Why no accurate computers?

[NAM001]

I'm pretty sure he was talking about a multi-level dive dive with a maximum depth of 100 feet.

then you don't have a 60 minute dive at 100 ft, which is what my reading of your comment implied. 100 ft is only good for 20 +/- min and then it turns tech because of involvement of deco protocols. I like many have made many hour plus dives which included but a few minutes at 100+ ft. I do a wreck dive and go to depth to explore the bottom of the ship for a few minutes and then beet feat to less than 60 for the remainder of my allowable air time at the top of the ship. depending who wants to get picky you can get techy and call it a prolonged uncalled for, deep stop at 40 ft or just a multilevel dive depending on ones tech/rec perspective.

 

[boulderjohn]

I have dived with a few people who use two identical dive computers with two different GF setups.
Computer 1 - Normal diving, GF's
Computer 2 - Very aggressive GF's 100/100

Under normal circumstance, the dive is carried out using computer 1.
If there is an incident, gas loss, decompression gas loss, bailout, medical issue, etc. And the diver needs to exit ASAP, i.e. before running out of gas or because of some other emergency. Then computer 2 is followed, with the expectation that a bend is likely.
This follows the rule that "we can fix a bend, we can't fix drowning".
It is also known as the KYAGB (kiss your ass good bye) plan.

I don't see the point of this, especially now that some computers have the SurfGF feature.

Let's say I have set my GF high to 75 and have a problem that makes me want to surface sooner. The SurfGF factor tells me what my surfacing GF will be at that time, and I can decide on the fly what number will be OK for me to surface.

 

[boulderjohn]

then you don't have a 60 minute dive at 100 ft, which is what my reading of your comment implied.

What you are saying is you misread my comment.

 

[OTF]

There are no accurate computers because there are no accurate models of decompression.

We need to use conservatism in computers or tables because we have no idea what is actually going on in a given person on a given dive. Don't let the complexity of the math, decimal places on the tables, or price of the computers convince you that any of them are anything other than statistical guesswork mostly based on how many goats and sailors we boiled in the 1950s.

If you ride the edge of a super liberal table or computer you are statistically setting yourself up for a bad time, even if it is just one out of one thousand eventual dives.

Dive conservatively, and gain back those precious extra 5 minutes of bottom time with nitrox, multilevel planning, or a little bit of hangtime.

 

[Gareth J]

I don't see the point of this, especially now that some computers have the SurfGF feature.

Let's say I have set my GF high to 75 and have a problem that makes me want to surface sooner. The SurfGF factor tells me what my surfacing GF will be at that time, and I can decide on the fly what number will be OK for me to surface.

Thats OK if you have a SurfGF Factor (which I assume is a feature on a particular computer or series of computers). Some computers allow you to change the GF on the fly. But, the whole purpose was to allow the diver to exit as quickly as possible in an emergency, whilst stressed, without the delay or stress of changing the settings in water.
It also allows the diver to check the 'conservative' TTS, or the aggressive TTS, and potentially choose something in between.
It's not a practice I have personally used, but I understand the why.

You could say I do something slightly similar, because I always dive an OC computer alongside the CCR (computer).
The OC computer gives me a worst case TTS if I bailout. The real bailout TTS will be less because the CCR will have accurate PO2 data from the CCR dive prior to the OC recalculation when I switch the CCR computer over to bailout. (assuming I have time to do this or that the reason I've bailed out isn't because the electronics have just failed.)

 

[Twoblues]

Hello, boulderjohn
My Oceanic Oci computer gives me the choice of Dsat and Pelagic z. Are both closer to PADI's than Navy?
Mike

 

[Gareth J]

There are no accurate computers because there are no accurate models of decompression.

We need to use conservatism in computers or tables because we have no idea what is actually going on in a given person on a given dive. Don't let the complexity of the math, decimal places on the tables, or price of the computers convince you that any of them are anything other than statistical guesswork mostly based on how many goats and sailors we boiled in the 1950s.

If you ride the edge of a super liberal table or computer you are statistically setting yourself up for a bad time, even if it is just one out of one thousand eventual dives.

Dive conservatively, and gain back those precious extra 5 minutes of bottom time with nitrox, multilevel planning, or a little bit of hangtime.

Thats why we use GF's!

It's also why we use the computer between our ears (I hope).

If I have been working hard throughout the dive, my gas loading will be higher than 'normal' so I'll pack extra time on the end of the dive to allow for this.
If I'm cold on the stops, and my circulation is reduced, I will in all probability add time on the decompression to allow for this (assuming I'm not going hyperthermic).
Similar for stress, fatigue etc.

The computer between your ears is the most important one!

 

[boulderjohn]

Hello, boulderjohn
My Oceanic Oci computer gives me the choice of Dsat and Pelagic z. Are both closer to PADI's than Navy?
Mike

The DSAT algorithm is essentially the same as the PADI tables. DSAT stands for Diving Science and Technology, and that is the PADI-affiliated group that created the PADI tables. Pelagic Z+ will be close to Buhlmann. Neither will be close to the Navy tables.

 

[boulderjohn]

Thats OK if you have a SurfGF Factor (which I assume is a feature on a particular computer or series of computers).

It's on all Shearwaters that have updated to current firmware.

 

[scubadada]

Hi @Twoblues

The USN tables are not designed for sport diving repetitive dives First dive NDLs are significantly longer than DSAT or PZ+. The PADI recreational dive planner was specifically designed for repetitive dives, with a shorter surface interval. As per @boulderjohn the DSAT computer algorithm was derived from the same dataset and is extremely similar. PZ+ is a proprietary version of Buhlmann, with changes not available in the public domain. DSAT 1st dive NDLs are longer than Buhlmann with a GF high of 95, however, on repetitive dives, Buhlmann tends to become relatively more liberal, often having NDLs as long or longer than DSAT, particularly at shallower depths. PZ+ is more liberal than Buhlmann GF high of 85 on the 1st dive, but they are more similar on repetitive dives.

First dive NDLs are relatively easy to come by and compare, but they don't reflect how computers behave on repetitive dives. I have been diving DSAT since 2002, about 1950 dives. I have been diving Buhlmann in parallel since 2016, nearly 800 dives.