Thread: How big is a micron?...some things to compare with.

The micron rating for a filter is a limited piece of information.

When a manufacturer makes a claim that something is rated at 5 micron that effectively says it will capture (some) 5 micron (or larger) particles but it does not say what what minimum proportion of the 5 micron particles it will capture, or more importantly what proportion of particles of other sizes the filters will capture.

Micron ratings are a bit like HP ratings or worse. The efficiencies I have measured for filters rated at 5 micron range from 50% for a room air filter to 99+% for my needle felt filtered DC.

What is really needed is the entire particle size versus efficiency curve like this


Likewise dust exposure levels in a single figure mg/m^3 are also limited.

European and Canadian medical authorities are now providing dust exposure for specific particle sizes.
These ratings are called efficiencies for "Particulate Matter" or PMx ratings where x refers to a range of particle sizes.

To compare particle Sizes I use this graphic in my presentations on this topic.
The source is Basic Information | Particulate Matter | Air & Radiation | US EPA


The third critical piece of information is the actual dust loading since if the loading is too high even a 99.5% filter may let too much dust through.

I agree that the whole "micron size thing" is over simplified, and people have far too much confidence in the quoted filtration efficiency.

The media are always making size comparisons that mean very little.

"About the diameter of a human hair", the acutal size of a human hair can vary greatly.

You have figures of around 50 to 70 microns, and that may be sort of average for some sample taken somewhere and probably from the head. But if all hair grown on humans is taken this may explain the range of figures I quoted as 40 to 600 microns.

Nothing ever comes in convienient sizes and there is always a range.

The same goes for filters of any type, acoustic, electronic or mechainical, they never exclude all stuff outside their range and they always have a curve of effectivness.

One thing that many fail to think about is certain persentage of large, long particles going length ways thru sieves or filters.
Which is one of the issues with asbestos and fibreglass, their relativly large particles can pass thru realtivly fine filters.

When I look at what size fibres area available for filter media, I wonder how it can be possible to make a truly effective filter finer than the available fibres.....and the answer is you can't.

Back in the days when the asbestos issue was especially hot, it was said that the only material you could make a good filter for asbestos out of was asbestos....even then it would not be 100% effective.

Ya know even as recently as the 80's they where still filtering beer thru asbestos filters, and it was one of the last uses of asbestos to be replaced.
It took chill filtering to achieve the filtration effectivness needed without asbestos.

cheers

Originally Posted by soundman

I agree that the whole "micron size thing" is over simplified, and people have far too much confidence in the quoted filtration efficiency.

The media are always making size comparisons that mean very little.

Nothing ever comes in convienient sizes and there is always a range.

The same goes for filters of any type, acoustic, electronic or mechainical, they never exclude all stuff outside their range and they always have a curve of effectivness.

One thing that many fail to think about is certain persentage of large, long particles going length ways thru sieves or filters.
Which is one of the issues with asbestos and fibreglass, their relativly large particles can pass thru realtivly fine filters.

When I look at what size fibres area available for filter media, I wonder how it can be possible to make a truly effective filter finer than the available fibres.....and the answer is you can't.

cheers

you are right about size range, which reminded me of this graph published in a document on mine dust by the NSW govt's Environmental Defender's Office:

particles.jpg

With two open cut mines nearby, the railway to the world's largest coal port even closer, and nearby industrial plants, chicken processing plant, turf farms and even a crematorium, the outside air is dirty enough that me venting outside won't make much difference to all that, so how effective filters are doesn't concern me much

Originally Posted by soundman

When I look at what size fibres area available for filter media, I wonder how it can be possible to make a truly effective filter finer than the available fibres.....and the answer is you can't.

Can't is a big claim. Maybe can't do it at the price weekend warriors are prepared to pay but practical absolute filters are a reality.

The issue with particles and particle sizes is that in terms of lodging in human airways and causing problems it's only dust between about 20 and 0.3 microns that currently seems to matter. The bigger stuff usually falls to the floor or is blocked but the nasal passages while the stuff smaller than 0.3 microns behaves increasingly like a gas and is breathed straight back out again with surprising efficiency. This is why there is little attention paid below 0.3 micron for respirable material.

Something called an absolute filter has been available for 50+ years they are called High Efficiency Particulate Arrestance (HEPA) or absolute filters developed to handle highly dangerous radioactive dusts like plutonium. They use surprisingly thick fibres but by stacking these fibres in a thick mat the particles become embedded or arrested in between the fibres. Extremely high efficiency rates are obtained by stacking these filters one on top of the other or passing the air multiple times through the filters it is possible to truly scavenge every single particle of stuff of 0.3 microns or larger out of an air stream. Of course they also grab a lot of smaller particles as well but not as efficiently.

