Thread: Particle Visibility

I will be watching this thread with interest!!

Lornie

Originally Posted by Chris Parks

We all know that if you backlight dust particles they are more visible. I have some questions....

1. What size particles can be seen reflected in the light

These will be mainly >5 microns.
If you can see these then you will almost certainly have a heap (10X - 100X) more smaller ones - the question is how many
If you can see the wood dust then you will already be well over the OHS standard of 1 mg/m^3 - the question then is how much?

2. Are those the particles that the dylos measures......

Dylos will measure smalerl than this, ie down to 0.5 microns

3. Can the dylos measure particles that cannot be seen in the light beam.....

yes

4. is there an advantage to having the dylos that I am not getting by looking at the light beam?

The dylos can provide a number so you can make a quantitative assessment of any changes in a situation which you cannot determine just by looking at dust.
The dylos does not rely on having a light source at the right angle shining into the dust cloud every time you want to assess the dust.

The problem with the dylos is it just shows a number and while it can be sort of converted to # particles/per cubic ft the reading is not calibrated so is not necessarily accurate, however it is very useful for relative measurements.Because the dyslos is not calibrated, two dylos machines will not necessarily give the same results so comparing results across machines is not reliable.

Ordinary air contains a lot of dust - even crystal clear air contains as much as 100,000 particles/m^3, clean urban air contains around one million particles/m^3 - this background dust cannot usually be seen at all. Without knowing the dust back ground a reliable dust measurement cannot be be subtracted from any dust reading, so if you "see dust" how do you know how much background to subtract? The dylos will at least allow you to do this. Most people using a dylos do not do this background subtraction so have no clue what there real dust levels are and often make incorrect decisions because of this.

Thank you Bob (and Chris)....

What I have learned from this thread is that I need to upgrade the MERV 6 filters in my air-purifiers to MERV 15 filters. The MERV 6 filters cleaned the air of everything I could see, which I now know were >5 micron particles (and also as per MERV 6 specs).

The light beam was useful for seeing dust that was not otherwise visible.

Seeing dust when using an air hose to blow off a cabinet....

Seeing dust when sweeping.....

Seeing dust when brushing off my jacket or patting my stomach.....

I think most of us are underestimating how much dust we breath, even with a working dust extraction system.

Lornie

Originally Posted by Lornie

....

Seeing dust when using an air hose to blow off a cabinet....

Seeing dust when sweeping.....

Seeing dust when brushing off my jacket or patting my stomach.....

I have done measurements of all of the above (except for the stomach patting) and the dust levels easily reached 10 - 15 times above OHS levels of 1 mg/m^3 and they stayed above that level for at least 30 min for the first two activities and 10 minutes for the last activity. In the overall scheme of things these relatively short term events are not that bad, its sanding or turning in a fog of dust for hours on end that will end up being the problem.

Another trick Bob teaches is to use our dusty or cyclone to clear the air after (say) blowing dust off a cabinet in preparation for lacquer. (I don't sweep; I vacuum with a 100 mm line from the cyclone)

Did the calcs. My cyclone turns over the equivalent of all the air in my shop in about 5 minutes. After blowing off a cabinet I can see dust in the air, but that disappears very quickly after I turn on the cyclone. Far better than relying on even a good fan to do the job.

Even a great dust extraction system with excellent hoods/guards/shrouds cannot grab everything at the source, which is desirable. Blowing off a cabinet is one example, so too are some routing jobs. Our exposure can be reduced dramatically if we clear the air quickly after/during a dusty job.

Originally Posted by John Samuel

......
Did the calcs. My cyclone turns over the equivalent of all the air in my shop in about 5 minutes. After blowing off a cabinet I can see dust in the air, but that disappears very quickly after I turn on the cyclone. Far better than relying on even a good fan to do the job. . . . .

If you are using 6" ducting the cyclone will be pulling at least 1250 cfm, so over 5 minutes thats 6250 cf of shed volume.
If your shed is 12 ft high inside, that's then a area of ~520 sq ft or about 7 x 7 m?

The air turn over figure is theoretical since this can only physically happen if there are no restrictions to flow anywhere around the walls and in fact only applies to a shed without walls. As soon as walls, and preferred pathways like windows and doors are involved, the very best one can hope for is for about half the actual shed air to be evacuated during the equivalent shed venting time - in your case the five minutes. This is because some of the incoming fresh air is exhausted before it mixes with the remaining dirty air.
In reality quite strong air currents are set up between small inlets (like a door or window) and even less of the dirty air is expelled. The sorts of experiments I have done for labs show somewhere between one quarter and one third of the actual air is removed when using preferred inlet vents and a point extraction system like an exhausting duct/port.

