Science Quiz - Oxygen tank filling

[DonMuncy]

First off, bear in mind, this is strictly a theoretical mental exercise, and has no practical importance. 

For those not familiar; "cascade filling" of the on-board O2 tank is done by attaching a lower pressure cylinder to the airplane tank and letting the pressure between them to equalize. Then disconnect it, and attach the higher pressure cylinder, and let them equalize. Each time the airplane tank needs filling this procedure is repeated, until you are not satisfied with the amount of O2 you can put in. At that point, you have the lower pressure cylinder refilled and it now becomes the higher pressure one.

I have two oxygen cylinders, one about 150 cu ft and one about 125 cu ft. My large one is full, and today I discovered the valve on the smaller one must have been cracked open and it is now empty. I will get it refilled tomorrow. Now with two full cylinders, (assuming they both show the same pressure), would I eventually get more useful oxygen by using the smaller or the larger one, as the "lower pressure cylinder". Or would it make no difference.

[kortopates]

Math Quiz - run the numbers in a spreadsheet


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[Hank]

The smaller tank will last a lesser number of refills in either position. Si ce yiu run them bith at each position, it won't matter in the long run.

Figure 125 ft³ x 2000 psi = 250,000 ft³ of oxygen. The 150 ft³ tank similarly holds 300,000 ft³ of oxygen. I'd use the smaller tank first for the low pressure fill, it'll empty faster, leaving more for the high pressure tank to fill over time. Done the other way, the smaller tank will fun down faster, as every cubic foot taken from it will reduce the pressure more than taking the same cubic foot from the larger tank.

[N201MKTurbo]

I have a Haskel compressor. I don't need math. Which one is it Charles or Boyle?

[DonMuncy]

3 hours ago, kortopates said:

Math Quiz - run the numbers in a spreadsheet


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If I was smart enough to run the numbers in a spread sheet, I would probably have already known the answer  

[carusoam]

The ideal law of economics...  acquire Haskel compressor on the cheap...

OK this is an interesting question....

Using the low pressure tank usually fills the plane’s tank more than half way....

Topping off requires the higher pressure in the new refilled tank. But still uses a large amount, killing off the peak pressure...

In the end, you find out, the more high pressure O2 you have, the better off you are.

Sooner or later, all your tanks end up Lower than you want.  Economics determines when you are going to refill one.

Keep refilling the larger tank.

 

Some other economics to consider...

If you refill a half full tank, do they give you a discount?

If you load the plane’s tank with the low pressure source, are they too close in pressure, and nothing transfers.....?

How low do you empty the plane’s tank before refilling...?

 

There isn’t a lot of math involved, because the pressure always flows from highest to lowest.

As long as your highest pressured tank is higher than the other two you have something to work with...

Now we can get into the math if you always know what the tank in the plane’s V, T, and P are... and same with the other two tanks...

 

Bottom line...

If you are only going to fill one tank, always Top up the larger tank for best flexibility...?

If the smaller tank gets emptied accidently, this sends a monkey wrench into the calculations regarding the cascade principles... that tank needs to be filled to a pressure significantly higher than the ship’s tank, near half way full to make the system actually workable...

In the end, the cascading principle works pretty well.  It requires one tank to be near the highest pressure available. And the ship’s tank to be as low as possible.  The middle tank gets the flexibility of being anywhere in the middle...

In the end, the middle tank goes too low to be usable... it goes off to be filled as the high pressure source. And the larger tank is doing duty as the low pressure source...

It really isn’t going to make a difference all three tanks play their given roll.  Having different sized tanks is a little less ideal for the planning...

If a third tank became available, how would that effect your cascade?  More effort to flip another valve and remember to put it back....

PP thoughts only, ready to do the calcs if you supply PVT data for your tanks...

Best regards,

-a-

[smccray]

I hope someone has a definitive answer.

My guess is that if we're talking about the two tanks alone you're better off setting up the small tank as the low pressure tank first but I don't really know. 

[carusoam]

The cycle is continuous...

the large tank that starts as the high pressure source, becomes the low pressure source on the next refill...

Problem solved for today, but passes the buck for the next time through the cycle...

Something to think about...

