Middlesex Gases


Tech Tip - What Constitutes the "Grade" of a Gas?

How do I know what “Grade” to order?

What “Grade” is the right one for my application?

Is Medical Grade the cleanest?

These are all good and common questions from end users of gases. Gases are graded by the number of nines (9) in the purity level. A gas that is considered a grade 5.0

  • is 99.999% pure, “five nines.”
  • A gas that is grade 4.8 is 99.998% pure, “four nines and an eight.”

What makes a gas a certain grade is the purity level of the gas. Gases will be dropped into purity categories according to the parts per million (PPM) levels of particular impurities. Using Helium as an example two common categories are as follows:

  • Grade 5.0, 99.999% pure will have less than 2 PPM of Moisture, less than 1 PPM of Oxygen and less than 0.5 PPM of Total Hydrocarbons
  • Grade 4.7, 99.997% pure will have less than 3 PPM of Moisture, less than 3 PPM of Oxygen and less than 1 PPM of Total Hydrocarbons

Most contaminants travel in moisture molecules and moisture is commonly the most unwanted contaminant in the “Life Science” industry.

In choosing the right grade of gas for your application, ask yourself what contaminants you would like to keep to a minimum in the application. Having answered that, ask yourself what is the tolerance level of those contaminants in PPM. With those questions answered you can order your gas within the limits of your application. Simply advise your gas supplier of the PPM level of the contaminants in the gas you are ordering and they will provide the right “Grade.”

Do not assume that Medical Grade gas is the cleanest. To be considered a Medical Grade gas, the gas must meet the requirements of the FDA. The FDA requires Helium to be 99.0% pure. That is miles away from our 99.999% pure grade. If you are not sure about the right Grade of gas for your application, call me at 617-733-5946 or send me an email at Mike Lee.