To give you an idea of their efficiency, if just 3 x 99.9997% efficient (@ 0.3 microns or greater) filters are used, the efficiency = 0.999997 + 0.000003*0.999997 + 8.9999959E-12 , not an easy calculation to do with a calculator or spread sheet. Now recycle the air 3-4 times at each filter stage and the efficiency goes close to the 50th decimal place.

By doing this the air in some parts of our ultra clean labs at work is less than 1 particle of 0.3 micron or larger per cubic metre (ie our detection limit is 1 particle of 0.3 micron or larger per cubic metre). In theory the limit is much less, around 1 particle of 0.3 micron or larger per cubic kilometre. The result is dust is a non-issue for contamination - instead the impurity of the chemicals and the containers starts to dominate our analyses.

So some truly remarkable and effective filters are available but they are well our of the $ ball park of the weekend warrior.

BTW none of the above is really needed for woodworking. A system that can provide 1000 cfm @ a minimum of 4000 fpm that vents outside a shed is all that is needed.

Originally Posted by richmond68

With two open cut mines nearby, the railway to the world's largest coal port even closer, and nearby industrial plants, chicken processing plant, turf farms and even a crematorium, the outside air is dirty enough that me venting outside won't make much difference to all that, so how effective filters are doesn't concern me much

Crikey, with all that going on around you, you might want to consider installing an efficient filter to filter the air coming into your shed .

Oh yes I am aware of HEPA filters, and they have been arround for quite a while.
If it was not for purpose engineered synthetic fibres and foams, they would not be possible.

And HEPA filters are generaly matrix filters rather than membrane filters..where the particles get trapped because they get confused and cant find their way out more than the particles being bigger than the holes.

The problem with HEPA filters common with many fine filters is that they don't flow real well even when new and clean clog redily.

All that being said, there have been domestic vacuum cleaners available with HEPA filters for quite some time.

Our latest household vac is a wonder, it has this expensive dusposable bag that looks to have a layer of dacron fibre sandwedged and matrix welded between two layers of what looks like fine dacron needle felt....and there is what I seem to remember is a two part "HEPA" filter pad follows that.
Ya certainly can smell any dust comming out of it and on full power I recon it would suck the spots of ya dog.

A far cry from my mothers Hoover constelation with its optional paper bag and open toped cloth bucket.

I tell ya what would be worth testing is a "Rainbow" vacuum cleaner, a machine that has been arround since the 50's and is still sold virtually unaltered, it has no bag, no cyclone and no cloth or paper filter, it works on a water bath, that should be filled and emptied every time the vac is used.
I understand they recently ( well since the 50's) added a HEPA filter to its output.

Interesting that no one seems interested in water or oil bath filters these days.....all those old cars had oil bath air filters, sure they where messy to clean, but not much got past them.


cheers

Originally Posted by soundman

Oh yes I am aware of HEPA filters, and they have been arround for quite a while.
If it was not for purpose engineered synthetic fibres and foams, they would not be possible.

The materials are not all that fancy the ones we have have are either paper or fibreglass, hardly exotic materials. The cost and key to their success is the effort required in fan folding the materials without cracking at the bends and making air tight seals all the way around the sides/edges of the folds but the materials themselves are not all that special.

And HEPA filters are generaly matrix filters rather than membrane filters..where the particles get trapped because they get confused and cant find their way out more than the particles being bigger than the holes.

Does that really matter?

The problem with HEPA filters common with many fine filters is that they don't flow real well even when new and clean clog redily.

For something small like a vacuum cleaner that is a relevant statement but if they are made large enough huge amounts of air can be put through them.
At work we have a 3 m x 3 m wall of HEPAs that filters external at @ 24,000 cfm - this lasted for 24 hours x 7 days x 8 years before it needed to be replaced.
These HEPAs are pre filtered using a 100 mm thick layer of polyester fibre similar to that used in domestic pillows. The pre filters were washed monthly and are often dark gray mainly from car exhaust.
Inside buildings we use many dozens of pre-filtered 1200 x 600 x 200 mm HEPA filters each at ~800 cfm with a very low <100 Pa resistance. We expect the building to be demolished before many of these need to be replaced.
In woodworking when pre-filtered with a decent cyclone, baring disasters, a HEPA could last for as long as the motor. The speed with which the HEPA filter blocks up on a HEPA filtered cyclone is a good but expensive test of the cyclone.

All that being said, there have been domestic vacuum cleaners available with HEPA filters for quite some time.

