pescador775
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I'm not a tech diver but a freediver who also has an MS in physics. I got into physics because I like to build stuff and to play with gas equations. I ended up doing electronics engineering which is a drag.
I read the Oxygen Hacker's Companion, fourth edition, and came up with several new ideas. I refer you to pages 46 and 110 in which various design ideas and techniques are offered for building and using trimix "sticks". I feel these are unnecessarily complicated and expensive so offer you a simple, foolproof device at lower cost. Only one stick need be built and only one O2 analyzer is necessary. Forget the He analyzer, the flowmeters, etc, not needed. Forget the compressibility coefficients, not needed.
Build your stick in the usual way except make it about 3 feet long and, instead of one, install two gas ports on the side near the top. Make the intake filter by drilling holes in a round PVC pipe cap of appropriate size. Insert an 1/4", slightly oversized dacron filter pad (used in air compressor filters), in the cap. Insert the filter pad so that it conforms to the round shape of the cap. The baffles are made by stacking several stainless wire pads on top of a foam filter. The foam filter is made by cutting a thick round section from some packing foam, the permeable type. These items rest against some sheet metal screws installed in the side of the PVC pipe. Install the O2 sensor as shown in the oxyhacker, in line at the bottom.
Hook the oxygen and helium lines to the fittings near the top of the pipe. Hook up the supply tanks with Harris oxygen regulators (EBay, 10 bucks) which have been modified by drilling the restricter ports with a #60 bit. Start up the compressor with filter drains open.
For purposes of discussion, assume that a 17/45/38 mix is needed. Start the helium and gradually increase the flow. Watch the oxygen analyzer while adjusting the helium. Initially, it should read about 21% or a bit less (check your calibration chart). When the O2 level drops to 11.5% lock the helium knob in place (or just keep everybody away from it). Remember, the helium fraction is 45% leaving 55% for air (or enriched air)-----thus, 0.55 X .21 X 100 = 11.5%. Turn on the oxygen and slowly increase flow until the analyzer (remember, only one is needed) reads 17%. Close the compressor drains and fill the SCUBA tank(s). Watch the O2 level and tweak as needed. When tanks are full, close the SCUBA valve, shut off oxygen, shut off helium, stop compressor, in that order. Check oxygen percent in the SCUBA. You are done.
The Harris regulators are the 50 psi type and are ideal for nitrox when modified as above. They are usually military surplus, chrome plated brass, single stage. Of course, the trimix stick can also be used for nitrox fills. I carry a shorter version on trips for nitrox fills from the back of my truck or on my boat. I like to SCUBA dive occasionally.
Pesky
I read the Oxygen Hacker's Companion, fourth edition, and came up with several new ideas. I refer you to pages 46 and 110 in which various design ideas and techniques are offered for building and using trimix "sticks". I feel these are unnecessarily complicated and expensive so offer you a simple, foolproof device at lower cost. Only one stick need be built and only one O2 analyzer is necessary. Forget the He analyzer, the flowmeters, etc, not needed. Forget the compressibility coefficients, not needed.
Build your stick in the usual way except make it about 3 feet long and, instead of one, install two gas ports on the side near the top. Make the intake filter by drilling holes in a round PVC pipe cap of appropriate size. Insert an 1/4", slightly oversized dacron filter pad (used in air compressor filters), in the cap. Insert the filter pad so that it conforms to the round shape of the cap. The baffles are made by stacking several stainless wire pads on top of a foam filter. The foam filter is made by cutting a thick round section from some packing foam, the permeable type. These items rest against some sheet metal screws installed in the side of the PVC pipe. Install the O2 sensor as shown in the oxyhacker, in line at the bottom.
Hook the oxygen and helium lines to the fittings near the top of the pipe. Hook up the supply tanks with Harris oxygen regulators (EBay, 10 bucks) which have been modified by drilling the restricter ports with a #60 bit. Start up the compressor with filter drains open.
For purposes of discussion, assume that a 17/45/38 mix is needed. Start the helium and gradually increase the flow. Watch the oxygen analyzer while adjusting the helium. Initially, it should read about 21% or a bit less (check your calibration chart). When the O2 level drops to 11.5% lock the helium knob in place (or just keep everybody away from it). Remember, the helium fraction is 45% leaving 55% for air (or enriched air)-----thus, 0.55 X .21 X 100 = 11.5%. Turn on the oxygen and slowly increase flow until the analyzer (remember, only one is needed) reads 17%. Close the compressor drains and fill the SCUBA tank(s). Watch the O2 level and tweak as needed. When tanks are full, close the SCUBA valve, shut off oxygen, shut off helium, stop compressor, in that order. Check oxygen percent in the SCUBA. You are done.
The Harris regulators are the 50 psi type and are ideal for nitrox when modified as above. They are usually military surplus, chrome plated brass, single stage. Of course, the trimix stick can also be used for nitrox fills. I carry a shorter version on trips for nitrox fills from the back of my truck or on my boat. I like to SCUBA dive occasionally.
Pesky