The differences between carbon fiber tanks and fiberglass tanks

by Harley Ayre

Before we begin, I want to thank all of you who responded to last week's survey. Your answers were thorough and enlightening. The folks at Airhog read them and will be thinking of ways they can better meet your needs.

Let's look at two similar air tank technologies - carbon fiber and fiberglass. They seem very similar, and they are in some respects; but there are also some important differences you should be aware of.

Hydrostatic test
All high-pressure vessels over 2" in outside diameter are periodically tested by hydrostatic test. The purpose of the test is to determine if the vessel has been work-hardened through expansion and contraction. If it has, it is condemned and cannot be filled by a dive shop. For aluminum and steel tanks, the hydro is the final word, unless a fill station spots an obvious flaw during an annual visual inspection. The common hydro schedule is every five years, though there are exceptions.

The age of the tank
But fiberglass and carbon fiber tanks are different than metal tanks. The U.S. tanks do have an aluminum reservoir inside (called a bladder), but it's reinforced by tight wrappings of either fiberglass or carbon fiber. The wrapping gives strength to the tank, and both fiberglass and carbon fiber tanks can be pressurized to either 300 bar (4351 psi) or 310.3 bar (4,500 psi). These tanks also require a hydro test, but they have a definite lifetime, as well. After 15 years from the date of manufacture (when they received their first hydro), carbon fiber and fiberglass tanks are condemned. That is different than aluminum or steel tanks, whose lifetime depends only on the results of the hydro.

Look at the date!
Refill stations have their personnel trained to read the date stamps on all air tanks. They know when a tank is supposed to be filled and when it isn't. An old aluminum or steel tank can be out of hydro and require another test before it can be filled again, but a fiberglass or carbon fiber tank can be condemned on the basis of the date, alone. It is possible, though highly unlikely, that a fiberglass or carbon fiber tank could have never been used one time and still be condemned because of its date of manufacture. That makes a big difference to you, because there are some places selling tanks with a lot less than their full life remaining.

Learn to read the date!
Here's a brief explanation of all the information found on a typical carbon fiber tank. A fiberglass tank would have similar information. Unlike aluminum and steel tanks that have this information stamped into the metal of the tank, fiberglass and carbon fiber tanks have stickers permanently attached under the clear coat of the tank. It will look like the sample shown below.


The label on a carbon fiber tank contains a lot of information relating to the testing of the tank. Fiberglass tanks have similar labels.

Here is the problem
Fiberglass air tanks are an older technology. Once carbon fiber came to market, it had a weight advantage that made fiberglass tanks unpopular, and they were soon replaced. A fiberglass tank weighs as much as 75 percent more than a carbon fiber tank of the same capacity. Emergency services that use the tanks want them to be as light as possible, so fiberglass fell out of favor. They are just as strong as carbon fiber tanks, but they're so heavy that the market for them collapsed.

Therefore, you may find a brand new fiberglass tank with very little life remaining. This is common, in fact, because fiberglass tanks are difficult to sell. Emergency services that were once their biggest users now buy only carbon fiber tanks, so a lot of fiberglass tanks never sold originally. They are now popping up for sale, so you need to assess the life they have remaining if you're interested in buying one.

Carbon fiber tanks should also be inspected with this in mind. They continue to be made, so you are unlikely to find one that has sat on the shelf for years. There are used tanks for sale everywhere. Find out how much life remains before you buy! At Airhog, all tanks are fresh and new, or you will be told their age before you buy.