We have tested many so called HEPA filtered vacuums at work most of them are not even close to perform up to their claims. Like HP on motors anyone can claim something is a HEPA filter when it is not. Many are very poorly made with the primary problem being seals and being too fragile to withstand normal domestic wear and tear.

Interesting that no one seems interested in water or oil bath filters these days.....all those old cars had oil bath air filters, sure they where messy to clean, but not much got past them.

Both require too high a pressure to overcome the oil/water heads required to filter.

When you talk of HEPA filters on a large industrial scale, that is completely irrelivent to anything we are interested in and they can have as many square whatevers they want to do the job.

HEPA filters remain low flowing and easily blocked for a given surface area, the use of additional prefiltering simply acts to reduce the flow, which makes them impractical for high flowing, restricted space and budget uses that we are interested in.

As for "paper" that can be anything and consist of a mixture of many fibres, the use of paper does not preclude it being a purpose made fibre or being treated or impregnated with almost anything.

OH and a HEPA filter made of Fibreglass, well that is almost as bad as a filter made of asbestos.....people are getting all narky about fibreglass too

The small HEPA filters I have seen have been made of ultra fine foam blocks or compressed packs of dacron fibres.
Dacron and it relatives are some of the finest fibres we have available to us.

When you talk of high flow rates in breathing air filtration, you are in a situation with a very low dust load, both existing in the air and due to the massive total surface area....if most of waht ya getting in ya prefilters is diesel soot, ya not in a dusty situation.

Again compltely irrelivent to workshop dust collection, where we have very high (orders of magnitude higher) dust loads and relativly small filtration areas.

As a HEPA filter in a cyclone.....don't make me laugh.....a cyclone can be very efficient.....but it can change from being incredibly efficient to being a near straight thru dust path if the air flow is disturbed or falls below the critical rate......all it takes is for someone to suck a big rag up the spout and you just cloged ya filter.
Even when the blower is winding up and spinning down the cyclone can be well below critical airspeed to allow dust to flow thru completly unempeeded.

This problem is one of the reasons large scale industrial dust extraction has gone away from cyclones and to large shaker bag houses, they have less preasure and flow loss than cyclones and they work equally well at all possible airspeeds.

As for HEPA filters in domestic Vacs......of course you will get some that don't come up to spec, but the single biggest problem with domestic vacs is that people do not empty or replace the bag nearly often enough and they certainly do not change the secondary filters when they should, if they actually have them fitted.

BUT I can tell you our new wonder vac is much cleaner machine than the one it replaced, which was one of the cleanest machines available at the time when it was baught, my nose tells me that.

As for oil and water filtration....if it has the massive areas that are alocated to hard filtration media it can be very high flowing indeed...no heads to push almost no back pressure at all.
But one of the main reasons we have gone away from water scrubbers on flues, spray booths and other places is "environmental correctness".
A water filtration system produces effluent, and that is a bad word, ya cant put that down the drain.

The fact that the crap has to go somewhere does not seem to register, if the dust landed on the floor and was swept up by a cleaner or moped up and poured down the drain, or blew out the window and was washed into the creek by the rain, that is acceptable, but putting filter effluent ( in most cases simple dust) down the drain is environmental vandalism

So what do we do, we use disposable fabric media and bury it in land fill.


There is sooo much BS involved in this whole subject, and except in particular cases ( usually toxic substances or up to ya armpits in $#@T dust levels) people are not dying in their droves or comming down with cancer from dust exposure, inspite of many decades of exposure at much higher rates than now.

cheers

Originally Posted by soundman

When you talk of HEPA filters on a large industrial scale, that is completely irrelivent to anything we are interested in and they can have as many square whatevers they want to do the job.

So room air filters suspended from home workshop are completely irrelevant?
These room air filters are of similar size and the same cost as Industrial level HEPA filters but are 10+ times less effective.
It's just that home workshop technology just hasn't caught up with what is out there in the industrial workplace place.
These HEPAs do not over load if they are pre-filtered properly - I have one of these in my shed at home - I'm obviously not pouring the dust from my table saw through, it but it very quickly scrubs the air in my shed as clean as an operating theatre by running it for 10-15 minutes. I don't normally recommend these things as a regular way to remove dust from air but every now and then they come in very handy.

As for the prefilter media being restrictive and the rest of your points I'll just disagree with most of them, and instead of just talking about it I'll go and test some more air filters.

