What's the point of these 3 gas computers?

[FishOutUvH2O]

For teh not-so-hard-core diver that has the skill and training to dive more than one EAN mix the gas switch is useful. Point: If you do a 90' dive for 29 minutes (USN No-D Table Rev. 6) breathing air or 32% EAN and during the ascent you ****ch to a pony/deco cylinder of 40% EAN at 70' you are now off gassing Nitrogen at a faster rate than if you ascended and decompressed on your original gas of either 32% or air.

Using a computer that can switch between the two planned gas mixes during this dive plan will allow the computer to compute your decompression and Nitrogen loading based on the decompression on EAN 40 and it will provide a lower repetive group designation and possably a shorter required surface interval time between dives. Otherwise diving more than one Nitrox mix just pads your safety based on one table but does not provide any other benifit.

Let me see if I've got this straight. And please bear with me on this one.

Let's say there's a multi-level, no-deco dive I want to make that has different things I want to see at different depths. With a computer that has gas-switching capability would I be able to extend my time at each thing I wanted to see by switching to a different gas mixture with a higher O2%? For example:

I want to see a wreck at 90fsw and I use EAN32 to maximize my time-on-target and this gives me X minutes of NDL. I then want to see a part of the reef wall at 70fsw. Are you saying that I could switch to a higher EAN mix and the computer would recalculate my NDL, giving me even more time at that depth than I would've had with the EAN32? Or am I just way off?

 

[RJP]

In marketing don't you generally have a "target consumer" to aim that marketing at?

I've already said too much!

 

[RJP]

Let me see if I've got this straight. And please bear with me on this one.

Let's say there's a multi-level, no-deco dive I want to make that has different things I want to see at different depths. With a computer that has gas-switching capability would I be able to extend my time at each thing I wanted to see by switching to a different gas mixture with a higher O2%? For example:

I want to see a wreck at 90fsw and I use EAN32 to maximize my time-on-target and this gives me X minutes of NDL. I then want to see a part of the reef wall at 70fsw. Are you saying that I could switch to a higher EAN mix and the computer would recalculate my NDL, giving me even more time at that depth than I would've had with the EAN32? Or am I just way off?

Yup, you got it in theory. Though the 90' vs 70' is such a small change in depth that it wouldn't be worth carrying a second gas with you to make the switch. You might get 2-5min more NDL time.

A better example might be a dive here off the coast of NJ on the wreck of the USS Algol where you could dive 28% at 130' feet to explore the bottom of the holds or get some scallops out in the sand for 20min and then ascend to 70' and switch to 50% to tour around the top of the superstructure for another 40min. This approach lets you do a really nice 60min dive as a no-stop dive instead of just spending 12min on the bottom and coming back up, which is what you'd have to do if you only had 28% available.

 

[gcbryan]

Well, given that Tech consists of many aspects and the use of He is only one of them. So, it is incorrect to state that a 3 gas, non-He, computer is not valid.

However, if you had expressed a desire to progress onto Trimix diving and wanted to buy a suitable computer to run those dives, then it would be fair advice to counsel you towards a more capable and 'future proof' computer.

For me, I just don't have enough regular access to depths that require Trimix, but my bottom times do require accelerated deco. That could change in the future... but today's market choices of He computers could also be redundant by the time that does....so I am not rushing out to buy something that may be antiquated before I can make the best use out of it.

I understand the usefulness of a two button non-He computer but just how useful is the third button. For example at the depths that you are talking about are you really using two deco gases? I know you could use 100% and 50% but at non-He depths is there much point?

 

[RJP]

I understand the usefulness of a two button non-He computer but just how useful is the third button. For example at the depths that you are talking about are you really using two deco gases? I know you could use 100% and 50% but at non-He depths is there much point?

Absolutely. Refer to the example I gave above of the USS Algol. If you wanted to do that as a deco dive here's what it looks like with 3 gasses vs 2:

3gas:

130' for 40 min on 28%
90' for 30 min on 40%
15' for 10 min on 100%

2gas:

130' for 40 min on 28%
90' for 30 min on 40%
15' for 21 min on 40%

Importantly, on the 2 gas version you'd need approx 100cf of the 40% mix. How ya gonna carry that?

You might say, "Do it with 28% and 100%..." but then you'd still have 21min of deco, and now you'd need 222cf of 28%.

This may not be the best example (and I'm not factoring 1/3rds or rock-bottom gas planning) but for longer multi-level deco dives within "recreational depths" there's a realistic role for 3 gasses.

 

[ianr33]

I understand the usefulness of a two button non-He computer but just how useful is the third button. For example at the depths that you are talking about are you really using two deco gases? I know you could use 100% and 50% but at non-He depths is there much point?

That depends what you consider non He depths to be

A profile such as 150 for 30 minutes justifies a second deco gas (or at least it does for me) On a dive such as that though any of the 3 gas nitrox computers would only be a backup,whereas a "real" tech computer could be the primary tool during the dive (backed up by contingencies for every unlikely possibility of course )

 

[DevonDiver]

That depends what you consider non He depths to be

Very true!

 

[FishOutUvH2O]

Wow! Fantastic info. Thanks to everyone, especially the OP for asking.

It would appear from the responses that the multi-gas switching function can be a useful one. But, only for a certain type of diver. If you agree with the computers decompression model, algorithm, data tables, required deco stops, etc. Then these "sort of" tech computers would be just fine. It would also appear, that most of the tech divers here would not fit that criteria. And for good reason. You have the knowledge, information, and training to make far better and more informed decisions than these types of computers can.

If knowing just about everything there is to know about an activity causes a lack of interest in it. It would appear that I'll be diving for a very long time.

 

[Rainer]

A better example might be a dive here off the coast of NJ on the wreck of the USS Algol where you could dive 28% at 130' feet to explore the bottom of the holds or get some scallops out in the sand for 20min and then ascend to 70' and switch to 50% to tour around the top of the superstructure for another 40min. This approach lets you do a really nice 60min dive as a no-stop dive instead of just spending 12min on the bottom and coming back up, which is what you'd have to do if you only had 28% available.

Awesome suggestion to push your CNS clock above 100%.

Any other great suggestions?

 

[ianr33]

A better example might be a dive here off the coast of NJ on the wreck of the USS Algol where you could dive 28% at 130' feet to explore the bottom of the holds or get some scallops out in the sand for 20min and then ascend to 70' and switch to 50% to tour around the top of the superstructure for another 40min.

Apart from the O2 issue that Rainer pointed out,anyone using 50% is presumably advanced nitrox/deco certified (regular nitrox maxes out at 40%)
If deco certified why not just do it as a deco dive and stop messing about trying to do an extended NDL dive?