How it works

[Vodkaman]

First of all, I have not even built my separator yet, due being temporarily fiscally challenged, so everything in this article is pure theorizing on my part and open to discussion and criticism.

But before improvements can be designed/invented, it would be a good idea to understand exactly what is going on inside the chamber, how it works and why it works. What follows are my thoughts on how it works, based on observation of your videos and my engineering background.

How does it work - There are two air rotations going on at the same time inside the chamber. The first is the obvious clockwise rotation (viewed from above) that we all know about. But there is a second axial vortex going on that as far as I am aware, no one on this forum has discussed. This second vortex, I will refer to as the ?axial vortex? in future, is key to the whole operation of the separator.

Take a look at the excellent video that Pitbull posted:

http://www.youtube.com/watch?v=QZSK88Hjl3A

A definite anti clockwise axial vortex is visible as the debris enters the chamber (2? 10?), but the flow appears more laminar when the chips are rotating around, but with a downward trend. This is because the visible particles are too heavy to be carried by the axial vortex. Here is a snap shot from the video, the arrow shows the axial vortex direction. The picture does not show the axial vortex, but if you view the video, it will be clear:



Explanation for the axial vortex - The central exhaust port is forcibly drawing air out of the chamber and this action sets the axial vortex in motion, by drawing in air from the bottom of the chamber. When this drawn air is replaced, the axial vortex is set in motion. Here is a diagram showing how I think it works:



Axial vortex function ? from the above diagram, it can be seen that the axial vortex air at the outer wall of the chamber is moving in a downward direction. This is driving the debris downwards towards the slot. Without the axial vortex, the debris would just continue to rotate around the chamber until pure chance put the particle in the ?dead zone? above the slot. Gravity is totally overpowered by the strength of the centrifugal forces acting on the particles and the velocity of the main rotation.

Here is another excellent video by Bulldog8:

http://www.youtube.com/watch?v=SswUX_keN1M&feature=mfu_in_order&list=UL

One minute into the video, the flow can be seen to be moving in an angled downward direction. Here is a snap shot taken from the video, the arrow indicates the particle flow direction.



Large particle movement ? we know already that the particles get thrown to the outer wall of the chamber and eventually exit through the slot. The reason for this is that the larger particles have more mass and are strongly affected by the centrifugal forces. They also have more inertia and don?t change direction very easily. The axial vortex imparts a small downwards force on the larger particles, but can not carry the particles, hence they do not swirl with the axial vortex.

Small particle movement ? having a very small mass, the smaller particles are less affected by the centrifugal forces and affected more by airflow direction. As the particle mass reduces, there comes a transition point, when the effects of airflow overcome the effects of the centrifugal forces. The finest particles are carried in the axial vortex and eventually caught by the exhaust flow and end up in the filter bag or embedded in the filter itself.

Dead zones ? these are areas in the chamber that airflow is reduced or inefficient. There are four main ?dead zones? in the chamber: directly under the exhaust duct mouth, top centre adjacent to the exhaust duct, top outer corner of the chamber and above the exit slot. Here is a diagram labeling the ?dead zones?:



The only ?dead zone? that serves any purpose is above the slot. When the particles enter this zone, the reduced flow allows the particle to decelerate and to drop through the slot, by the gravitational force. This completes the function cycle.

Improvements ? just about everyone who has adapted this design to their needs, has asked themselves the question, ?Can the design be improved?? Phil Thien came up with this novel design of efficiently removing most of the debris before the filter. He no doubt spent countless hours and lots of money perfecting the design. Even Phil would probably admit that there are improvements, but if the improvement does not reap tangible, quantifiable results, then why complicate the design for such a small gain. Keep things simple is design rule No1.

Here follows my thoughts and ideas, again up for discussion and criticism:

Drop slot ? I have never really understood why the drop slot does not extend to 360 degrees. Phil obviously did a lot of testing on this feature and established the best working solution and I accept that solution. The only vague explanation that I can come up with, is that the flow settles before the slot is introduced. The last thing you want is air blowing through the slot and disturbing the already filtered fines.

Dead zones ? the dead zones that do not serve any purpose could be ?faired? out. This would improve the flow and efficiency. Whether the improvement would be large enough to justify the not inconsiderable extra work, is in doubt, but in theory, it should provide improvements. Here is a diagram of what I had in mind:



Angular rather than curved fairings, not as good, but would still improve the flow, but still a very complex and time consuming modification. Input from a fluids engineer would be valuable here.

