Trimix blending without O2

[Wookie]

This approach assumes, of course, that you have access to banked nitrox. Believe me, that does not happen in the Rocky Mountain west.

Truth. The OP, however, may have access to different resources. I'm surprised that there isn't nitrox where you are. Without thinking about it at all, wouldn't nitrox help with altitude diving?

 

[boulderjohn]

Truth. The OP, however, may have access to different resources. I'm surprised that there isn't nitrox where you are. Without thinking about it at all, wouldn't nitrox help with altitude diving?

Sure it would, but not when your deepest local diving is 35 feet.

In order to get the depth that you need to make nitrox worthwhile (about 85 feet max), you have to drive 6 hours from the Denver area to the town of Santa Rosa, NM. Driving across town one day, I made a casual count of 24 abandoned buildings on the couple mile length of the main street--old Route 66. We are not talking about a bustling metropolitan area. There is one place to get your tanks filled in town, and that is at the shack (quite literally) at the Blue Hole where a wonderful woman named Stella will give you a fill when she is there on weekends (or during the week if she is available and you give her a call at her home.) She does maintain her filters well and will fill on top of whatever mix you put in your own tanks, but she wants nothing to do with nitrox on her own. The nearest place for a fill other than that is in Albuquerque, about 2 hours away.

There is another lake in that town that is much deeper, but it is on private property and only a handful of people are allowed to dive it. After this month, no one will be allowed to dive it.

The shop with which I do my business in Boulder goes there a little less than once a month to conduct classes, and when they do, the instructors do a lot of diving for different classes, and they like to use nitrox. They have 6 O2 clean tanks they use for that, and when they want to get them refilled, I come by with a transfill whip and an O2 supply bottle and put the required O2 in them. In return, I get to have my tanks topped off when I need it.

 

[decompression]

I don’t have a compressor but I have access to air and banked Nitrox and the shop where I work. I have access to O2 as well but just hoping to add He at home, drop of my tanks for a top off of 32% to make things simple and easy. I’m not sure that I would loose a lot of He to atmosphere (as John mentioned) as it would seem to go through the K cyl reg, to the whip and into an empty tank to then be topped up with 32%. As the He PSI is quite low the cyl should last for quite a few fills.

 

[boulderjohn]

I’m not sure that I would loose a lot of He to atmosphere (as John mentioned) as it would seem to go through the K cyl reg, to the whip and into an empty tank to then be topped up with 32%.

I just want to make sure I am understood.

Let's say you fill empty LP 108 doubles with 18/45 to a 3,000 PSI fill, using a 300 cubic foot helium bottle with a starting pressure of 2400 PSI. You will need 108 cubic feet of helium, leaving you with about 1500 PSI in the supply bottle, depending upon the temperature (which makes a huge difference). You do the dive and finish with 1200 PSI--meaning you still have 540 PSI of helium in your tanks. For the next dive, you will go shallower, and you will only need 21/35. That means you need 1,050 PSI of helium. You will need to add 510 PSI of helium to your tanks, but that is not possible, since your gradient from supply bottle to tanks is only 300 PSI. The best you will be be able to do is add maybe 180 PSI. That means that in order to get the fill you want, you are going to have to vent a lot of gas, including that expensive helium, into the atmosphere to get your doubles down far enough. If you have a booster, that is not necessary.

Here's a tip--do not put any more gas in each fill than you will need for the planned dive (with reserve, of course). That way you will have to vent out less of it before filling for the next one.

Meanwhile....

I am in despair. I had what I thought was a minor problem in my booster, and I sent it in for repairs. It turns out to be a major problem, and they are having trouble getting the parts. It looks like I will not get it back in time for a major technical diving trip planned for the weekend of the 15th. Lots of people were depending upon that booster to give them their fills. Not having that booster is being rightfully perceived as catastrophic.

 

[decompression]

Thanks John, now I understand what you were meaning with venting.