Deco Question

[takis_s@bigpond.net.au]

Hi to all and also would like to say what a fantastic pool of information this site is!!
Just a question that i also posted on the gavin scooter list.
If i would use a tx 50/25 mix instead of the normal Nx 50 what would be the time difference frrom the 21m to the 9m stops using a 18/45 backgas and a bt of 20 min in a depth range from 48 to 54m.
Thanks in advance for any replies regarding this.

[boomx5]

i wouldn't change anything up myself. the only time i add some helium to my 70' bottle is if i'm using an al80 to make it a bit on the lighter side.

[Bob Cooper]

Quote: (Originally Posted by takis_s@bigpond.net.au)

Hi to all and also would like to say what a fantastic pool of information this site is!!
Just a question that i also posted on the gavin scooter list.
If i would use a tx 50/25 mix instead of the normal Nx 50 what would be the time difference frrom the 21m to the 9m stops using a 18/45 backgas and a bt of 20 min in a depth range from 48 to 54m.
Thanks in advance for any replies regarding this.

It wouldn't add anything to the deco.

[Ahmed Adly, Marlin Inn DC]

Quote: (Originally Posted by takis_s@bigpond.net.au)

Hi to all and also would like to say what a fantastic pool of information this site is!!
Just a question that i also posted on the gavin scooter list.
If i would use a tx 50/25 mix instead of the normal Nx 50 what would be the time difference frrom the 21m to the 9m stops using a 18/45 backgas and a bt of 20 min in a depth range from 48 to 54m.
Thanks in advance for any replies regarding this.

I think the main reason for the 25% He would be to make the tank ride better. Also, it would make for easier breathing of course. But when you have multiple stages, the least tanks you have sagging, the better.

Ahmed

[davy]

I also don't change anything and in fact feel better with the 50/25 after a dive on a high helium content bottom mix.
for 21/35 as bottom mix I usually don't bother with helium in the decogas other than to lighten up the tank maybe if I have to use an AL80 for example.

From 18/45 and up I like helium in the decomix in the way the gas switch gradually lowers both helium and nitrogen. instead of just taking away the helium and potentially spiking the nitrogen.

[Brian Allen]

Quote: (Originally Posted by Ahmed Adly, Marlin Inn DC)

I think the main reason for the 25% He would be to make the tank ride better. Also, it would make for easier breathing of course. But when you have multiple stages, the least tanks you have sagging, the better.

Ahmed

I can't think of anything worse, if it were for diving in the UK. I look forward to the switch onto 50%, i start to really warm up instead of breathing cool cold helium. I don't look forward to the backgas breaks at all before the 100% switch or after 12 mins... brrrrr.

[Hassan Adly]

Hi,

Looks like you're diving in the wrong lattitude, Brian!

I would not change anything in the deco. The deco will actually be better than with 50% nitrogen. It's not that easy to quantify though, so I wouldn't try to actually reduce the deco time.

As mentioned above, an aluminium 80 stage will ride better with 25% helium in it.

[Mark Chase]

If you look at the off gassing profile of the Helium in the bottom section of this link (where they discuss 50/50 deco mix)

http://www.hhssoftware.com/v-planner/decomyths.html

You will see that its still the Nitrogen that controls the overall length of the deco. Using Trimix deco as you describe will slow down the off gassing of the Helium because it will reduce the difference in the gradients but it should increase the off gassing of the nitrogen a bit.

So with the DIR preferred curve I cant see any problems ignoring the he in the deco mix. This should be done in the full knowledge that it puts much greater emphasis on controlling the stops above 21m and is totally unsuited to Buhlman type ascents or people whos stops look like a rip saw blade when you print out the profile..

ATB

Mark Chase