In the 2022 ‘Tools and Shop’ issue of Fine Woodworking, the magazine published a comparison article titled “HEPA-Rated Shop Vacuums“. They tested and compared seven different HEPA-Rated shop vacuums. The article is testing shop vacuums under new and “clogged” conditions, however they missed a key factor in shop vacuum performance.
The article completely overlooks the most important aspect of a testing shop vacuums: Flow. The article overemphasizes static pressure which, although relevant, has much less to do with the effective performance of a shop vacuum. In fact, the author even mentions it in the opening paragraph of the article, “Shop vacuums don’t move enough air (measured in cubic feet per minute, or CFM) to handle big chip producers like table saws…”. That was the first and last word on flow in the entire article.
Asa Christiana was the author and is credited to have conducted the tests for this article. As a former editor of the magazine of 15 years, he is well know in the Fine Woodworking community. According to his LinkedIn Profile he also reports to have attended 4 years of technical college as well as 2 years engineering. He now works as a freelance writer for various woodworking and homebuilding magazines, as well as producing content for several tool manufacturers.
Why I wrote this response
My reason for this article is not to try and criticize the author or Fine Woodworking Magazine; In my opinion Fine Woodworking Magazine is the premier woodworking magazine on the market. It’s the only magazine I subscribe to, and I’ve always found it to have the highest quality content. I also enjoy listening to the Fine Woodworking podcast – Shop Talk Live, sometimes more just for the entertainment value, than the educational content, but it’s a good listen and I recommend checking it out.
The reason for writing this article is because I when I read Fine Woodworking Magazine I have high expectations of accurate and well though out articles, with good research and testing to back up their recommendations, especially when the article is giving guidance on where to spend my hard earned money. I am disappointed that this article, which made some strong conclusions and recommendations, missed such an important point, which ultimately led to some suspect advice.
I did reach out to Fine Woodworking through their Facebook Page for any comment or clarification, but as of publishing this article I haven’t received a response. If I do, I will update this article accordingly. Also, I do realize that this issue came out a year ago, but it was under a pile of magazines and I never got around to opening it until now!
What the article got right
Range of Products
Asa did a good job of selecting a wide range of shop vacuums to evaluate, ranging from $400USD (For the Dewalt DWV010) to $975 (for the Festool Cleantec CT 48 E AC HEPA). These vacuums range in capacity from 8 to 12.7 gallons nominally.
Comparing on Cost
It’s sometimes hard to look past the initial purchase cost of a tool like this, especially when it’s in the hundreds of dollars, but it’s important to consider the total cost of ownership when maintenance and service is required.
If you use you shop vacuum for dust collection in woodworking or carpentry, or in any trade for that matter, you will find quickly that the cost of replacing filters and bags can add up quickly. Not only did Asa evaluate the cost of replacement bags and filter, and educate the reader that universal and reusable bags are available for all models, but by evaluating the performance empirically, he determined the true effective bag capacity.
By using the relative bag capacity as a multiplier, you can calculate the comparative cost of operating each vacuum. For example, if the effective capacity is 4.5 gal. (as in the example of the Bosch VAC090AH) and the disposable bag cost is $35 for a pack of 5 (7$ each), then the cost of sucking up a gallon of dust is $7/4.5 gal. = $1.66/gal.
The reusable bags from each shop vacuum manufacturer are significantly more expensive than the disposable units, but if they can be reused more than ~10 times then they start to pay off. Although it is unclear if there is further degradation in performance over time considering it will be impossible to fully empty and clean the reusable bags.
While I state in the opening of this article that static pressure is not the most important characteristic of a shop vacuum, it is still relevant. It is an indication of how effecting the pump is at overcoming the pressure drops present in a real situation, where there is a dirty filter, a long hose and a tool attached, all acting as restrictions for the blower to act against. However it is impossible to evaluate the efficacy of a shop vacuum on pressure alone.
What the Article Missed
Measure pressure without relating it to flow
The Bosch VAC090AH is advertised as capable of 97 inches of water lift. Inches of water lift is really just a descriptive term for negative pressure (vacuum). It can be converted to a more common unit, such as PSI (Pounds per square Inch). 97 inches of Water lift is equivalent to 3.5psi of negative pressure. This alone doesn’t sound like a lot, but keep in mind when you’re talking about creating a vacuum, you can’t exceed 14.7 psi because this is atmospheric pressure. There is a longer explanation of why this is true, but for now just take it as fact.
