Thread: Testing mask filter media

Originally Posted by dmorse

Thanks Bob. I was interested because of the limited filter area on the printed masks.

I'm surprised you didn't connect your manometer to the box and check pressure drop too.

Nah - I just used the "put it on and try it out" method.
It's very easy to tell if something is going to be too restrictive.

Thanks again Bob. So if the flannel cloth plot improved at the lower flow rate should one conclude that the Miele bag material would also improve in a similar manner to land somewhere between the cloth and the P2 materials? Swiffers are going back to the floor where they belong.

I do have a little request if possible. As one with colour deficient eyes it would be easier to spot the material line on the graph if the points along the plot were different shapes or at least the starting point. Round dot for one material, square for another, triangle and so on.

Pete

Originally Posted by QC Inspector

Thanks again Bob. So if the flannel cloth plot improved at the lower flow rate should one conclude that the Miele bag material would also improve in a similar manner to land somewhere between the cloth and the P2 materials? Swiffers are going back to the floor where they belong.

That is a reasonable assumption but Vac filters are designed for higher flow rates so there may not be as much of an improvement as flannel cloth.
Anyway will test later today.

I do have a little request if possible. As one with colour deficient eyes it would be easier to spot the material line on the graph if the points along the plot were different shapes or at least the starting point. Round dot for one material, square for another, triangle and so on.

Here is the graph with all the data so far.
NB the Vac bag and TJK have not been remeasured at slower air speeds.

Screen Shot 2020-04-14 at 6.50.12 am.png

Below is a contribution from the Old Tools forum.

Regards from Perth

Derek


In case you missed it, please listen to the first 13 minutes of
yesterday's Quirks and Quarks podcast. It's sobering, but might help you
stay healthy.

Quirks and Quarks is a radio science digest produced by the Canadian
Broadcasting Corporation.

https://22163.mc.tritondigital.com/C...Q-20200409.mp3
I suppose masks are an old tool.
+++++++++++++++++++++++
Don and Assorted Porchizens,

This is exactly what I've been trying to tell people for the last 2
months. I've published a simple effective method of making N95 near
equivalent masks on the web in various places and handed out the plans.
It's a 6 mb file, if you want it, let me know. I sent it to the Surgeon
General directly after he put out the 45 min instruction on how to make
an absolutely worthless mask. He hasn't seen fit to tell people how to
make better masks. YOU are your own 1st responder.

This is very abridged version. Not much is old tool related other than
following the instruction if you wish to become one.


CDC?s masks are simple spit hoods.

Get a PURPLE Filtrete filter from Walmart (furnace filter) and take it
apart, you will have a HUGE amount of the white filter material from the
inside. A single layer, flattened out, inside your mask turns it into a
real mask with near N95 capability. You also have enough material to
make them for your friends and relatives. It?s a LOT of material. If you
make the mask out of microfiber hand towels from the dollar store,
rather than cloth, you'll be happier, they are comfortable and the whole
thing can be disinfected repeatedly with either soap and water or
alcohol. Let it air dry in the sun. Strong UV light is your friend.

Fold the hand towel in half and you have a mask that is large enough to
cover from the top of your nose to the below your chin. Sew cord to the
corners and you have a better way to attach it than loops behind your
ears. Sew the wide, double-wire, twist-tie strip from the top of your
bag of coffee just below the top of the mask in the middle and you have
an excellent nose bridge, like the one in a procedure/surgical mask. Sew
or glue the edges shut through the filter material as well as both
layers of the hand towel. Repeat as needed for friends and family.

Thanks Derek. Who wants to ask Bob to go into his cleaning supplies closet to test micro fibre cloth?

Pete

PS. Thanks for the changes to the charts.

Bob, I have some Cleanspace P3 cartridges if you'd like one for comparison purposes (or just for curiosity)

Thanks that would be great - one is plenty. I'll PM you

Hi Bob

I am assuming that the best filter material is a true HEPA filter. This would capture down to 0.03microns. Simply add a layer of this inside a washable mask, and then wash after use. Yes? No?

If yes, where does one get true hepa filters? Which vaccuum cleaners use this? Or which overhead filters ditto?

Regards from Perth

Derek

Originally Posted by derekcohen

Hi Bob

I am assuming that the best filter material is a true HEPA filter. This would capture down to 0.03microns. Simply add a layer of this inside a washable mask, and then wash after use. Yes? No?

Maybe start by re-reading post number 7 earlier on in this thread. This explains what particle sizes the greatest viral loads ride on.

