The demand for effective fume containment devices is truly international. Many manufacturers of such devices have a global customer base. These manufacturers must demonstrate effectiveness of their products through successful fume hood containment testing.
There are, however, two widely used and quite different fume hood containment tests. These two tests are ASHRAE 110-2016 and EN17145 Part 3. The former test has been developed in the US; the latter is Pan-European.
This fact frequently confounds international comparison of two products whose effectiveness is reported using different tests. This situation is even more serious if these products are participating in the same tender evaluation process.
The worldwide map shown below depicts this distribution of containment tests. Countries where both standards are used (orange stars) present unique problems for fume hood manufacturers.
The map confirms each containment test is widely used in the region where the standard was created. More distant areas (orange stars) use both standards. When the two containment test standards come into conflict in these regions, comparing containment results of two products tested differently creates severe commercial and competitive issues. Since both tests are so different, we should really try to reach a consensus on how to select products evaluated using such dissimilar criteria. Let’s first look at these standards more closely.
The Two Containment Test Approaches:
A. ASHRAE 110-2016
The ASHRAE 110-2016 test traces its roots to research published by the American Industrial Health Association (Knudson and Caplan) in 1982. 1
Hood face velocity is measured using a thermal anemometer and a sash plane velocity grid specified in section 6.2 of the standard. Sulfur hexafluoride (SF6) is diffused into a fume hood and the quantity that escapes into the mannequin breathing zone is measured using an Infrared Spectrophotometer or Ionization Technology. An illustration of the basic setup is shown below: 2
The ASHRAE 110-2016 test procedure employs 100% sulfur hexafluoride as the tracer gas. The gas diffuser (tall circle) is set at a supply pressure of 30 PSI with a diffusion rate of 4 lpm. Tests are run with a mannequin for 5 minutes and SF6 concentrations in the mannequin breathing zone (wide circle) recorded. An SME (sash movement effect) test is run for a total of two minutes and includes opening and closing the vertical sash twice in 30-second intervals over the two-minute run. Tests are run and SF6 concentrations in the mannequin breathing zone recorded.
Although ASHRAE 110 does not define a pass-fail level for the test, AIHA Z 9.5 sets forth a suggested pass/fail level of average breathing zone tracer gas of 0.05 PPM 3.
B. EN 14175; Part 3
While EN14175, Part 3 and ASHRAE 110 both measure fume hood containment, they do this quite differently using very different equipment and calculations.
En14175 has a total of six sections:
*Hood safety requirements
*Hood containment testing
*Testing on-site methods
*Installation & Maintenance
*VAV performance testing
We will focus only on Section Three which defines hood containment and the evaluation of this performance characteristic.
While the EN14175, part 3 test method evaluates and reports levels of escaped tracer gas, it also generates a series of unitless numbers called Containment Factors which represent hood tracer gas flow rate divided by the product of extract rate times escaped tracer gas concentration. In most reports written today, either escaped tracer gas concentration, and/or the containment factor are used to quantify containment performance.
The EN 14175; Part 3 test uses a 10% sulfur hexafluoride – nitrogen mixture with a total delivery rate of 2 LPM for interior plane test (using a 9-point diffuser sampling array); a rate of 4.5 LPM for the exterior plane and robustness test, and a [(5 to 8) / 1,000,000]) fraction of the hood exhaust rate flow for the air exchange efficiency test.
These challenge rates (CR’s) are much smaller than the challenge rate of 4 LPM of 100% SF6 used in ASHRAE 110; see chart below:
The 2 to 6 LPM flow rate of 10% SF6 converts to a pure SF6 challenge rate of 5% to 15% of the ASHRAE 110 challenge rate of 4 LPM of 100% SF6!
Details of the Actual EN 14175 Tests:
1)Gas diffuser array for Exterior Containment, Robustness, and Air Exchange Efficiency Tests.
2) Exterior sampling array & samplers:
“Exterior Plane” is defined as the outermost portion of the hood frame housing the sash and consists of a variety of sampling points depending on the dimensions of the sash opening.
3) Interior Plane sampling array & diffuser for interior plane containment:
The nine interior sampling points and single diffuser are located on a single device that is placed in several locations at the hood sash plane. The diffuser (circled) for this test is in front of the sampling grid on the same assembly. The sampling ports are on the sash plane.
(SF6 TRACE GAS LEAK DETECTOR Q200/USON QUALICHEK 200 is for the testing.)
(The circle marked SF6 LEAK DETECTOR Q200/USON Q200 is for the testing and now it is produced ONLY by D-Industrial.)
The “Gas Analyzer” used in EN14175 is generically described in section 5.3 and has these requirements:
a) A detection level of 10-8 volume fraction or less (10 ppb by volume, or 0.01ppm)
b) A time constant of less than 15s
c) A data recording capability of 1 reading every two seconds or less
d) For the Flow Sciences test, an ionization leak detector compliant with points a-c (shown above) was used.