To reduce this effect in labs, vents equally spread and multiple point extractions are used on pairs of opposite walls, or ceilings and floors, producing a cross ventilation effect. Something similar one could do in a in a shed would be opening up a roller door on one side of a shed and open all DC vents on all machines

If only one exhaust duct is used and say one door or window is used for incoming fresh air for a shed that theoretically takes 5 minutes to fully vent itself would be something like this
During the first 5 minutes 25% of the original air is removed, the resulting duct concentration is now down to 75% of the original
During the next 5 minutes 25% of the 75% will be removed, this leaves 56% of the original dirty air
5 more minutes, now its down to 42%, another 5 minutes and its 32%, another 5 minutes will be down to 24%
This means it will take 40 minute to get down to 10% of the original value and this may still not be enough to meet OHS levels.

This demonstrates the need to open all vents in a syste (ie use multiple collection points) and to have correspondingly several inlets to allow fresh air to come in.
This will then approach the theoretical half value model
During the first 5 minutes 50% of the original air is removed, the resulting duct concentration is now down to 50% of the original
During the next 5 minutes 50% of the remaining 50% will be removed, this leaves 25% of the original dirty air
5 more minutes, now its down to 12.5%, another 5 minutes and its 6.25%
So it takes somewhere between 15 and 20 minutes to get below 10%.

This highlights the need to leave the DC running for some time after any dust making activity.

I had read many years ago that part of the reason fine dust floated in the air for so long was because it was all (mostly) positively charged, and therefore bounced off each other (hanging in the air) instead of falling to the ground.

..... that the fine dust particles were so light that the (magnetically) positive charge was a force to be reckoned with (equal to (though working in every direction, not just 'down')) the same as the force of gravity....

I took that to mean that adding an ion generator to the shop would help balance the static charge in the air, and consequently help the dust particles drop out of the air.

I already owned a large ion generator, and using it in the shop 'appeared' to help (I have no measuring tools, except for the 500 watt light beam. (I am also aware that there are health hazards to too much ion, so I use it very little, only when sanding a lot.)) The ion generator 'appears' to clean the air about twice as fast as not using it.

Does anybody else use one? Have you heard of this? What can you tell me about this?

Lornie

edited to fix closing parenthesis

Originally Posted by Lornie

I had read many years ago that part of the reason fine dust floated in the air for so long was because it was all (mostly) positively charged, and therefore bounced off each other (hanging in the air) instead of falling to the ground.

The main reason they float is due to Brownian motion, ie there enough molecules of oxygen and nitrogen moving at very high speed to impart some momentum to the bigger ones to keep them floating around in the air for a while. The positive charge effect is usually minor compared to this.

I took that to mean that adding an ion generator to the shop would help balance the static charge in the air, and consequently help the dust particles drop out of the air.

I already owned a large ion generator, and using it in the shop 'appeared' to help (I have no measuring tools, except for the 500 watt light beam. (I am also aware that there are health hazards to too much ion, so I use it very little, only when sanding a lot.)) The ion generator 'appears' to clean the air about twice as fast as not using it.

Does anybody else use one? Have you heard of this? What can you tell me about this?

The problem with an ion generator is its effect will be quite local so recirculation is needed to make it effective. And in the end the particles are not collected but end up on surfaces and the floor and the first time they are disturbed - POOF - they're floating around in the air again. Even though they are now neutral, do they stay suspended for the same length of time ? Yep - they more or less will. It is far better to direct energy to expel them from the shop, or preferably collect them at source.

Originally Posted by John Samuel

Did the calcs. My cyclone turns over the equivalent of all the air in my shop in about 5 minutes. After blowing off a cabinet I can see dust in the air, but that disappears very quickly after I turn on the cyclone. Far better than relying on even a good fan to do the job.

Bob,

Exactly.

That's why I used the term, "the equivalent of all the air in my shop" rather than "all the air in my shop". I am lucky to have a shop layout such that when I open all gates quite good coverage of the shed is achieved by the inlets. Typically, I leave the cyclone running for only a few minutes after using most (not all) machines, but only when I am very happy with the capture at source. When in doubt, it is allowed to run longer. Usually, a couple of times a day I let it run for about 20+ minutes.

One acid test is the accumulation of dust when I am away. If the surface dust in my shop (on bench tops etc) is no worse than in my study after I have been away a couple of weeks (it generally is) I am a happy chappie.

These days I run the cyclone when spraying. It is far superior to the fan at clearing the air in the shop allowing me to re-enter the shop without a mask much sooner.

Bill Pentz, BobL and others have been saying for a long time that there is no substitute for capture at source. By my reckoning, they are right. If we do that well, our overall exposure will be reasonably low. I have not given my shed a "proper" clean-up in a month. There is no need. Most is captured at source. Any minor spillage is vacuumed immediately, so I am not walking it into the air.

Originally Posted by Lornie

I had read many years ago that part of the reason fine dust floated in the air for so long was because it was all (mostly) positively charged, and therefore bounced off each other (hanging in the air) instead of falling to the ground.

