A helium tank can have as much as 99% pure helium in it, but in order to be certain– it is best to ask your helium supplier what percentage of helium is in the tank. Most likely the tank will be between 90 percent to 99% helium.
Helium suppliers use other gases to fill the rest of the tank. These gases are heavier than helium and closer to the weight of air; typically nitrogen is used and it weighs only slightly less than air. Float times and balloon lift will not be the same if your tank has other heavier gases. Some disposable tanks have as much as 20 percent air added to them.
Bargain Balloons has tested the float times of disposable tanks and on average it is much lower. It is so much lower, that some customers even think the balloons used are dective or poor quality. A helium tank never contains 100 percent helium.
Companies will often advertise helium used in the balloon industry as balloon gas. These companies are honestly telling you the tank has a lower purity rating, but you would have to ask them what that rating is.
BOC advertises on its website that balloon gas is 97 percent helium. A percentage of air or nitrogen is okay. There is even a technique called 60/40, when a balloon is inflated with 60 percent helium and 40 percent air. The balloon can still float, but you need to know what you are doing to make sure it does. As helium prices rise, we as concerned helium suppliers may add more nitrogen to keep prices down.