Question Clarification on using Perdix @/delta + 5

[ScubaSamScubaStevesFather]

According to this shearwater article (Evolution of Dive Planning - Shearwater Research), it states:

Let’s assume you are on a deep reef and you notice that your TTS is approaching your maximum TTS. You ascend a few metres and you notice that your Δ +5 is now +1. This means that you are still incurring additional decompression, although at a much slower rate, and so your TTS will continue to increase. If you come up a few metres more, you can now see that your Δ +5 is zero. This means that you are neither ongassing or offgassing and you can stay at this depth without increasing your TTS. If you ascend slightly shallower and your Δ +5 changes to -1 then you can see that you are now offgassing and you can stay at this depth almost indefinitely as your TTS will slowly reduce.

My issue with this is the last sentence. Specifically, if my delta +5 is showing a -1, then even though that might mean I’m off gassing, it does *not* mean that I am on or ahead of my TTS schedule. In other words, if I stay at a depth with delta+5 showing -1, for 5 minutes, then I will have effectively increased my deck obligation by 4 minutes. In other words, in order for me to “keep to my schedule”, the delta+5 should show -5 to indicate that 5 minutes later at my depth, my TTS will decrease by 5 minutes.

Am I understanding this correctly?

 

[inquis]

in order for me to “keep to my schedule”, the delta+5 should show -5 to indicate that 5 minutes later at my depth, my TTS will decrease by 5 minutes.

It's indicating -1 because TTS will only decrease by 1. You're making progress toward the deco obligation, but not as much as if you were to ascend to the specified stop. If you move to the stop depth (and assuming the stop time is greater than 5 min), then delta+5 will equal -5.

 

[Manatee Diver]

You are on schedule when your delta five is -5 until your stop time is less than five minutes.

 

[ScubaSamScubaStevesFather]

That’s what I thought! Thanks for the sanity check.
I know a lot of people generally use @+5, however if im using a +5 to see if ill stay within my runtime, then I feel like the delta+5 will be more helpful since I can immediately see if its -5 or more negative <- would you guys agree, or do you generally use @?

 

[Manatee Diver]

I use delta five as it gives me a quicker look at what I want to know, how fast I am on or off gassing. Outside of rounding that might give you a -6 for a brief minute, you should never see it less -5.

 

[ScubaSamScubaStevesFather]

I use delta five as it gives me a quicker look at what I want to know, how fast I am on or off gassing. Outside of rounding that might give you a -6 for a brief minute, you should never see it less -5.

Well wouldn’t you expect to see it more negative (less) than -5? For example, you plan a 150ft 15min bottom time dive, but you end up spending the last 5 minutes at let’s say 130 <- since you’ve reduced your planned deco time, you should expect to see a more negative number than -5 at 130ft I.e. you’ve reduced your TTS

 

[Manatee Diver]

No, it doesn't know your plan it simply works up a plan based on what you dove. So you should never see less than -5.

 

[inquis]

I use @ and am looking for it to match the planned TTS. This gives me notice the ascent should start very soon. Gather the team's attention, blow a bag, whatever.

 

[ScubaSamScubaStevesFather]

No, it doesn't know your plan it simply works up a plan based on what you dove. So you should never see less than -5.

Sorry I’m having trouble understanding this. Let’s say you dive to 200ft for 10 minutes. Your tts let’s say is 20 minutes Your delta+5 would be something positive since you’re on gassing at this point, which would increase your tts.
Now let’s say at minute 11 you hypothetically shoot to 70ft (ignore ascent rates for the sake of this illustration). In that scenario, your delta should be considerably more negative than -5, because if you were to stay at 100ft for 5 minutes, that should definitely reduce your deco obligation by more than 5 minutes.

A more real world example would be what I said earlier:
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. 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”

At minute 11 I decide I’m not comfortable with the narcosis and rapidly (again, ignore ascent rates for now) come up to 130ft to finish out my bottom time before I start deco. Because I am now at 130, I would expect to be able to stay at that depth for *longer* than minute 15 because I would have reduced my bottom time <- my delta plus must show a more negative number than -5 (eg -8) because I’ve reduced my deco obligation.

Hmm, actually as I write this, I think what you mean is that it should be impossible to have more than -5 because that would mean you are decompressing above your deco stop? And as for the examples I have above, the number would not be negative because you would be ongassing at those depths.

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.

^ am I understanding you now?

 

[inquis]

Now let’s say at minute 11 you hypothetically shoot to 70ft (ignore ascent rates for the sake of this illustration). In that scenario, your delta should be considerably more negative than -5, because if you were to stay at 100ft for 5 minutes, that should definitely reduce your deco obligation by more than 5 minutes.

Once you're at 70 ft, it assumes you'll stay at 70 ft for the purpose of Delta+5. It is negative(*), but it won't be more negative than -5, because TTS itself assumes you will ascend all the way to the stop (20 ft in your example). As I mentioned before, you are making progress, but not as much as you "should" be (which is implicit in TTS).

(*) I put this profile into Subsurface, and the gradient factor is greater than 0 at 70 ft. That means you're offgassing, so Delta+5 will be negative.