If you work in the field of fume hood performance testing (FHT), chances are, you’re already familiar with sulfur hexafluoride. More commonly known as SF6, sulfur hexafluoride is used as a tracer gas to help determine the effectiveness of fume hood containment performance and help keep lab personnel safe.
NEBB FHT Certified Professionals (CPs) know SF6 due to its application in Tracer Gas Containment Tests. Per NEBB FHT Procedural Standards, commercial grade SF6, or other acceptable gas of approximately the same molecular weight, stability, and grade as SF6 is required in order to properly test fume hood containment and offer Tracer Gas Containment Test Data Reports to clients. This standard—and its specific request for SF6—comes directly from the ANSI/ASHRAE 110-2016, Method of Testing Performance of Laboratory Fume Hoods standard.
In the arena of fume hood performance testing, NEBB CPs perform testing in regard to average face velocity, airflow visualization and tracer gas containment. Some clients may opt for one test while others request data reports related to all three. A Tracer Gas Containment Test, however, is the only procedure that involves SF6. It is also the most important test when it comes to life safety.
“The ASHRAE 110 standard outlines these three tests and users can choose to do one, two, or three of them and still say ‘We did an ASHRAE 110 Standard test,’ but if something is leaking out of the hood, this is the only method that can catch a leak. Your face velocity can be right on and that hood can still be leaking out contaminants,” explains Elizabeth Blankenship, P.E., Managing Member of BPI Testing. “Lab safety is really important. Every time I go into a lab setting, I see improper uses of fume hoods. The fact that there isn’t a better tracer gas right now makes it hard for me to go out and sell clients on this type of testing.”
With issues related to cost, equipment, and the environment, the search for a suitable replacement to SF6 is nothing new for veterans of FHT. In fact, attendees of the 2014 NEBB Annual Conference in Fort Lauderdale, FL may recall former NEBB Technical Director Don Fedyk giving a presentation on FM-200TM called “Proposing a New Tracer Gas for Fume Hood Testing.”
Equipment Availability
Seeing that NEBB’s Tracer Gas Containment Test standards are based off of the ASHRAE 110 standard, professionals must use metering equipment that records at a one-second interval and is able to detect tracer gas measurements of ten parts per billion. While reading measurements once per second may sound like a lot, it was determined necessary to ensure continuous sampling.
“Back in the 1995 standard, samples were recorded every ten seconds and now the 2016 standard is every second. That’s because a surge of tracer gas could escape the fume hood and if you’re not measuring every second, you will miss it.” Explains Michael Kelly, Project Manager of Air Filtration Management and Chair of NEBB’s FHT Committee. Coupled with the fact that SF6 is the only tracer gas approved by name in the standard, NEBB CPs are left with few equipment options.
“The toughest question I get asked by new people at the fume hood seminars I do, is what equipment they should own. There are only two meters available—the MIRAN SapphIRE which has been taken over by Thermo Fisher Scientific and the USON-Q200 Leak Detector which is now no longer made in the US,” explains Kelly. “We’ve been talking about changing the standard for a long time, so regardless of when it hits, we know it’s coming. Those meters are intended for the current standard based on SF6, so it’s very hard for businesses to make the investment today when we don’t know what’s going to happen tomorrow.”
Cost Limitations
“There’s no good option. They are both outdated, with a minimum cost near $30,000. The USON-Q200 uses an argon tank to clean out the meter too, so then you have two tanks to lug around. The SapphIRE has an AC to DC converter that is around 6 by 4 inches to charge a 3-inch cube, making it heavy too,” says Blankenship. “The MIRAN Sapphire has been discontinued. They don’t make it anymore. They may still calibrate it, but they won’t service it anymore, likely because it’s ancient. I can’t fault them for it, but it’s expensive equipment and it’s frustrating knowing if it breaks, you are just out of luck.”
In addition to the hefty price tag and availability issues associated with metering equipment, the cost of SF6 itself is not exactly low. For example, filling a 40-pound high-pressure cylinder tank with commercial grade SF6 can easily cost $550, if not more. And as with anything, the higher the material costs, the higher the cost to end users.
