Portable Oxygen: A User's Perspective

Compressed Oxygen

Schlepping Your Oxygen
"Best" Size

Portable Oxygen: A User's Perspective
Compressed Oxygen

What is compressed oxygen? How many sizes of cylinders are available? Is there a "best" size for me? How much does it weigh and how long does it take to empty?

The information here provided is for educational purposes only and it is not intended nor implied to be a substitute for professional medical advice. Always consult your own physician or healthcare provider with any questions you may have regarding a medical condition.   

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Oxygen is stored in two ways. Oxygen can be frozen and stored in containers that resemble thermos bottles. Oxygen can also be compressed and stored in cylinders under several thousands of pounds of pressure. This section discusses compressed oxygen. The next section discusses liquid oxygen.

Compressed oxygen for medical use is stored in silvery aluminum cylinders with dome-shaped tops of green. If you use compressed oxygen, your cylinder is probably one of those pictured in Figure 1.

Figure 1
Aluminum Oxygen Cylinders

Cylinder Photo

(Photograph courtesy of Advanced Aeromedical at www.outletmedical.com)

Two sets of names are used to specify cylinder sizes. The older set identifies them alphabetically from A, the smallest, to E. the largest. You may be most familiar with the lightweight B cylinder or the E cylinder, which is usually pulled along in a cart.

Each of the newer set of names starts with the letter M to denote "medical," followed by a number, which specifies the amount of oxygen in cubic feet that can been compressed into the cylinder. For example, the B cylinder is also called the M-6 cylinder.

In Table 1, you will see that the most common and oldest of the cylinders carry both of these names.

Table 1
Aluminum Cylinder 


Diameter (in.)
Height (in.)
Capacity (liters)
Weight (lb.)**
A or M-4
B or M-6
C or M-9
D or M-15
E or M-24
*Specifications vary slightly among manufacturers
**Empty weight--without valve or oxygen
Source: Catalina Cylinders at wwwcatalinacylinders.com

To make this article of greater interest to you, determine the size cylinder you use.

If your cylinder's base is about the size of a 1 liter bottle (3.21 in. diameter), it is one of the first three listed in Table 1.

If its base is about the size of a 2 liter bottle (4.38 in. diameter), it is one of the remaining five cylinders in Table 1.

With a  12 inch ruler, measure the height of your cylinder, up to the base of its valve. Knowing its height and base, you can find your cylinder in Table 1.

Chances are that you use a portable which has an M-6 cylinder (image), the one that appears bolded in the above table. The "6" tells you that a maximum of 6 cubic feet of oxygen can be stuffed into it. An M-9 holds 9 cubic feet and an M-15, 15 cubic feet.

What does 6 cubic feet of oxygen look like? Pretend you are holding a square pizza box in front of you that is two feet wide and two feet deep. Visualize its height at 1.5 feet. That pizza box can hold 6 cubic feet (2 x 2 x 1.5) or about 164 liters of oxygen.

How can 6 cubic feet, or 164 liters of oxygen be crammed into an M-6 cylinder, which is the size of a liter bottle? The answer is by applying increasing pressure to the oxygen. The first liter into the cylinder needs only 15 psi of pressure, an amount just slightly more than the atmospheric pressure around the cylinder. Each liter to follow needs a little more pressure until finally, the last liter enters the cylinder under pressure of about 2,000 psi.

Later in this article you will learn about cylinders, called, "composite" cylinders, that can store oxygen under 3,000 psi.  You have done your math right if you figured out that a composite cylinder the same size as your aluminum M-6 is an M-9 because it holds a third more oxygen.

Some folks feel more comfortable talking about oxygen measurement in liters, rather than cubic feet. To change to liters, multiply the cubic feet by 28.33.

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Schlepping Your Oxygen

It was Dr. Tom Petty who we all know as the Father of Oxygen Therapy, that first used this term "schlepping" as it applies to pulling an oxygen cart (image).

Often the solution for folks who just cannot carry a cylinder is an E cylinder, to drag along behind them on a cart. You, like I, have seen folks, schlepping their E cylinders, who are too frail to control it

I suggest that these folks are not being well-served.
  • Simply switching an empty eight pound E cylinder for a full one requires more energy than doing the same with a five pound portable. The task of moving E cylinders in and out of a cart is even more demanding.

  •  A cart with an E cylinder on it weighs 15 pounds and requires an inordinate amount of energy to move them into and out of a vehicle.

  • A cart is so top heavy that it is dangerous, particularly on sloping and broken walkways, over rough terrain and up stairways. Folks who don't have the strength to control the carts when they tip and fall, expose themselves to personal injury.

  • An E cylinder stores enough oxygen to last a 2 Lpm (liters per minute) user more than five hours--and that's when its set to continuous flow. If a conserver were used, the oxygen would last at least twice as long. Most of us are out for about five hours. Why do we need a cylinder to carry both today's and tomorrow's oxygen with us?
Okay, I know there is a place for E cylinders--but not on carts.
  • Carry several in the back of your car to use while traveling.

