Bottom Timer & Depth Averaging ?

[giles45shop]

I just bought one of the new Uwatec bottom timers w/ the depth averaging function as a back-up for my Cressi Archimedes II.

My question is whether I can use the average depth function along w/ tables to determine NDL's on dives that are not square profiles. In other words, say I do a dive to 80' for 5 minutes and then spend 20 minutes at shallower depths. The average depth per the bottom timer is 40' for a total time of 25 minutes (for example).

Is it reasonable to use the table data for 40' and 25 minutes as opposed to the 80' for 25 minute table data which is what you would normally use in calculating your NDL for the next dive.

Any input welcome.

Thanks,

John

 

[DA Aquamaster]

No. Tissue loading is not linear and you will eventually end up with a profile that requires deco, but does not look that way accoriding to average depth and time. The curves on a PADI Wheel are a pretty good visual illustration of the non linear nature of the calculations and the PADI Wheel also has some specific rules to make it work.

The average depth does however work well for SAC rate computations (assuming the workload is more or less evenly distributed across the dive) as that is a linear equation.

 

[giles45shop]

Thanks! I don't have a PADI Wheel (SSI Trained), but I understand what you are saying.

 

[Charlie99]

My question is whether I can use the average depth function along w/ tables to determine NDL's on dives that are not square profiles. In other words, say I do a dive to 80' for 5 minutes and then spend 20 minutes at shallower depths. The average depth per the bottom timer is 40' for a total time of 25 minutes (for example).

Is it reasonable to use the table data for 40' and 25 minutes as opposed to the 80' for 25 minute table data which is what you would normally use in calculating your NDL for the next dive.

Depth averaging will work for some profiles, but not others. DIR-diver.com - Average depth for deco? discusses using average depth for mandatory decompression rather than NDL calculations, but the same concepts apply when using average depth to estimate NDL.

Basically, it works for those cases where one starts deep and then each level after that is shallower than the previous ones. For example, treating 20 at 80' + 20 at 40' as 40 at 60' will work. It does NOT work if you do 20 at 40' first and then 20 minutes at 80'.

DA Aquamaster --- you can run some examples on the wheel and easily verify my statements above. It does work for some types of profiles --- for deep-then-shallow profiles depth averaging is a tiny bit more conservative than the result of the wheel.

===============

On a more general note, a "slight" amount of depth averaging is helpful on many dives planned and executed on tables. For example, if one plans a dive to 60', but you stray down to 62' for a couple of minutes, the by-the-book response would be to consider the dive as all at 70'. A reasonable bending of the rules would be to compensate for a slight excursion beyond the planned depth moving a bit shallower for a while. For example, if one went to 65' for a few minutes, then moving on up to 55' for the same mount of time or longer would effectively put the dive back onto the 60' plan.

There are various decompression planning programs such as V-Planner that confirm the above -- the above free advice is worth what you paid for it.

Charlie

 

[giles45shop]

Thanks! i've been thinking about getting a Deco program like V-Planner so I can play around with different scenarios.

John

 

[DA Aquamaster]

Depth averaging will work for some profiles, but not others. DIR-diver.com - Average depth for deco? discusses using average depth for mandatory decompression rather than NDL calculations, but the same concepts apply when using average depth to estimate NDL.

Basically, it works for those cases where one starts deep and then each level after that is shallower than the previous ones. For example, treating 20 at 80' + 20 at 40' as 40 at 60' will work. It does NOT work if you do 20 at 40' first and then 20 minutes at 80'.

DA Aquamaster --- you can run some examples on the wheel and easily verify my statements above. It does work for some types of profiles --- for deep-then-shallow profiles depth averaging is a tiny bit more conservative than the result of the wheel.

===============

On a more general note, a "slight" amount of depth averaging is helpful on many dives planned and executed on tables. For example, if one plans a dive to 60', but you stray down to 62' for a couple of minutes, the by-the-book response would be to consider the dive as all at 70'. A reasonable bending of the rules would be to compensate for a slight excursion beyond the planned depth moving a bit shallower for a while. For example, if one went to 65' for a few minutes, then moving on up to 55' for the same mount of time or longer would effectively put the dive back onto the 60' plan.

There are various decompression planning programs such as V-Planner that confirm the above -- the above free advice is worth what you paid for it.

Charlie

We are pretty much in agreement in terms of multi-level diving rules and how they would apply to a depth averaging feature. The PADI wheel for example requires the deepest portion of the dive first with the subsequent depths to be at least 20 ft shallower to ensure you avoid a reverse profile and that the overall profile keeps the various on and off gassing of compartments safely away from the critical limits.

I also agree to a point that averaging small deviations in a dive at a more or less constant depth is not going to cause potential problems - unless you are pushing the NDL's. One of the other tenents of multilevel diving - whether a wheel or a computer is used - is that multi-level NDL's are much more conservative as they are in effect much closer to square profiles where there is little or no margin of safety in the table.

If you are diving standard air or nitrox table limits and your averaging puts you close to a max depth/time situation for the table, it is a good idea to build in a safety (in this case it should be regarded as a mandatory deco stop) stop for 3-5 minutes at 10 or 20 feet just to hedge your bets and a longer stop would not be a bad idea. A few minutes of prevention is worth an awful lot of cure in terms of DCS.

I use both D-Plan (a bubble gradient model) and Palm VPM (basically a VPM-A model) and agree with your conclusion that bubble gradient and variable permeability models would probably confirm your example above (I have not checked), but they also tend to be a bit more conservative than the average recreational table and certainly more so than a US Navy table.