Exhaust height adjustment ? this idea could reduce the amount of fines that make it to the filter bag, but it will not eliminate completely. I could be wrong on this one, but I believe the rotation of the axial vortex could be slowed down by raising the height of the exhaust duct mouth. By slowing down the axial vortex, the particle size threshold will be reduced and smaller particles will make it to the slot dead zone and be collected. Here is a diagram of what I had in mind:



Again, I am sure Phil experimented with this adjustment and I will gladly bow to practical proven experience. I am not convinced that raising the exhaust mouth to the roof of the chamber would give the desired result, but maybe there is an optimum height that works best. I throw it out for discussion anyway.

Rotation speed ? rotation speed can be increased by reducing the cross sectional area of the chamber. This increase in speed would increase the centrifugal force and affect finer particles, lowering the threshold of escaping fines. This would also reduce airflow efficiency, but  I am sure there is an optimum cross section for any given system. An adjustable false top would reveal the answer to this one, requiring very little extra work.

Filter removal ? seeing as the debris that makes it through the exhaust and into the filter bag are so few and so fine. Could these not be exhausted to the outside atmosphere? They are so fine and light that they would be diffused in the slightest of breeze and cause no problem. Maybe this is a definite NO, but just thought I would mention the idea. By removing the filter bag, airflow efficiency would be greatly improved, due to lack of back pressure.

Any more ideas? ? these are not the only ideas that I have had, but they have been discarded either because of reduced efficiency of airflow or they did not address the fines problem, which is the only problem that exists.

I invite you now to submit your own ideas, no matter how wacky they are, for discussion.

Dave

[pitbull]

Dave-

Now thats what I like to see. Awesome write up and some great visuals.

I am not sure exactly where to start but I will throw this out-

Using my design as an example, I believe that there is a margin of error that is acceptable in the where the outlet is placed respective to height of the bottom of the tophat. Mine for example has an interior height of about 6-3/4" if I remember correctly. However my outlet port does not extend down exactly halfway. It is about 2-1/2" inches or so. However, I think that going up to near the top would have hurt the cyclonic action.....would going lower increase. imho only if you went to low. I thin somewhere in the middle is key.

Now I do think you are on to something with curving out the dead zones....although I am not sure if the risk(time) vs reward is worth it. I can assure you that my top hat is scrubbing all but everything I can not see on a consistent basis. I would hat to throw an unscientific number up here but it seems to be getting close to 100% of everything. Bulldog posted an excellent pic of the scrubbing his DC did with his drum sander. Now keep in mind, he and I are both not using the Harbor Freight DC so there is a bit more power at play to make up for a "non" efficient dead zone recovery.

But go ahead and give it a shot and test it. Possibly using the foam "noodles" for the pool (sliced in half, pulled to contour and glued around the edges like crown molding) would be a low cost material to test with.

[WayTooLate]

Dave -
Just a few comments on your write-up... 
You made several keen observations and have deduced much of what is happening inside the separator. 

I would however be quite curious about any empirical tests you would do with your first build. 

I have worked with a few 'true' cyclone systems from small 1/2 hp to large, 10hp units.  So I understand more of what happens inside those than what is going on in Phil's Separator.  Applying some of my cyclone 'hard knocks', here is my take on things... 

Phil's device is accurately called a 'Separator'.  It functions on two levels: 1) to separate as much heavier debris from the airstream as possible; 2) To keep that debris 'idle' so it does not re-enter the airstream. 

Various 'trash can' separators have been around since the original Shop-Vac.  I made my first one in 1976.  All of them made an improvement over doing nothing.  However, they had one common trait: The best designs create the best 'whirlpool', when empty - but worked the worst when they started getting full.  The rotating air stream was being used to stir up more debris - which then made it out the exhaust. 

Phil's Baffle makes this amazing contribution - once the debris has dropped below the baffle, it is isolated from the cyclonic action above and cannot re-enter the airstream.  It also isolates that centrifugal action from the amount of debris and/or air below, so it performs quite consistently regardless of how full the can gets. 