All other issues related to SF6 aside, a client may opt to forego a Tracer Gas Containment Test based on cost alone. Imagine a client with a hundred or so fume hoods to be tested, and it’s conceivable that testing of that proportion may just not fit their budget. While manufacturers may foot the bill for some fume hood tests, it is really the ‘as used’ testing that is vital to the health and safety of personnel.
“In most cases, the lab environment is what causes a fume hood to lose containment. The characteristics of the lab like cross-airflows, a supply diffuser right above the fume hood, and other equipment can all affect the air characteristics of the lab and that is what will cause the fume hood to lose containment. That is why new installations certainly want to do this test and if the lab is retrofitted, you want to do it as well. When I do work at universities, I test 100 percent of the fume hoods in those labs. Safety should outweigh cost,” recommends Kelly. “Additionally, some of our clients get a reduction in their insurance premiums to perform tracer gas containment testing.”
Environmental Impact
Perhaps most importantly of all, SF6 is detrimental to our environment. As the most potent greenhouse gas, the effect of SF6 on the environment is nearly 24,000 times worse than that of carbon dioxide. According to the United States Environmental Protection Agency, SF6 is a very stable, synthetic chemical with a lifetime of 3,200 years once emitted into the atmosphere, meaning even small traces have long-lasting effects.
The environmental impact of SF6, alone, has led to the state of California phasing out its use since 2011. While these restrictions stemmed from the energy industry—the biggest culprit of SF6 use, with gas-insulated switchgear for electrical grids totaling 80 percent of total SF6 consumption—leaders in the building industry are stepping up to initiate changes, too.
In fact, organizations like ASHRAE and NEBB have been changing their standards and requirements to reflect best practices in environmentalism and occupant safety for decades. In 1995, the ASHRAE 110 standard was revised to identify SF6 for use in fume hood testing because the dichlorodifluoromethane gas (also known as refrigerant R-12) previously prescribed in ASHRAE 110-1985 was identified as a harmful pollutant.
“ASHRAE has been looking into alternatives for a while, but as far as I know, they just couldn’t find a suitable gas with the same characteristics as SF6. Some companies use nitrous oxide as an alternative right now. Neither ASHRAE nor NEBB have approved that. Nitrous oxide is also considered a greenhouse gas and it has its own safety hazards,” states Kelly. “SF6 is nonflammable, it’s just a greenhouse gas. So we’re protecting the environment by banning SF6, but as a result may not know if workers are safe using their fume hood.”
“When it’s not good for the environment, but at the same time you have life safety issues, what is the solution?” Blankenship inquires.
Case Study: Research Project 1573
“I am waiting and hopeful that the standard changes, which ASHRAE has been discussing because SF6 is such a terrible environmental hazard,” mentions Blankenship. The search for an eco-friendly, cost-effective alternative has been tricky both due to the challenge of finding a gas with the same chemical makeup of SF6, as well as identifying readily available equipment to eject and detect an alternate gas.
Such hope, however, may lie in ASHRAE’s Technical Committee 9.10, Laboratory Systems sponsored Research Project 1573 focused on determining a suitable replacement of SF6 when used as a tracer gas in accordance with ANSI/ASHRAE Standard 110. Beginning back in March 2017, the research project was led by principal investigator Tom Smith, President and CEO of 3Flow and voting member of ASHRAE 110 for Revision of the Standard, Special Projects Committee as well as Technical Committee 9.10.
“SF6 was great. We could control it, it was odorless, colorless, nontoxic. It just happens to be a really bad environmental hazard. And cost was also a major problem,” comments Smith.
The goal of Research Project 1573 was to determine a replacement to SF6 that produced equivalent or superior results. Zeroing in on isopropyl alcohol (IPA) as a potential replacement for SF6, the team’s investigation spanned approximately two years.
IPA as an Alternative Tracer Gas
“It’s clean, it’s green, and it serves a natural disinfectant when aerosolized. So, it has some great properties, it’s easily detectable, and at the concentrations we’re using it, it’s relatively low hazard,” explains Smith. “But essentially what gave us the idea is that it had been used previously in indoor air tracer studies, so we already knew that we could generate it, we could detect it, and it was relatively inexpensive and available.”
