Gauge Pressure Question

[Xizang]

I was answering an old DM exam last night and encountered this question

What is the Gauge pressure at 16 meters in fresh water?

I answered 1.6 ata, but the answer key said it is 1.55 ata. The formula for gauge pressure is simple depth / 10 meters right? Is there a different formula for fresh water?

 

[jonnythan]

Fresh water is less dense, therefore the pressure at a given depth is less in fresh water than salt water.

 

[Xizang]

Fresh water is less dense, therefore the pressure at a given depth is less in fresh water than salt water.

ok so whats the formula to arrive at 1.55 ata?

 

[Kim]

Freshwater is 3% less. That means that 1.6 in salt water is 0.048 (1.6X3%) rounded to 0.05 less - i.e 1.55.

 

[Xizang]

Freshwater is 3% less. That means that 1.6 in salt water is 0.048 (1.6X3%) rounded to 0.05 less - i.e 1.55.

Oh so 3% is the factor. I don't recall reading this in the PADI Encyclopedia for Recreational Diving. Anyway, thanks very much!

 

[jonnythan]

You can divide any salt water number by 1.026 to get an approximate fresh water value. A depth with a pressure of 1.6 ata in salt water is 1.6 / 1.026 ~ 1.55 in fresh water.

55 fsw / 1.026 ~ 54 ffw.

 

[Kriterian]

I was answering an old DM exam last night and encountered this question

What is the Gauge pressure at 16 meters in fresh water?

I answered 1.6 ata, but the answer key said it is 1.55 ata. The formula for gauge pressure is simple depth / 10 meters right? Is there a different formula for fresh water?

One atmosphere (atm) of sea water is 33 ft, and the formula is depth/33. For fresh water it's 34 ft, so it's depth/34. 16 meters is about 52.5 ft, so....

52.5 / 34 = 1.54 atm

 

[Canadian_Diver]

I was answering an old DM exam last night and encountered this question

What is the Gauge pressure at 16 meters in fresh water?

I answered 1.6 ata, but the answer key said it is 1.55 ata. The formula for gauge pressure is simple depth / 10 meters right? Is there a different formula for fresh water?

Directly from the SSI EAN Manual:

Note: All of the formulas appearing here use depth values for salt water. To solve these formulas using depth values for fresh water, substitute 34 feet for 33 feet when working with imperial measurements. Substitute 10.3 metres for 10.0 metres if working their metric equivalents. For simplicity, these formulas assume that 33 feet or 10 metres of salt water exert a pressure precisely equal to either one atmosphere or one bar.

In your case..... 16m / 10.3m per ATA = 1.553 ATA

Also, another handy way of calculating:

Saltwater adds .445 lbs of pressure per foot of depth to the ambient atmospheric pressure.

Freshwater adds .432 lbs of pressure per foot of depth to the ambient atmospheric pressure.

(Depth x Water Pressure Per Foot of Depth) + 14.7 PSI = PSI @ Depth
(Depth x Water Pressure Per Foot of Depth) / 14.7 PSI = ATA

eg; At 52.49 feet (16m) depth of freshwater ------ (52.5 x .432) / 14.7 = 1.54 ATA

The calculations are very close. Hope this helps!

Cheers!

 

[partridge]

Axua:

Canadian is right. In salt water it is 1 atm every 10 meters. In fresh it is 1 atm every 10.3 meters.

Good luck on the IDC

Paul

 

[Xizang]

Axua:

Canadian is right. In salt water it is 1 atm every 10 meters. In fresh it is 1 atm every 10.3 meters.

Good luck on the IDC

Paul

should take note of that 34 and 10.3 for freshwater thanks everyone!

Partridge - thanks man! hope to be diving again there soon!