1) The middle tank gets emptied accidently....

2) the CB pilot is only allowed one refill...

3) what does he do to get the most mileage out of the system as it stands....?

4) I (being a certified CB) would seriously consider the following steps....

• use the big tank to refill the emptied tank the best it can.
• empty as much O2 out of the big tank as possible, including filling the ship’s tank to the highest level possible....
• Finally refill the big tank to it’s highest practical pressure.
• cycle restarts...
• wait until the big tank’s pressure is too low to be the top of the cascade.
• swap it’s position to the middle.
• now a slightly smaller tank is doing the duty of the top of the cascade.  Less than ideal, but not imperfect...

5) For the non-cb...

• go refill both tanks and start the cascade cycle anew.
• equilibrium will occur over time....
• You always refill a tank that has some O2 left in it.... no way around this unless you just allow it to escape...
• 

Engineer=practical scientist...

Best regards,

-a-

[DonMuncy]

59 minutes ago, carusoam said:

1. Keep refilling the large tank

2. If you refill a half full tank, do they give you a discount?

3.  If you load the plane’s tank with the low pressure source, are they too close in pressure, and nothing transfers.....?

4. How low do you empty the plane’s tank before refilling

5. If a third tank became available, how would that effect your cascade?  More effort to flip another valve and remember to put it back....

1. In my opinion, no! No matter what the size, you re-fill the lowest pressure one.

2. No, you take your cylinder in and give it to them. They give you a full one. Makes no difference how much or little is in the trade-in.

3. If your plane's tank had a higher pressure than the cylinder, it would flow back into the cylinder. This is not a problem, as you will always be filling your cylinder(s) long before that point. When your high pressure tank gets low enough that it won't fill your plane to your satisfaction, you refill your low pressure tank and start over. "To your satisfaction" is a relative term depending on how big a CB you are, and where your next flight is going. If you are on your way to CA from TX, you fill your lowest cylinder and fill up. You check the chart, and if you have enough pressure to handle you and your passenger(s) for the anticipated flight, go for it.

4. See 3

5. An infinite number of cylinders would be ideal, but too expensive. It doesn't take but a moment to turn off one valve and open another, but there are still limits. If you go to the FBO and pay them their $75 or so, they will use (generally) a 6 tank cascading system. Unless they have one of the $5000 pumps which will suck all the O2 out of a tank and pump it into yours. 

All in all, two tanks works pretty well. I can usually get about 4 fills before I have to pony up the $25 of so to fill one cylinder.

[Hank]

33 minutes ago, carusoam said:

The cycle is continuous...

You always refill a tank that has some O2 left in it.... no way around this unless you just allow it to escape...

You could always huff some on the way to the welding shop to refill. A snort or two when working on the plane would be nice, too. What does extra oxygen do for tired muscles? Does it work the same as for tired brains?

Edited March 5, 2018 by Hank

[gsxrpilot]

Another CB option here is to install a MountainHigh O2D2 unit and quit filling your cylinder. Well, you'll still have to fill it, but you'll use about 80% less O2 and will therefore be filling much less often.

It only takes three or four O2 fills away from home to pay for the O2D2 system. After installing, I never fill away from home.

[carusoam]

Picture me jogging down the trail dragging my extra O2 bottle... 

My heart keeps me O2 limited to about 7mph long term (8:00)... multiple miles.

the first mile with O2 at a good saturation level throughout... I get about 8mph (7:30)...

being short of oxygen while running can be similar to that of flying.  You may recognize the usual patterns we discuss around here.

The leg muscles start to slow down as the O2 levels drop, (harder to notice in the cockpit)... intentionally slow up a bit and the O2 levels return, the legs continue on...

The magic of measuring the human body.  Actual data collected over time.

What would be interesting would be having O2 while jogging...  right now I know I run lower on O2, but is it blood flow limited, or O2 saturation limited..?

I’m not raising my HR to experiment, i’m Breathing fully as it is, I expect having additional O2 available while jogging would raise the available O2 to the leg muscles allowing their nice long extended strides... somebody with more blood / O2 saturation knowledge could be helpful here....  

1) what percent of the blood leaves the lungs fully O2 saturated?  At sea level,

2) what percentage of fully O2 saturated blood is possible with additional O2 availability.

