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The shape of a dive table

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The shape of a dive table

Postby eDiver » Mon Mar 12, 2012 5:51 pm

When you took your Open Water course, among other things, you learned that, when we dive, our body accumulates excessive Nitrogen and then when we surface this extra Nitrogen gets released.

You also learned that this intake/release of Nitrogen is somehow a function of depth and time and most likely you were taught to track it by using a dive table:
..... select the depth column .... go down until you find your bottom time (rounded-up) .... go right to find the pressure group at the end of the dive ..... look for your surface interval time (range) .... go down to find your pressure group at the end of the surface interval ..... and if you are doing a repetitive dive ..... turn the dive table around ..... find the column with the Pressure group at the beginning of the dive .... look for the row with the depth of the repetitive dive .... find the cross-section .... write down the Residual Nitrogen ..... turn the table around and repeat procedure ... remembering to add the RN to the bottom time ......

But, have you ever wondered how a dive table really works?
Do you know that a dive table has a shape that tracks the nitrogen loading during the dive and is not just a large collection of boxes with numbers in it?

Let's see an example together.
In the figure below we have a dive profile of a real multi-level dive with a maximum depth of 90ft and bottom time of 40 minutes (including a safety stop of 3 min).
Image

The first thing we notice is that this dive would NOT be possible using a dive table (40 minutes at 90ft is way into deco) .... this is because dive tables assume squared profiles .....

The second thing we notice is that this dive is actually feasible for most AOW recreational divers - equipped with dive computers - as it is NOT a deco dive.

In fact, if we analyze this dive using divePAL (ZH-L16C - moderate conservatism), we find out that the maximum value of nitrogen loading during the dive was ~88% and the N2 load at the end of the dive was ~76%.
See figure below.
Image

So, since this dive is feasible, what should its Pressure Group be?
Or even better, how does the Pressure Group change during this dive?
What is the shape of the dive table for this dive?

In the version 0.12 of divePAL Windows, we introduced the feature Estimated Pressure Group that visualize how the PG progresses (changes) during a dive. Please note that this tool should NOT be used for planning real dives as its main intent is for education.

In the image below we can see that 10 minutes into the dive the EPG was B.

At half point into the dive it was I, then it peaked at Q at about minute 33 and adjusted at that level till the end of the dive.
Image

When I look at the dive tables in this way I almost like them :D
Last edited by eDiver on Tue Aug 19, 2014 8:04 am, edited 1 time in total.
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Re: The shape of a dive table

Postby eDiver » Tue Mar 13, 2012 10:06 am

Of course we run the shape of the table also during the surface interval.
See image
Image
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