FAQs About Filling Cylinders

Diver Certification
Cylinder Qualification
Filling Procedures

Gas Quality
Contents & Labeling
Oxygen Service

Diver Certification
Do I need a diving certification in order to get a sport diving gas fill?
Yes. To receive an Air fill, we require a SCUBA diving certification. To receive a fill with any Nitrox mixture of 40% or less Oxygen content, we require a Nitrox Diver certification.
What type of certification must I have to get a technical diving gas fill?
To receive a fill with any hyperoxic mixture greater than 40% Oxygen content (including pure Oxygen), we require a Technical Diver or equivalent certification. To receive a fill with Helium mixtures, a Trimix Diver or equivalent certification is required. We will observe any specific mixture restrictions that appear on your C-card. We interpret Normoxic Trimix to mean a Helium mixture with an Oxygen content between 18% and 24%.

What if someone else is getting fills on my behalf, and they don't have the appropriate certification?
Unless you have made prior arrangements, we will not allow anyone who lacks appropriate certification to receive a cylinder. In particular with breathing gases other than air, because divers should always analyze and label their own cylinder contents, those divers should be the ones picking up the cylinders.

Cylinder Qualification
When exactly does my visual inspection ("VIP") or hydrostatic test ("hydro") expire?
Annual VIPs are a voluntary program with the SCUBA industry and are considered to expire one year (12 months) following the inspection. By law, a US Department of Transportation (DOT) required hydrostatic test expires five years (60 months) following the test. Because neither a visual inspection sticker nor hydro retest mark show the day of the month performed, a long established and widespread practice in the SCUBA industry has been the valid period expires on the first day of the month shown. Recently, the DOT has issued a clarification statement that the hydrostatic test officially expires on the last day of the month shown. However, we recommend re-qualifying your cylinder before the first day of the expiring month, because most dive shops continue to observe the more conservative expiration date. Fill Express will not fill any cylinder after the last day of the month in which the visual inspection or hydrostatic test expired.
Will Fill Express fill my older aluminum 6351-T6 alloy SCUBA cylinders?
Fill Express will fill an aluminum 6351-T6 alloy SCUBA cylinder only if Fill Express has performed the annual visual inspection on the cylinder including an eddy current test. Read our article on aluminum 6351-T6 alloy cylinders for more information regarding this fill policy.

What cylinders require an interior inspection before Fill Express will fill them?
Our fill station operators are trained to perform a cursory examination of each cylinder before it is filled. We require an interior inspection whenever we encounter a cylinder with:

Leaks

Rattles, sloshes, or other unusual noises

Abnormal weight

Unexplained zero pressure, i.e., completely empty

Foul-smelling contents

What cylinders does Fill Express refuse to fill?
Regardless of what appears to be a valid evidence of inspection sticker or hydrostatic test stamp, we consider some cylinders unsafe to fill. We will not fill any cylinder with:

Heavy corrosion, especially line corrosion at the boot

Exterior damage, such as a dent, gouge, bulge, or deep pit

Evidence of exposure to high temperatures, such as scorching

Vinyl or other unusual or unrecognized coatings or paints

Valve using a lead plug overpressure burst disk

Attributes that would cause it to be permanently removed from service per DOT or PSI standards

Are there any exceptions to your cylinder qualification requirements?
We fill the small 2L, 3L and 4L Faber steel cylinders used in Evolution, Inspiration and Dräger rebreathers. Although these cylinders lack a DOT stamp, we fill them to a maximum of 200-Bar (3000 psi) if they have a current hydro test and evidence of visual inspection sticker. We will NOT fill the stainless steel welded ball pressure vessels used in the Mark 15 rebreathers.

Will you fill an SCUBA cylinder over 15 years old?
Yes. If it meets all our other cylinder qualification requirements, we will fill any SCUBA cylinder regardless of its age or composition.

Filling Procedures

What is the procedure for leaving a cylinder to be filled?
When you drop off one or more cylinders to be filled, we will prepare a work order tag to indicate your desired final contents and pressure along with your name and contact information. The work order tag is attached to the cylinder and, upon request, you receive a numbered claim check.

What if I don't have my claim check when picking up cylinders?
If a claim check was issued, we will not release the cylinders until the claim check is produced. If you can't find the claim check, we will accept your photo identification as long as it matches the name on the work order tag. Please make prior arrangements if someone else will be picking up the cylinder and they won't have the claim check.
How fast do you fill my cylinder?
Our target fill rate is 500 psi per minute. This means that the standard aluminum 80-cubic-foot cylinder with 500 psi residual takes approximately five minutes to fill. In addition to extending cylinder life, at this fill rate, there is not enough heat created to significantly affect the final pressure.
Why don't you put my cylinder in a water bath when you fill?
Fill Express can give you a FULL FILL without incurring the problems of wet fills. The counterproductive practice of wet filling in the SCUBA industry is declining, but still common even in the face of strong arguments against the practice. Read what the experts say about wet filling at these links:

"The Case for Dry-Filling Scuba Tanks" by Fred Calhoun, PE

"Filling Cylinders in Water--Time to Review" by Bill High, President of Professional Scuba Inspectors, Inc.