Most (even radioactive filter grade) HEPA filters are rated at 0.3 microns.
The reason for this are;
- its quite difficult to measure particle sizes smaller than this
AND
- as indicated bask in post number 7, the smaller a particulate is the less it usually matters and the same applies to viruses under normal exposures (ie NOT a HD COVID19 ward or the labs that handle the live viruses where a space suit type outfit may be needed)

The terms HEPA and True HEPAs don't mean very much - what matters is the filters efficiency rating at a given particle size - usually 0.3 microns.
A standard hospital operating theatre filter typically filters 99.97% of particles >0.3 microns.
This filter will also filter out some 0.1 micron and even a few 0.03 micron particles but no filter ratings for these particle sizes are usually give for the reasons above

If a filter is rated at 99.99% at 0.3 microns, it should also filter out more at 0.1 and 0.03 micron stuff than a 99.97% filter etc
At work we used filters rated up to 99.997% at 0.3 microns. I have seen some that were 99.999.

I would not believe any claims on a packet or box by any filter manufacturer as no one checks their claims.
The thing to look for is a certificate of testing of a product that is usually attached to the product itself - see below.
This usually means that individual filter has been tested and certified.

If yes, where does one get true hepa filters? Which vaccuum cleaners use this? Or which overhead filters ditto?

See comments above.

HEPA filters with a credible certificate usually come in large sizes ie 600 x 600 x 150 mm (or larger) sizes to fit into an air filter unit and as such cost an arm and a leg. They material is usually made of borosilicate glass and so don't suit face masks.

It's somewhat pointless fitting a HEPA insert into a conventional face mask as the leaks around the side will immediately defeat the HEPA properties.

If you want top notch face filtration use a P3 cartridge mask - it will be as good as you are going to get for a face mask. A P2 will be almost as good at larger particle sizes but not quite as good at small particle sizes but as I say above this does not matter as much in terms of viral load although if you wanted a full belt and braces then by all means go for a P3. Faced with a lack of space suits and suitable PPE and being exposed to high viral loads some ICU doctors at Royal Melbourne went and bought themselves full face P3 face masks.

Interestingly, about 10 years ago I offered for free on these forums some 20, slightly smoke damaged 600 x 1200 x 200 mm serious (99.99% at 0.3 microns) HEPA filters. These had been in lab fire at uni and had a small amount of smoke contamination, deemed unusable in our super clean clean rooms and were replaced by the UNI's insurance. I tested half a dozen of the filters and they worked just fine - you just had to run them for a while to get rid of the smokey smell. Replacement cost for these filters was around US$400 plus US $300 for the enclosure and fan unit. Over several years we imported about 70 of these direct from the US. Here in AUS these cost over AUS$2000 for the combined filter and enclosure.

One member from these forums took two filters and I took 4 of them - the rest ended up in SKIP ! I installed one oft these HEPAs as my room air filter in my shed - I hardly use it because dust extraction system works so well.
Hepa1.jpg


Here is the test certificate. I have retested the filter and it still meets it rating

HepaSpecs.jpg

Oh and don't forget ventilation - it's actually potentially more useful than a mask as it reduces the amount of virus that settles or sticks on surfaces which them leads to a heap of other problems.

Some more test data here.

Bob, I know you are busy on other matters at the moment and are no doubt already aware of these developments at CSIRO. The production is still at the scaling up stage and yet to go through accredited testing.

Face masks for Covid-19 - CSIRO

Looks like you would probably have to buy one of those bulk industrial sized rolls, ie if you could buy just one. So, not an option for the DIY-er at home.

I also hear that Flinders and SA universities are setting up a joint mask testing facility to overcome the current situation where masks have to be sent to the US for certification. I'm surprised you haven't been headhunted, Bob.

Flinders drives Aust-first face mask test facility – News

If we get a second and third wave of this virus then perhaps we will all be making our own and one of those industrial sized rolls will be the next forum bulk buy...

Originally Posted by NeilS

Bob, I know you are busy on other matters at the moment and are no doubt already aware of these developments at CSIRO.

Yep I have been following this.

I also hear that Flinders and SA universities are setting up a joint mask testing facility to overcome the current situation where masks have to be sent to the US for certification. I'm surprised you haven't been headhunted, Bob.

Unis are waaaay over extended due to the OS student problem so I'm sure they will find plenty of their own under or unemployed people to do this. QUT also has a whole research department on Air quality.

I've become sort of distracted away from air filters efficiency measurements and about to dismantle and put away all the gear for it as I don't have enough room to keep it all out in my workspace.

I was going to wait until I had finished my Dispersed Oil Generator (DOP Dispersed Oil Particle generator) to generate enough fine particles so I could accurately test the P3 filter that EJ sent to me but this morning there was a bit of smoke in the city from forest burning off in the Perth Hills so I though that might be enough fine dust for testing.

