ASHRAE Journal - November 2009 - 74

Research Topics 5.0 4.0 3.0 2.0 1.0 0.0 0 SF6 (Face) N2O (Face) 1,000 1,500 Time (s) 2,000 2,500 3,000 Figure 1: Tracer gas test. Using N2O as Tracer Gas Molecular Weight. Standard 110 allows alternative tracer gas; however, it requires a “similar molecular weight.” SF6 has a molecular weight of 146 while N2O has a molecular weight of 44. The momentum of the tracer gas induces airflow into the ejector and results in an air-gas mixture leaving the top of the ejector. At a release rate of 4.0 L/m, the induced volumetric flow was approximately 500 L/m for SF6 and 400 L/m for N2O. While the molecular weights are different, the size of the plumes differ by less than 25%, resulting in similar challenges to the hood. Concentration (ppm) Single Gas Tests We tested 30 laboratory hoods at a leading university with Standard 110-1995 using SF6 as a tracer gas and then using N2O. We used the university’s acceptance level AU 0.1 ppm. The conclusion, based on an acceptance criterion, was independent of the tracer gas used. Twentyeight hoods “passed” both tests, and two hoods “failed” both tests. By Martin Burke, Associate Member ASHRAE; Larry Wong; And Gerhard Knutson, Ph.D., Life Member ASHRAE NSI/ASHRAE Standard 110, Method of Testing the Performance of Laboratory Fume Hoods, provides a quantitative Dual Gas Tests The single gas tests had excellent agreement between the tracer gasses, yet the preponderance of tests near the limit of detection did not give the comparison data needed to evaluate N2O against SF6. To do that, we devised a “dual gas” test. We used two ejectors, placed side-by-side, and split the sample to report two infrared spectrophotometers (one for each gas). This allowed simultaneous tests with both tracer gases. For eleven hoods, the sash was full open (28.5 in. [724 mm]) and used a nominal face velocity of 60 fpm (0.30 m/s). For 18 hoods, we challenged the hood with a small cooling fan blowing from the front (15 hoods) or the side (three hoods). The fan turned on for 10 seconds every two minutes. For each hood, we tested 50 minutes and parsed the data into 10 five-minute paired tests. Figure 1 shows a typical 50-minute test with the sash full open. The figure shows remarkable correlation between the test results of the two tracer gases. In fact, the variability between each five-minute test is more significant than the variability between the simultaneous tracer gas readings. measure of hood performance, so it is influential in design and development of hoods. Because of global warming concerns, the tracer gas used in the standard is in jeopardy. Standard 110 releases tracer gas in a prescribed manner within a laboratory hood and monitors the concentration in the breathing zone of the mannequin, simulating a chemist or technician at the face of the hood. The average concentration provides a quantitative hood rating that allows comparison to other hoods or laboratory situations. Sulfur hexafluoride (SF6) is the tracer gas used. It is ideal because it is nontoxic, non-reactive, odorless, and absent in the environment. Also, the gas is easily detectable with a low (0.01 ppm) minimum detection level with readily accessible portable infrared spectrophotometers or electron capture devices. However, adsorption of infrared radiation and chemical and physical stability contribute to its high global warming potential of 23,900.1 Consequently, SF6 is imminently subject to California regulations that will restrict or remove it from fume hood testing. While looking for an alternative, we examined nitrous oxide (N2O). Conclusion Tests show N2O is a viable candidate for an alternative tracer gas for SF6, and the 30 single gas tests were essentially indistinguishable. For the 11 tests with full open sash at a nominal 60 fpm (0.30 m/s), face velocity showed remarkable similarity for the two gases. Tests with the fan challenge showed the influence of the fan. In addition, graphs of tracer gas readings clearly showed similar response with both tracer gases. Toxicity. Using the TLV as a rough measure of toxicity, N2O is more toxic (20 ppm) than SF6 (1,000 ppm). However, a recent review of the toxicological data2 concluded that the ASHRAE performance testing, using N2O as a tracer gas, presented negligible increased risk to testers, room occupants, or maintenance workers. Background Level. N2O is present in the atmosphere with variable background levels fluctuating around 0.32 ppm. However, the fluctuations are slow and the test results can be background corrected. 74 ASHRAE Journal Comparison of SF6 and N2O References 1. EPA. 2006. High Global Warming Potential (GWP) Gases, www. epa.gov/highgwp/scientific.html. 2. Vernon, R., S. Larsen. 2009. “Review of Nitrous Oxide Toxicity Data as it Applies to ASHRAE 110-1995 Procedure.” http://ucih. ucdavis.edu/docs/N2OforASHRAE110.pdf. Martin Burke is engineering manager with TSS in Berkeley, Calif. Larry Wong is environment, health and safety manager at the University of California in San Francisco. Gerhard Knutson, Ph.D., is president of Knutson Ventilation in Edina, Minn. ashrae.org November 2009

ASHRAE Journal - November 2009

Table of Contents for the Digital Edition of ASHRAE Journal - November 2009