View Full Version : Ambient and Absolute Pressure

Andy P
21-10-2016, 05:18 PM
Evening all,

During a club evening, the subject of pressure was brought up. Ambient and absolute that is. There was a distinct spilt in the 'discussion' and to say things got a little heated was an understatement.
It started with 'what is the water pressure at 15m', ' answer is 1.5bar' , that i managed to follow. THen it turned to atmospheric pressure, easy thats 1bar (at sea level excluding weather systems).

As i understand it, absolute pressure is 'atmospheric pressure at sea level (in this case 1bar) plus water pressure (in this case 15m which is 1.5bar) making a total of 2.5 bar.

Then ambient pressure was introduced......

It was suggested that ambient pressure was related to the medium that was being referred to. So, the 'ambient water pressure' at 15m was 1.5bar. This made sense to me as the definition of ambient pressure suggested this.
However, it was then said that this was not correct when related to diving and that ambient and absoulte pressure are the same.

Ive looked at this and they both seem to be correct.

Can anyone help to clear this up!!!

Thanks in advance

21-10-2016, 06:18 PM
Ambient means "immediate surroundings". Ambient-pressure is the surrounding pressure, in your example at 15m. Hence hydrostatic-pressure (1.5bar) plus atmospheric-pressure (1bar) is 2.5bar ambient-pressure at 15m.


21-10-2016, 07:26 PM
are we talking 15m true depth from the surface and if so is it fresh water or seawater and if seawater what time of year and which particular bit of 'the sea'? strictly speaking the answer to the original question is water density x g x 15m.

must be Friday:)

nigel hewitt
21-10-2016, 08:45 PM
These are two different concepts.

Absolute pressure is the alternative to gauge pressure.
Absolute pressure is the 'absolute' total pressure from all sources.
Gauge pressure is a convenience that resets the reference point not to zero but to atmospheric pressure.
We use gauge pressure to measure our cylinders but need to transfer in and out of absolute pressure to do things like Nitrox calculations.

Ambient pressure is the surrounding pressure at a point.
This can be measured using absolute or gauge pressure.