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[pescador775]

Senario #2 - Nitrox membrane
That would mean I would need to carry 4 nitrox tanks.
125 lb for compressor and 208 lb for 4 tanks = 333 lb
Add another 52lb for a safety O2 tank = 385 lb

My math:

Nuvair Traveller II-385 lbs
4 SCUBA tanks-208 lbs
O2 tank-30 lbs
5 gal gas-40 lbs

Total: 663 lbs

 

[Waterskier1]

Pesky,

You are correct. I got tripped up by the total = 385 lbs from the OP and coincidentaly, the weight of the Traveler II being 385 lbs also.

Now, I'm not sure where the OP got the weight of the compressor at 125 lbs. The only weight I can find for the lightest system, the Traveler, is 140 lbs shipping weight (they don't say the net weight). But that system does appear to require another drive compressor: "Uses Your Installed Grade E Air".

$7000 can buy a lot of O2 bottles, and refills, and probably not a lot of extra weight. I haven't checked into it yet, but wouldn't a O2 bottle handle more cubic feet per pound than using a normal tank?

 

[Jimmer]

Ray, can you add a membrane system to any existing scuba compressor? How much do those systems cost?

Thanks,
Jim

 

[Waterskier1]

I know you asked Ray, but I'll take a stab at this. The way I understand it, and Ray or others please correct me, is that the membrane systems is a separate way of creating oxygen enriched air. You use a low pressure, higher volume compressor to force air through a membrance that actually separates the nitrogen out.

These were orginially used for nitrogen generators. But the air that is left over is reduced in nitrogen, or enriched in oxygen. That air is then compressed to scuba operational pressures (typically 3300 pounds).

In my limited experience, that final compression is done by the "membrane system", although I guess you could construct your own system if you could get the membrane, and then use your original compressor to compress the resultant oxygen enriched air to scuba pressures. But, most of the prebuilt systems would not require or use your existing scuba compressor, but most do require a source of low pressure air (150-175 lbs) to "feed" the membrane system.

I'll let Ron comment on the costs.

Hope I haven't overstepped my bounds by responding here.

 

[rcontrera]

Yes, a membrane system can be added to any compressor that is rated for Nitrox. All ColtriSub compressors are rated to 40% at 3000 psi.

You just need an air source. It can come from bottles or a low pressure compressor. That air is regulated to the proper working pressure, filtered, heated to the proper temperature and then run through the membrane. Nitrogen is vented off to atmosphere while the O2 rich air is mixed with atmosphere to the proper percentage and then fed to the compressor inlet. One other thing you will need is an electrical source for the heater. Standard 110 household is fine.

Again, Nuvair makes these for Coltrisub. A Nitrox stick system runs about $5750 for up 3-6 cfm/32-40%.

One thing to remember ... the percentage of O2 that comes out of the membrane system can vary from the percentage that actually gets pumped into your bottles. the reason is that some of the O2 is scrubbed out in the filtering process. So, it is good to have an O2 analyzer in the fill line somewhere. Or else, use trial and error to figure out just what percentage of feed air you need to have to get your required end use gas mix.

 

[Jimmer]

How high output of LP compressor would be required for a given output of HP compressor? For example if I had a 5CFM@4500 psi compressor, how many cfm LP compressor do I need?

 

[rcontrera]

For a 5 CFM HP compressor, you will need 175 psi at 15 CFM to give you 40%. And screw style LP compressors are a LOT quieter than piston ones.

These are just round numbers since the specs are figured out to ColtriSub/Nuvair compressor capacities.

 

[pescador775]

About the 02 readings and discrepancies thereof. Water is scrubbed in the compressor filters, not oxygen. O2 partial pressure of hot, humid air is less than that of dry air. This is because water displaces some of the oxygen molecules and because hot air is "thinner". The sensor gives a relative reading in percent for various reasons but the oxygen PP actually is lower in humid air versus dry air. To compensate, the sensor calibration must be lowered thus forcing the system operator to raise the oxygen output of the membrane by a few tenths to get the desired percentage reading. Once most humidity is removed by the compressor the water vapor no longer exerts its own partial pressure. At that point, the oxygen content of air exiting the compressor is the same in absolute and relative terms and the concentration read by the sensor will be correct if the O2 sensor(s) were calibrated using a compensation chart or by blowing dry air to it. That is, before taking O2 readings at either end of the membrane/compressor system, the sensors should be individually calibrated. The oxygen sensor at the membrane needs to be calibrated downward according to the published humidity and temperature chart, I say again, to prompt the operator to increase the oxygen flow (vent more nitrogen) into the compressor. The sensor at the compressor output should be calibrated using dry air from a tank but it can be done in wet air with a compensation chart. All this is true if a LP compressor is used to drive the membrane but is not true if air is decanted with a reducing regulator from HP tanks to the membrane. This air is presumed dry and calibration should be done with dry air or use the compensation chart if it is inconvenient to do otherwise. Either way, the sensor will read the correct oxygen percentage in the membrane and compressor output at "normal" (not increased) oxygen flow. But, (there is always a but);

Temperature alone, even in dry air, may have an effect on a sensor. However, the Teledyne (and other) sensors appear to be temperature compensated, I guess that is done by using a thermistor in the bridge circuit. In any case, compensation charts show no calibration offset is needed when calibrating for dry air in the range 32F-100F. I don't know how hot the membrane air gets but it would have to be more than 100F to affect calibration more than a smidge in dry air. Otherwise, with humidity above 10% RH use the standard calibration chart for humidity and temp combined as mentioned before. If your sensor is reacting to temperature alone then trial and error is in order. Good luck and don't sweat the small stuff.

 

[Scuba Duck]

Keep in mind that for the membrane systems you need a low pressure supply of air (130-160 psi) at about 3X the flowrate of your high pressure air compressor. For the traveler system, you fill Scuba tanks with air, and then use the scuba tanks regulated down to low pressure to provide the gas supply for the membrane. The adiabatic expansion from high to low pressure means that gas drops temperature across the regulator and needs to be heated back up to about 105 degrees F before sending into the membrane. If you don't heat the gas back up properly, or if you over pressurize the membrane, you can pop some of the tubes/fibers inside the membrane which is a seriously expensive mistake (I'm attempting to fix a membrane presently with burst tubes, the replacement membrane alone is $8K). Best to have a dedicated low pressure air supply so that you don't even have the ability to pop any fibers or inject the membrane with really cold air. You also need to have proper filtration for them so that you don't clog the little fibers with dirt, which is not an issue when you are feeding with grade E air from a Scuba cylinder or bank, but is an issue when using an oil lubricated low pressure air compressor to feed the membrane. Membranes are great for large scale production of Nitrox and for their independance from any oxygen logistics, but you need to make sure you take proper care of them, or the costs add up fast.

Additionally, heating the air to feed the membrane results in heating the oxygen sensor down stream of the mixing section. Most oxygen sensors are affected by temperature: the chemical reaction inside the sensor speeds up as the sensor heats up, indicating a higher percentage of oxygen than actually present. An oxygen analyzer at the outlet of the air compressor, can be used to re-calibrate the oxygen sensor downstream of the mixing section (after the system has heated up to working temperature), which will yeild much more accurate results.

 

[ucfdiver]

UCF diver. You are in my area. Where are you getting your fills?

The only place I know that cheap is Fun To Dive Scuba in Sanford.

Cave Excursions East. Most of my diving is done around that area anyways, I just get filled before I go, and before I come home. Rich is a great guy and I highly recommend doing business with him. $0.08/cu ft for 32%.