5) Robustness Test
(The SF6 LEAK DETECTOR Q200/USON Q200 is for the testing and now it is produced ONLY by D-Industrial.)
To carry out this test the sash is set to a specified sash opening position. After a period of 60s of tracer gas flow, the movements of the black rectangle wheeled cart (see left of illustration above) across the fume hood front completed for six crossings. The path of movement should start and end at a point 600 mm on each side of the fume hood. The time between each crossing should be 30s. The test gas concentration is measured and recorded. The measuring signal of the gas analyzer is recorded for 30s after the rectangle movement stops.
6) Air Exchange Efficiency Test:
The air exchange efficiency test uses a flow rate which yields a fume hood interior gas concentration of 5 – 8 PPM measured at the device duct collar. Gas is shut off when steady-state concentration is reached. Concentration / time data is recorded until interior fume hood SF6 concentration reaches 20% of the steady state value ~ (1 – 2 PPM). Time for this to occur is noted.
An efficiency number is then calculated as a ratio of observed and ideal air change rates of the device. Because this number does not evaluate containment, and has an unclear relationship to hood efficiency, it will not be discussed further in this white paper.
The average reading of all containment testing techniques mentioned above is recorded and plugged into the following formula for Containment Factor:
8) Pass-Fail levels for EN 14175 Data
Note, data on this German chart uses a comma instead of a decimal point which is the custom in many European countries.
It should be noted that most published containment values for commercial products are far lower than the limits expressed in these two sources cited above. Most fume hood manufacturers publish data having no detected values whatsoever for escaped tracer gas during any test. This is Flow Sciences experience with this test as well!
9) Springer6 expresses several issues with the formula for calculating Containment Factors. Remember, the larger the CF, the “better” the containment is supposed to be. Consider the following:
a) To most reviewers, the term “Containment Factor” implies a number whose magnitude indicates degree of containment. In actuality, CF combines the ability to contain with how much exhaust the system is using.
10) What has actually been seen in the field when data are obtained and CF is calculated? Here’s a data table using the BS EN14175 Part 3 procedure on an actual fume hood 4:
The exhaust rate, Q, is in the denominator of this calculation. Therefore, the equation for Containment Factor gives a larger (better) number with smaller exhaust rates.
You can see how the CF “magic numbers” 5613 and 3426 pop up everywhere because the detection of SF6 is zero, but the “zero” has been replaced by the instrument detection limit of 0.01 PPM! Also note the non-standard use of “>” to dignify the four significant digits in the containment factor.
On this table, the lower face velocity 0.3 m/s shows a “better” Containment Factor than 0.5 m/s. This number has been shown higher simply because the exhaust quantity has gone down while the escaped SF6 remains undetectable. This anomaly gives a misleading impression that containment characteristics have improved at the lower face velocity.
Ali Bicen and others have attempted to stratify the containment Factors into a banding scheme shown below: 5
Note that outer plane Containment Factors (Protection Factors) trend lower the lower the face velocity (ie class) of fume hood. (Class 1, Class 2, Class 3, the lowest class) This banding scheme shows the expectation that higher face velocities will yield higher Containment Factors (larger numbers), but as we have seen from the example in # 10 above, experiments and observations have shown that this is not usually the case!
Waldner’s SteffenSpringer states quite clearly6 the inappropriateness of the Containment Factor using an analysis which I restate below, starting with the formula for calculating the factor defined earlier:
He makes the following comments:
1) The structure of the equation is very simple.
2) Theoretically, the result means: the higher the factor the higher the containment of the fume hood.
3) The magnitude of Containment Factor is dependent on three values:
a) Flow rate of tracer gas
b) Extract flow rate of fume hood
c) The mean tracer gas concentration inside and outside the hood (the latter is the only value in this equation that relates to the ability of fume hood to contain).
4) According to EN14175, CFR should be rounded to the nearest integer and it must be indicated “if the result is limited by the detection limit of the instrument”
5) Now here is where it gets interesting! With q, the tracer gas flow rate, being always constant (stipulated in EN 14175) and an assumed detection limit of 0.01ppm of the measuring equipment, the only variable remaining is the extract volume of the fume hood since no detectable tracer gas escapes. Is this result (CF calculated as described before) suitable for an objective comparison of fume hood containment?
Springer does not believe the containment factor is suitable as an objective indication for fume hood containment unless all models in question are tested under the same conditions, with the same extract volumes and with same test equipment. If only one of these variables is not the same, different Containment Factors for the exactly same fume hood performance will be calculated! For this reason, the Containment Factor does not enable the user to select the best hood based on containment performance.
