Disclaimer: these posts might not be suitable for people who have physica-o-phobia ;)
This is the next installment in the series on dive master physics. Today, I am going to write about how calculate pressure per at specific depths. There are some important things to remember when working with pressure. There are two types of pressure that is referred. There is absolute (ATA) and gauge pressure. The two very similar and it is very easy to confuse. Be very careful when reading the question to ensure that you know whether you are working with ATA or gauge.
The easiest way to remember the difference between the two is to think about what a gauge is. The gauge pressure is what you see on the pressure gauge. The other takes ambient (surrounding pressure) into account. So when the question asks you to determine ATA remember to always add in the ambient air pressure at sea level. Lets do a simple example to demonstrate the difference.
How much ata pressure is at 90 feet/30 meters?
90 feet divided by 33 = 2.73 atm + 1 atm = 3.73 ata
30 meters divided by 10 = 3 atm + 1 atm = 4 ata
Note that you always add the ambient pressure of 1 atm to the answer to calculate the ata pressure. So when you see "ata" think "add 1 atmosphere".
Gauge pressure is the same except that you don't add the 1 atm. The answer is what you see on the gauge. So when you see gauge - think what I see on the gauge.
Some other things to remember:
33 feet / 10 meters of sea water = 1 atm
34 feet / 10.3 meters of fresh water = 1 atm
Lets do another example.
What are the gauge and absolute pressures 67 feet / 22 meters in fresh water?
First calculate the atmospheres at depth
67 feet / 34 = 1.97 ata
22 meters / 10.3 = 2.14 ata
Remember that gauge pressure is what you see on the gauge so therefore the gauge pressures those we have just calculated.
To calculate the absolute pressure (ata) remember to add 1 atmosphere (the effect of the air column on the water). There the absolute pressures are
1.97 + 1 atm = 2.97 ata
2.14 + 1 atm = 3.14 ata
To add a little complication to the matter, the questions sometimes ask you to determine the psi or kg/cm2.
To do that you need to remember that 1 atm is equal to 14.7 psi (pounds per square inch) and 1.03 kg/cm2.
Imperial: 2.97 ata x 14.7 psi = 43.66 psia (pounds per square inch atmosphere)
Metric: 3.14 ata x 1.03 kg/cm2 = 3.23 kg/cm2