  • Place one in the bracket of your wheelchair or scooter to use when you tool about.

  • Keep them in reserve for emergencies.
If you are one of the folks who cannot carry your oxygen, first, select a cylinder smaller than an E cylinder and then find a way to transport it. Here are two suggestions.
  •  Value Medical Supplies advertises an oxygen carrier for walkers and rollators (image). With a rollator, you also always have a seat with you.

  •  Search through your luggage store for wheeled luggage for your portable system, as Kathy and Sally did.
Kathy and Sally are members of the Pulmonary Rehab group at Mary Washington Hospital in Fredericksburg, Virginia. Both have been on oxygen for a short time and continue to lead active lives that requires them to be out of their homes and dependent on their portable oxygen systems.. Kathy runs the household for her husband and adult daughter. Sally is church historian, is a board member of her county's historic commission, and participates in the local garden club. In addition to doing all the household shopping, both regularly attend Rehab three times a week.

Both use portables with M-6 cylinders. Until recently, each always carried her portables over the shoulder, Kathy's in a bag that held the cylinder in a vertical position and Sally's in a bag where the cylinder lay on its side.

Both complained of the weight of carrying oxygen, along with the full handbags each had always carried. Each purchased a wheeled (or rolling) backpack (image) they fill with both oxygen and personal things. Kathy's filled backpack weighs 8 pounds and Sally's, with a backup M-6 cylinder, weighs slightly more. "I carry an extra cylinder," Sally says, "because my doctor wants me on continuous flow when I walk, so I use up more oxygen." Kathy says, "The backpack holds all I need to carry and rolls along easily behind me." Sally says, "It's great for most things, but I still use my carry bag for church."  If you buy one, Kathy suggest you get one with a handle that locks in position. "When you stop," she says, " you have something to lean on."  Sally adds, "The plastic wheels are noisy, so either get one with rubber wheels or pick places to walk that are carpeted."

These rolling backpacks have several advantages over an E cylinder on a cart, with a combined weight of 15 pounds. The rolling backpacks weigh half as much, have a lower center of gravity, and carry enough oxygen to last most folks five hours or more.. You can find inexpensive rolling or wheeled backpacks by doing a search on http://www.google.com.

Before purchasing a rolling backpack, check with your oxygen provider. Some portable systems cannot be enclosed.

After reading the above, the manufacturer of the Handi-Air Tote contacted me and asked if Sally would try out and comment on his product. Here is Sally's report.

My name is Sally. I am on 2 Lpm and use portable oxygen with an M-6 cylinder. Until about 6 months ago I carried my portable oxygen over my shoulder.  When that became difficult I purchased a rolling backpack, which I used until I received the Handi-Air Tote. Here are my comments after two weeks using it.

Carries a single cylinder better (than the rolling backpack). I can tie it down so it doesn't shift about in the bag and tie it in a diagonal position so the weight is distributed evenly.
It carries two M-6 cylinders well, although that's a bit much for me to carry or pull.
The extra pockets help me organize the other things I carry, so I don't have to put them where the cylinder will roll over them.
I can quickly open and look at the content's gauge when I need to.
The handle doesn't retract under pressure, so I can lean on it when I stop to rest.
The rubberized tires make less noise on hardware floors than do the plastic ones on my rolling backpack.

The biggest benefit of the Handi-Air over the rolling backpack is that I can lift it from the passenger's seat in my car, bring it across my body, and set it on the ground outside the driver's door with little difficulty. When I was using the rolling backpack, I had to disconnect from my oxygen, walk around to the passenger's door and reconnect to my oxygen. That walk without oxygen is not fun. Now, that I have the Handi-Air that walk is a walk I no longer have to do.

My only question is about the dual tires. I wonder why. Stones get caught between them and I find it more difficult to make sharp turns.

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"Best" Size

You want the best of both worlds--lightweight and reasonable duration. If you pick too light, you may select cylinders that empty too frequently. If you pick heavy, you may find you are carrying two days worth of oxygen.

The necessary parts of a compressed oxygen portable system that contribute to its weight are a cylinder and its valve, a regulator, for continuous flow, or conserver, for pulsating flow, and a carrying bag. To estimate the weight of your system, you need to determine and add up the weights of its parts.
  • The weight of your aluminum cylinder is provided in Table 1.

  • All cylinders use a CGA 870 type valve. The most common and lightest one weighs 11 oz.

  • A regulator or conserver weighs about 1 pound. To find out the weight of yours, see Table 2, below. Should you not know your conserver's name, look for it on your conserver's nameplate, or click on the names in the table until you find one that looks like yours.
Table 2
Conserver Weights and Savings Ratios
 Weight (lb.)
Savings Ratio
Pulse Conservers*

Oxymatic 401A

Impulse Elite

Oxymatic 411A

Wave 6200

Pneumatic Conservers


Cypress 0.94

O2N Demand II

O2Xpress 1.0

EasyPlus 0.79

OPC 830 1.4
*Weight does not include weight of required battery.
**Plus the weight of its regulator.
***At setting 2, only.

