Protocols 101: Other Calibration Gases

Other calibration gases—what’s the difference?

Based on questions from customers like you, we have gathered the most common inquiries about other calibration gases, all in one place.

Why do I need a zero gas?

Setting a zero point in your analytical instrumentation is critical, and so is the purity of your zero gas. The EPA requires the use of zero gas to meet the requirements of 40 CFR 72.2 in EPA CFR 40 Part 60 EPA test methods, and EPA CFR 40 Part 75.

Contamination of your zero gas can influence the results of your measurements. Airgas ensures your zero gas meets 40 CFR 72.2 in EPA CFR 40 Part 60 EPA Test Methods, and EPA CFR 40 Part 75. For each zero gas, you will receive a certificate verifying this and that your gas does not contain the following:

  • Concentrations of NOx, SO2 and/or total hydrocarbons above 0.1 parts per million (ppm)
  • A concentration of CO above 1 ppm
  • A concentration of CO2 above 400 ppm


Talk to an Airgas specialist about achieving cleaner air, regulatory compliance and more.


Talk to an Airgas specialist about achieving cleaner air, regulatory compliance and more.

Defining the true value of zero

Absolute zero is the minimum point on the thermodynamic temperature scale: 0°K, -459.69°F or -273.16°C.

What is the right tolerance for an EPA protocol gas?

An effective EPA protocol gas mixture features the right combination of blend tolerance, analytical tolerance and traceability. Look for EPA protocol gases that feature two additional tolerances: preparation and certification.

  • Selecting the right combination of blend tolerance, analytical tolerance and traceability is not a complex task. Airgas offers six grades, covering most specification requirements:

    The mixture specifications table below outlines available grades from Airgas and their respective tolerances. All of our six mixture grades have two tolerances—preparation and certification. If your requirements are different from those listed, tap into our network of specialty gas experts by simply calling our technical hotline at 877-ASG-4-GAS or ask your Airgas Specialty Gas Specialist to review your needs. In addition, and with the exception of batch-certified mixtures, each calibration gas standard comes with a Certificate of Analysis. All mixtures are filled to 2000 psig at 70°F unless otherwise noted.

    Available Mixture Grades from Airgas

    View the Airgas Specialty Gas catalog for more information.

    Have a question? Ask an Airgas specialist.

What is the difference between a NIST-Traceable Reference Material (NTRM) and Gas Manufacturers Intermediate Standard (GMIS)?

A NIST-Traceable Reference Material (NTRM) is a gas reference material (or gas mixture) manufactured by a commercial gas supplier but certified by NIST. NTRM gas standards are equivalent to SRMs and are used to analyze and certify commercial gas mixtures, such as EPA protocol gases.

A Gas Manufacturers Intermediate Standard (GMIS) is a gas reference standard made by a gas supplier and certified according to the U.S. EPA protocol rules for GMISs. This term is frequently used in the gas industry to mean any generic gas reference standard, but Airgas restricts the meaning to only those standards (gas mixtures) made by the U.S. EPA GMIS procedure.

When should I use CEM Daily Calibration standards instead of EPA protocol gases?

Defining the difference between EPA protocol gases and compliance gases is simple. Where EPA protocol gas standards are not required by regulatory guidelines, Continuous Emissions Monitoring (CEM) Daily Calibration standards are blended to the same exacting standards, are NIST traceable and have an analytical accuracy of ±2%. In addition to those components and combinations of components listed as EPA Protocol Standards, additional gas mixtures and components are available as daily calibration standards.

What defines a toxic gas?

We define a toxic substance as one that has the ability to produce injurious or lethal effects through its chemical interaction with the human body. Toxic gases can be high pressure, reactive, flammable or nonflammable and/or oxidizing in addition to their toxicity. The degree of toxicity and the effects will vary depending on the gas.

US government agencies have issued guidelines and regulations for the use of toxic gases. Emissions from hazardous air pollutants that are considered toxic by the EPA are regulated by compliance guidelines. OSHA has also issued guidelines and safety precautions that must be implemented when your team is handling toxic gases used to measure these types of emissions. All are critical to protecting your team and the surrounding community where you work.

Here are some important safety steps you should perform when working with toxic gases:

  • Strictly adhere to permissible exposure levels
  • Read the Safety Data Sheet (SDS) thoroughly before use
  • Refer to your local building code for storage and use requirements for toxic gases
  • Never work alone with toxic gases—inspect the system that will contain the gas and thoroughly test it for leaks with an inert gas before use
  • Purge all lines with an inert gas before opening the cylinder valve or breaking connections
  • Ensure work area is well-ventilated
  • Have gas detectors, self-contained breathing apparatus on hand and wear protective clothing
  • Store your breathing apparatus in a safe area immediately adjacent to the work area, so that in the event of an emergency you can go directly into that area, close the door and safely use it
  • Full body showers, eye washes, fire alarms and fire fighting equipment should be readily accessible
  • Use gas cabinets and manifolds to help eliminate safety hazards, and preserve concentrations during use

As a best practice, keep your inventory of toxic or poisonous gases at a minimum. When a project is completed, return any leftover cylinders to Airgas—they should never be stored for possible future use. Storing cylinders that contained toxic gases could result in accidental removal of cylinder labeling, making it an unnecessary hazard and greatly increasing the cost of proper disposal.