Trimix blending question

[cool_hardware52]

I could still use those prices if you have them handy, so I can do the math and see what would be worth doing.

I am almost tempted to just get a fill whip, partial pressure mix my own helium/o2 content (w/ou a booster) and then top off the tanks at a buddy's compressor. So that's what...a $250 investment?

If all you have available is a transfill whip you can blend Trimix, but you will not recover all the expensive He you are buying. If you choose to first decant the O2 and then He you will leave a lot of He in the supply bottle unused. If you first decant He, then O2, the O2 pressures you will be dealing with will be higher. This is prefered by some as the O2 is being added in the Middle of the gauge range where in theory the gauge is more accurate. If you are blending 21/35 you will get a few fills from a supply bottle of He. 35% of 3000 is ~1050. If you are trying to blend higher He mixes the yields get dismal fast. A cascade of He bottles will help, but cascades involve more tanks to rent and more plumbing.

You also will be dumping let overs. If you can't boost you can't pump more He and O2 on top of the gas left in your cylinders.

Any way you look at it costs $$


Tobin

 

[Rick Murchison]

... Two stage boosters can do much better, but they are more complex and expensive. ...

Guess I should have mentioned that ours is a three (six, depending on your point-of-view) piston, two stage Haskel. Very effective (I almost said efficient, which is untrue); no problem bottoming the gauges on the input side. (No, it isn't really zero, but it's low enough that the gauge reads zero).
Rick

 

[pescador775]

Tobin, you forgot to add quotes to that article. Scuba Dream, you have your answers. A compressor and stik are user friendly, fewer calcs and tricks to compensate for helium's funny compression ratio. Also, a stik is cheap to build. Rick is right, a Haskel two stage (15/30) will draw down a supply cylinder but it takes more drive air when the storage tank gets low. No biggie if you have a shop compressor which will put out 110 psi and about 10 cfm. Start thinking about a Helium analyzer. Weight the cost/benefit of a compressor/stik/analyzer against booster scenario. Weigh those against plain old PP filling for trimix. All the penny pinching about the cost of filling a set of doubles might be missing the point. What you should be asking is where can I get the basic mixing equipment cheap. The other stuff is expendable. Helium will be costed out regardless of how you pump it. But, it's nice to be able to use it all. Ask where can I obtain the necessary hardware at the lowest price and BTW, what is the necessary hardware? I'll repeat the answers given by others, cascades can be used for filling emergency O2 and stage bottles. Use those for that and PP filling until you save up for a booster and/or compressor. A few weeks ago, several new, Haskel boosters were sold by govliquidation.com for $1200 each. I estimate the retail at $5000 each minimum. Happy hunting but unless you are wealthy like most of these guys don't dive in until the water's warm.

 

[cool_hardware52]

Tobin, you forgot to add quotes to that article.

What article?

Tobin

 

[Gilldiver]

When I do my trimix fills I use a 3 tank cascade of 300 cubic foot bottles and then have a 4th bottle for final top off of high % mixes. Doing 50%+ mixes are tough as your supply pressure is only 2200 psi or so. So filling 3500 psi tanks gets to be tricky.

You can use you O2 whip with an adapter.

If you know your mix formula (psi of O2, He/Air) and have a good gauge (mine ran about $300 and is worth ever cent) you can get just about all of the He out of the tanks - at least down to 100 to 200 psi.

If you want to read up on boosters, get the Airspeed press book on O2 bosters. The book will show you what to look for, and look out for, and will show how you can make your own booster as a DIY project.

What I will do is drain my tanks to be filled fully. Add whatever psi my lowest bottle of He has in it - 300-700 usualy.

Now add all of the O2 you need to make your mix. If I need 300 psi of O2 aqnd I have 300 psi of He in the tank, fill to 600 psi. Acctualy I'd go to about 330 for compressability reasons, which you will learn about in your gas mixing class.

Now go back and add the rest of the helium you need to make the mix.

When I do this I am usualy making between 3 and 6 sets of 100-120 doubles and do it over 3 or 4 days.

As for the Helium tanks - lease them it will be cheeper over the year. I lease my four for 120 a year and pay about 80 and 300 in CT.

I personaly have a O2 whip with the gauge and a Helium whip. When filling a twin set I have both whips on the tanks and use the gauge for both gasses. But I have a valve imediatly upstream of the gauge and always have the needle valve and main tank valves on the O2 side shut off and the storage tank gauge has to be at zero before filling the helium.

