Trimix blending question

[ScubaAndDreams]

Good day,

I used to blend my own Nitrox in my garage a couple of years ago. Partial pressure w/a fill whip and then just top it off with clean air. This method was relatively cheap as all I really had to pay for was the whip.

Now, if I want to go about blending my own trimix (yes I will take an adv mix course...I just want to know what $$$ i am getting in for) I am assuming I can do the same thing with the whip, but will need some sort of booster to boost the Helium storage bottles so I can use most of the helium in the bottles.

I know nothing about boosters...how much are they and more importantly how complicated and how much of a pain are they (I am assuming that the booster and actually procurring the helium bottles is the only additional cost I would have).

Thanks

 

[cool_hardware52]

Good day,

I used to blend my own Nitrox in my garage a couple of years ago. Partial pressure w/a fill whip and then just top it off with clean air. This method was relatively cheap as all I really had to pay for was the whip.

Now, if I want to go about blending my own trimix (yes I will take an adv mix course...I just want to know what $$$ i am getting in for) I am assuming I can do the same thing with the whip, but will need some sort of booster to boost the Helium storage bottles so I can use most of the helium in the bottles.

I know nothing about boosters...how much are they and more importantly how complicated and how much of a pain are they (I am assuming that the booster and actually procurring the helium bottles is the only additional cost I would have).

Thanks

Sherman,

A nice Haskel can be had for ~1500-3000, not sure about the local Isalnd prices.

In addition you need a source of drive gas, i.e. expendable gas to operate the booster. You can run a booster off a scuba tank with a first stage to drop the pressure, but if you are buying air fills that gets expensive fast. A better choice is either a low pressure " shop air" compressor (the drive gas need not be clean) or an electric booster. I don't know much about the electric boosters, but somebody with more info will be along here shortly I'm sure.

As long as all you want to boost is Inerts you don't need a O2 clean booster, and that will save you $$. Home builts start to look interesting here.

You may also want to consider a home HP compressor and continous blend both your Nitrox and Trimix. It's being done, lots of good info regarding this at TDS.

You either need an expensive booster and shop air compresor to run it or a home scuba compressor and nitrox / trimix stick.

If you go the compressor route you would no longer need dive shop air tops either.

OTOH hand if you go the full booster route, and get a O2 clean setup, you will have ready source for O2 deco bottle fills. Boosting O2 to high pressure carries more risk than boosting He however.

If you are already buying O2, one strategy is to trans fill your deco bottles from new O2 supply bottles, and then continous blend using the remaining O2 in the supply bottles. You won't be getting full fills in your deco bottles, best is 2400-2500 from a fresh supply bottle, so you might need a couple more deco bottles, but they are cheap compared to a O2 clean booster.

There's no one perfect answer.


Tobin

 

[Scuba-Jay]

interesting question ill be interested to hear more responses!!

 

[rjack321]

With a 2 cylinder cascade, I get my 300cf/2400psi "Ts" of helium down to about 400psi before returning them. Boosters really won't get you lower than that, but you will have to dump gas in your scuba cylinders to put it in.

Miniboosters are slow but fairly cheap. I'm looking for a used on right now.

 

[ScubaAndDreams]

rjack...

How many fills of say 21/35 or 18/45 would you say you can get off that cascade assuming you;re using Al80 dbls (which you prolly dont given your location).

Oh and if people wouldn't mind posting how much they are paying for their Helium storage bottles (and grade) so I can get an idea of going this route is worth it. The dive shop I call my LDS (and really do like a lot) is charging 2 bucks/cf.

 

[do it easy]

With a 2 cylinder cascade, I get my 300cf/2400psi "Ts" of helium down to about 400psi before returning them. Boosters really won't get you lower than that, but you will have to dump gas in your scuba cylinders to put it in.

Miniboosters are slow but fairly cheap. I'm looking for a used on right now.

Are the mini boosters rated for the kind of duty cycle that filling doubles would require?

Also, not a direct question for Richard, are there any precautions for boosting helium that will be used as a breathing gas? I've seen some people boost argon through a hydraulic piston/gizmo, but I certainly wouldn't want to breathe from it or from the tank it was pumped into. Are Haskels that aren't O2 clean, clean enough for breathing gas?

 

[Rick Murchison]

With a 2 cylinder cascade, I get my 300cf/2400psi "Ts" of helium down to about 400psi before returning them. Boosters really won't get you lower than that, but you will have to dump gas in your scuba cylinders to put it in.

With a booster you can suck that He tank right down to zero, and there's no need to drain your scuba cylinders other than making the math easier.
Rick

 

[rjack321]

With a booster you can suck that He tank right down to zero, and there's no need to drain your scuba cylinders other than making the math easier.
Rick

I don't have much experinec with boosters other than a masterline at a shop I know. It struggled about 400 psi and just couldn't get below 250psi.

The haskel miniboosters are better suited for RB bottles but they can handle 20-40cf pushes here and there. They can be had for <500 used.

Yes you can breath their gas without the O2 cleaning.

 

[ScubaAndDreams]

I don't have much experinec with boosters other than a masterline at a shop I know. It struggled about 400 psi and just couldn't get below 250psi.

The haskel miniboosters are better suited for RB bottles but they can handle 20-40cf pushes here and there. They can be had for <500 used.

Yes you can breath their gas without the O2 cleaning.

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?

 

[cool_hardware52]

Air driven boosters are nothing more than an intensifier. A large diameter piston driving a small diameter piston. Large volumes of low pressure drive gas can pump smaller volumes of higher pressure gas.

A booster has a couple key specifications. One is maximum output pressure based on drive gas pressure. This is a function of the ratio of the area of the large piston to the small piston. For example if the large piston has a cross sectional area of 40 sq inches, and the high pressure piston has a cross sectional area of 1 square inch then the ratio is 40:1. With 100 psi drive gas you can generate 4000 psi at the output.

The second spec is the compression ratio of the HP side of the booster. This is a ratio of the volume of the HP cylinder when the piston is retracted vs the volume when the piston is fully extended. "Dead space" or volume in the hp cylinder that cannot be filled by the moving piston such as the volume between the check valves and end of the cylinder, the compression ratio is always less than infinite. (If there was zero dead space the volume of the retracted piston would be divided by zero, and the math wizards out there know ya can't divide by zero)

It is this ratio, the compression ratio of the HP cylinder that determines how far the supply bottle can emptied to.

For example if the compression ratio is 5:1 If you have a supply gas bottle at 500 psi you can at best expect to compress this gas to 2500 psi. If the bottle you are trying to fill is below 2500 psi then you will succeed in pumping at least some volume of gas. If the either the supply gas pressure falls below 500 psi, or the bottle being filled rises above 2500 psi all you will do is squeeze the supply gas into the "dead space" but you will never open the outlet check valve. Minimizing dead space is a key design criteria in compressors.

The solution to this problem is a multistage booster. That's exactly why scuba compressors typically have 3 or 4 stages. A scuba air compressor has "supply gas" a 1 ata and an output at maybe 300 bar.

There are many variation in air driven boosters. Single stage boosters will have a very hard time using low pressure >500 psi supply gas, just as the supply bottle pressure drop the pressure in the tank is rising, conspiring against draining the supply bottle.

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



Tobin