Water pressure question

[Tortuga James]

I have been doing a lot of studying this winter in preparation for much deeper dives this coming season off the coast of NC. One of the things I learned this past summer was that I needed to improve my gas management planning skills for the 100-120' depths I will be doing this year.

Here is a question I can't get past. ATA is based on increments of 33' + 1. So, by that formula, the pressure is labeled as 4 ATA at both 67 and 98 feet. Or as 5 ATA at 101 and 126 feet. I understand the concept of conservative planning, but using the above assertions, would I be doing something wrong if I planned my gas management strategy as 4.5 ata at a depth of 115'?

 

[Thalassamania]

I have been doing a lot of studying this winter in preparation for much deeper dives this coming season off the coast of NC. One of the things I learned this past summer was that I needed to improve my gas management planning skills for the 100-120' depths I will be doing this year.

Here is a question I can't get past. ATA is based on increments of 33' + 1. So, by that formula, the pressure is labeled as 4 ATA at both 67 and 98 feet. Or as 5 ATA at 101 and 126 feet. I understand the concept of conservative planning, but using the above assertions, would I be doing something wrong if I planned my gas management strategy as 4.5 ata at a depth of 115'?

Ken, the air pressure is one atmosphere, then each 33 feet of water adds another atmosphere of pressure. At 4 ATA, you're under one atmosphere of air and three of water, so that's 99 feet. 5 ATA is one atmosphere of air and 4 of water or 132 feet. 115 feet is 3.49 atmospheres of water plus one atmosphere of air or 4.49 (call it 4.5) ATA.

 

[TeddyDiver]

No, not wrong at all. Actually you can do it with the true depths too if you want to. It's just one calculation to determine the actual pressure. However all roundings are allways done to the safer direction...

 

[Tortuga James]

right, so at 99 fsw you are at 4 ata, 99/33= 3 ata + 1 for the air on top of the water. Right?

My thinking is that each foot of depth is 1/33rd of an ata. So wouldn't a depth of 116 fsw be 4.5 ata instead of 5 ata?

 

[H2Andy]

would I be doing something wrong if I planned my gas management strategy as 4.5 ata at a depth of 115'?

no ... you are rounding up .02 ata which would make your calculations more conservative

i assume you divided 115 by 33 and added one, to arrive at your ATA, and then rounded up to your nearest 10th?

 

[Thalassamania]

right, so at 99 fsw you are at 4 ata, 99/33= 3 ata + 1 for the air on top of the water. Right?

correct

My thinking is that each foot of depth is 1/33rd of an ata. So wouldn't a depth of 116 fsw be 4.5 ata instead of 5 ata?

You were confused, the "1" is an atmosphere not a foot of water. 1 atmosphere is 33 feet of water, one foot of water is 1/33 of an atmosphere.

 

[SteveAD]

there is absolutely nothing that says we have to work in whole ata.

ambient pressure (in ata) is (depth/33)+1

so at 115' it would be (115/33)+1

or (3.4848 repeating)+1

or 4.4848 repeating

or close enough to 4.5 as to make no difference

so no you wouldn't be doing anything wrong.

on the other hand always rounding up and calling it 5 ata could be considerd an extra buffer zone.

 

[Divin'Papaw]

I think you need the PADI Deep Diver Course.


(JUST KIDDING, JUST KIDDING!!! I couldn't resist due to several recent threads!)

 

[Meng_Tze]

Do pressure calculations metric and then go back to feet if needed.


1AT= 10M

30m = 4 ATA (3 for deth and 1 atmosphere of "the atmosphere")
30m=30*3.3= 99 ft

115ft = 115/3.3 =34.8m (I call that 35)
35m=4.5 ATA

There are rounding factors in this, but they do not pose any dangers since your guages (depth and pressure) probably have bigger tollerances. I would however not round up to 5 ATA since that is a 5m (16.5ft) difference.

 

[Thalassamania]

there is absolutely nothing that says we have to work in whole ata.

ambient pressure (in ata) is (depth/33)+1

so at 115' it would be (115/33)+1

or (3.4848 repeating)+1

or 4.4848 repeating

or close enough to 4.5 as to make no difference

so no you wouldn't be doing anything wrong.

on the other hand always rounding up and calling it 5 ata could be considered an extra buffer zone.

I apologize for the interjection, I think it a very bad idea to round in that manner. Add a buffer at the end if you need to, but know exactly where you are, because that rounding might work for you or against you depending on just what you were calculating.