 

[cool_hardware52]

When I do my trimix fills I use a 3 tank cascade of 300 cubic foot bottles and then have a 4th bottle for final top off of high % mixes. Doing 50%+ mixes are tough as your supply pressure is only 2200 psi or so. So filling 3500 psi tanks gets to be tricky.

I agree that a multibottle cascade will allow you to empty / capture more of the gas in the supply bottles.

Proper use of a cascade is the key. A cascade is more than a "bank" of bottles connected to a common manifold. A cascade has two or more bottles, each with it's own valve that feed a common whip.

For example lets say you need to put 1050 psi of He into pair of empty Al 80 bottles to make 21/35 (This ignores the heating and compressibility issues)

Assume you have a 4 bottle cascade and the lowest pressure supply bottle in the cascade is at 400 psi, the next is at 800 psi, the next is 1500, and last one is fresh from the supplier at ~2500 psi. (if you have freindly supplier who doesn't mind what you are up to )

With the fill whip connected to the double 80's you would first open the valve on the lowest pressure tank, and wait for the pressure to equalize. Depending on the size of the supply bottle you will end up with maybe 250 psi on both the doubles and the supply bottle.

Now close the valve on the lowest pressure supply bottle and open the valve on the next lowest pressure, wait for it to equalize. Now you have something more than 250 and less than 800 psi in both the doubles, and the 2nd supply bottle.

Repeat with the next bottle. Here you need to pay close attention as you want to end up with 1050 in the doubles and your supply bottle is above that pressure. You might be done with bottle # 3. If not you can repeat with bottle number 4, i.e. the fresh one at 2500 psi.

Eventually the lowest pressure tank is returned to the supplier, and tank #2 becomes #1, #3 becomes #2 etc. The fresh tank is now #4.

If you are starting with scuba tanks that are empty, or close to it you can always get a few psi out of the lowest pressure in the cascade. This often not the case however, as it's far more common to end the dive with a 3rd of more of the orginal fill pressure in the tank. If you are starting with 1000 psi in your doubles you won't get any gas out of the first two supply bottles in my example.

The solution is to dump the rest of your tank and start over. In the end you either dump useable gas from your scuba tanks or return the supply bottles with substantial "unuseable" gas in them, either way there's He you paid for and didn't get to breathe.

Cascades are a great "middle solution" i.e. You can recover more gas than you would with a single supply bottle, but you have more expense in tank rental and fittings. OTOH a cascade is simple, requires no drive gas and is much less expensive than a booster.

I point out the operation of a cascade only because I've seen people walk up to a cascade of O2 bottles and start with the highest pressure tank, or worse yet open all the supply bottle valves at the same time.:11:


Tobin

 

[ScubaAndDreams]

Is there any specific reason (other than having to disconnect your whip) to have the cascade connected with a common manifold?

Thanks for all the great answers so far.

 

[cool_hardware52]

Is there any specific reason (other than having to disconnect your whip) to have the cascade connected with a common manifold?

Thanks for all the great answers so far.

Nope, you can do that but it's a lot more work, and you have to bleed the whip or part of the whip depending on how it's constructed, each time you move from supply bottle to supply bottle.

I used to refill ABO bottles in General Aviation Aircraft from a cascade, and moving the whip from bottle to bottle was an extra PITA that made the purchase of the pig tails and tee's worthwhile.

BTW it's worthwhile to have a means to mark the remaining pressure in each supply bottle. Hang a 3 x 5 card around the neck.


Tobin

 

[ageddiver]

If you're only blending for yourself, it's OK and probably less expensive to just go with the whip. Not only is the boosting equipment expensive to buy, but it can be expensive in $ and time to maintain. Unless the price of He there is very high, you'll only pay the extra equipment off if you're dividing the costs with others. The best investment I made was a compressor to top off the mix and keep me from having to drive to the shop for air.

You might check to see if any of the local shops will let you mix there. There's at least one shop around this area that will let you use their station if you take a blending course from them. All you pay for is the gas you use at their prices - it's a great deal.

 

[ScubaAndDreams]

that's the issue...without going into too much detail...the shop has its hands bound by the very fact that they are a scuba shop...the supplier knows it and they have to use 99.999...their costs is $1.50 cf...