Max Helium fraction continuous blend

[Tyler duran]

I’ve been doing some research on doing trimix stick fills and it seems folks with home made stick systems have a difficult time hitting the targeted helium fraction on the first go. Which in turn might also throw the O2 fraction off.

Most just seem to adapt to it by monitoring, analyzing, and adjusting on the fly. This can lead to the dreaded bleed down and top up which is really unfortunate with the current cost/availability of helium.

My thoughts were: why not just focus on helium first? Pump an empty tank up to the desired pressure with 100% helium, once that’s stabilized and analyzed just top off with the appropriate Nitrox mix to final pressure.

I understand I’m effectively using the compressor as a booster pump however I don’t know if that’s a bad deal. Helium isn’t a fire hazard like oxygen can be so the compressor doesn’t need to be oxygen clean.

So in short, what’s the max He fraction one can feasibly squeeze through a home compressor + diy stick set up?

 

[tbone1004]

You can pump 100% helium if you want to, but one of the big problems with pumping helium through the compressors is you get blow-by in each of the stages so it is rather wasteful. That is the main reason that you have issues blending trimix through them and why most people don't. If you're doing anything more than 30/30 and/or doing high volumes, it really is worth it to buy a booster.
There are some potential heat concerns with high He mixes on the pump though so it is critical that you monitor head temperatures while pumping

 

[Tyler duran]

Tbone, super glad you dropped in. Any ball park guess how much is wasted pumping 100% He? Maybe this isn’t even worth experimenting with.

He is expensive enough, if a booster is the only logical means of packing it, I’ll stay away all together.

 

[tbone1004]

Tbone, super glad you dropped in. Any ball park guess how much is wasted pumping 100% He? Maybe this isn’t even worth experimenting with.

He is expensive enough, if a booster is the only logical means of packing it, I’ll stay away all together.

if you look through the forum archives here and on "the deco stop" there is a lot of good information there, but I would expect 5-10% based on what I've heard about the levels dropping.

 

[Tyler duran]

Hey that might be acceptable, definitely close enough for me to take the gamble and see what I get.

I’ll mind build the helium plumbing a little more before I uncork a K of UHP I recently got on trade. I’ll report back, but I suspect the waste won’t pay off a decent booster in my lifetime.

 

[tbone1004]

@Tyler duran you won't pay for it in waste, but it is certainly infinitely more convenient to have one... granted I have 2 ;-)

 

[rjack321]

I pump 100% helium up to about 2400 psi and maybe 40-50mins runtime. More than that and the oil will get foamy and that's bad news lubrication wise.

There is no "loss", blow-by just ends up getting recirculated back into the 1st stage. 100% helium actually runs cooler than CBing trimix, at least at modest pressures. You still have to make an educated guess about how much is in your filter stacks, the larger your tanks the less that guess needs to be accurate. I just shut off the helium, pull the bag off my filter inlet, and push the residual helium through the pump with 32% as a top off gas. I get within 1% of target on helium this way and have been doing this for almost 10 years.

 

[tbone1004]

@rjack321 that's why I said depending on compressor. In the Rixes it is lost forever...

 

[Billy Northrup]

I focus on the O2 number and make sure I have enough HE. Small batch trimix is hard to hit but running into a bank allows for tweaking on the fly. When I worked at a dive shop, I'd hit my numbers running into a bank then start filling up 6-10 sets of doubles.

 

[Tyler duran]

Ok well I got a temporary setup pieced together and did some tests. The focus was just to see how much He I could affectively extract from an industrial K bottle and into empty/swept large OC tanks. After which, CB some Nitrox on top to make 21/35.

Before even starting I acknowledged that you can PP several bottles with enough He to accomplish a 21/35 mix so I eliminated that from my waste calculations. I let some locals with CCR’s grab the top off the He K for their little dil bottles. So I started with a K of He at just a smidge above 1k psi.

I also realized I didn’t really need my stick. I don’t need to blend or decant anything so why waste He filling it. I just put together a basic whip to connect the He reg to the compressor, and just used the reg to set flow control on the fly. This might be a little harsh on the compressor on first start and shutdown.

My process wasn’t a fast one but it did work! While pumping the 100% He I monitored compressor head, case, and oil temps closely with a calibrated thermometer and only noted a slight increase in the head of about 20 degrees compared to head temps while pumping air. My oil never got frothy/foamy and was also monitored closely.

After the He was in the tanks I let the tanks stabilize for a few hours and re-rigged the compressor for CB’ing nitrox. All my tank pressures were happy so I started pumping again. Once the first tank was done I was bummed at what the analyzer was indicating. Both O2 and He fractions were way off and the analyzer was constantly changing its mind.

I talked with the owner of the analyzer (loaner from one of lucky RB dil bottle owners) and they said to just let it sit for a few hours and stabilize and check it again. Well that did it. The analyzer was consistently indicating 21.5/37. I guess packing nitrox on top of pure He takes a little while to mix? Maybe stir it next time.

After all was said and done I wasted less than 5% of the He I started with and I’m sure most of that was in the whips. I highly suspect I waste way more O2 in the stick but that’s cheap and needs to be decanted for safety reasons obviously.

for now I believe I have continued to elude buying a booster. Only time will tell how hard this is on the compressor. Hopefully my trimix needs (minimal) won’t lean on it too hard. Last step is going to be to send a few samples off and see what the report sniffs out.