know the weight of your rig from Step 4 above so all we need to work out is the mass.
As we know the weight of the camera rig and it’s buoyancy the difference is the total displacement. i.e. for every gram, the rig displaces it represents a cubic centimeter of water which weighs 1g.
For example, a rig that weighs 1500g on land and 500g underwater displaced 1000g or 1000ml of water.
We also know that the 100ml of fresh water weighs 100g yet 100mls of salt water weighs approximately 102-103g, although this can be different for example the Red Sea is even higher at approximately 104g.
If we take the normal measurement of 102.5g per 100ml it will take an additional 2.5g of weight to reach neutral buoyancy per 100ml. So for the example above where displacement was 1000g the rig would in effect be 25g lighter underwater or would require an additional 25g of weight to achieve the same buoyancy as fresh water.
Weight of Rig on Land (g) – Weight of rig in fresh water (g) = Displacement (g)
Displacement (g) * 0.025 = Salt Water variation (g)
Weight of rig in fresh water (g) – Salt Water variation (g) = Weight of rig in salt water (g)
Although this is not much at all in the big scheme of things it is something to consider if you are trying to get your camera as close to neutral as possible.
Note: There are a number of assumptions here to achieve these results such as the temperature of the water, density of tap water etc… However, it’s close enough!