Typically, purity ratings for specialty gas products are focused upon high purity gases. However, purity ratings also apply to gas mixtures and specialty gas regulators with equal importance.
Gas Purity
Purity ratings are used to indicate the possible impurities in a measured amount of a pure gas, or gas mixture. Impurities are any substance other than the intended molecules or compounds in a mixture.
Impurities can be introduced in a number of ways, but primarily they are inherent in the raw material in its pure form. These impurities depend on the gas type and method of manufacture.
Purity Ratings for Specialty Gases
There are many terms used to indicate the purity of a given gas but these terms are not absolute and variance in the same term can be found across different industries.
In the specialty gas industry, a system has been adopted to simplify purity ratings so that they are easily compared. Since specialty gases are generally of a much higher purity than in other industries, purity ratings are based on the number of nines in the purity percentage.
This rating is sometimes prefixed with an upper-case N. Examples include;
- 2.0 or N2.0 = 99%
- 3.0 or N3.0 = 99.9%
- 6.0 or N6.0 = 99.9999%
The same system is used to indicate purities where one or more of the numbers are not nines. These numbers are indicated after the decimal point in the n-rating.
- 2.8 or N2.8 = 99.8%
- 3.5 or N3.5 = 99.95%
- 4.8 or N4.8 = 99.998%
Purity Ratings Vs Specialty Gas Concentration
Purity ratings are always relative to the concentration of the gas. In some cases it is useful to use this rating to determine the actual amount of impurities in a more relative unit like ppmv or mols.
To work out the total possible impurities, subtract the purity rating from 100%, and multiply this figure by the total number of units.
Example 1:
- Nitrogen with a rating of 5.0, equals 99.999% purity.
- 100% – 99.999% = 0.001% total possible impurities.
- To determine ppmv (parts per million by volume)-
We know that 1,000,000 ppmv = 100%
So we multiply 1,000,000 x 0.001% = 10ppmv
Therefore there can be up to 10ppmv of impurities in Nitrogen with a rating of 5.0.
Example 2:
- Carbon Dioxide with a rating of 3.0, equals 99.9% purity.
- 100% – 99.9% = 0.1% total possible impurities.
- To determine ppmv (parts per million by volume)-
We know that 1,000,000 ppmv = 100%
So we multiply 1,000,000 x 0.1% = 1000ppmv
Therefore there can be up to 1000ppmv of impurities in Carbon Dioxide with a rating of 3.0.
Gas Mixtures
Purity ratings can also be useful when dealing with mixed gases. Many of our specialty gas calibration mixtures contain components at such a low level that they become insignificant, but we can still work out the total impurities.
A mixture containing:
- Carbon Dioxide (3.0) 2.0% vol
- Methane (3.5) 2.5% vol
- Nitrogen (5.0) BALANCE
Using the same method to calculate ppmv as in the previous example, we know that:
- Carbon Dioxide (3.0) can have 1000ppmv of impurities
- Methane (3.5) can have 500ppmv of impurities
- Nitrogen (5.0) can have 10ppmv of impurities
Now, we take the total possible impurities and multiply that by the percentage of the mixture they constitute:
- Carbon Dioxide – 1000ppmv of impurities X 2.0% = 20 ppmv impurities
- Methane – 500ppmv of impurities x 2.5% = 12.5 ppmv impurities
- Nitrogen – 10ppmv of impurities x 95.5% = 9.55 ppmv impurities
Gas Purity’s Effect on Gas Instruments
It is important to note that impurities will not necessarily have any effect on your instrument. However, when selecting a purity grade for your application it is important to know what these impurities may be and whether they will have an effect. We can provide this information on pure gases and gas mixtures we supply.
For example: grade 5.0 nitrogen is commonly used for zeroing portable gas detection instruments because we know that none of the impurities (typically hydrocarbons) will have any effect on the other sensors.
High Pressure Specialty Gas Regulators and Gas Purity
When dealing with high pressure specialty gas regulators, the purity rating refers to the amount of impurities which a regulator may introduce into the pure gas or gas mixture. It also indicates the highest gas purity that the regulator can maintain. E.g. methane at grade 6.0 would require a grade 6.0 regulator.
Regulator impurities can be introduced in several ways, including:
- Residual materials inside the regulator can be absorbed into the pure gas or gas mixture.
- Leaks may allow ambient atmospheric gases to enter the regulator.
- Reactions between the pure gas or gas mixture and the regulator’s materials.
When you are selecting a regulator to use with pure gases or gas mixtures, you must determine an acceptable level of impurities which may be introduced.
Purity grades vary with different materials, purity of the material and quality of manufacturing.
Examples of Different Specialty Gas Regulator Purity Levels:
- Lab-Master Series 5.0 (99.999%) Up to 10ppm impurities
- Spec-Master Series 6.0 (99.9999%) Up to 1ppm impurities
- Chem-Master Series 6.0 (99.9999%) Up to 1ppm impurities
The selection of specialty gas regulators is therefore equally as important as the selection of the gas purity.
Selecting a N5 purity gas and then connecting an inexpensive regulator without any rating defeats the objective of the limiting or minimizing impurities.
Both gas purity and regulator purity must be considered when determining the best solution for specialty gas applications.
