PADI IDC Theory - Physics
Test yourself with these questions
PADI IDC Theory Physics
Ok, let’s take things one step at a time.
Here we’ll talk about THREE major topics that come up time and time again on PADI Physics exams.
In each of the areas, I’ll give a few tips, and show you a video.
Then you’ll get a chance to answer some questions.
Each of the videos are from my Online Preparation Course.
You can watch the video first, or you can steam straight into the questions.
Good luck!
1. How to calculate pressures underwater
Every 10 metres or 33 feet of seawater, Gauge pressure changes by 1 atm.
Total, or Absolute pressure includes both Gauge pressure and Atmospheric pressure.
E.G. Pressure at 10 metres or 33 feet = 1 amt of Gauge pressure and 1 atm for the atmospheric pressure. 1+1=2 atm.
Why this is important
You need to be able to calculate pressures in seawater to be able to answer all those quesions like:
“what size will a balloon become if you take it down to 30 metres/99 feet?
Test Questions - PADI IDC Physics - pressures underwater
What is the gauge pressure at 34 metres / 112 feet of fresh water?
3.3 atm / 48.4 psi
Is the gauge pressure for a given depth in salt water be the same/greater/less than the same depth of fresh water?
Greater than
What is the absolute pressure at 20 metres / 66 feet of fresh water?
2.94 bar / 42.6 psia
What is the absolute pressure at 34 metres / 112 feet of fresh water?
4.3 ata / 62.4 psia
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Extra information
Metric
For every one metre in sea water, pressure changes by 0.1 atm. Example: at 23 metres pressure is: 23 X 0.1 = 2.3 atm for gauge, +1 = 3.3 for absolute.
Imperial
For every foot of sea water, pressure changes by 0.0303 atm. Example at 76 feet pressure is 76 X 0.0303 = 2.3 atm for gauge + 1 = 3.3 for absolute
Fresh Water
In fresh water, it’s much the same, but for metric use 0.097 for every metre, and in imperial use 0.0294 for every foot.Effe
2. How to calculate - effects of pressure
As you get deeper, some things get smaller (e.g volumes) and some things get larger (e.g.density, breathing rates, gas percentages)
Use your practical diving experience to work out whether the answer will be larger or smaller. Then either multiply or divide the number by the absolute pressure.
Why this is important
Usually PADI Physics exam questions have more questions about the effect of pressure than any other topic.
All questions related to depth can be answered using the methods in my video:
- Volume of balloons
- Breathing rates
- Partial pressures of gasses
- How much air needs to pumped from the surface
Test Questions - PADI IDC Physics - effect of pressure
Imagine that the air in your cylinder was accidentally mixed with 0.5% carbon monoxide. If you took this to 20 metres / 66 feet, what would the surface equivalent percentage be?
1.5%
The actual percentage would still be 0.5%
But the effect would be equivalent to surface percentage of 1.5%
Approximately how much air must be pumped down from the surface to fill a 40 litre/1.4 cubic feet container if the container lies in 20 metres/66 feet of sea water?
120 litres / 4.2 cubic feet
Approximately how much air must be pumped down from the surface to fill a 50 litre container if the container lies in 40 metres/132 feet of sea water?
250 litres
If it takes a diver 90 minutes to breathe all of the air from a cylinder at the surface, appoximately how long will the air in that cylinder last at 20 metres / 66 feet of sea water if all of the other conditions remain the same?
30 minutes
A diver is using a cylinder filled with air (21% Oxygen, 79% Nitrogen) With respect to the oxygen, at approximately what depth would breathing this mix have the same effect as breathing pure oxygen at the surface?
40 metres/130 feet
During a wall dive in the ocean, you release a balloon at a depth of 24 metres/80 feet. The balloon contains one litre of air. What is the balloon’s volume at the surface?
3.4 litres
3. How to calculate PADI displacement questions
Metric
In fresh water, it’s often as simple as taking one number (litres of displaced water) from another (weight in kilos of the object).
In salt water, first divide the weight (kilos) by 1.03, and then go ahead as with fresh.
Imperial
Multiply the cubic feet of displaced water by 62.4 in fresh water, or 64 in sea water, then deduct that number from the weight (pounds) of the object. Divide that number by either 62.4 or 64 depending on freshwater or seawater to get the answer in cubic feet.
Why this is important
Back in ancient Greek times, Archimedes said: “a body immersed in a fluid experiences an upthrust equal to the weight of the fluid displaced.”
But he didn’t need to take PADI IDC Exams on diving physics!
The good news is that it’s SO much easier than some people think! Take a look at the video before you attempt the questions.
Test Questions - PADI IDC Physics - displacement
Your dive boat bumps into a large box floating just under the surface of a freshwater lake. After bringing it aboard, you determine the weight to be 75 kilograms/165 pounds. How much water does it displace?
75 litres / 2.64 cubic feet
A 200 kilogram / 440 pound anchor that displaces 127 litres / 4.5 cubic feet of water lies on the sea floor in 17 metres/56 feet. What is the minimum amount of water that must be displaced from a lifting device to bring the anchor to the surface?
67 litres / 2.38 cubic feet
If a diver weighing 75 kilograms / 165 pounds in neutrally buoyant in fresh water, the same diver with the same equipment would _____ in salt water.
Float
What is the minimum amount of water that must be displaced from a lifting device to bring a 453.5 kilogram / 1000 pound anchor to the surface from 40 metres/ 132 feet of salt water in the anchor displaces 254.8 litres/nine cubic feet?
185.5 litres/6.6 cubic feet
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PADI Dive Theory - Physics
I’ve met loads of people who tell me that they’re hopeless when it comes to Physics.
I’m hoping that these videos and test IDC questions have helped.