How to find the pressure at a given depth in FRESH water.

Everything I have shown you so far will also work for questions in fresh water.  All you need to do differently is calculate the pressure at the depth in the question differently.

Here is how you do that:

Gauge pressure.

Questions may just ask for the gauge pressure at a given depth.  We know how to find the ambient or absolute pressure from the table above, but what about gauge?

The gauge pressure is one ata less than ambient. We are basically talking about your depth gauge. We know that at the surface at sea level we are under 1 ata of pressure yet our depth gauge shows 0 ft of M. So our depth gauge is designed to show pressure less the 1 ata of surface pressure.

What is gauge pressure at 10m fresh water?

• In salt water the ambient is 2 ata

• So gauge in salt is 1 ata

• Divide by 1.03

• 0.97 ata

What is the gauge pressure at 30m salt water?

• In salt water the ambient is 4 ata

• So gauge in salt is 3 ata

• Divide by 1.03

• 2.91 ata

Now lets look at using the First depth / second depth method.

This is a different method to the 1,2,3 approach we have used so far. I actually prefer it, but some people find it more confusing. I think it is quicker.

This method is VERY useful when the question you get has fractions in the question and answer. You will see what i mean when you get to example 4........

Example 2

Example 3

Example 4