  • A carrying bag's weight can vary. A camera-style bag, one that carries the cylinder in a horizontal position, weighs about three-quarter pounds more than a standard vertical bag.

  • Add to this the weigh of batteries, including a spare, if your conserver is electronic, as well as washers, wrenches, and extra cannulas.
Adding up all the above will tell you how much you are carrying to support your oxygen requirements.

For example, Table 3 shows the weight of a system that uses an M-6 cylinder.

Table 3
Weight of an M-6
Portable System
Weight (lbs.)
M-6 Cylinder
Regulator or
Total Weight
*Weights vary from just under to just over a pound. See "Conservers" for exact weights.
The actual carrying weight may be slightly more if you include a carry case, batteries, and cylinder wrenches.

You can use Table 3 to determine the weight of a system if other cylinders are used. For example, an M-9 cylinder weighs 3.7 pounds (see Table 1), 1.5 pounds more than the M-6. So a portable system with an M-9 cylinder will weigh 1.5 pounds more than one with an M-6 cylinder.

The duration of your cylinder depends on the setting you select on the regulator or conserver. If you use a conserver for pulsating flow, duration also depends on its savings ratio (see Table 2) and on your breathing rate. The following table should give you an idea about the duration of your aluminum M-6 cylinder.

Table 4
Duration (in hours) of an M-6 Cylinder

Saving Ratio*
*For users whose breathing rate is 20 bpm
**Continuous flow
If you use a cylinder other than the M-6, you can determine your cylinder's duration using Table 4. Suppose you have a conserver with a 3:1 savings ratio that you use at the 4 setting. If your cylinder is an aluminum M-6, your duration, according to the Table 4, is about 2.7 hours. If your cylinder is an aluminum M-9,  M-15, or M-24, multiply the 2.7 hours by 9/6, 15/6, or 24/6, respectively, to discover the duration of your cylinder.

You may complain that your portable system with an aluminum M-6 cylinder is too heavy. You can ask your provider to get you an aluminum M-4 cylinder. It is 27 percent lighter. You might expect the duration to also be 27 percent less, but no, its duration is 33 percent less. Doesn't sound like a good deal to me.

You may also complain about the duration of your M-6, as shown in the Table 4, and plan to seek out a larger cylinder for a greater duration. Table 5 shows two options with aluminum cylinders.

Table 5
Weights* & Durations of an
Aluminum M-6 Cylinder*

as Compared to

Two Larger Aluminum Cylinders*

Weight, as a percent of an Aluminum M-6 Duration, as a percent of an Aluminum M-6
*Filled to 2,000 psi

As Table 5 shows, the additional duration you get is complemented by an equivalent increased in weight. The moral is--and if you have been following this, you know what I am going to say--There is no free lunch when you are dealing with aluminum cylinders. If you want greater duration you pay for it with far greater weight than you might expect.

When you can convince your oxygen provider to acquire composite cylinders and fill those cylinders to 3,000 psi, then we in the U.S. will have what oxygen users in Europe and the Far East already have--lighter cylinders with longer duration.

Just to give you a "peek over the fence" at what you have in store when your provider stocks composite cylinders, I will replicate the above table, but this time in Table 5 comparing your aluminum M-6 cylinder with composite M-9 and M-15 cylinders.

Table 5
Weights* & Durations of an
Aluminum M-6 Cylinder*

as Compared to

Two Larger Composite Cylinders**

Weight, as a percent of an Aluminum M-6 Duration, as a percent of an Aluminum M-6
*Filled to 2,000 psi
*Filled to 3,000 psi

 Here it is, in simple language.

  • When you trade in your 2.2 pound aluminum M-6 for a 1.7 pound composite M-9, your new duration is 150 percent of your old aluminum M-6.

  • When you trade in your 2.2 pound aluminum M-6 for a 2.6 pound composite M-15, your new duration is 250 percent of your old aluminum M-6.

Now, take either number--150 percent for the composite M-9 or 250 percent for the M-15, and multiply it by your number from Table 3. That would be your duration using these composite cylinders.

How quickly can you get some of the composite cylinders? Your oxygen provider is not reimbursed for conservers by either insurance companies or Medicare. Your provider is willing to pay the cost because the cost is small when compared with the revenue it generates.  A conserver that can stretch oxygen duration by a factor of three means the provider needs just a third of his work staff and a third of his cylinder inventory.

So expect to see composite cylinders when your provider becomes aware that the same kind of savings will occur by both decreasing the weight and increasing the capacity of cylinders.

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© 2001 Copyright
Revised September 2003
Peter M. Wilson, Ph.D. 
Founder of PortableOxygen.org

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Last Modified: December 26, 2013

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