I have these comments on your observations. 
a) I don't think there is as much 'axial rotation' as you suspect.  I can't prove or disprove it, but I suspect much of what you observed in Pitbull's video is due to the rotational airstream intersecting his round duct - I think it has to deflect and follow the circumference.  I have a square inlet on one separator and observe none of that phenomena.  Although, it does have a neutral van to divert the rotating air past the inlet. 

b) If there is substantial axial rotation, I don't know if it is a 'bad thing' to have the 'dead zones'.  Again, my experience is with tapered cyclone designs, but I suspect that a reason Phil's design works so well is that the baffle does NOT go all the way around.  As the rotating column of air gets across the open slot, there is a slight decrease in air pressure and the air flow wants to drop into the slot.  The end of that slot cuts off the downward flow and then it rotates over the slot again.  I suspect that this 'pumping' action gives the debris a pathway below the baffle. 

Remember those amusement park rides where you spin around and the floor drops out?  You don't fall down because centrifugal force is greater than your friction against the side wall.  If you had a marble and a bowling ball in there with you, they both would still fall together to the bottom at the same speed as when it was still.

c) I would not be so quick to remove the 'dead zones' and streamline the exhaust path.  I think that will also ease the ability of debris to flow out with the air.  With cyclones, I learned that if the exhaust duct was 'short' and near the top (or flush to top), we exhausted many fines that had not had a chance to be forced out of the airstream.  If we extended the duct too close to the bottom, we could start sucking out fines that had been forced out by centrifugal force, but now were to close to the exhaust.  Unlike Goldilocks, the 'just right' zone was pretty large as long as we stayed away from the top/bottom 20%. 

I don't know if anyone has built a clear acrylic or polycarbonate version of Phil's design, but they could perform the same tests we did with cyclones...  We use a variable speed motor and smoke.  By running the system much slower, you can observe much more.  As the speed is increased, you can observe where flow changes and what doesn't change. 

Again, the beauty of Phil's design is its simplicity and practical results.  It does not take much to get substantial benefits.  It also scales quite well. 

Perhaps Phil can chime in with the results of his many prototypes and his test results...

- Jim

[Upnorth]

...about filter removal and "greatly improved airflow efficiency due to lack of back pressure".

I have recently posted preliminary drawings of an outside vent system I am planning to build. I still have doubts about turbulence and back pressure with that rectangular outlet but this is the only one I can think of building, a 15" - 18" 90* tube being just about impossible to find and perhaps would not be that much more efficient ???

Richard

[Vodkaman]

Thanks Pitbull and Waytoolate for taking the time to reply to my rambling thread. You have provided valuable feedback and thoughts, which I will take into deep consideration when designing my test rig.

Although my cave is in dire need of a dust collection system, I feel that it is going to be way beyond the house electrical supply (I live in Indonesia). Example, I have to switch off every single electrical item in the house, if I want to use the table saw, which is an 2.15HP 1.6KW 11 ampere machine. It cost me ?150 to extend the supply just to achieve this. The normal small house supply is 0.75Kwh. To extend to the next level is going to cost ?400. I have thought about petrol generators, but they only go to 2.2KW, after that, they get way too expensive.

I have a fairly good idea for the test rig. Adjusting the exhaust height is simple, as is adjusting the height of the chamber, with a false top. I like the idea of reducing the flow rate for visibility and will incorporate that idea also, with a sliding vent sleeve after the exhaust. I can monitor velocity (pin pong ball). I will probably be able to monitor relative pressure, as I am currently designing a pressure sensor for another project. Collection will be measured with a digital gram scale (in and out). I have an endless supply of fines, from a duplicator machine that I built. It uses an angle grinder fitted with a circular saw and is used to cut low density wood, providing perfect material for this experiment. The test rig will be scaled down, as I only have a shop vacuum to play with.

?Why re-invent the wheel?, I hear you ask. Well, I have a lot of spare time on my hands and this is the kind of project that I really enjoy. This is not about inventing anything new, Phil already took care of that for us. This is about understanding how it works and what parameters can be adjusted to get the most out of the machine. It may very well just confirm what everyone is doing, but there is also a chance that something good will come of the work.

If I am wrong about the presence of an axial vortex, then fairing of the dead zones is probably pointless. My first job will be to determine if the vortex exists or not. I liked the bowling ball analogy, made a lot of sense.

Upnorth - this thread started after I read your thread, but I got carried away. Nice CAD work by the way.

