Calibration Gas for Nitric Oxide, Nitrogen Dioxide, and Nitrogen Oxides

Nitric oxide (NO), nitrogen dioxide (NO2), oxides of nitrogen or nitrogen oxides (NOX) are by products of several processes including incomplete diesel combustion, power plant production & incineration, just to name a few.

There are often questions regarding which calibration gas to use, what is the correct balance gas, and/or what regulator and tubing should be used.

Let us get the information on these calibration gases correct.   

Here are some recommendations to follow:

• It is not recommended to mix NO and NO2. It is possible but the mixture is highly reactive and unstable resulting in a change in concentrations of the two gases. It is strongly recommended to keep them separated.
• Always use a balance of nitrogen for NO mixtures. An air balance will react with the NO and create NO2.
• Always use a balance of air for NO2 mixtures. The oxygen (20.9%) in the air balance maintains the NO2 stability by providing an excess of oxygen.

Nitrogen dioxide process of dissociation:  2NO₂ → 2NO + O₂
An excess of oxygen in the mixture will favour recombination.

• NO is typically used to represent NOX in most gas mixtures. EPA Protocol mixtures will list NO certified valve and the NOX as a reference value.
• NO2 cannot be mixed with SO2. The combination is unstable and NO2 can oxidize SO2 to form SO3. Where NO2 and SO2 are required keep them separated.
• For underground mining applications such as coal mining you can mix NO2/CO/CH4/O2 together but the gas mixture shelf life will be based upon the NO2.

Recommendations for regulators and tubing:

• No matter whether the regulator is a flow regulator or pressure regulator ALWAYS use stainless steel
• Use one regulator for each mixture. Don’t mix and match regulators. For example, if you have one mixture of NO/CO/SO2//N2 and one of NO2//Air, you need to purchase two regulators. Label them and only use them for the correct gas mixture. Trace amounts of SO2 could find their way into the NO2 cylinder and contaminate the gas mixture.
• If you are using tubing to deliver the gas mixture, always use Teflon or Teflon-lined tygon tubing.  Do not use standard tygon tubing. The NO2 and NO will adsorb onto the tubing.
• Try to keep the tubing as short as possible. All tubing has a permeation level and these gases are expensive.

Cylinder size:

• Calculate the total volume of gas which you will use in a year. From that determine the best size cylinder which will maximize your efficiency and lower your cost/ litre of the gas.

Today CAC GAS offers a 12 month shelf life for NRC’s (60 litre and 112 litre) and a 36 month shelf life for larger high pressure cylinders (900 litre and 4000 litre). The larger the cylinder the lower the cost per litre of the gas.