PADI Instructor Course, Divemaster Practice - Physics 2

Questions moving between two depths

Now we will consider questions that ask about 2 different depths. Until now everything has been compared to the surface. When we are asked a question that involves moving from one depth to a second depth. We just need to do the three step process twice, with a little twist.

There is another way to do it, which I call the first depth/second depth method. I will show this alternative method later in the series, but for now it is probably best to stick with the three step method you are already becoming familiar with.

I am using the British spelling this time!

Let’s look at the steps.

Step 1a – 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 2a – Think about the question.

·        IGNORE THE SECOND DEPTH RIGHT NOW. In this step you are going UP to the surface. That is a BIG difference to how you used the three step process before.

·        What is the question asking about. Do you expect the answer to be less or more given you are GOING UP?

Step 3a – Finding the pressure.

•    Scan the question and find the FIRST or more importantly and correctly STARTING depth (in meters.)

•    Use the table to find the pressure at that depth.

•    Write the pressure down.

Step 1b – Take your answer from the steps above.

Step 2b – Think about the question again.

·        Remember in the first three steps you calculated what would happen at the surface. So now you are going DOWN from the surface.

·        What is the question about? Are you expecting an increase or decrease compared to the SURFACE?

·        Choose multiply or divide.

Step3b - Finding the pressure.

•    Scan the question and find the SECOND or more importantly and correctly the ENDING depth (in meters.)

•    Use the table to find the pressure at that depth.

•    Write the pressure down.

 

Example Questions

BaLLOON / Sealed Flexible Container question

1.      A balloon with a volume of 9 Litres is taken from a depth of 20 Metres to a depth of 10 Metres. What will the volume of the balloon be at this new depth if it is in salt water?

a.      The same

b.      13.5 Litres

c.      6 Litres

d.      18 Litres

Step 1a What is the unit of measurement? Look at the answers. It’s Litres. Take that unit from the question. 9 Litres. Write down the number 9

Step 2a Multiply or Divide? Use you scuba knowledge. Will the balloon be bigger when it reaches the SURFACE or smaller. A balloon going UP from 20 Metres to the surface will get BIGGER. We need to  multiply.

Step 3a What is the pressure at the FIRST depth. The pressure at 20 Metres is 3 ATA. So write down the number 3.

9x3=27 - We have now calculated the size of the balloon at the surface.

Step 1b Take your answer from the three steps above. What is it? It’s 27.

Step 2b Multiply or Divide? We are taking the balloon DOWN from the surface. Will it get bigger; or will it get smaller? Its gets smaller as it goes down to depth, so we divide.

Steb 3b What is the pressure at 10 Metres? From your depth pressure chart it is 2. Write down the number 2.

What have we got:

9x3=27

27÷2=13.5 Answer B

Air consumption in minnutes

2.      If a diver takes 45 minutes to breathe through a tank of air at 5 Metres, how long will take to consume the same tank of air at 35 Metres?

a.      40.5 mins

b.      19 mins

c.      15 mins

d.      135 Mins

Step 1a What is the unit of measurement?  Look at the answers. It’s Minutes. Take that unit from the question. 45 Minutes. Write down the number  45.

Step 2a Multiply or Divide? Use you scuba knowledge. Will your air last you longer AT THE SURFACE than at 5 metres, or will you breathe through it faster? Your air will last longer at the surface than at depth. So we need to make 45 Minutes BIGGER. We multiply.

Step 3a What is the pressure at the FIRST depth. The pressure at 5 Metres is 1.5 ATA. So write down the number 1.5.

Now we have 45x1.5= 67.5 - We have now calculated how long their air will last at the surface

Step 1b Take your answer from the three steps above. What is it? It’s 67.5

Step 2b Multiply or Divide? We are thinking about how much air a diver would breathe at the second depth of 35 Metres, compared to the SURFACE. Will the air last longer at depth or will you breathe through the air faster? You will consume the air faster. So we need to make 67.5 minutes smaller.We divide.

Steb 3b What is the pressure at 35 Metres? From your depth pressure chart it is 2. Write down the number 4.5.

What have we got:

45x1.5=67.5

67.5÷4.5=15 Answer C

Air consumption in psi

3.      A diver consumes 10 PSI of air, every minute at 50 Metres, how many PSI of air will they consume per minute at a depth of 20 Metres? They are in salt water.

a.      20 PSI

b.      5 PSI

c.      30 PSI

d.      4 PSI

Step 1a What is the unit of measurement?  Look at the answers. It’s PSI. Take that unit from the question. 10 PSI. Write down the number  10.

Step 2a Multiply or Divide? Use you scuba knowledge. Will your air last you longer AT THE SURFACE than at 50 Metres, or will you breathe through it faster? Your air will last longer at the surface than at depth. But this isn’t a question about time, it is asking about the quantity of air we will breathe. So having decided the air will last us longer that is because we are breathing less air molecules with every breath. We will breathe a lower PSI per minute at the surface than at depth. We need to make 10 SMALLER, we divide.

Step 3a What is the pressure at the FIRST depth. The pressure at 50 Metres is 6 ATA. So write down the number 6.

Now we have 10÷6= 1.6666 - We have now calculated how many PSI the diver will breathe through per minute at the surface.

Step 1b Take your answer from the three steps above. What is it? It’s 1.666666 PSI

Step 2b Multiply or Divide? We are thinking about how much air a diver would breathe at the second depth of 20 Metres, compared to the SURFACE. Will the air last longer at depth or will you breathe through the air faster? You will consume the air faster. You will breathe MORE PSI per minute at 20m than at the surface. So we need to make 1.6666 bigger. We multiply.

Steb 3b What is the pressure at 20 Metres? From your depth pressure chart it is 3. Write down the number 3.

What have we got:

10÷6 = 1.6666

1.66666x3 = 4.99999 Answer B

 

Now you can take a quick quiz to test yourself