3) There is always a limit, too much O2 has some ill effects... 100% saturation we measure is based on the usual concentration of O2 available in air....

4) Yes I have... taken my O2 saturation sensor out on the trail... got my HR meds adjusted after reporting the findings...

5) for background, I used to run 7:00 miles. I tried to string as many of those together as possible for 5k runs... I don’t do that anymore... reminder, don’t keep up with high School athletes.  Not good for your health in some cases...

6) supplemental O2 is intended to use as a turbo normalizer... boosting O2 on the ground would be more like a supercharger... sounds dangerous for an old tired motor... 

Fun life lessons learned from the MS community.

Best regards,

-a-

[ilovecornfields]

16 minutes ago, gsxrpilot said:

Another CB option here is to install a MountainHigh O2D2 unit and quit filling your cylinder. Well, you'll still have to fill it, but you'll use about 80% less O2 and will therefore be filling much less often.

It only takes three or four O2 fills away from home to pay for the O2D2 system. After installing, I never fill away from home.

I bought my O2D2 several months ago and haven’t filled the tank since then and I have it set to automatically go on at 5000 ft. It’s easily 10x more efficient than the oxysaver cannula. I almost bought a cascade system before someone posted about their O2D2 system and now I’m really glad I didn’t!

[kortopates]

 

17 hours ago, DonMuncy said:

If I was smart enough to run the numbers in a spread sheet, I would probably have already known the answer  

I keep a Hobbs log spreadsheet where I pretty much track everything in the plane. One page of the spreadsheet is for Transfilling where I track refills for the aircraft. technically, servicing the O2 bottle on the plane is preventative maintenance and I track it my spreadsheet. I use a cascading system of 2 bottles, each 225 cuft

Since the transfilling process equalizes the pressure between the ships bottle and the filling bottle, the basic formula (from Boyles Law: P1V1=P2V2)  for calculating the pressure of both afterwards is simply to add up the total O2 as P*V from both plane and bottle and then divide by total volume:

((Plane starting psi* Plane volume) + (Bottle psi*Bottle Vol)) / (Plane Volume + Bottle volume)

One would have to take into account Darcy's Law for the drop in temperature in the source bottle and the increase in temperature of the plane bottle to get real accurate but I find empirical values will be within 75 psi keeping it simple.

With my 115 cuft kevlar tank I have never needed to get O2 away from home either except when travelling really long distances out of the country. 

[jaylw314]

2 minutes ago, kortopates said:

 

I keep a Hobbs log spreadsheet where I pretty much track everything in the plane. One page of the spreadsheet is for Transfilling where I track refills for the aircraft. technically, servicing the O2 bottle on the plane is preventative maintenance and I track it my spreadsheet. I use a cascading system of 2 bottles, each 225 cuft

Since the transfilling process equalizes the pressure between the ships bottle and the filling bottle, the basic formula (from Boyles Law: P1V1=P2V2)  for calculating the pressure of both afterwards is simply to add up the total O2 as P*V from both plane and bottle and then divide by total volume:

((Plane starting psi* Plane volume) + (Bottle psi*Bottle Vol)) / (Plane Volume + Bottle volume)

One would have to take into account Darcy's Law for the drop in temperature in the source bottle and the increase in temperature of the plane bottle to get real accurate but I find empirical values will be within 75 psi keeping it simple.

With my 115 cuft kevlar tank I have never needed to get O2 away from home either except when travelling really long distances out of the country. 

Temperature will not make a difference since you are allowing time for the air in the ship tank to cool to ambient.  Unless, of course, you aren't   Realistically, the temperature change is probably a small fraction of 300 degK, so probably accounts for little difference in overall capacity.

[kortopates]

4 minutes ago, jaylw314 said:

Temperature will not make a difference since you are allowing time for the air in the ship tank to cool to ambient.  Unless, of course, you aren't   Realistically, the temperature change is probably a small fraction of 300 degK, so probably accounts for little difference in overall capacity.

