Am I understanding gauge pressure and absolute pressure correctly?

[boulderjohn]

I am going to guess that this is a DM course you are taking, but your profile says you are both NAUI and PADI so I don't know which. If it is PADI, you should be using the Encyclopedia of Recreational Diving as a resource. The following answers are exactly what you are looking for.

The gauge pressure is the pressure exerted by the water at that depth. At 66 FSW, your gauge pressure would be 2 atmospheres.

The absolute pressure (ambient is the same) is the pressure of the water plus the pressure of the atmosphere. At 66 FSW, the absolute pressure is 3 atmospheres.

Here is an example of how this differentiation can become important. If you are at 102 FFW at sea level, you are at 3 atmospheres gauge and 4 atmospheres ambient. Do the same dive at 6,600 feet elevation, and you are at 3 atmospheres gauge and 3.8 atmospheres ambient--only a small difference in terms of percentage. Go to 34 feet as you ascend at sea level, and your ambient pressure is now 2 ATA, but at altitude it will be 1.8--a 10% difference. Go to the surface at the end of your dive, and you are at 1 atmosphere ambient at sea level but 0.8 ambient at altitude--a 20% difference. So you will see that during a dive at altitude, you will on-gas just about as much during the dive as you would at sea level, so your tissue pressure will be close to the same. As you near and reach the surface at altitude, the ambient pressure is much less, so you have a much greater concern about making a safe ascent.

 

[lowviz]

...//... I have never heard of a pressure gage being used to determine water pressure in recreational scuba. ...

That is my issue with the OP's confusion with regard to this question.

Yes, physically, it is completely defensible, but way too technical. It only serves to confuse. Maybe it was an attempt at a deeper understanding, but not every diver is a physicist.

 

[boulderjohn]

I have never heard of a pressure gage being used to determine water pressure in recreational scuba. Is it used in some commercial or military diving situations?

The student is learning about pressure. It is an instructional exercise to that end, and that end is learning the relationship between these terms.

 

[lowviz]

Distance requires two values. Point A and point B. You can now measure the distance between them.

Similarly, pressure requires two values. Easiest way to think about this (in diving) is that one is a reference and the other is the measured value.

Gauges:
PSIA uses a vacuum as its reference.
PSIG uses one standard atmosphere as its reference.
Differential pressure is not referenced.

Pressure is important to divers as it controls on and off-gassing. Here is the problem. Divers experience a combined pressure that comes from both the air and water above them. So PSIA gauges would be really nice and end all our confusion. Problem is, that gauge would read 14.7psi or 1 bar when disconnected at the surface. That messes everybody else up.

So we get used to taking a gauge reading and adding an atmosphere...

 

[EFX]

I'm taking an advanced course and learning about gauge pressure vs. absolute pressure. A take home test question is as follows:

"The gauge pressure at a depth of 59 feet in fresh water is ____ psig and the absolute pressure is ___ psia."

Is this another way of asking about ATM vs ATA? I know that 59 ffw is 1.7 ATM (depth of 59' divided by 34'). I know that ATA would then be 2.7.

But seeing "psig" and "psia" I assume its asking for a different computation that I don't understand. I've tried to find online and I'm lost.

Any wisdom on this?

33 ft of salt water exerts a pressure of 1 atm. 34 ft of fresh water exerts a pressure of 1 atm. The reason it takes less depth for salt water versus fresh is that salt water contains more minerals which adds weight to the water. You have half of the problem solved. You just need to add the conversion of atm to psig. Once you've got gauge pressure add the pressure of the atmosphere at sea level (assumed) to get absolute pressure psia.

pressure of the water column: 59 ft x 1 atm / 34 ft x 14.7 psig / atm = 25.5 psig
pressure of the atmosphere at the surface: 1 atm x 14.7 psig / atm = 14.7 psig
absolute pressure of water column: 25.5 psig + 14.7 psig = 40.2 psia

To get the math right work with the units first. Then add the numbers to the units and calculate.

 

[gopbroek]

I have never heard of a pressure gage being used to determine water pressure in recreational scuba. Is it used in some commercial or military diving situations?

Very few divers ever dive without an absolute pressure gage measuring the water pressure. A depth gauge is nothing more than an absolute pressure gage with the pressure indication value expressed in units of depth.

 

[60plus]

But a depth gage is not an absolute pressure gauge, if set correctly it reads zero at the surface which means the absolute pressure (assuming you are diving in the sea) is actually one atmosphere.
For most diving i would think the difference between PSIG and PABS does not matter, however if you drive a steam engines its quite a significant difference.

 

[Flynn Colorado]

Thank you all for the thoughtful responses!

It appears my answer is what I had ultimately came up with.

Again, I'm very grateful for all you guys chiming in with such detail.

Great board!

I believe I may have figured out the answer for myself since it wants the answers expressed in psi.

Fresh water is .432 psi per foot. At 34' feet (1 ATM), that's 14.7 psi. At 59', its 25.5 psi. Absolute would be 39.7 psi.

Perhaps the answers are 25.5 for gauge pressure, and 39.7 for absolute pressure?

 

[Zef]

Would you mind explaining either here or in PM the calculation and reasoning behind the 39.7 for absolute pressure?

Thanks,
-Z