A great way to come prepared to an IDC is know your dive Theory. The Course Director in the IDC is not there to teach you Physics or any other subject for that matter. Of course we offer a refresh day and you are more than welcome to join to see if you are up to date.
Making your IDC less stressful is really to know your dive theory before you enter the class. A good way is reading the encyclopedia and study your knowledge workbook, but PADI has a better tool to prepare you for the dive theory, dive theory on line, a PADI eLearning product.
Video support will help you understand Physics, Physiology, Equipment, Skills & Environment and the RDP so much easier.
One common problem is weight of water.
Salt water weighs 1.03 kg and Fresh water weighs 1.0 kg per liter.
In order to bring an sunken object to the surface we need to calculate how much extra water we have to displace with a lifting device.
First we need to know the weight of the object and the weight of the water it displaces. (don’t get thrown off when they mention a depth, this is irrelevant)
If an object is in fresh water it makes life easy as fresh water weighs 1 kg/ltr
Example fresh water:
Weight object: 600 kg This represents a downward gravitational force.
Water displacement 300 Liter; each liter of displacement represent 1 kg of upward buoyant force.
So we need to displace 300 Kg of water to make this object neutral buoyant. (300 x 1 kg/ltr = 300 kg)
So we have an upward buoyant force of 300 kg counteracting the downward gravitational force of 600 kg.
600 – 300 = 300 kg. We must add 300 kg of buoyancy to the object to make it neutral buoyant, so displace 300 liters of water. (Since fresh water weighs 1 kg per liter)
Now in salt water we need our calculator
Example salt water:
Using the same object, but now each liter of displacement represents 1.03 kg
To convert kg to liters we need to multiply the 300 Liters with 1.03 = 309 kg this is the total upward buoyant force
Now we can subtract: 600 kg – 309 kg = 291 kg So we know we have to add 291 kg of buoyant force to the object, if we use a lift bag each liter of air added to it will provide an additional 1.03 kg of buoyancy.
Therefore, we have to convert back to liters 291 divided by 1.03 = 282.52 Liters, which is needed to make the object neutral buoyant.
Hope this helps some of the divemasters or IDC candidates, check more blogs for more solutions.