Switching Nitrox mixes underwater

[Deac in the Wake]

Preface: I do not dive doubles and (currently at least) do not do any tec diving.

I inquired with Suunto about the two Nitrox mix settings available on the Cobra3. You can set Mix1 and Mix2 for different Nitrox blends. I was curious how you can switch from Mix1 to 2 during your SI between dives. The reply was that this is only possible underwater and that if (for example) you're doing a two-tank dive with different mixes, you'll need to reset the value for Mix1 when switching to your second tank on deck.

So my question then became: if this is an air integrated computer, is switching underwater from Mix1 to 2 only possible if you're diving a twin? And if so, would you really switch Nitrox mixes during the same dive? I know when you're tec diving you have travel and bottom gas and can also use 100% o2 during deco but this is in context of rec diving.

 

[muddiver]

I use the Cochran Gemini which has the same feature. The way it works is that during the dive if you stop drawing gas from the cylinder with the sending unit on it, the computer knows that you have either switched to your alternate cylinder (and 2nd mix) or you stop breathing. The computer will continue to caculate your dive based on the ambient pressure, the change in ambient pressure and the new FO2 of the 2nd gas mix. The only really useful case I have found for this function is if I make a deep dive on one mix and then at a shallower depth on ascent I switch to a richer % O2 mix to off-gas faster. It is a way to use the theory of decompression diving, without doing a decompression required dive. I do not plan the dive outside the No-Deco range of any table or the No-Deco range of the computer, but I get a shorter surface interval. In theory there is also less Nitrogen loading.

 

[vjanelle]

If you're technical diving you wouldn't use one of these computers, you'd use a bottom timer / depth gauge(combined into the same device, a lot of higher end computers will do the same) and you'll pre-calculate your dive plan before hand(including all of your decompression obligations) and dive that plan. The chances of you and your buddy's computer both giving you the same results for a given dive are quite low.

Usually with gas switching computers you press a button (at least with my vytec I would press the mode button underwater to switch gases) which normally wouldn't function underwater since it would allow you to change the settings.

With twin tanks in a manifold, you'd have the same mix in both tanks since the partial pressure of the oxygen & nitrogen would be the same across both.

In a recreational dive setting, I guess you could use it if you wanted to shorten your surface interval and had a bottle of 40-99% and the education to use it, but an hour is hardly a long time to wait between dives.

 

[Superlyte27]

Lots of tech divers use computers just like this to switch their gas management. Blanket statements like "If you're technical diving you wouldn't use one of these computers, you'd use a bottom timer / depth gauge" simply is not the case across the board.

I'll give you an example of where a multi-gas computer comes into play. Last month I did a dive on the Spiegel Grove in Key Largo. I took 4 bottles of gas with me. My first tank was an AL80 filled with 30% Nitrox. I took two backmounted Steel 95's filled with 32% and I took an AL50 with 100% Oxygen. My dive plan consisted of going down to the propellers at 135' on 30%, switching to back gas on 32% checking out the deck and a few swim throughs and penetrations, then deco'ing at 20' on 100%. Total run time surface to surface was 1:40 with :27 minutes of deco.

My computer runs 3 gases which can only be switched from bottle to bottle underwater. A simple button push to switch bottles, then a button push to confirm selection. I don't own 4 transmitters, but my computer doesn't care. I use brass and glass on all my regulators as a backup to my transmitters on back gas and stage bottles.

 

[vjanelle]

Were you diving solo then? And did you plan you dive out including your deco before the dive?

(I'm not trying to be a dick, I'm just curious what some people practice outside what I've experienced. I know some people that fly by wire for deco, but they use tables for backup.)

 

[fnfalman]

If technical divers don't use dive computers then how come Dive Rite have the Nitek X? How come Suunto has the HelO2? How come Shearwater have a computer out? How come Liquivision have a computer out?

It's pretty funny how a 200-dives "technical diver" would presume to make broad statements about technical diving.

 

[ianr33]

If technical divers don't use dive computers then how come Dive Rite have the Nitek X? .

But have they ever sold any ?


Lots of "tech" divers use computers. I have yet to meet one that uses a gas integrated computer.

 

[vjanelle]

Never claimed I was a technical diver(starting the training, but whatever...). Just repeating what I've been told time and time again from experienced technical divers & my instructors, and what I've seen from mixed group diving(my computer telling me I had ~2 minutes of NDL when my buddy's computer was reporting 24 minutes of NDL left. We opted to ascend and follow my more conservative computer as per training).

IMHO, Shearwater & liquivision tend to aim towards the rebreather crowd, which is a different situation (ie, when you could blend your loop mix on the fly to do accelerated deco all the way to the surface if you so desired). As I said, I know people that do fly by wire deco, but they advocate (custom) tables + pre dive planning beforehand.

Don't know anyone personally using the HelO2. It's an interesting product but for it's price I'd probably spend the couple extra hundred on the liquivision or the pursuit.

But yeah.. again... Plan the dive, dive the plan. I'm not sure why in a mixed situation you'd rely on a device to calculate your decompression solely for you without any pre-planning.

 

[ianr33]

IMHO, Shearwater & liquivision tend to aim towards the rebreather crowd,

The Shearwater was aimed at rebreathers when it first came out, but has become very popular for OC diving. I know 5 people with Shearwaters. Only one of them has a rebreather. Only know 1 person with an X-1. He dives OC .

 

[vjanelle]

The Shearwater was aimed at rebreathers when it first came out, but has become very popular for OC diving. I know 5 people with Shearwaters. Only one of them has a rebreather. Only know 1 person with an X-1. He dives OC .

Ah, from their marketing literature that's the opinion I got of their products (that they tend to lean towards the RB crowd more than the OC crowd).

My original point still kinda stands though that you should still get the training and plan your dives vs (solely) depending on a computer that no one else in your team might have, but I drank that koolaid apparently a while back.