It is hard to imagine that venting direct to the outside atmosphere would not have a significant improvement. A filter encrusted with fines has to provide more back pressure, I would have thought. BUT, there is a lot to HVAC systems, as I am learning from reading all the ducting and venting threads on this forum. In car design (my background), HVAC is an entire department unto itself.

There is probably a right way and a wrong way to vent to atmosphere. Your solution of venting over a large area seems reasonable to me.

Dave

[Bulldog8]

In my build, I considered the effect of an elbow in the dust stream above the baffe. In your drawings you call this area the axial vortex. I used a tophat style baffle that removes this concern due to the inlet pipe stopping at the edge of the baffles outer edge. With the other popular style of baffle, the inlet elbow sits in the middle of the airstream and would have to slow the rotation speed. Therefore, I would have to conclude that if rotation speed aids in scrubbing, a tophat is the way to go. However, if a slower rotation speed provides better scrubbing, the inlet elbow style is better.

Thoughts?

[WayTooLate]

Bulldog -
I agree that your 'side inlet' design provides the least obstruction for incoming debris and maximizes the area for separating particles from the airstream.  This is the way cyclones are designed.  Bill Pentz's research and designs have a neutral vane that extends into the cyclone to improve rotational airflow.  Since cyclones use a larger vertical height and the debris spirals down, that neutral vane doesn't see the same air/debris coming past it again... 

Since a wide variety of inlet configurations exist and most seem to achieve very good results, it does not seem to be a critical factor in success.  Since most of the 'baffle-builders' are hobbyists and not commercial shops, I suspect that the difference between 90% effectiveness to 95% or from 95 to 98% is not important.  It is the 0-90% improvement that has helped us. 

When commercial shops begin using more Thien devices, then I think that the 'science' of what works best will become better known and developed.    For them, they have two driving factors:  Power efficiency - the 5% savings of running 10, 20 or 50hp motors for a whole shift becomes significant.  Air Quality - when OSHA requires the shop air to improve and you are facing everyone switching from dust masks for full respirators.  These issues become driving factors to squeeze the most effectiveness and will justify the detailed testing to determine what works 'slightly better'... 

Looking forward to what happens next!
- Jim

[BernardNaish]

How the Thien Baffle Works.

We do not yet know how the Thien baffle removes wood chips and dust from an air flow. We do have considerable evidence that it does work from the posts in this excellent blog together with some information, some data and a lot of very innovative ideas. We might also look towards the evidence available from the use of cyclones both those used by wood workers and those developed for the removal of household dust such as the Dyson vacuum cleaner.

I have written here a description of how I suggest it might work. I have drawn extensively from evidence presented in this blog and from the science of fluid dynamics that I have gleaned from Wikipedia and elsewhere, interpreted in the light of my own knowledge and experience gained in my former work as a technologist. I have used some wording from these sources and I make no claim to them.

Nothing I state here has any validity until it has been shown experimentally to be correct. I do hope, however, that my contribution stimulates the experimental work that I think needs to be done. Man is inquisitive as well as essentially practical.

The Thien Baffle Top Hat.

I am considering here only the Thien baffle when made up in the "Top Hat" design. It may or may not be equally applicable to other ways of using this baffle. I have chosen this because I consider the Top Hat to be a superior method of construction and use. I have built two crude cyclonic drop can separators for use with shop vacuum cleaners that work extremely well without a baffle but stopped working when connected to a high volume dust collector. Hence my thinking here is more concerned with HVDC than HPLV shop vacuum cleaner based systems.
Bernoulli's principle states that for any flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure and vice versa. Swiss scientist Daniel Bernoulli published this principle in his book Hydrodynamica in 1738
Bernoulli's principle relates to the principle of conservation of energy. This states that, in a steady flow, the sum of all forms of mechanical energy in a fluid along a stream is the same at all points on that stream. Of course this is theoretical and assumes no losses due to friction from the stream container.
If a mixture of air and dust is flowing horizontally in a section of pipe or tube, particles are subject only to their speed and their weight, if the speed increases it can only be because the fluid in that section has moved from a region of higher pressure to a region of lower pressure AND if its speed decreases it can only be because it has moved from a region of lower pressure to a region of higher pressure. Consequently, within a fluid flowing horizontally, the highest speed occurs where the pressure is lowest, and the lowest speed occurs where the pressure is highest.
The gases in air have very little weight compared to the wood chips or even the tiny grains of extremely fine dust. As our flow enters the circular chamber of the separator the wood debris will be thrown against the outside wall because they are heavier.
These heavier parts of our flow require more energy than the air does to flow at the same rate and as the energy input is essentially fixed they will not move as fast as the air so will fall downwards and this will be increased as these particles will lose some of their energy because of the friction against the chamber wall against which they have been thrown because of their greater weight. The rough nature of the wood debris is not as slippery as the air so the effect of this friction will be greater on them than on the air.

