Is nitrox worth it for deeper rec dives?

[Storker]

The extra O2 is really just offsetting the amount of Nitrogen in the gas.

This.

The point with nitrox is to reduce the FN2 and thus the pPN2 in your breathing gas, reducing your N2 uptake. And the easiest way to do that is to increase the FO2, since you already have O2 in there. Messing about with introducing another gas (like He) in the mix is another can of worms.

 

[markmud]

I've going to have some deeper dives (110-130ft) this coming season. I'm wondering if nitrox is worth the cost for the wee bit of extra bottom time! I ran the numbers on my Perdix (medium conservatism). At 130ft with 28%, you only get 3 more minutes of NDL time than on air.

At my shop, I'd pay $14/tank (up to 100cft tank) for custom mix (you can get banked 32% for $12/tank). In addition, I'd have to have a pair of tanks O2 cleaned ($50/tank, so $100).

Most of my diving is only 2 dives a day. Occasionally I'll do 2 Lake Michigan charters in 1 day (4 dives), but most of those are shallower (less than 90ft, most under 70ft). I have a trip to the Straits of Mackinac in June (3 days of 2 dives each) and Toby in July (1 day of 4 dives 1 day of 2 dives). I'll be taking my own tanks on the Straits trip, and am up in the air about the Toby trip (Divers Den only has 6 HP100 tanks for rent) - I refuse to dive aluminums now. Screws with my weighting.

I got a pair of tanks O2 cleaned last summer, but I really only used the O2 cleaning when I was on my Alpena, MI trip. I could have easily done those dives on air.

I've debated this with several friends and they all just do deeper rec dives on air. I didn't get my new doubles setup (tanks I already had) O2 cleaned, either.

Hi Marie13,

I use Nitrox whenever I can. On a dive to 130 fsw I will use 28%. Usually, I use 32% with an MOD of 111 feet. The difference occurs not at depth, but while I am "multilevel diving" up toward the safety stop. And, when doing multiple dives in one day, I am not close to NDL while air divers are starting to hit staged decompression.

Think in terms of the long game, not the specific dive, and Nitrox really pays off.

Have you thought about getting a 32% fill and then bleeding off some gas and trans-filling with air using a whip? The calculations are linear and quite easy to accomplish. You will need an O2 sensor to test your final mix.

markm

 

[KenGordon]

3 extra minutes is a lot on a 45 - 50 minute dive. 3 - 6 extra photos worth perhaps?

A dive to 40m/130ft is likely to have a no stop time of 6 to 8 minutes, so 3 more is a lot. Whether a 10 or 11 minute dive to 40m is worthwhile is a question though.

A bit shallower, say 30m, the difference might be extending a 20 minute NDL to 28 minutes. That is nearly the sweet spot for Nitrox.

 

[StefinSB]

Hi Marie13,

I use Nitrox whenever I can. On a dive to 130 fsw I will use 28%. Usually, I use 32% with an MOD of 111 feet. The difference occurs not at depth, but while I am "multilevel diving" up toward the safety stop. And, when doing multiple dives in one day, I am not close to NDL while air divers are starting to hit staged decompression.

Think in terms of the long game, not the specific dive, and Nitrox really pays off.

Have you thought about getting a 32% fill and then bleeding off some gas and trans-filling with air using a whip? The calculations are linear and quite easy to accomplish. You will need an O2 sensor to test your final mix.

markm

I don’t get that. Why not ask the fill station to only fill the tank to the desired pressure with 32% and then have them top off with air. No transfill whip required. The mix needs to be analyzed either way!

 

[Centrals]

A dive to 40m/130ft is likely to have a no stop time of 6 to 8 minutes, so 3 more is a lot. Whether a 10 or 11 minute dive to 40m is worthwhile is a question though.

A bit shallower, say 30m, the difference might be extending a 20 minute NDL to 28 minutes. That is nearly the sweet spot for Nitrox.

25-30m range is best on Nitrox 32.
I don't think I had ever used Nitrox 36 on that range because I would need a much bigger tank to last the pace.