So again, I'd advise caution in using this approach for recreational divers who may not have a very expansive understanding for the ins and outs and factors affecting decompression.

I think it is worth noting that there is a difference between the practice of depth averaging such as the OP described being "possible" versus "reasonable". It may be both possible and resonable on a particular dive profile on a 1 day two dive per day outing, but it may not be reasonable at all in the middle of your multiday multi dive per day dive trip.

Personally, if it is just a back up for a computer failure, I'd complete the dive based on max depth and time information and then just extend the surface interval a bit to build in some safety margin prior to the next dive (essentially back up and follow the rules of the table on dive 1 and 2.) This would ensure you exit safely after dive one without having to do in water mental gymnasitics to figure out if your profile is going to work with the average depth and max time on the bottom timer. And for the second dive it ensures you stay on the safe side of the line in terms of RNT when planning and executing the dive.

If you had a Wheel or were familiar with the procedures for using a standard dive table for multi-level diving, I'd reverse engineer the first profile based on the depths and times for the levels, assuming you remember them accurately and they fit within the Wheel/mulit-level parameters (and if they don't a depth averaging approach would also be a very bad idea) and then use that RNT outcome to plan the second dive.

 

[limeyx]

Depth averaging will work for some profiles, but not others. DIR-diver.com - Average depth for deco? discusses using average depth for mandatory decompression rather than NDL calculations, but the same concepts apply when using average depth to estimate NDL.

Basically, it works for those cases where one starts deep and then each level after that is shallower than the previous ones. For example, treating 20 at 80' + 20 at 40' as 40 at 60' will work. It does NOT work if you do 20 at 40' first and then 20 minutes at 80'.

Charlie

Not going to go into it in any more details, but you can definitely use depth averaging for the profiles you claim it cannot be used for.

I have used a form (and my buddies too) of it for almost every single dive I've done (excepting my very first few out of open water)

Cave divers use it all the time in caves that vary up and down way more than 20-80 feet.

Having said that, there are rules and procedures and it's not something one should "just start doing" without some good guidance and mentoring.

 

[Charlie99]

Not going to go into it in any more details, but you can definitely use depth averaging for the profiles you claim it cannot be used for.

I can achieve any desired result by adding enough fudge factors to the depths and times. While you may call this depth averaging, it is not what I call using "average depth".


The original poster was talking about simply using the true average depth, which IMO is NOT suitable for shallow-to-deep multilevel profiles, either to calulate mandatory deco or the somewhat more difficult task of figuring out NDL.

Do you disagree with the observations of Peter Steinhoff at DIR-diver.com - Average depth for deco? that

However, doing "Profile 3 - Shallow to deep" we can't use an regular average is this will NOT give us enough decompression.

I agree that if one starts adding additional weighting factors to either time, depth or both, that one can adjust your calculations for any arbitrary profile, but that is not the same as using the average depth function of a bottom time. In particular, Peter Steinhoff give an example of an adjustment that uses a depth other than actual depth when calculating the "average":

The solution to the problem above is to take the profile into account and using a weighted average where we weight towards the deeper side when we have profile going shallow to deep. Having experimented with different profiles I found that using 75% of the difference a good rule of thumb.

I don't know whether this sort of weighting is suitable for NDL calculations, or if different fudge factor(s) need to be applied.

Charlie

 

[limeyx]

I can achieve any desired result by adding enough fudge factors to the depths and times. While you may call this depth averaging, it is not what I call using "average depth".


The original poster was talking about simply using the true average depth, which IMO is NOT suitable for shallow-to-deep multilevel profiles, either to calulate mandatory deco or the somewhat more difficult task of figuring out NDL.

Do you disagree with the observations of Peter Steinhoff at DIR-diver.com - Average depth for deco? that

I agree that if one starts adding additional weighting factors to either time, depth or both, that one can adjust your calculations for any arbitrary profile, but that is not the same as using the average depth function of a bottom time. In particular, Peter Steinhoff give an example of an adjustment that uses a depth other than actual depth when calculating the "average": I don't know whether this sort of weighting is suitable for NDL calculations, or if different fudge factor(s) need to be applied.

Charlie

You can use a straight average for a lot of dives.
You can use a straight average(either from a computer or your head) + some weighting for all dives.

Did the original poster state that they would *only* use the computer average ?

I do not use Peter's tables or exact method of calculating avg. depths (not hugely far off but not the same either). Peter's methods are his own, and I know very few people that use them honestly.

I ignore shallow to deep or deep to shallow almost entirely.

 

[giles45shop]

All,

Thanks for the discussion on this. I'm definitely not an expert at decompression planning, although I do have some understanding. The background around my question was related to a dive I did in Redondo Beach Canyon about a year ago.

I was diving a watch & analog depth gauge, my buddy was diving a computer. Dive profile was 70' for ~ 15 minutes, then 40-60' range for 45 minutes, finally a 3 minute safety stop at 20'. All on air. Going strictly by the tables, I was over the max NDL limit of 40 minutes, but by my buddies dive computer, we were fine (note when I dive with someone, I'm not more than 5' away at any time).

Intutitively, it would seem that some type of averaging would be appropriate for this dive, and I was thinking my new bottom timer would provide some valuable info. I definitely would go primarily by my computer, but w/ a computer failure in a dive as described, the bottom timer average would seem beneficial in figuring out what to do next.

Obviously this is a case-by-case type issue, depending on profiles of individual dives. I'm headed back to SoCal this weekend, and will be diving the canyon again, so just getting prepared

Thanks,

John