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Physics of diving Part 1
In Part 1, I start with the basics and EASY questions. Later parts will use the same fundamentals to answer more complicated questions. I will also start by focusing on questions where pressure is the key component and then look at other topics.
There are four simple steps to answering most of the physics questions the come up in a Divemaster or Instructor exam that requires calculation.
Step 1. Find the unit of measurement that the question is asking about, and take the number in the question that is in that unit.
Step 2. Using your diving knowledge, think about the answer you would expect in general terms. Do not focus on the numbers yet, leave your calculator alone! Based on your conclusions, decide if you should be multiplying, which will make the number bigger, or dividing to get a smaller number.
Step 3. Find the pressure at the depth given in the question.
It’s that simple!
Before we start, there is a table you need to be able to write down on the scrap paper you are given in the exam. When the exam starts, just jot this table down before you read any of the questions. Then when you get a pressure question, refer to it.
Learn this table, its pretty easy. 0,10,20 etc 1,2,3 etc
Watch DIVE Physics VIDEO 1 to see it done!
Step 1 – Find the unit of measurement.
· Look at the answers; it is multiple-choice after all.
· Is the question calling for an answer in Minutes, PSI or BAR, Liters?
· Find the number from the question that is in that same unit and write it down
Step 2 – Think about the question.
With your existing knowledge of diving you know:
• In terms of time. The deeper you go, the faster you breathe your air. If you ascend to a shallower depth, you will consume your air less quickly.
• Regarding PSI/BAR. The deeper you go, the more air you breathe with every breath. The amount of air you consume in PSI/BAR will increase. When you ascend to a shallower depth, your air consumption will decrease; the amount of air in PSI/BAR will decrease.
• Regarding volume in liters. If you take a balloon down, it gets smaller. If you take it up, it gets bigger
Using this knowledge, you should be able to look at the question and without concerning yourself with the numbers, decide:
• Will the correct answer be more minutes or fewer minutes? If the correct answer is more minutes you multiply, if you know the answer should be less or fewer minutes, then divide.
• Will it be more PSI/BAR (multiply) or less PSI/BAR (divide)?
• Will it be a bigger balloon – more liters (multiply), or a smaller balloon - fewer liters (divide)?
This is by FAR the most important step. It seems like the most obvious, but the greatest number of answers I mark as incorrect are because people haven’t taken this simple step.
Step 3 – Finding the pressure.
• Scan the question and find the DEPTH in meters.
• Use the table to find the pressure at that depth.
• Write the pressure down.
A few practice questions should make this much clearer:
Air Consumption question in minutes
1. It takes 55 minutes to breathe through a tank of air at the surface. If all other factors remain unchanged, how long will it take to breathe through the same tank at 40m?
a. 275 mins
b. 55 mins
c. 13 mins
d. 11 mins
• Step 1 - The question is asking how many minutes – so we look for minutes and find 55 minutes stated in the question. Therefore, 55 is the first number we write down.
• Step 2 - Think about it. Will you breathe through you air faster at 40m than the surface, or will your air last you longer? As an experienced diver, you know you will breathe through your air WAY faster at 40m than you would at the surface. So you know your answer should be less than 55 minutes. You can put the division symbol ÷ under step 2.
• Step 3 – The depth is 40m, so the pressure we write down is 5.
• Step 4 – We should now have 55 ÷ 5 written on our paper. 55÷5=11minutes. The answer is d)
Sealed flexible container (balloon) Questions
2. If we have a balloon with a volume of 8 liters at the surface what would its volume be at 30m?
a. 32 ltrs
b. 24 ltrs
c. 2 ltrs
d. 2.6 ltrs
• Step 1 – The question is asking about liters, so we look for the number of liters provided in the question. We get the number 8.
• Step 2 - Think about it. Will the balloon get bigger as we take it deeper, or will it get smaller? Your diving knowledge tells you, as you take an air filled flexible object such as a balloon to depth, the air inside becomes denser and the object gets smaller. So we are looking for an answer smaller than 8. We will write down ÷
• Step 3 – The depth is 30m, so we write down a pressure of 4.
• Step 4 – We now have 8 ÷ 4 written on our paper. 8÷4=2 Liters. The answer is c)
Air consumption in BAR or PSI
3. A diver breathes through 50 BAR of air in 40 minutes at the surface. If all other factors remain unchanged, how many BAR would the diver breathe in 40 minutes at 20m?
a. 25 Bar
b. 16.6 Bar
c. 100 Bar
d. 150 Bar
• Step 1 - The question is asking how many Bar – We look for Bar in the question and find 50. The first number we write down is 50.
• Step 2 - Think about it. Will you breathe through more air at 20m than at the surface, or less air? The deeper you go, the more air molecules you breathe with every breath. Meaning you will breathe MORE air in 40 mins at 20m than you would at the Surface. We want a number that is more than 50 Bar. We write x
• Step 3 – The depth is 20m, so the pressure we write down is 3.
• Step 4 – So we have 50 x 3 =150 Bar. The answer is D)
Pumping air from the surface
4. How much air must be pumped from the surface to fill a container with a volume of 100 Ltrs. The container is at a depth of 40m in salt water.
a. 100 Ltrs
b. 400 Ltrs
c. 500 Ltrs
d. 20 Ltrs
• Step 1 - The question is asking how many Ltrs – We look for Bar in the question and find 100. The first number we write down is 100.
• Step 2 - Think about it. The air is going down. Just like a balloon, it will get denser and the volume of the air will decrease. So we need to pump more than 100 Ltrs at surface pressure for it to have a volume of 100 Ltrs at 40m. We want anumber bigger than 100. We want to multiply.
• Step 3 – The depth is 40m, so the pressure we write down is 5.
• Step 4 – So we have 100 x 5. 100x5=500 Ltrs. The answer is C).
There are a few reasons this seemed so simple.
· Hope fully one of them, is the simple and methodical approach I have taught you!
· However, all the examples used above had nice round number for the depths 10m, 20m etc.
· Also all the questions were comparing a single depth with the surface.
In part 2; I will look at depths that are not round numbers 23m, 37m and so on. I will also introduce questions that ask about changes when you go from one depth to another depth.