Update April 2016    dissolvedgas.com

Trace Measurement Ltd

The difference between PPM O2 as a dissolved gas unit and PPM O2 quoted by gas suppliers

When a gas supplier says, for example, the have a maximum of 2 ppm oxygen in their Nitrogen it means exactly what is claimed… there are two molecules of oxygen for every million molecules of Nitrogen.

Expressing this 2 ppm in percentage terms = (2/1,000,000) x 100% = 0.0002% O2   

Now consider the case of dissolved oxygen (DO)….

DO (PPM) means mg/L, DO (PPB) means µg/L so 1 PPM = 1000 PPB ( mg/L = milligram/litre, µg/L = microgram/litre)  

A typical dissolved oxygen table will show the amount of oxygen that will dissolve in water at a given temperature and pressure, for example at sea level and 15.5°C (60°F) the DO table shows that 10 mg/l of oxygen will be in the water at equilibrium (i.e. the water has become fully saturated with the air)


However what is also known from general science is that air at sea level contains 20.94% oxygen

Hence 20.94% oxygen (gas phase) ~ 10 ppm  (mg/l) ~ 10000 ppb dissolved oxygen!!

To extend this imagine a 10 fold reduction: a gas containing 2.094% O2 would equilibrate in water at 1 ppm.

Looking at the table below one can see this argument extended

Understanding dissolved gas units

Oxygen %

DO (ppb)













The table shows that cylinder gas containing 2 ppm oxygen (ie 0.0002%) would give rise to 0.1 ppb dissolved oxygen

In our experience it is unnecessary to use ultra-pure gases when checking the zero of oxygen analysers - we believe the standard production Nitrogen supplied by gas companies will do a perfectly good job for far less cost