Will you fill a cylinder in water bath upon request?
Upon request, we'll gladly:

Completely fill your cylinder in a dry environment

Immerse the cylinder in a clean fresh water bath for 5 minutes to rinse and check for leaks

Dry the valve

Check the pressure

Will you fill my cylinder to more than its rated maximum pressure?
Fill Express does not overfill any aluminum cylinder. Overfilling shortens cylinder life, as well as increases risk of cylinder failure. Increased pressures also can cause burst disks, O-rings, and 1st stage regulator seats to unexpectedly fail prematurely. Willing to accept these risks in exchange for the increased gas volume, technical divers sometimes overfill their recently manufactured chrome-moly steel cylinders.

Do you partial-pressure blend in my cylinder?
Infrequently. We have the capability of partial-pressure blending in your cylinder. However, at Fill Express, it is very rarely required even when you request something other than our premix. We can produce Nitrox using a process known as membrane separation. We generate our Trimix using a process known as continuous atmospheric entrainment. This allows us to deliver the final mix directly into your cylinder without any need to partial-pressure blend.

Gas Quality
What is the quality of your Air and Nitrox?
Our gases are analyzed quarterly by an independent laboratory to ensure that our Air and Nitrox meet Ultra Pure standards for carbon monoxide, methane, moisture, oil, particulates, and odor. Our compressors incorporate a variety of filter systems that allows us to produce and store gases that meet what is known as the Grade E quality verification level as defined by the Compressed Gas Association (CGA). At each of our fill stations, just prior to our fill whips, we have a special final hyper-filter system which allows us to deliver gases that meet an even more stringent purity specification known as "Oxygen-Compatible" as defined by the International Association of Nitrox and Technical Divers.
What is the quality of your Oxygen?
Our Oxygen meets a mil-spec standard known as Aviators Breathing Oxygen (MIL-0-2721OD) and is required to be 99.5 percent pure. The water vapor content must not be more than 7 ppm with a dew point of -81°F. The very low moisture content required of ABO is to ensure proper operation of the airplane oxygen system at the extremely low temperatures encountered during flight. Our oxygen consistently tests at 100% as delivered to us from our supplier. However as we transfill the ABO into customer cylinders there will be small amounts of other breathing gases introduced from the plumbing of the boosters and fill whips. Particularly where the cylinder being filled is small, the final content should be considered slightly less than 100%. For the purposes of setting dive computers or decompression software, we recommend using a value of 99% for the actual purity.

What is the quality of your Helium?
There is no widely recognized gas purity standard for Helium. We purchase a highly refined grade of Helium our specialty gas supplier calls Ultra High Purity, which they define to be 99.999% pure. Similar purity specifications from other gas suppliers are termed Laser Grade, Zero Grade or Semi-Conductor Grade. This level of purity is sometimes referred to with the generic term "five nines pure". According to our gas supplier, the price of Ultra High Purity Helium is about double the price of what are known as industrial, welding and balloon grades of Helium. We chose not to use those lesser grades of helium because they are commonly contaminated with methane, nitrogen, and sometimes with argon.

Cylinder Contents & Labeling
How can I be certain about my cylinder contents?
We have a simple but effective system to assure you know what is in your cylinder. A work order tag is placed on the cylinder to indicate the desired final contents and pressure. The work order tag provides the fill station operator with a clear visual indication of what gas to put in the cylinder. Once the cylinder is filled, we assist you in performing an analysis of the cylinder contents in your presence. Finally, we affix a strip of biodegradable non-residue adhesive "tank tape" marked with your analysis. This is true for all Air, Nitrox, Trimix and Oxygen fills.
What cylinder contents labels do you recommend?
Unlabeled cylinders are assumed to contain air. For cylinders containing a gas other than air, our recommendations differ depending upon the intended use and type of gas.

In sport diving, we recommend (but do not require) cylinders containing Nitrox with oxygen concentrations of 40% or less should be labeled with a color-coded, 6-inch-wide band. The top 1 inch and the bottom 1 inch of the band should be yellow. The middle of the band should be green with the word Nitrox in yellow.

In technical diving, the cylinder labeling has become somewhat controversial with some training agencies specifically recommending against contents labeling and others requiring contents labeling. If you choose to label your cylinders, we recommend the following:

Cylinders containing Nitrox less than or equal to 40% should be labeled with the words Nitrox or Breathing Gas Other Than Air.

