Magazine article Occupational Hazards

Tracer Gas Testing Applications for Industrial Hygiene Evaluations: Tracer Gas Techniques Offer a Versatile Tool for Evaluating Airflow in Ventilation Systems, Rooms and Buildings

Magazine article Occupational Hazards

Tracer Gas Testing Applications for Industrial Hygiene Evaluations: Tracer Gas Techniques Offer a Versatile Tool for Evaluating Airflow in Ventilation Systems, Rooms and Buildings

Article excerpt

Tracers have been used extensively in various types of research and diagnostic activities. Examples include the use of radioactive fluids in the human body, dyes in water streams and even the use of tags on birds, deer and other wildlife.

[ILLUSTRATION OMITTED]

Tracer gases are being used increasingly in the field of industrial hygiene in a similar manner. This article reviews how to apply proven tracer gas techniques using sulfur hexafluoride (S[F.sub.6]) as an investigative tool in industrial hygiene evaluations. Some of the more common and easily conducted applications include:

* Determining air exchange rates and patterns within rooms or buildings.

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

* Assessing the degree of short-circuiting/reentrainment of exhaust discharges back into buildings.

* Verifying the effectiveness of local exhaust ventilation systems.

Why Sulfur Hexafluoride as the Tracer Gas of Choice?

Sulfur hexafluoride (S[F.sub.6]) is the most common gas of choice used in tracer gas testing. This is due to the following factors:

* S[F.sub.6] 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.

* S[F.sub.6] is normally not found in the environment. It is man-made; therefore, environmental background levels are essentially non-detectable.

* There are numerous instruments available that can detect S[F.sub.6] concentrations in the parts per billion (ppb) range, and some which are reported to be in the parts per trillion (ppt) range. Therefore, tracer gas applications do not require the use of large quantities of gas.

* S[F.sub.6] is readily available through most compressed gas suppliers.

Air Exchange Rate Determination

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, S[F.sub.6] 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 S[F.sub.6] (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 S[F.sub.6] are used to calculate air exchange rates.

_ = [ln C ([t.sub.2]) - ln C ([t.sub.1])] / ([t.sub.2] - [t.sub.1]) (Formula 1)

_ = Air exchange rate (number of air exchanges per hour)

ln = Log normal (natural log)

C = Concentration (dimensionless); 1 ppm = 0.000001

[t.sub.1] = Time at start of measurement period (hours)

[t.sub.2] = Time at end of measurement period (hours)

The smaller the volume of the ventilated building/room being tested and the less complicated the ventilation system, the easier this method is to apply. More complex ventilation systems with multiple air inlets and exhausts may require multiple injection points and monitoring points. …

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