Frequently Asked Questions about Tracer Gas Testing by Dan Napier, CIH

How do I find out if the ventilation system is not bringing in dirty air or bringing the exhaust air back into my building?

Well first you need to check the system for it's ability to either obtain clean air or filter and clean the outside air. Often the fresh air intake is located in an area where clean fresh air is not available.  The "Classic Architectural Design\Actual Use" disaster was on a high rise building that had Indoor Air Quality issues.  Fresh air was introduced into the building from an outside ceiling at the second story.   Unfortunately the City Buses used the area immediately adjacent to the building to correct the bus schedule.  So buses would sit at idle for ten to twenty minutes at a time, directly underneath the "Fresh Air Intake".  Basically directing diesel exhaust directly at the building fresh air intake plenum.  Beyond the location and source of the fresh air --There are specific rules for how much air can be re-circulated and how much needs to be fresh outside air. These situations can usually be found by carefully checking the building and using direct reading instrumentation.  

After that determination, if you still have problems - Usually you do what is called tracer gas testing. It is used to do the following things:

Measuring airflow in ventilation systems where pitot tube or hot wire anemometer measurements are not practical or accurate.

Measuring the amount of short-circuiting/re-entrainment of exhaust air back into buildings

Finding out the air exchange rates and patterns within rooms or buildings

Checking the effectiveness of local exhaust ventilation systems

How exactly do you do tracer Gas testing?

Well one thing you can measure is the airflow rates in an HVAC (Heating Ventilation and Air Conditioning) System. That is done by putting a constant concentration of a known marker gas into the ventilation system and measuring the marker gas level in the air stream at some distance downstream from where you started. A downstream location sampling point at least 25 times the average duct diameter is the best place to start. But go back to all the other reasons to do tracer gas testing to see all the other possible reasons for testing, there is a method for each of the reasons to test your HVAC or exhaust system.

Are there any other reasons to use tracer gas testing?

Tracer gas testing can show airflow rates in ventilation systems. The advantages of tracer gas testing is that elbows, junctions and other turbulence-producing components of the system do not wreck your tests. Pitot tubes or hot wire anemometers must be done in the laminar flow zone of the ventilation systems, away from elbows, junctions and other turbulence-producing components. Most of the time HVAC systems do not have many laminar flow zones. If there are any they are usually unreachable except with some star-trek kind of elevation system. On the other hand the more complex an HVAC System is the more places that tracer gas will need to be placed to determine the system effectiveness.

A CIH is a professional who has completed College/University educational requirements, work experience and passed a difficult test. For the long answer go to the ABIH Web Page for a complete definition and a list of CIH's in your area.

What is Exchange Rate?

Sometimes we need to know how much outside air is being provided to a building or room or how airtight a room is (such as when a control room in a chemical plant is used as a safe haven during a chemical release/spill event). The American Society of Testing Materials (ASTM) International has developed Method E741-00, "Standard Test Method for Determining Air Exchanges in a Single Zone by Means of a Tracer Gas Dilution," for determining air exchange rates in buildings.

The test method specifies several different ways to determine air exchange rates. However, the simplest method requiring the least amount of equipment is the "concentration decay" method. To employ this method, marker gas is injected into the space of concern. Once a uniform concentration is achieved throughout the space (fans may be necessary to achieve this), the level of marker gas (decay rate) is monitored over a period of time, usually between 15 minutes and 4 hours (longer time periods are required for lower air exchange rates). The initial and end concentrations of marker gas are used to calculate air exchange rates.

What is ventilation effectiveness?

The effectiveness of a ventilation system's ability to either contain or capture a chemical contaminant is what is called effectiveness.

How do I measure Effectiveness?

The effectiveness of a ventilation system that is used for controlling air contaminants/employee exposures can be measured with  Marker Gas. First you must put marker gas directly into the ducting of the ventilation system, then note the marker gas levels inside the same duct at a distance of at least 25 duct diameters downstream. Next you must put marker gas in the same amount where the offending material is being released outside of the ventilation hood. You then compare the two levels and draw conclusions as to the effectiveness of the system.

How do I check to see if the exhaust vents are not going back into the work area?

Put marker gas in at the exhaust hoods and measure inside the building to see if marker gas is getting back into the building. Measure the levels in the hoods, and in the work area. If the hoods are 100% effective you should not find any marker gas in the air. Be very careful when you set up because you can spill some marker gas very easily.

Check out the Wikipedia Page for more information.The California Air Resources Board’s (CARB) regulation for Reducing Sulfur Hexafluoride Emissions from Gas Insulated Switchgear took effect in 2011 and is codified in the same subchapter as CARB’s mandatory GHG reporting and cap-and-trade regulations. The regulation applies to all owners of SF₆-insulated switchgear. It imposes an annual maximum rate of SF₆ emissions that is reduced by 1 percent over a ten-year period from 2011 to 2020.  From 2020 onwards, the maximum emission rate is not to exceed 1 percent. CARB is also proposing regulatory amendments to phase out use of SF₆ in gas-insulated equipment (GIE) starting in 2025.  The phaseout schedule limits the GIE owners’ ability to acquire new SF₆ GIE without an approved SF₆ phaseout exemption. More information on California’s regulation can be found here  DNA Industrial Hygiene has stopped using SF6 as a tracer gas because of the impact it has on the ozone layer.

Dan Napier, CIH

© 8/31/2022