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 Busses used the area immediately adjacent to the building to correct the bus schedule.  So busses 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 SF6 into the ventilation system and measuring the SF6 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.



Why do you use Sulfur Hexafloride?


Sulfur hexafluoride (SF6) is the most common gas used in tracer gas testing. It does sound very technical and possibly dangerous, but it is currently the best available. Here are several reasons why it is used.


SF6 is relatively low in toxicity. The Occupational Safety and Health Administration (OSHA)Permissible Exposure Limit (PEL) and the American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Value (TLV) for this gas have been set at 1,000 ppm. Its primary health hazard is asphyxiation.


The gas is virtually odorless, with no odor threshold published by the ACGIH or other professional organizations. Therefore, its use in buildings does not needlessly alarm occupants.


SF6 is normally not found in the environment. It is man-made; therefore, environmental background levels are essentially non-detectable.


There are many instruments available that can detect SF6 concentrations in the parts per billion (ppb) range, and some which are reported to be in the parts per trillion (ppt) range.


Because you can detect SF6 at very low levels tracer gas applications do not need very much SF6 to be released.


SF6 is easy to purchase from compressed gas suppliers, however it is not inexpensive and you have to have a special regulator designed for SF6.


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.


What are the down sides for SF6?

It is an ozone depletor, it is highly regulated and eventually will not be available.  It is difficult to obtain in some areas.

Can I just start tomorrow to do this testing?


Not really, tracer gas testing is a complex and difficult task and it needs to be carefully planned. Your best bet is to find a CIH who has the equipment and SF6 available. The instruments are specific for SF6 and the SF6 is costly and usually is only available in shop tank amounts. The results of the testing need to be evaluated and are complex. The CIH will be able to conduct the test, evaluate, explain what the results mean the results and make recommendations if changes are needed.


What is a CIH?


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, SF6 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 SF6 (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 SF6 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. SF6 is a good choice for effectiveness measurements.


How do I measure Effectiveness?


The effectiveness of a ventilation system that is used for controlling air contaminants/employee exposures can be measured with SF6 Tracer Gas. First you must put SF6 gas directly into the ducting of the ventilation system, then note the SF6 levels inside the same duct at a distance of at least 25 duct diameters downstream. Next you must put SF6 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 effective ness of the system.


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


Put SF6 in at the exhaust hoods and measure inside the building to see if SF6 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 SF6 in the air. Be very careful when you set up because you can spill some SF6 very easily. You cannot smell it or see it - Remember.


Dan Napier, CIH

© 2010