At half the environmental impact of carbon dioxide, IPA is relatively inexpensive and easy to obtain. While SF6 may cost anywhere between $15-50 per pound, the same amount of IPA costs a mere $1.25.
After a few different trials, the research revealed that isopropyl alcohol could be vaporized and mixed with air before being discharged from an ASHRAE 110 outlet diffuser. Through this process, Smith’s team also recognized the need for an alternative testing method that would reliably create a plume of similar proportions each time.
“That’s what we’re working on now. We have a new ejection system and we’re going to send the specifications to a number of groups in the committee so that they can run their own tests and confirm—or dispute—the results we got with our testing methods. We’re putting out a method to do that. It’s looking a little more consistent now and should be closer to something that could be fabricated—and if not purchased commercially, constructed in the same way that we do it now with the ASHRAE 110,” confirms Smith.
Although the ejection system is still being fine-tuned, Smith’s research did identify a low-cost, readily available leak detector. Photoionization detectors (PIDs) were used to detect the IPA in tracer gas form.
“We looked at RAE PIDs as a way to detect the IPA because we’ve used these many, many times in indoor air quality studies and they’re used for all kinds of different atmospheric detection. They’re about $3,000—so nearly one tenth the cost of what we were using previously,” details Smith. “They’re very rugged, they’re very stable and they’re reliable. We hooked everything up and did comparison tests—SF6 versus IPA.”
Because the detection equipment is designed to measure total volatile organic compounds, there is a chance that PIDS could pick up trace amounts of IPA designated for other applications elsewhere in the lab. For that reason, background measurements prior to conducting the tracer gas testing is imperative. Smith states, “We’ve been doing air tracer tests for 30 years both as an ASHRAE 110 fume hood testing method, but also indoor air quality, odor migration, concentration migration, ventilation effectiveness studies, so there’s always going to be a potential for difficulties with any air tracer. Background concentrations are always a potential cause of concern.”
A Solution in Sight
“This is a method that’s not perfect, but it’s darn good. We still have to get over the finish line. The project and report are done, but we have to get the committee to accept it. We also must keep in mind that IPA is classified as a flammable material so caution is advised and proper training in handling and use would be required for safety. Ultimately, it will be up to the ASHRAE 110 committee to decide if IPA is suitable as a tracer for ASHRAE 110 tests,” affirms Smith. “They’re actually doing a parallel thing in the European Nation with [standard] EN14175, considering the use of IPA, as well. We preceded their work though, so we didn’t have a path to follow, but it’s nice to see it’s going the same direction.”
Although the ASHRAE 110 Standard is not yet revised, it appears a long-awaited change is on the horizon. For many FHT CPs uncertain about equipment investment or struggling to sell the current process of tracer gas containment testing to clients due to environmental concerns, an answer is coming.
“The solution is to find the alternative and find manufacturers that can manufacture the equipment, but that’s been the case for 15 years. It’s likely going to be a joint effort as they reach out to manufacturers,” reasons Kelly. “As a buyer, I’d want to know if there are going to be any suppliers in the states, or how many different manufacturers would exist. It’s always nice to see competition and have different purchase options.”
“The standard is under revision right now. We are working on the tracer gas test methodology with SF6 because, based on the results of our study, we learned a lot about the technique. We’re updating the standard for SF6 and we’re going to provide an alternative. But the alternative won’t be provided until we have some validation from others that it provides equivalent or better results. First, the research must be confirmed and, of course, we will also need specifications to ensure any necessary equipment can be machined and made available,” concludes Smith. “We have a two-year timeframe in which we’re looking to complete. We have our eye on 2022. That is when we would like to have the approval of the new draft.”
It appears everyone in the field of fume hood performance testing is ready to say good riddance to SF6 and the search for related detection instrumentation. With IPA as a potential tracer gas replacement and PIDs used to detect it, an eco- and budget-friendly solution may be just around the bend. Slowly but surely, fume hood testing—specifically, tracer gas containment testing—is making its way to the future, with added protection and advantages for our planet, people, and pockets along the way.
