by Rex W. Moore, LIH, CIH, CSP
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.
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 (SF6) 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/re-entrainment 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 (SF6) is the most common gas of choice used in tracer gas testing. This is due to the following factors:
- 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 numerous 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. Therefore, tracer gas applications do not require the use of large quantities of gas.
- SF6 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, 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.
_ = [ln C (t2) - ln C (t1)] / (t2 - t1)(Formula 1)
_ = Air exchange rate (number of air exchanges per hour)
ln = Log normal (natural log)
C = Concentration (dimensionless); 1 ppm = 0.000001
t1 = Time at start of measurement period (hours)
t2 = 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.
Airflow Measurements in Ventilation Systems
Tracer gas testing can accurately determine airflow rates in ventilation systems. The advantages of tracer gas testing versus traditional means of measuring airflow rates in ventilation systems is that elbows, junctions and other turbulence-producing components of the system have less impact on the accuracy of the measurements. Traditional means of measuring airflow utilize pitot tubes or hot wire anemometers (measurement locations for these types of instruments need to be in the laminar flow zone of the ventilation systems, well away from elbows, junctions and other turbulence-producing components). Quite often, ventilation systems do not have many laminar flow zones, thereby making traditional measurement techniques inaccurate.
Measuring airflow rates in ventilation systems with tracer gas testing is achieved by injecting a constant concentration of SF6 into the ventilation system and monitoring the SF6 level in the air stream at some distance downstream of the injection point. A downstream location sampling point at least 25 times the average or equivalent duct diameter is recommended.
Plugging the dose rate and SF6 measurement into the following formula will result in an accurate airflow determination.
Q (ft3/min) = Dose (ft3/min)/C (ft3 SF6/ft3 air) (Formula 2)
Q = Airflow
Dose = Amount of SF6 injected in ft3/min
C = Concentration of SF6 (dimensionless);
1 ppm = 0.000001
Sometimes we need to check the effectiveness of a ventilation system that is used for controlling air contaminants/employee exposures. The effectiveness of a ventilation system's ability to either contain or capture a chemical contaminant can be accurately determined using tracer gas test methods.
To measure the effectiveness of a ventilation system's ability to capture a chemical contaminant, two different steps need to be conducted. The first step involves injecting a constant known quantity of SF6 gas directly into the ducting of the ventilation system, followed by recording of SF6 levels inside the same duct at a distance of at least 25 duct diameters downstream of the injection point. The second step involves the injection of SF6 at the same constant known quantity at some location of concern (an area of contaminant generation) outside of the ventilation hood, coupled with the recording of SF6 levels inside the ventilation ducting at the same location as in the first step.
The effectiveness of the ventilation hood in capturing air contaminants generated at a location of concern can be calculated by inserting SF6 levels measured in the first and second steps into the following formula.
Hood Efficiency % = [(SF6) (Step 2)/(SF6) (Step 1)] x 100 (Formula 3)
To measure the effectiveness of ventilation of an exhausted enclosure to contain chemicals generated during a process or piping failure, SF6 gas can be released into the exhausted enclosure at a location of concern followed by SF6 measurements outside of the exhausted enclosure at a location of concern. Inserting the SF6 measurement data, plus data relative to the gas of concern in the exhausted enclosure, into the following formula will allow for calculating the equivalent release concentrations outside the enclosure:
ERC = [(Pc)(Qp)(Tm)] / [(Tc) (Qt)] (Formula 4)
ERC = Equivalent Release Concentration in ppb
Pc = Concentration of Process Gas/Vapor of Concern in %
Tc = Tracer Gas Concentration in %
Qp = Chemical Emission Rate (liters per minute)
Qt = Tracer Gas Injection Flow Rate (liters per minute)
Tm = Maximum level of tracer gas measured outside enclosure in ppb
Evaluating Re-Entrainment of Ventilation Exhaust
Tracer gas testing can also be used to determine the degree or amount of re-entrainment of a ventilation exhaust from a building. This is achieved by injecting a constant amount of tracer gas into the exhaust duct upstream of the exhaust fan while simultaneously measuring the SF6 level at a location downstream of the exhaust fan and within the building at a location of concern. The ratio of the concentration of SF6 in the area of concern within the building to the concentration in the exhaust duct gives us the percent of re-entrainment of exhausted air back into the building (see Figure 4).
% Re-entrainment = (Co / CF) x 100 (Formula 5)
Co = SF6 Concentration in Fresh Air Intake Duct
CF = SF6 Concentration in Exhaust Air Duct
This example is based on one ventilation unit. Large buildings often have multiple ventilation units. Therefore, to adequately determine the degree of re-entrainment, testing needs to be conducted on all ventilation units.
There are several different instruments for measuring SF6. Some are reported to measure in the parts per trillion range, while most are reported to measure in the low parts per billion range. The Foxboro Co. has several different instruments, which operate on the principle of infrared spectrometry (they provide direct readings of SF6 in the low parts per billion range). California Analytical has a series of instruments that operate on the photoacoustic principle (they provide direct-reading measurements in the low parts per billion range). Both manufacturers offer devices that weigh less than 18 pounds and provide almost real time direct-reading measurements. Grab samples of air are periodically analyzed for SF6 (approximately every 45 seconds)
Several companies manufacture gas chromatograph instruments equipped with electron capture detectors, which are reported to detect SF6 in the parts per trillion range. However, they are generally large, bulky, laboratory-type instruments, and are not easily transported. Field samples must be collected in syringes within areas of concern and then later injected into the instrument.
There are several manufacturers of instrumentation that works on the principle of Fourier Transform Infrared Spectrometry (FTIR), which also are reported to measure SF6 in the parts per trillion range. However, these instruments typically are heavy, weighing 50 to 100 lbs, and often require the use of liquid nitrogen as a coolant.
When conducted properly, tracer gas testing techniques can provide useful and accurate data for resolving industrial hygiene concerns.
Rex W. Moore, LIH, CIH, CSP is manager, Occupational Health and Safety for the Chicago Regional Office of Clayton Group Services Inc. He has 28 years of experience providing professional safety, industrial hygiene and environmental consulting services to a wide variety of industries at both plant and corporate levels. Moore specializes in design and management of industrial hygiene and indoor environmental quality (IEQ) surveys; industrial ventilation systems design and evaluation; asbestos inspection, management and project design; and noise measurement and control. He has extensive experience developing and implementing OSHA compliance programs addressing such issues as lockout/tagout; confined-space entry; hazard communication; hearing conservation; lead, asbestos and cadmium management; personal protective equipment (PPE) and respirator protection; and fire and medical emergency plans.