OMG, my SI was less than 60 minutes!

[KeithG]

Will I die? Well no, but let's discuss Surface Interval length rationally and either establish or dispel some myths?. What happens to your body during the SI?

Please forgive my feeble attempt at theatrics in the thread title, but a recent thread highlighted that some (many, most, all?) people have an obsession with the magic SI number of 60 minutes. It appears that some people consider anything less than a 60 minute SI will break some fundamental law of diving (and you will die?).

I understand that there is a general guideline for a minimum of 1 hour SI - I do not have any handy references to this, so please feel free to link them in and provide supporting narrative.

So let's discuss shorter vs longer SI and their effects on multi-dive days.

The things I am aware of:
- PADI tables start at surface intervals of 0-3hrs and very quickly progress down to 0-7 minutes and end at 0-2 minutes.
- I have been led to believe that Suunto computers penalize divers by artificially reducing subsequent NDLs if a SI is less than 60 minutes
- I am aware of (and have used) 2 very popular Caribbean dive ops that routinely do 2 tank dives with less than a 60 minute SI

Let's talk Surface Intervals!

 

[doctormike]

Two things.

One, it's more nitrogen loading for the second dive since your compartments are more saturated. Nothing magic about 60 minutes, but the longer they are, the less N2 loading on the second dive. Specifics obviously depend on the mix, depth and time of the previous dive.

Two, multiple ascents in the same dive are a risk factor for DCS according to the bubble pumping hypothesis. So the shorter your SI, the more this is a factor. If the SI is short enough, then you are basically doing multiple ascents in the same dive. According to this theory, you are at increased risk for DCS by doing that even controlling for actual tissue loading. That is, even if you don't break your NDLs, you are at increased risk for an "unexplained hit". Ask me why I believe that...

 

[boulderjohn]

In the PADI version of the OW course that teaches tables, there is a section where students have to learn to compute the minimum surface interval needed before a second dive. In one of the problems they have to do, the answer to the imperial question is 4 minutes, and the answer in the metric version is 0 minutes.

So there is no official minimum surface interval. If you do a short shallow first dive, you can surface, wave to the crowd, and head back down.

On the other hand, there can be a practical reason. On typical 2-tank recreational dives I do in South Florida, the operator I use the most does indeed to a 1-hour surface interval. Do it enough times and you will see why. If you do the typical dive sequence on air, you will be pretty close to NDLs at the end of the second dive. If you are a paying customer and you have to do a shortened second dive because of a short surface interval you will not be happy.

 

[HantsDiver]

On the other hand, there can be a practical reason. On typical 2-tank recreational dives I do in South Florida, the operator I sue the most does indeed to a 1-hour surface interval. Do it enough times and you will see why. If you do the typical dive sequence on air, you will be pretty close to NDLs at the end of the second dive. If you are a paying customer and you have to do a shortened second dive because of a short surface interval you will not be happy.

You need to stop suing people.

If you don't like their surface interval then just chose another dive operation.

 

[boulderjohn]

You need to stop suing people.

If you don't like their surface interval then just chose another dive operation.

I fixed it. When I type the word "use," my fingers get excited and get out of order. Since it results in a real word, I don't get a spell check warning. I can't begin to estimate how many times I have made that typo.

 

[RainPilot]

I believe the 60 minutes came from the choice of controlling compartment for the development of the DSAT tables which is pretty much the PADI tables.

IIRC the navy used the 60 minute compartment since they don't need repetitive dives but that really played havoc with rec divers. DSAT then used the 30 minute compartment so 2 half lives is 60 minutes.

There is nothing magical about the number, especially with computer use now being de rigeur. Agreed though that very short SI over multiple dives are adding some element of risk. How much? No idea.

 

[HantsDiver]

I fixed it. When I type the word "use," my fingers get excited and get out of order. Since it results in a real word, I don't get a spell check warning. I can't begin to estimate how many times I have made that typo.

Sorry - it was my silly little joke. It's the second time I've made the same one. Although I do think everyone would know what you mean.

 

[boulderjohn]

I believe the 60 minutes came from the choice of controlling compartment for the development of the DSAT tables which is pretty much the PADI tables.

IIRC the navy used the 60 minute compartment since they don't need repetitive dives but that really played havoc with rec divers. DSAT then used the 30 minute compartment so 2 half lives is 60 minutes.
.

The Navy used the 120 minute compartment to determine surface intervals, but that fact is not obvious when looking at them. You can tell mostly by the fact that 6 iterations of a half time is considered a complete washout of that compartment, and the Navy tables washed out in 6 X 2 = 12 hours. PADI's research indicated that the 40 minute compartment could be used for most recreational dives, but they chose to use the 60 minute compartment. The PADI tables wash out at 6 X 1 = 6 hours.

Neither of those tables have any restriction for doing shorter surface intervals when appropriate.

The PADI tables require a minimum of a one hour surface interval if the diver is doing 3 or more dives in a day and any of them end in the W or X pressure group.

For reasons I do not understand, nitrox training frequently calls for a minimum of one hour surface intervals

I believe that the RGBM algorithm used by Suunto and Cressi and some others does penalize for surface intervals under an hour.

 

[KeithG]

Two things.

One, it's more nitrogen loading for the second dive since your compartments are more saturated. Nothing magic about 60 minutes, but the longer they are, the less N2 loading on the second dive. Specifics obviously depend on the mix, depth and time of the previous dive.

Two, multiple ascents in the same dive are a risk factor for DCS according to the bubble pumping hypothesis. So the shorter your SI, the more this is a factor. If the SI is short enough, then you are basically doing multiple ascents in the same dive. According to this theory, you are at increased risk for DCS by doing that even controlling for actual tissue loading. That is, even if you don't break your NDLs, you are at increased risk for an "unexplained hit". Ask me why I believe that...

So let's talk bubble pumping (but keep it simple based upon the thread being in the basic forum).

Do the tables account for this? Which (if any) computer algorithms account for this?

The manual for my very old Uwatec Aladin discusses simulation of microbubble formation while in surface mode. They do not explain what this means, but it does seem to imply that the ZH-L8 ADT algorithm from way back in the early 90's understands and accounts for shortish surface intervals?

 

[KeithG]

...
The PADI tables require a minimum of a one hour surface interval if the diver is doing 3 or more dives in a day and any of them end in the W or X pressure group.

Interesting - we have identified a source of the 60 minute golden rule, but also unearthed the 180 minute rule.

I will attempt to retype the paragraph here from an image of a PADI table I found on the interweb:

Special Rule for Multiple Dives
If you are planning 3 or more dives per day: Beginning with the first dive, if your ending pressure group after any dive is W or X, the minimum surface interval between all subsequent dives is 1 hour. If your ending pressure group after any dive is Y or Z, the minimum surface interval between all subsequent dives is 3 hours.

The interesting take away's for me are:
- the 1 hour rule applies to all subsequent dives, regardless of the ending pressure group on any subsequent dive
- there is a 3 hour SI rule!