Question Clarification on using Perdix @/delta + 5

[Manatee Diver]

I'm quite confused with your scenarios.

Let's take a look at a real dive.

In the first valley I have a Delta five of +5-8, as I ascend to the top of the breakdown pile my delta five will start dropping going to around +1-2, as I descend down the valley again it will start raising probably to the same amount. As I ascend again to the top of the breakdown pile it once again lower to +2-3.

As I make my ascent to the surface it slowly lowers to zero where I am close to neutral for the controlling compartments. As I ascend it will slowly lower probably to -1-2, until I make my gas switch at 70ft to my 50% bottle where it will probably jump to -3-4, only finally getting to -5 will I hit my stop depth at 30ft, Once the stop is cleared it will probably raise up to -3-4, only again going to -5 once I reach the 20ft stop and switch to oxygen. There it will remain at -5 until my stop time is less than five minutes and it will tick down until cleared. The ten foot stop doesn't matter on oxygen, you off gas at the same rate, at least under SRI's implementation of the Buhlmann.

I then continue my stop at 20ft for extra safety margin, as I am probably at least an hour from a hospital, and a few hours from a chamber.

 

[inquis]

I plan a dive to 180 for 15min bottom time, and it gives me a total run time of let’s say 50 minutes. When I hit 180 feet at minute 10 (slow descent), my TTS is something like 40 minutes with a delta+ of let’s say 10minutes.

Assuming the plan also included that slow descent, you will have a TTS of 35 minutes at minute 15. If you leave at T=15, ascend at 30 fpm, hit the stops as dictated, then you will surface after 35 more minutes for a total runtime of 15+35 = 50 minutes. So no, TTS won't be 40 minutes at minute 10.

At this point I can say “this is acceptable because if I stay at 180 for five more minutes, my TTS will be 50minutes, which is what I planned for”

You're mixing up the terms. TTS is Time To Surface from the current time. It is NOT the Runtime when you surface. If you stay down until TTS equals the planned runtime before starting your ascent, you will surface way later than your planned surfacing time. (In fact, you will be over by the time from splash to starting your ascent.)

 

[inquis]

Therefore, maybe the more accurate example would be (forget about dive plan):
Dive to 180ft, check delta+, and see delta+ is +15minutes, then immediately ascend to 150ft, check delta+, and it should show a less number than 15, possibly more than 5.

Yes, at 150 ft, Delta+5 will be less than 15. It might even be negative, but I can't say for certain since you didn't give a time at 180 ft. The only way it will be more negative than -5, however, is if you are shallower than the mandatory decompression stop it wants you to do. You will be off-gassing faster than it thinks is safe. Don't do that.

 

[ScubaSamScubaStevesFather]

The only way it will be more negative than -5, however, is if you are shallower than the mandatory decompression stop it wants you to do. You will be off-gassing faster than it thinks is safe. Don't do that.

Yep, this right here was the root of my confusion. This makes sense why delta+5 shouldn't be more than -5.

You're mixing up the terms. TTS is Time To Surface from the current time. It is NOT the Runtime when you surface. If you stay down until TTS equals the planned runtime before starting your ascent, you will surface way later than your planned surfacing time. (In fact, you will be over by the time from splash to starting your ascent.)

(Yes, my descent is included in bottom time)

I'm not sure I agree with this. According to the perdix manual, TTS is defined as:

The time-to-surface in minutes. This is
the current time to ascend to the surface including the ascent plus all required deco stops.
Assumes:
Ascent rate of 33 feet per minute
(10 meters per minute). Decompression stops will be followed. Programmed gases will be used as appropriate.

Irrespective of my dive plan, TTS will tell me how many minutes at the current depth it will take me to surface w/ deco stops. If I planned my gas management (part of my manual written backup dive plan) for a 180ft dive for 15 minutes, with a total run time of let's say 50 minutes, then at minute 15 if my TTS is less than 50-15=35 minutes, I will know that I have enough gas to complete my deco*. Continuing with this example, if at minute 5 I hit 180 and for whatever reason I decide to come up to 130ft, then I know that at minute 15 my TTS will definitely be less than 35 minutes, which means I'll be able to stay at that depth past minute 15. In other words, I traded deeper depth for an increase in bottom time at a shallower depth, without increasing my deco obligation.

Please let me know your thoughts on this ^

*I purposely wrote deco, with the assumption that my deco is being done on some variation of oxygen and not using backgas, since variations in depth/time will obviously change how much backgas/bottom gas I am breathing <- for the sake of this discussion let's just say I have more than enough backgas to extend my bottom time

 

[inquis]

In other words, I traded deeper depth for an increase in bottom time at a shallower depth, without increasing my deco obligation.

The obvious conclusion of your scenario is that you'd like to stay at the shallower depth until TTS reaches the original planned TTS. You're using TTS as a surrogate for the required amount of deco gas, and that practice has both supporters and critics.

I would urge you to plan both cases with your particular depth and gas choices. Every time I have done such analysis, the deco gas for the shallower+longer dive did not exceed that of the deeper+shorter dive. However, my analysis is hardly exhaustive, and there is more than a little comfort that comes from having actually planned the profile variations that you might make.

 

[inquis]

I'm not sure I agree with this. According to the perdix manual, TTS is defined as...

FWIW, this latest post used the terms correctly. Previously, you postulated a scenario where you made TTS equal to the runtime rather than (runtime-bottomtime).