Originally Posted by soundman

Interesting that no one seems interested in water or oil bath filters these days.....all those old cars had oil bath air filters, sure they where messy to clean, but not much got past them.


cheers

that's because nobody is really interested in removing cylinder heads and doing a decoke and valve grind every 20 thousand miles either. Of course you can still buy oiled foam air filters for your car, but the paper air filter cartridges not only filter better, in most cases they flow better as well. It's a foolish idea bringing up old technology in motor vehicles where emissions are concerned, because to do so means you'd have to be unaware of the enormity of progress made. A 1968 Mustang will release more pollutants parked in your driveway than a 2008 Mustang does at 70mph on the freeway. How did it happen? Lots of very small incremental improvements, and lots of testing, measuring, and comparing.

Thinking about the amount of optimisation done in the automotive arena, where they've improved paper filter cartridges to the point of them lasting 50,000km for air and 30,000 for oil, I have no doubt what is possible to achieve.

btw, due to surface tension it's quite possible for fine particles to land and release from liquids, not unlike they would from a solid surface.

Originally Posted by BobL

So room air filters suspended from home workshop are completely irrelevant?
.

Pretty well, the only people who buy them are those with plenty of money 2 spare.

From the start they where a product aimed at a percieved need rather than to achieve a result..some bright spark thaught they could slap a common standard furnace filter in a box with a fan and charge....how much?

That is unles you are tellin me anybody can walk in an buy an industrial grade HEPA room filter for $300.

If that is the case tell me where.....I wont be wasting it in my shed I'll put it in my house, because that is full of "Invisable Dust"......along with visable dust and cat hair.

Speaking of cat hair, I would be interested to see a properly researched comparison of the respiratory health effects of cat ownership V woodworking...believe me there would not be a cat in the house if I had a say in the matter.

Cheers

Originally Posted by richmond68

that's because nobody is really interested in removing cylinder heads and doing a decoke and valve grind every 20 thousand miles either. Of course you can still buy oiled foam air filters for your car, but the paper air filter cartridges not only filter better, in most cases they flow better as well. It's a foolish idea bringing up old technology in motor vehicles where emissions are concerned, because to do so means you'd have to be unaware of the enormity of progress made. A 1968 Mustang will release more pollutants parked in your driveway than a 2008 Mustang does at 70mph on the freeway. How did it happen? Lots of very small incremental improvements, and lots of testing, measuring, and comparing.

Thinking about the amount of optimisation done in the automotive arena, where they've improved paper filter cartridges to the point of them lasting 50,000km for air and 30,000 for oil, I have no doubt what is possible to achieve.

btw, due to surface tension it's quite possible for fine particles to land and release from liquids, not unlike they would from a solid surface.

Don't ever underestimate the posibilties of "old technology", most of the problems of oil bath technology could easily have been overcome in vehicles, if there was a will to do so.
The old oil baths used plain and simple engine oil, same as in the sump.....arround the same era we where putting simple ordinary mineral oil in two stroke engines.....if specificly designed low residue, low ash oils where developed for filter baths like have been developed for two stroke engines, much of the suposed cokeing problem would not exist....in fact low ash two stroke oil would probably do the trick a treat.

If the oil was pumped over the filter eliment rather than the preassure diferential method, the issue of flow restriction would pretty well be solved

Funny I don't remember the families 1954 Morris Oxford having a problem and needing a decoke or a valve grind in the nearly 20 years that we owned it.......it most certainly had an oil bath filter.

The main reason for the disaperance of the oil bath filter is that they where expensive to make and the car company made no money out of them..a throw away paper filter was far more profitable.

As for oiled foam filters, thay have a checuered past.....the old RAMFLO filters where pretty ordinary and where known to let large grains of sand thru, but some of the modern oiled foam filters spec up pretty damn well...but that is an entirely different technology.

A good wet filter design can cope with extrordinary quantities of particulate matter, can be self cleaning and deal with a wide range of particle sizes in a single stage AND it can effectivly filter particles many times smaller than the mesh size.

OH yes if they put effort into wet filters the old technology can work very well.

cheers

Originally Posted by soundman

Don't ever underestimate the posibilties of "old technology", most of the problems of oil bath technology could easily have been overcome in vehicles, if there was a will to do so.

And maybe not? - there appears to be issues with new aftermarket oil impregnated air filters causing failures with hot-wire mass airflow sensors fitted to a lot of newer vehicles.

Originally Posted by Vernonv

And maybe not? - there appears to be issues with new aftermarket oil impregnated air filters causing failures with hot-wire mass airflow sensors fitted to a lot of newer vehicles.

No doubt...but if you fart in the wrong direction you will cause a failure in mass airflow sensors.

We have now swung the argument from the stoneage to the beyond reason technical age where if you change any thing on a car it wont work properly.

cheers