Lornie,

BobL is correct. Brownian motion is the principal reason very small particles float. This is well established in science, a Nobel Prize was won for work in this field about 100 years ago.

Whether dust particles are charged or not, Brownian motion will keep them suspended for a long time. If I walk into my shop after a few days away, electro-statically discharged dust collects on the bench tops. When swept off, even some of the visible particles remain suspended for quite a while, so it is no stretch to believe the smaller particles will be in the air much longer.

Adding to the debate.....What effect does air temp/hum have on the suspension of particles? Does dry air allow more floating of particles? Does moist air have the opposite effect? or Is there no difference?



Pete

Yes, both humidity and temperature have an effect.

Temperature impacts on relative humidity.

Very dry air helps particles to become and remain charged. But the bigger issue is Brownian motion, and humidity does play a part. Particles that become moist become heavier, and more easily fall out of "suspension". Spraying water has for ages been a dust control measure. Blokes that apply liquid window tint spray the room with a fine water mist to clear the air of particles before applying the tint.

But for practical purposes in a workshop other factors have a far greater impact. If we are primarily concerned with health we do well to recall that Total Exposure = Concentration X Time of exposure. So we should be concerned primarily with total exposure. How well we capture dust at the source is nearly always the no 1 issue.

I think the goal of collecting dust at the source is a noble one, and I have put more effort into it these last two years than in the previous thirty. But there are so very many woodworking operations where that just isn't possible.....

Sometimes it is 'not possible' because the cost of collection overwhelmes the cost of performing the woodworking operation..... So I resort to air filtration within the shop (in addition to the best economical dust extraction I can do at the tool).

My 1200 square foot shop with 9 foot ceilings has three (650-1000 cfm) air filters mounted to the ceiling moving (and filtering) the air in a circular pattern. I don't think I have seen or read of other shops using more than one, and that seems inadaquate to me.

Shouldn't we spend as much time talking about filtering the air, as we spend talking about (the impossible job of) collecting dust at the source?

Lornie

Originally Posted by Lornie

.
.
.Sometimes it is 'not possible' because the cost of collection overwhelmes the cost of performing the woodworking operation..... So I resort to air filtration within the shop (in addition to the best economical dust extraction I can do at the tool).

My 1200 square foot shop with 9 foot ceilings has three (650-1000 cfm) air filters mounted to the ceiling moving (and filtering) the air in a circular pattern. I don't think I have seen or read of other shops using more than one, and that seems inadaquate to me.

Filtration is of significance in countries where folks want to retain warm air inside their shop on a cold day. In countries like Australia this is far less of an issue although as increasing numbers of Aussie woodworkers install air conditioners for cooling the same will start to apply.

On this forum and elsewhere I recommend where possible to vent shed air to the outside of the shed for two reasons.

The first is that the wood worker does not have to the worry about the quality of the filters being used on their DC. This means that even cheap cloth bags can be used so wood workers do not need to purchase expensive pleated filters or room filters.

The second reason is DC leaks. Most DC setups I have assessed have large visible leaks that literally pour fine dust back into the shed. I then wonder how many are invisible and can only be detected with a particle counter. Some of these leaks look minor, such as a small fold in a plastic dust bag seal against the DC rim, but because a DC is a major particle generator ie it grinds big bits into little bits, a small leak can emit a lot of small particles. This suggests that if a DC is located inside a shed a DC should be checked for leaks every time it is emptied/cleaned but this does not matter if it is vented outside the shed. The more mobile the DC is inside a shed the more likely it is to have a leak probably because it and the plastic collection bag gets knocked and holes get punched in it, yet another reason to use ducting and locate the DC out of the way so it is less likely to get damaged.

I would only recommend locating a DC inside a shed or using a room air fliter to scavenge dust from shed air if a woodworker cannot vent outside their shed. A particle counter like a Dylos would then provide some assurance that the filters were efficient and to identify and remedy leaks. A room air filter (that is not leaking) is a useful device to have in addition to venting outside. I also recommend running a DC for some time after the last dust making activity but noise caused by air moving through air ducting inside a shed can still be irritating (mine is 70 db at 1 m from an inlet) so what I do is run the DC for just a couple of minutes after the last dust making activity and then switch the room air fliter on for 10-20 minutes after that. The room air filter is much quieter (58 db at 1m) and uses only 80W as opposed to 2200 W.

I have only tested 3 room air filters and one of them was my laboratory type filter which was so efficient I could not accurately measure its efficiency but it is >99.9%. The two others were the consumer level filters and were only about 50% efficient at the smaller particle sizes which is consistent with the published filter efficiencies of 70% at 1 micron for most of the commercially available room air type filters, so it is a good thing that you have 3 filters. As well as checking for leaks a Dylos would enable you to measure your filter efficiencies and if necessary make an appropriate correction.