Petrel average depth calculation

[rhwestfall]

I need to go look at that data some more.... didn't catch a difference from the Petrel and Desktop, but wasn't looking for it......

 

[Dr. Lecter]

My Petrel is in surface mode now as I am 10 days after my last dive. It shows max depth 42m and average depth 21m, so it seems to me that it is not weighted average but just max depth divided by 2.

It may seem that way to you, but as someone who has had max depths past 100m and average depths way shallower than 50m due to all the deco time, and cases where I was more aggressive on deco and wound up with an average depth deeper than half my max depth, I can assure you you're wrong. Which is not to say I know for sure how a Petrel calculates average depth - but it's not that.

Your earlier request about "on/off" for so-called deep stops suggests you're not doing technical diving and probably have the Petrel in the OC Rec mode. You have to decide what a "deep stop" is before you can make them a binary, "on/off" thing. Normally we set our first/deepest stop using VPM conservatism or the low GF setting...so it's not a set type of stop so much as it's a choice about where to start your hangs.

If you're in OC Tec, you should know how GF and/or VPM works and set your conservatism where you want it to get the deepest deco stop you want. By the same token, there's little value to seeing your next series of deco stops since you know they can change based on how you handle your current deco stop and the ascent from it to the next one.

If you're in OC Rec, the Petrel manual for that mode says "The Petrel does not do “deep safety stops”. That is, there are no extra stops added around 50ft to 60ft (15m to 18m) when ascending from a no-deco dive." So, if you want a deep safety stop, you'll have to plan and execute it yourself - say, two minutes at one half your max depth. The Petrel's GF info can be valuable there, as it will show you how much you're offgassing (if at all) when you're holding at a deeper depth. You can't really tweak the LO GF in OC Rec, but you can pick something that's going to be more conservative and call for a deeper 1st stop if you do go into deco:

Conservatism

The decompression model conservatism can be set to three fixed levels:

•
Low (45/95)

•
Med (40/85)

•
High (35/75)

The corresponding gradient factors are shown for

reference, but cannot be edited directly.

Low conservatism means longer NDL times and less

decompression. High conservatism means shorter NDL

times and more decompression.

The Low setting is similar to PADI and NOAA no-stop time

tables for air and nitrox diving.

Safety Stop

The Safety Stop setting can be set to the following values:

•
Off

•
3 minutes

•
4 minutes

•
5 minutes

•
Adapt

When using the Adapt setting, a 3 minute safety stop will

be used, unless the dive exceeds 100ft (30m) or the NDL

falls below 5 minutes, in which case a 5 minute safety stop

is used.

 

[tcoen]

Hello,

Can someone explain me how is the average depth calculated on Petrel?

Hi Insomnia,

The average depth is calculated during a dive using the standard running average (or cumulative moving average) calculation. See:
Moving average - Wikipedia, the free encyclopedia

There is no weighting. However, the average is not updated when in surface mode, or when in dive mode and shallower than 1.6m (to prevent surface intervals from dragging down the average).

btw. Do you plan to include list of mandatory deco stops in new Petrel firmware? It would be nice to see complete list of all deco stops and not just the first deco stop.

During a dive, if you run the Dive Planner in the Dive Setup menu, you will see a list of all future stops based on the current conditions.

I guess there is no chance to add a feature to ON/OFF deep stops in the future?

I'm not quite sure what is meant by this question. If like Dr. Lecter mentioned, you mean the deep "safety stops" on no deco dives, then this is not a feature we offer. For deco dives, the Buhlmann ZHL-16C with Gradient Factors decompression algorithm does allow adjusting the deep stop behavior by changing the GF Low setting. I recommend some reading or discussions with a tech instructor if changing the GF settings from the defaults. Note that the Petrel OC Rec mode has a few presets to simplify this. A few resources are:

Clearing up Confusion about Deep Stops - by Erik C. Baker
http://www.shearwater.com/wp-content/uploads/2012/08/Deep-Stops.pdf

Deco for Divers - by Mark Powell

Video on Petrel Tissues graphs
Petrel Tissues Demo - YouTube

I hope this helps answer your questions.

Best regards,
Tyler Coen
Shearwater Research

 

[Dr. Lecter]

During a dive, if you run the Dive Planner in the Dive Setup menu, you will see a list of all future stops based on the current conditions.

Cool, learn something new every day.

 

[insomnia]

As I said previously, it seems that Petrel calculates weighted average depth but strange is that average depth is diffrent on Petrel and Shearwater desktop.

I am using Petrel in Tec mode as I dont like Rec mode look and no availability to choose GF based on the dive conditions (just 3 levels of conservatism mentioned in your post).

 

[tcoen]

but strange is that average depth is diffrent on Petrel and Shearwater desktop.

I'll talk to the Desktop developer about this. I imagine that the Desktop might be handling the near-surface behavior differently.

Best regards,
Tyler

 

[insomnia]

Thank you very much for all your answers, help and support - really appreciate that.

 

[rhwestfall]

A little data crunching:

PDC: average depth reported: 61'
Desktop: average depth reported: 59.3'
Xcel: (1) average function applied to all depth data: 59.344'
(2) average function omitting zero values at end: 60.475'
(3) average function omitting first value and zero values at end: 60.528'

 

[HIGHwing]

Cool, learn something new every day.

I did not know this either...

 

[Stoo]

You really should know exactly what stops you are going to make before you even begin a decompression dive, and ideally, they should be written on your fins...

Awesome idea... with my increasing farsightedness, I could probably actually read my plan.

I'm waiting for Shearwater to come out with a Senior's version of the Petral... about the size of an iPad, with BIG numbers...