Dumb Question Pressure changes in the 30 ft Range

[Pook-60]

Howdy Folks

Please forgive if this question is covered in another thread...I couldn't find it.

Is it true that the pressure changes in the 0 - 30 foot range are greater than changes in the 30 - 60 or 60 - 90 ranges?

If that's true...why? And how much?

Thanks ahead of time for your answers.

Pook:06:

 

[Scububbs]

That question was addressed in my OW class, which I completed just a few months ago. As it was explained to me, from surface to 30 feet is a full atmosphere, so the pressure on your body has just doubled. The next atmosphere down, 60 feet, only adds a 50% increase in pressure, and from 60 to 90 feet the increase is only another 30%.
In actual practice, I don't notice any difference between atmospheres anyway, although I'm sure there are physiological changes of which I'm not aware.

 

[jtivat]

Yes it is true between 0 and 33' is one ATA difference, at the surface you are at one ATA and at 33' you are at two ATA so you have doubled your pressure. Now to double your pressure again you must go to 99' or 4 ATA. Not sure if I am getting my point across or not. In water every 33' is another atmosphere of pressure so at the surface you are at 1 then 33' you are at 2 now to double that you must go to 4 atmospheres or 99' then too double it again you wust go to 231' or 8 ATA.

 

[ronbeau]

Howdy Folks

Please forgive if this question is covered in another thread...I couldn't find it.

Is it true that the pressure changes in the 0 - 30 foot range are greater than changes in the 30 - 60 or 60 - 90 ranges?

If that's true...why? And how much?

Thanks ahead of time for your answers.

Pook:06:

The pressure is 1 atmosphere at the surface.
If you go down 33 feet your are at 2 atmospheres.

At this point you doubled the pressure from the surface to 33 feet.

Now if you go from 33 feet to 66 feet you go from 2 atmospheres to 3 atmospheres of pressure.

At this point the pressure went up by 50% by going from 33 feat to 66 feet.

In other words the pressure doubled between the surface and 33 feet. By going down an additional 33 feet the pressure only went up 50%.

As a result the pressure difference between the surface and 33 feet (200% differential) is 4 times greater than going from 33 to 66 feet (50% differential) .

In both cases you went down 33 feet deeper.

Your instructor is really lacking if he/she did not emphasize this within the SCUBA open water course.

 

[partridge]

Not only is it important that you know this but you should also know the effects on the body. 33 feet to the surface is the greatest expansion of air in your body and so is the most dangerous time. You should be watching your rate of ascent more carefully during this time. This stresses one point of importance of the safety stop as a method to stop your ascent and give the gas one last chance to escape.

On the other side of the coin is the descent for this portion of the dive. It is very easy to do damage to your ears during this part as the change in pressure is the greatest. So go down slowly enough so that if youo feel discomfort you can still stop and ascend instead of shooting down.

 

[Walter]

Is it true that the pressure changes in the 0 - 30 foot range are greater than changes in the 30 - 60 or 60 - 90 ranges?

If that's true...why? And how much?

Yes and no. The pressure change is constant all the way down, so - no. OTOH, the % of pressure change is greater the shallower you are.

 

[kidspot]

You'll probably find you have to equalize more frequently during the first 30 ft. and you'll find the greatest bouyancy changes in this range. That was actually why one of my classes was done in 15ft of water, so we could see the changes in bouyancy much more easily.

Aloha, Tim

 

[keyshunter]

Howdy Folks

Please forgive if this question is covered in another thread...I couldn't find it.

Is it true that the pressure changes in the 0 - 30 foot range are greater than changes in the 30 - 60 or 60 - 90 ranges?

If that's true...why? And how much?

Thanks ahead of time for your answers.

Pook:06:

This is definitely not a dumb question. A knowledge of the relationship of depth and pressure, along with its ramifications is extremely important. If a person completes an open water class without at least a basic knowledge of this relationship, the instructor is not doing his/her job.

 

[pete340]

As a result the pressure difference between the surface and 33 feet (200% differential) is 4 times greater than going from 33 to 66 feet (50% differential) .

In both cases you went down 33 feet deeper.

Minor correction: the change from the surface to 33 ft. is 100%, which is 2 times greater than going from 33 to 66 feet (50%). The rest of the numbers are right.

 

[JMcD]

I'll take a shot at addressing a bit more of the "Why", but am open to correction:

1 atmosphere is the pressure exerted by the weight of one square inch of the atmosphere pressing down on the earth. Water is heavier than the air/smog/vapour of the atmosphere, so it takes less of it to exert the same pressure. 1 square inch of 33 feet of water weighs the same as one square inch of the atmosphere. (Ignore the different weight of salt vs fresh water - it is minor for this example) SOooo while pressure is not only linear (up and down weight), it may help to think that at 33 feet you have the pressure of the atmosphere pressing on you as well as the weight of the 33 feet of water above you. This is twice the amount of weight you were bearing at the surface. At 66 feet you have the atmosphere, the first 33 feet and the next 33 feet - 3 atmospheres worth of "weight".

We notice change in pressure more than the absolute amount. Think of a piece of rubber in the middle of a glass of water - water above and below - no pressure exerted on the rubber - no "feeling" of pressure. Now put the rubbber across the middle of the glass so air is underneath and water above - the weight of the water is greater than the force of the air holding it up - so the rubber will streatch - "feeling" the difference in pressure. In our bodies, it is the air pressure that offsets the pressure of water in our ears, etc. We get the air from our tanks at the same pressure as the environment around us - so when we equalize we are putting the same pressure on oether side of "the rubber" therfore no strain.

Just a few more thoughts for you...

(Any correction/ input welcome)