 

[markmud]

I don’t get that. Why not ask the fill station to only fill the tank to the desired pressure with 32% and then have them top off with air. No transfill whip required. The mix needs to be analyzed either way!

Hi StefinSB,

Yeah, you are right. I thought of that and then guessed maybe the fill station refused to do that. Your post is the logical way to do it for sure.

markm

 

[Barnaby'sDad]

I don’t get that. Why not ask the fill station to only fill the tank to the desired pressure with 32% and then have them top off with air. No transfill whip required. The mix needs to be analyzed either way!

Yup. That's what I did with one of my tanks this weekend. I had 1,100 psi of 32% left in the tank from a fill that I used for a short dive that I did with the shop a few days earlier. I asked the shop owner to top the tank off with O2 compatible air ("hyper" air). He didn't bat an eye at that. Topping the tank off with air got the tank down to 26% O2.

I would think that the only reason a shop would have an issue with doing that is if they do not have "hyper" (O2 compatible) air.

 

[markmud]

Yup. That's what I did with one of my tanks this weekend. I had 1,100 psi of 32% left in the tank from a fill that I used for a short dive that I did with the shop a few days earlier. I asked the shop owner to top the tank off with O2 compatible air ("hyper" air). He didn't bat an eye at that. Topping the tank off with air got the tank down to 26% O2.

I would think that the only reason a shop would have an issue with doing that is if they do not have "hyper" (O2 compatible) air.

Hi Barnaby'sDad,

I only have one tank that is O2 clean. All the rest have nitrox in them. You don't need "hyper" air if a tank is filled from a bank or membrane EANX. Your shop must use partial pressure blending.

My point is:
The OP's shop may use a bank system or membrane filter system which eliminates the need for O2 clean tanks, and the process for topping off becomes much simpler. Either system, it is an easy task to make 32% into 28 percent.

markm

 

[Barnaby'sDad]

Hi Barnaby'sDad,

I only have one tank that is O2 clean. All the rest have nitrox in them. You don't need "hyper" air if a tank is filled from a bank or membrane EANX. Your shop must use partial pressure blending.

My point is:
The OP's shop may use a bank system or membrane filter system which eliminates the need for O2 clean tanks, and the process for topping off becomes much simpler. Either system, it is an easy task to make 32% into 28 percent.

markm

My shop uses partial pressure blending. The OP has said that most of the shops that she will use do not have banked Nitrox either.

My bit about the use of O2 clean air was relevant to the OP, as she's in the same boat that I am. If she wants to dive Nitrox and use the shops in her area...she'll need to get her tanks O2 clean and keep them that way.

 

[Nigel Bryant]

I've going to have some deeper dives (110-130ft) this coming season. I'm wondering if nitrox is worth the cost for the wee bit of extra bottom time! I ran the numbers on my Perdix (medium conservatism). At 130ft with 28%, you only get 3 more minutes of NDL time than on air.

At my shop, I'd pay $14/tank (up to 100cft tank) for custom mix (you can get banked 32% for $12/tank). In addition, I'd have to have a pair of tanks O2 cleaned ($50/tank, so $100).

Most of my diving is only 2 dives a day. Occasionally I'll do 2 Lake Michigan charters in 1 day (4 dives), but most of those are shallower (less than 90ft, most under 70ft). I have a trip to the Straits of Mackinac in June (3 days of 2 dives each) and Toby in July (1 day of 4 dives 1 day of 2 dives). I'll be taking my own tanks on the Straits trip, and am up in the air about the Toby trip (Divers Den only has 6 HP100 tanks for rent) - I refuse to dive aluminums now. Screws with my weighting.

I got a pair of tanks O2 cleaned last summer, but I really only used the O2 cleaning when I was on my Alpena, MI trip. I could have easily done

those dives on air.

I've debated this with several friends and they all just do deeper rec dives on air. I didn't get my new doubles setup (tanks I already had) O2 cleaned, either.

I think it all depends on what is finishing your dive. If you are coming up due to getting to your air limit then there is little point in nitrox. If it’s on NDL then there is.
I am normally on my air limit prior to NDL but have dived on nitrox. It meant I was less tired but as I was on a liveaboard at the time I actually wanted to sleep at night !