It was more of a comment on the challenge of getting the two tanks to actually equalize. Its a very minor point I was trying to make, that because the plane's tank is warming up, (it gets noticeably warmer to touch while filling), while the source tank is cooling, after you disconnect the filling tank, the planes tank is then going to cool to ambient and stabilize at a bit lower pressure as it cools.  But agreed, its not significant.

 

[jaylw314]

1 hour ago, kortopates said:

It was more of a comment on the challenge of getting the two tanks to actually equalize. Its a very minor point I was trying to make, that because the plane's tank is warming up, (it gets noticeably warmer to touch while filling), while the source tank is cooling, after you disconnect the filling tank, the planes tank is then going to cool to ambient and stabilize at a bit lower pressure as it cools.  But agreed, its not significant.

 

I assumed you don't just crank open that valve all the way.  That would be interesting to see how hot the aircraft tank gets!

[kortopates]

10 minutes ago, jaylw314 said:

I assumed you don't just crank open that valve all the way.  That would be interesting to see how hot the aircraft tank gets!

No, it take me about 20 min with my two cascading tanks.

[Yetti]

23 hours ago, carusoam said:

Picture me jogging down the trail dragging my extra O2 bottle... 

My heart keeps me O2 limited to about 7mph long term (8:00)... multiple miles.

the first mile with O2 at a good saturation level throughout... I get about 8mph (7:30)...

being short of oxygen while running can be similar to that of flying.  You may recognize the usual patterns we discuss around here.

The leg muscles start to slow down as the O2 levels drop, (harder to notice in the cockpit)... intentionally slow up a bit and the O2 levels return, the legs continue on...

The magic of measuring the human body.  Actual data collected over time.

What would be interesting would be having O2 while jogging...  right now I know I run lower on O2, but is it blood flow limited, or O2 saturation limited..?

 

I don't think lactic acid works quite that way.  Glycogen stores and the such are about 45 minutes for most people

[jaylw314]

12 hours ago, Yetti said:

I don't think lactic acid works quite that way.  Glycogen stores and the such are about 45 minutes for most people

 

On ‎3‎/‎4‎/‎2018 at 9:34 PM, carusoam said:

What would be interesting would be having O2 while jogging...  right now I know I run lower on O2, but is it blood flow limited, or O2 saturation limited..?

up to about 5000 feet, O2 flow is limited by blood circulation, not O2 ventilation in the lungs.  People exercising in Denver feel short of breath because their rib muscles get tired from having to breath more frequently, but as long as they do, the amount of oxygen getting to the muscles is maintained (which always cracks me up when I see football players sucking down oxygen like their life depends on at sea level.  Not gonna make a difference, guys!).

Glycogen stores still do require oxygen to metabolize aerobically.  It's the bottleneck of blood flow to the muscles that causes lactic acid to be produced temporarily anaerobically.

Above 5000 feet, of course, O2 flow starts getting supply-limited in the lungs.

[jlunseth]

We (divers) use a cascade system when filling dive tanks.  The tanks sit in cold water to keep the temp. from going too high.  The tanks still warm up, and the difference in pressure between a hot and a cold tank can be 200 or 300 psi.  That's out of a nominal 1800 for most tanks, so the heating issue is not insignificant.  If you fill a tank hot to 1800 pounds and take it diving in cold water, you will immediately lose 300 psi or even more if the water is really cold.  That's fifteen percent.  Same thing if you fill an aircraft tank and let it get to hot, or it is just naturally warm outside, then you come back on a cold day, or you go up to the flight levels and wonder where all that O2 went.

[N201MKTurbo]

4 hours ago, jlunseth said:

We (divers) use a cascade system when filling dive tanks.  The tanks sit in cold water to keep the temp. from going too high.  The tanks still warm up, and the difference in pressure between a hot and a cold tank can be 200 or 300 psi.  That's out of a nominal 1800 for most tanks, so the heating issue is not insignificant.  If you fill a tank hot to 1800 pounds and take it diving in cold water, you will immediately lose 300 psi or even more if the water is really cold.  That's fifteen percent.  Same thing if you fill an aircraft tank and let it get to hot, or it is just naturally warm outside, then you come back on a cold day, or you go up to the flight levels and wonder where all that O2 went.

Huh, I always fill Aluminum 80s to 3000 PSI.