What is the meant by the term Grade 5.0, or Grade 5.5?
You can never have 100% absolute with a gas or chemical. Because of this gas purity is measured by the number of 9’s, and is based on reported impurities. With typical specialty pure gases, you begin with 99% (2 nines), followed by the decimal point. This allows us to scale up purity to our best level, which –depending on the product specification – may be 99.99999%.
Consider an example of a high purity Helium for instrumentation. You may be offered Grade 5.0 (99.999%), where the total maximum level of reported impurity will not exceed 10 ppm, or 0.001%. If your instrument required a higher purity, you may choose a Grade 5.5 (99.9995%), where the reported impurity level would not exceed 5 ppm, or 0.0005%. Individual impurity specifications are defined in your supplier catalogue.
The best practice is to review the published impurity levels offered to ensure the product meets your expectations.