Cylinders containing Nitrox between 41% and 74% should be labeled with the words Decompression Mix or Breathing Gas Other Than Air.

Cylinders containing oxygen concentrations of 75% or greater should be labeled with the word Oxygen.

Cylinders containing Trimix should be labeled with the word Trimix or Breathing Gas Other Than Air.

Cylinders containing Argon should be labeled with the word Argon AND the words DO NOT BREATHE.

All cylinders containing a breathing gas other than air should have a label or tag indicating the oxygen percentage currently in the cylinder and the maximum operating depth (MOD). In addition, cylinders containing mixtures with MODs less than 100 feet should have the MOD marked in 3-inch-high numbers such that the MOD is clearly visible during the dive.

What cylinder contents labels do you require?
We think labels and stickers on SCUBA cylinders are in some ways counter productive because they encourage and obscure corrosion. We require all cylinders containing a breathing gas other than air to have a label or tag indicating the oxygen percentage currently in the cylinder. We require Argon cylinders to be clearly marked with the words DO NOT BREATHE.

What cylinder contents labels do you obey?
Filling a cylinder with a breathing gas other than is labeled can create dangerous situations where the contents might be used under the assumption it's actual contents match the labeling. If a cylinder has been dedicated to a specific breathing gas with permanent MOD or contents labels, we will fill that cylinder only with the labeled gas. For example, if the cylinder is labeled "Oxygen 20 ft" we will fill only with 100% Oxygen or if the cylinder is labeled with "70" maximum operating depth we will fill only with 50% Oxygen. If you wish a fill with other than the permanent contents label, the label must be removed or obscured (i.e. covered with tape).

Do you sell Air?
Yes, but we call it Normoxic Nitrox (also known as Oxygen-Compatible Air). We use the same oxygen-compatible compressors, filters, and gas-handling procedures to make our Air as we do our oxygen-enriched Nitrox mixtures. Our Air and Nitrox both meet the same Ultra Pure quality standards. Although the CGA G-7.1 standard for Grade "E" Air states that it may contain from 20% to 22% Oxygen, Air is normally expected to have 20.95% Oxygen content. Because our Air travels through some of the same plumbing as our other gases, it may analyze as high as 22% Oxygen content. Because of the chance that we might accidentally introduce Nitrox into an Air cylinder, we handle all cylinder fills with the same procedures. This means that we analyze our Air fills for Oxygen content in the same manner as we do our other breathing gas fills. The price for our Ultra Pure Oxygen-Compatible Air is the same as that of our other sport Nitrox Premixes.

Will you fill my Nitrox cylinder with Air?
Yes. We can fill your Nitrox cylinder with Normoxic Nitrox (also known as Oxygen-Compatible Air). This means we will follow our policies and procedures as for any other Nitrox mixture: the cylinder contents must be analyzed and a contents label affixed to the cylinder. The price for the fill is the same as for our Nitrox sport premix. The diver must also handle the cylinder as Nitrox, meaning they should take care to always know the oxygen content and MOD of the gas they are actually breathing.

Oxygen Service

What do Nitrox Ready and Oxygen Service mean?
When the SCUBA diving community prepares an item for use with breathing gases containing high oxygen percentages or pure oxygen, they generally think in terms of washing it with detergents, replacing rubber parts such as O-rings and seals with oxygen-compatible equivalents and reassembling with oxygen-compatible lubricants. However, most other industries working with very high pressure oxygen have a very different standard known as oxygen service.

Oxygen service means the materials are both:

Oxygen Compatible -- compatible with high concentrations of oxygen.

Oxygen Clean -- free of hydrocarbon contamination and particulate matter.

True oxygen cleaning of an oxygen compatible component (often made from exotic metal alloys or other compounds) takes place in a special clean room, whose atmosphere is free of dust and contaminants. Once the component is free of hydrocarbons and other combustible elements, it is sealed within a sterile environment and never again exposed to normal atmospheric dust, moisture, and contaminants. Only then is the item said to be suitable for oxygen service.
Some manufacturers offer diving products labeled Nitrox Ready, whose oxygen-compatible components are free of hydrocarbons and other flammable contaminants. The metal components are usually stainless steel or brass, which are suitable for use with oxygen at the pressures encountered in diving activities. The lubricant used in assembly is Christo-Lube® or other oxygen-compatible lubricants. The O-rings are made from Viton® or other oxygen-compatible materials. These components are not, however, assembled in a clean room or sealed in a sterile environment. As a result, the manufacturers do not label the products as suitable for oxygen service, although many divers consider them to meet oxygen service criteria at pressures encountered in the diving community.