These problems with CF have caused evaluators using EN 14175 – 3 to evaluate a hood based on the ppm of escaped tracer gas(ϕ), rather than the Containment Factor CF! 9
The remaining problem is that a majority of EN14175 tests show NO tracer gas escape under any of the many tests outlined above. In other words, EN14175 has a great deal of trouble differentiating between fume hood performance from one design to another.
Andy Sinnamon echoes this sentiment on linkedin, saying:“(my Company) went through this process some time ago. The lack of a mannequin and the dilute tracer gas makes a pass virtually guaranteed for most any hood design.”11
I concur the test differentiates poorly between performance conditions at different face velocities, but I am not sure about the test’s ability to detect a faulty hood, since we probably would not see such tests published.
Summary of Significant Issues with EN 14175:
1) The complexity of EN14175 equipment:
The most accurate way of judging performance of fume hoods is by assessing the ‘containment’,ϕ, CF is not a useful factor.
Because of equipment complexity, the EN 14175-3 containment test is usually performed only at design stage (in a test lab). Actual construction practice has evolved into doing field commissioning of a delivered fume hood with simpler “tests” not part of EN 14175 7.
The only situations when the EN 14175-3 test is performed on an installed fume hood is when other lab conditions produce doubt that the hood is working. “For example, when smoke tests have suggested turbulent air movement within a hood (despite the average face velocity being acceptable) and instances of complaints of unpleasant smells emanating from a fume hood (despite the satisfactory face velocity measurements).” 8
2) The unclear meaning of the Containment Factor CF:
A fume hood is built to protect people from hazardous materials using exhaust. Depending on the design and the amount of extracted air, any containment device should be able to keep hazardous fumes inside and protect people in front of the device opening. How effectively a fume hood can “contain” is allegedly depicted by what EN14175-3 calls the Containment Factor. The better a fume hood can hold a certain containment with less air, the larger the Containment Performance Factor is supposed to be.
However, undetectable escaping tracer gas levels lead to a factor of indeterminate value which is not comparable to other instances where different detectors are used. Due to this, two identical fume hoods with the same amount of extracted air and the same opening in the front can have different Containment Factors if detectors of differing sensitivity are used.
In the selection of fume control devices, safety of the operator (low quantities of escaped fumes) is of the highest importance, not an algebraic combination of containment and exhaust volume data. In the writer’s opinion, a containment evaluation should not use a hybrid value like the Containment Factor to quantify hood performance.
The author therefore believes that the focus in EN 14175-3 should be on detected escaped tracer gas (ϕ) at a specified face velocity, not the containment factor (CF).
The University of Cambridge notes that the “type tests done according to EN14175-3 are clearly made in compliance with ideal conditions. This means that even regularly occurring disturbances are not taken into account and fume hoods with applicable very low face velocity provide outbreaks if there are any minor disturbance occurs.” 8
As discussed previously and noted by Egbert Dittrich, the Containment Factor is not suitable as an objective indication of fume hood containment unless all models in question are tested under the same conditions, with the same extract volumes and with same detector equipment. Only one of these variables not being the same, will lead to different containment factors. In practical terms, the Containment Factor does not enable the user to select the best hood based on the value of CF. 7
3) The inability to differentiate between fume hood performance.
Unless and until we can measure escaped tracer gas better, virtually all published data for hoods tested with EN 14175-3 yield the same result for all escaped tracer gas tests. Zero. (that is, 0.00 PPM)
My experience is ASHRAE 110-2016 generally shows measurable low tracer gas escape. Ability to produce finite readings has allowed developers to tweak designs to some degree and have data which show improved performance. 12
ASHRAE 110-2016 and EN14175 Part 3 Compared:
The author believes both tests could be improved. The ASHRAE 110-2016 test appears to have a much more substantial SF6 challenge, yields more differentiable results, and is an easier test to perform in the field.
The EN 14175-3 test measures more potential containment escape points. It also has a standardized robustness challenge employing a flying rectangle to simulate a walk-by. The table below summarizes many of these test comparisons.
The Impact of 2 Standards on Brand Competition.
Companies that do international business must be prepared to test hood performance with whatever containment test is specified on a case-by-case basis. Both containment tests have unique strengths and weaknesses.
Because the tests are so different, conversion of one test’s data into projected readouts from the other test’s procedures is a fool’s errand.
A comment on objectivity. The larger number of “good” ASHRAE 110 aspects in the above chart is a reflection of the author’s predominant use of ASHRAE 110-2016 test through his entire career. It should not be viewed as “proof” that one test is better than the other.
Nor should the opinions of others who are commercially committed to one test or the other.
Attempts should therefore be made to establish a single standard for the Twenty First Century world of lab construction. Such a standard would facilitate evaluation of containment products on an even playing field for all competitors, wherever the laboratory is located.