The amount of PM2.5 dust/smoke in the air is currently; ~200 ug/m^3 so its pretty grubby. this translates to ~25000/m^3 particles of >0.3 micron.

The Ps filter from EJ isa CleanSpace filter.
CSP3a.jpg

Below shows the P3 filter which is the "thing" under the 3 elastic bands and as it is a curved shape I had to make an adapter so that I could mate it to my test chamber.
It so happens that the curve matches the OD of 150 mm PVC ducting so I made up a 125 mm long filter holder chamber from 150 mm PVC ducting and put an end cap on each end of the duct and a hole on the middle of one of the end caps that sits over the hole/entry of the test chamber.
The filter holder chamber has a hole drilled in the side which is covered by the P3 filter firmly held in place with 3 elastic bands.
The P3 filter has a foam lip that seems top provide a good seal.
The white covering on the outside is some sort of prefilter.

CSP3.jpg

I then run the particle counter in the test chamber "with" and "without" the filter. The ratio of these two measurements gives me the efficiencies/

Once the air had cleaned itself of the filter and test chambers the P3 filter showed a total of <1 particle for 10 x 3 second counts for all 6 particles sizes (0.3. 0.5, 1, 2.5, 5 and 10 microns). This is awkward as it means I simply still do not have enough particles to make a meaningful measurement so I can only really put upper limits. In practice it doesn't really matter as it shows how bloody good these filters are.

I then went back and tried the P2 filter material and got much lower results than last time. Some of this is having a heap more particles available for testing but I also suspect I had a leak last time.

Below is a graph comparing the P2 and P3 filters.
NB: Y-axis scale is greatly expanded compared to previous graphs with all measured results being >~99.8%
P2M (red) is the measurements of the P2 mask - the (red) values for the 2.5, 5 and 10 micron efficiencies are the same as but buried under the P3 (green results), this is a measurement issue (see below)
P3M (green) is the measurements of the P3 mask - the values for the 2.5, 5 and 10 micron efficiencies are the same as P2m but these are a result of measurement issues (see below)
P3S (grey) is the 0.3 micron manufacturer's spec (>99.97%) also is buried under the P3Pm(blue symbol)
P2Pm (orange) is a measurement for 238 nm size dust from a referred journal (ref below) for P2 type masks
P3Pm ((blue) is a measurement for 238 nm size dust from same referred journal for P3 type mask
Not shown on the graph because it doesn't fit is P2S which is the 0.3 micron manufacturer's spec (>95%)

The strange (lower) efficiencies for the 2.5, 5 and 10 micron are artefacts of still not having enough counts to play with (<1 measured count for any of these particle sizes).
In practice these efficiencies are always higher in efficiency than the 1micron size, ie all would be above 99.99% for P3 and 99.97% for the 2


Screen Shot 2020-05-15 at 7.53.21 am.png

At 0.3 microns the P2 is 99.8% compared to the manufactures spec of >95%
For the P3 I get 99.9996% compared to 99.97%

Extra Nerdy stuff
The nominal measurement tolerance for the P2 value of 99.8% is +/- 4.1% so even the lowest possible value would be 95.7% is still with spec
The measurement tolerance of the P3 cannot be determined because of the <1 counts.

Bottom line is both P2 and P3 appear to easily meet spec with the P2 significantly better than spec. This is not unusual as most filters perform better than their rated spec.
Your specific filters may not be this good.

The dangerous sizes particles for wood dust are those between about 1 and about 10 microns. Given the P2 is rating about 99.97% above 1 micron this makes P2s very suitable for wood. The 0.3 micron wood dust is likely to be breathed out again which makes them less dangerous.

In contrast, some thing like asbestos where dust particles as smaller than 0.2 microns are a problem (not so readily breathed out) the super high efficiency of the P3 filters are definitely preferable.

Measurements have been made on efficiency of filtration of much smaller (30–60 nm) particles for several types of filter were we would expect significantly reduce filtration efficiencies, Comparison of Nanoparticle Filtration Performance of NIOSH-approved and CE-Marked Particulate Filtering Facepiece Respirators | Annals of Work Exposures and Health | Oxford Academic
N95 = 95.72%
P2 = 97.78
P100 = 99.991 %
P3 = 99.84%

I am starting to suspect that I had leaks in some of the measurements of other filter media - when I get my DOP generator running I will repeat all measurements again to see what I get.

With mandatory masks starting here in VIC, would my P3 be considered good enough for covid or are the surgical masks better for that sort of use?