How do I get my cylinder and valve to be Nitrox Ready?
Most cylinders and valves that have been in use have some level of hydrocarbon contamination. To make a cylinder and valve suitable for nitrox service, they must be disassembled and cleaned of hydrocarbon contamination and reassembled with oxygen-compatible O-rings and lubricant. This process requires training, special materials and is time-consuming. For a very reasonable fee, Fill Express can prepare your cylinder and valve for nitrox service. New cylinders and valves purchased from Fill Express are always prepared by the factory for nitrox service using facilities not available to local dive shops, these new cylinders and valves will never be any cleaner than the day they are put into service.

How do I know that my cylinder and valve are Nitrox Ready?
Once a cylinder and valve have been prepared for nitrox service, a special sticker (or often a special version of the evidence of visual inspection sticker) is affixed to the cylinder. Unless the sticker explicitly states that a cylinder and valve are suitable for nitrox service, they are not. If the cylinder is ever filled with anything other than Oxygen-Compatible breathing gases, it is no longer suitable for nitrox service, and the sticker should be removed. Even with the best quality fills, hydrocarbon contamination can build up over time. Fill Express recommends that the cylinder and valve should be prepared for nitrox service each time the cylinder is hydrostatically tested.

What does the large green and yellow Nitrox decal mean?
Per PSI standards, presence of a color-coded 6-inch-wide green and yellow NITROX band decal indicates only that the cylinder contents are Nitrox. The large NITROX band does not indicate the cleanliness of the cylinder, its suitability for partial pressure blending, or what method was used to fill the cylinder. It is only the evidence of inspection sticker that indicates if a cylinder and valve are Nitrox Ready.

When does my cylinder or valve need to be Nitrox Ready?
The recreational diving industry has widely followed what is often referred to as the "40% Rule" which states the cylinder and valve must be Nitrox Ready only when they will be exposed to a gas mixture containing more than 40% oxygen. While sport diving Nitrox and technical diving Trimix don't absolutely require that the cylinder and valve be Nitrox Ready, many fill stations blend directly in the tank by first adding 100% oxygen and then topping off with oxygen compatible air. This process, known as partial-pressure blending, requires that the cylinder and valve be Nitrox Ready. In practice, almost all Nitrox and Trimix cylinders are Nitrox Ready because partial-pressure filling is so common. You should keep in mind that not everyone agrees with the 40% rule. Luxfer has an extremely detailed statement of their position on aluminum cylinders and oxygen which argues any cylinder that contacts more than 23.5% should be oxygen clean.

Does Fill Express require that my Nitrox cylinder be suitable for nitrox service?
Fill Express recommends, but does not require, all cylinders used for breathing gases other than air should be Nitrox Ready. Fill Express follows the "40% Rule" guidelines of our training agency affiliation with IANTD. This means any Nitrox mixture, premix or custom blend, at Fill Express less than or equal to 40%, does NOT require that your cylinder and valve be Nitrox Ready because our Nitrox is premixed before entering your cylinder, not partial-pressure blended with 100% Oxygen in your cylinder. Any Nitrox exceeding 40%, including our premix 50% and 80% Nitrox, DOES require that your cylinder and valve be Nitrox Ready.

Does Fill Express require my Trimix cylinder be suitable for nitrox service?
Sometimes. Because our standard Trimix blends are produced using a continuous atmospheric entrainment blending system, hyperoxic gases do not come in contact with your cylinder. Thus, at Fill Express our Trimix fills with standard gases do NOT require that your cylinder and valve be suitable for nitrox service. Many custom Trimix gases would require that your cylinder and valve be suitable for nitrox service.

Does Nitrox generated using membrane separation or entrainment contaminate my Nitrox Ready cylinder?
Usually, but not at Fill Express. Most premix Nitrox produced using membrane separation or entrainment only meets what is known as the CGA Grade E quality verification level. Filling any cylinder with CGA Grade "E" quality Nitrox will introduce hydrocarbon contamination such that the cylinder is not considered O2 clean and may not later be used for partial-pressure blending. This is true even if the cylinder was previously Nitrox Ready. However, at each of our fill stations, just prior to our fill whips, we have a special final hyper-filter system which allows us to deliver our gases meeting the more stringent purity specification known as "Oxygen-Compatible". Because of the hyper-filter, Fill Express premix Nitrox will not contaminate your cylinder.

Will any fill or VIP at Fill Express contaminate my Nitrox Ready cylinder?
No cylinder filling or visual inspection procedure at Fill Express will change the cleanliness state of the cylinder. All our gases, premix and custom are Ultra Pure and oxygen compatible. Fill Express only uses service materials, such as o-rings and lubricants, that are oxygen compatible. If your cylinder was Nitrox Ready when it entered Fill Express, it will still be Nitrox Ready when it leaves Fill Express.