Am I understanding gauge pressure and absolute pressure correctly?

[Flynn Colorado]

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?

 

[KenGordon]

This is not a real question. It is designed to test that you have read the book.

It depends on what they mean by gauge pressure. Find that bit in the book and read it again.

In my world 59 feet is 20m and the absolute pressure is 3 bar. The odd 2m isn’t worth considering except if you are calculating air consumption rates after a dive.

BTW a metre is about 39 inches and 10m is about 32ft 10in. Where are you getting 34ft?

If you are early in your diving I suggest skipping the hard maths now and doing it all as metric.

 

[Flynn Colorado]

This is not a real question. It is designed to test that you have read the book.

It depends on what they mean by gauge pressure. Find that bit in the book and read it again.

In my world 59 feet is 20m and the absolute pressure is 3 bar. The odd 2m isn’t worth considering except if you are calculating air consumption rates after a dive.

Unfortunately I have to treat it like a real question and I can't refer to a book. Its from a lecture and my notes aren't sufficient on this topic.

I only wish, as well, I could round off measurements, meters or feet, whichever. But we're indeed calculating SAC (surface air consumption rates) as well as RMV (respiratory minute volume) with every dive.

 

[lowviz]

I'm not at all bought in to that question, as it is worded, as a learning experience.

The implication is that the gauge pressure is zero (anywhere) without any added pressure.

 

[Flynn Colorado]

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]

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?

I calculate 25.5psig but I am not sure how you calculate 39.7psia....my calculation is 40.3psia:

Per your statement of 34 feet = 1 ATM, I calculate the following:
59 feet /34 = 1.74 ATM
1.74 X 14.7psi = 25.578psig
(1.74x14.7)+14.7 = 40.278psia

Normally I calculate this using the metric system which is so much easier:
1ATM = 1Bar
1ATM=10m
18m /10 =1.8ATM = 1.8 Bar
1.8ATM+1 = 2.8ATA = 2.8 Bar

To calculate absolute pressure you just have to add the pressure of the air column above the surface of the water to your gauge pressure. So if you calculate 25.5psig then you just need to addd 14.7psi to the answer to get psia (40.2psia).

-Z

 

[swimlikethefish]

This is what I believe the answer to your question is:

Let's start with the difference between ATM and ATA. 1 atm (one atmosphere) equals 14.7psi. Just like standing on the beach. The air you breath is roughly 14.7psi. Now let's say we are under water. At 33ft, we add another atmosphere, due to the pressure of the water above us (14.7psi) so if you had a gauge it would read 14.7psi (because it is calibrated for 0 standing on the beach) this is what you are referring to as gauge pressure.

Now Ata is atmospheres absolute. At 33 ft you have the 1atm of water + 1 atmosphere of air above it = 2 atmospheres absolute.

 

[gopbroek]

PSIG or 'gaged' pressure is actually differential pressure between ambient and the applied pressure. And PSIA is absolute pressure and is the pressure differential of the input compared to a theoretical complete vacuume. With PSIG the value will always be zero if the input is at the same value as ambient
Example at 2 ATA depth, about 66', the ambient absolute pressure would be 3 ATA's, 2 water and 1 air for about 44 PSIA.
A standard PSIG gage would read 0 PSIG at any depth.

 

[KenGordon]

Unfortunately I have to treat it like a real question and I can't refer to a book. Its from a lecture and my notes aren't sufficient on this topic.

I only wish, as well, I could round off measurements, meters or feet, whichever. But we're indeed calculating SAC (surface air consumption rates) as well as RMV (respiratory minute volume) with every dive.

I tell my son that the purpose of tests at school is to check that he understood the lessons. If he fails the test they will know to make sure he understands the subject by helping him some more.

It sounds like you need to do that lecture again.

On the other hand, I tell my students that it is ok to get the question about calculating deco having changed elevation wrong because they will never use tables and certainly never the wild and crazy altitude stuff. Some diving education is worthless.

To calculate your gas usage rates you need to know absolute pressure. Unless your ‘gauge pressure’ refers to your cylinder pressure it is a red herring and pointlessly introduced.

I am particular worried by the 34ft bit and calculating psi per ft. Fresh water is 1kg/l a litre is a 0.1m cube. This means that a 10m column of water exerts a pressure of one bar (14.7 ish psi). Since 10m is 32ft10 or 394inches I figure fresh water is 0.448 psi/ft. Personally I would contend that is actually a less useful measurement than furlongs per fortnight and anyone using them ought to be sent away for re-education.

Where does the 34ft bit come from? It is almost as though you are getting the salt vs fresh water conversion backwards.

 

[Motojack]

Absolute pressure is a measure of pressure vs absolute vacuum (14.7 psi at surface under standard conditions)
Gage pressure is a measure of pressure vs ambient pressure (reads 0 at surface under standard conditions)
- gage pressure at 59' is 59/33*14.7=26.28 psi
- Absolute pressure at 59' = gage pressure + 14.7 = 40.98 psi

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?