Now in comparison, lets look at another type of vacuum pump. One used for a vacuum press (Those large plastic bags used to glue together veneers and bent laminations). These little pump can create a lot of vacuum. This little vacuum made by Accupress can create 25 in of mercury lift. This is equivalent to 340 in of water lift (12.3 psi) – This is 3.5 times as much as the Bosch Shop vac. But it only flow up to 1.5CFM (Cubic Feet per Minute). This is 100 times less than the rated flow of the Bosch Shop Vacuum. It is cheaper than all of the shop vacuums listed in the article, but while it create a lot of negative pressure, it would be utterly useless as a shop vacuum, because there is no air flow to draw in the dust.
Not mentioning flow at all
First off, let me state that measuring flow is much more difficult in this application than measuring pressure. The pressure gauge used in the test is quite inexpensive, accurate and easy to adapt with commonly available fittings and adapters. So I understand why this may have been the focus. But evaluating flow would not have been beyond reasonable expectations for an article of this type.
All of these shop vacuum manufacturers list the rated flow in the product specifications. This would have at least provided a good point of comparison, even if it meant trusting the manufacturers listed values. And for the sake of rounding out the evaluation, I have listed the rated flow for each HEPA-Rated shop Vacuum in the FWW article and the end of this article.
How to measure flow
Flow meters to measure in this flow range, for air, are available. They aren’t readily available at your local hardware store, and they require some additional equipment to setup and take measurement (Signal conditioner, Data-logger and a laptop), but it certainly isn’t beyond the realm of possibilities.
But even if setting that up wasn’t in the scope or budget for the article, there are better ways of testing shop vacuums than measuring static pressure. One method would be to use 2 vacuum gauges along a fixed length of pipe to create a simple differential pressure flow meter. This requires a bit of know-how and a bit of math, but it’s simple enough to setup.
An alternative to direct flow measurement
Another option to evaluate the flow capability would be to measure the dynamic pressure under varying flow restrictions. This would be directly indicative of the flow performance without actually putting a specific flow number on it. One simple way to do that would be to install the vacuum gauge at one end of a pipe (nearest the vacuum) and then create different restrictions at the opposite end. The measured vacuum pressure would be a direct measurement of the suction performance of the shop vacuum under real world condition. If the flow capability of the vacuum was reduced, then the resultant measured vacuum pressure would also be reduced.
If I was to conduct this test that is likely exactly how I would do it. With a bit of experimentation you could test each vacuum under realistic and repeatable conditions, and create a nice graph comparing their suction power over varying conditions.
Incomplete test draws wrong conclusions
The assumption that a static pressure measurement reflects the performance of the shop vacuum leads to incorrect conclusions.
- The article claims that because the static pressure in the 2/3 full condition remained at near new condition, the felt bags tested “Did their job”. This is misleading at best. Even if the felt bags were very clogged, the static pressure would be unaffected as long as the blower still had a flow path to the downstream pressure gauge; even if the flow path was partially restricted.
- Several of these shop vacuum models (all but the FEIN) feature a pressure reversal feature that intermittently reverses the flow inside the vacuum to knock dust of of the filters This is meant to extending the useable life of the filters. But by relying on static pressure again, the author incorrectly concludes that this feature is not useful, because the static pressure has not been significantly degraded. This would be much better assessed using the restricted flow test that I proposed above.
Comparison of Flows and Pressures
These flow values are all taken directly from the manufacturers websites. They range from 130CFM to 155CFM. These are all max. values, however, and would definitely be reduced as filters clog and lengths of hoses are attached. This is where a real-world performance test would be really valuable.
I added pressure numbers just for reference. I took these from the manufacturers sites. A few were not listed so I used the values from the FWW article.
Pressure: 97″ Water lift
Pressure: 47″ Water Lift
Pressure: 98″ Water Lift
Pressure: 97″ Water Lift
Pressure: 92″ Water Lift
Pressure: 78″ Water Lift
Pressure: 77″ Water Lift
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