A curved air stream will develop a pressure gradient perpendicular to the direction of flow, with the lower pressure on the inside of the curve. Bernoulli's principle predicts that the decrease in pressure is associated with an increase in speed. So that as the air passes around the separator it slows down towards the outside wall of the chamber. This means the chips and dust will tend to fall even faster than they would in a laminar flow.

Inlet Pipe Design.

Air travelling along a round pipe will tend to rotate particularly if it is made by welding or folding a length of steel sheet into a pipe with spiral walls. It will also occur due to the spin of the earth and if a rotary blower is used. A rotating flow is highly complex consisting of many smaller turbulent flows. When introduced to a cyclone separator such disturbed flows reduce the efficiency of the separation of the dust from the air. By changing a round pipe into a rectangular section tube, by means of a transition adaptor, the rotation and its associated turbulence can be halted and if the rectangular section is long enough a steady laminar flow achieved. This is presumably why ventilation ducts in buildings and fume hood trunking in laboratories are rectangular rather than round in section. Recently some dust collectors have begun to use a rectangular duct, between the blower and the simplistic system that directs the flow either to the filter or the debris collecting bag, for the same reason.

Bernoulli's principle as applied to the Thien Top Hat separator above assumes that the airflow is horizontal and axially laminar hence for it to work well the flow into the chamber needs to be as axial and laminar as possible. This means it must be as free of rotation and its associated turbulence as we can get it. Hence, in my opinion the inlet pipe needs to be rectangular in section.

We know that the optimal width of the drop slot for the Thien separator baffle is 1.125 inches. If the inside width of the rectangular inlet duct is also 1.125 inches then it will contain the highest possible concentration of wood debris immediately above the slot. Bernoulli effects as described above should then have the greatest possible opportunity of forcing all of the debris into the collection container under the baffle. Clearly the cross sectional area of the rectangular section needs to be the same, or slightly more, than that of the round pipe from which it flows.

Water Pump Effect.

Our wood burning stoves work well because a draught of air is drawn up the chimney stack by air currents passing horizontally across the top of the chimney liner. This can be improved by adding a rain deflector over the top of the stack because it creates a more laminar air flow. In other words we have a tube held at right angle to a laminar air flow.

Consider the humble Bunsen burner. Here we have a vertical flow of flammable gas passing up a burner tube past a hole in the side through which air is drawn because of the Venturi effect which is intimately associated with the Bernouilli principle. The water pump, another piece of common laboratory equipment, works the same way except in this case the fluid flow is water from the main water supply passing across the top of a tube at right angles to it that creates a considerable vacuum in that tube drawing in gas or fluids.

Similarly if we present a laminar flow of air and wood debris passing across the top of a tube held at right angles to that flow then a vacuum will be created in that tube and some of the mixture, that closest to the pipe will be drawn in. I suggest that the drop slot in a Thien baffle is just such a "tube" albeit rectangular and being much longer in one direction than the other. If the mixture closest to this tube is richer in wood debris then more of this will be drawn below the baffle than air.

I suspect that if we can accurately measure the pressure above and below the baffle then we would find that we have a vacuum in the collection container. This may explain why quite a number of contributors have noticed their tin collection can collapsing under the pressure of the atmosphere pressing against their outside walls. I suspect that this collapse occurs less when the system is dead ended as little or no air is passing the drop slot. The implication may be that the collection chamber IS critical after all in that it must be rigid enough to be able to withstand this pressure to achieve the greatest separation effect.

Other Possibly Relevant Effects.
Bernoulli's principle can also be derived directly from Newton's 2nd law. If a small volume of fluid is flowing horizontally from a region of high pressure to a region of low pressure, then there is more pressure behind than in front. This gives a net force on the volume, accelerating it along the stream. I do not yet see how this affects the Thien baffle but I suspect it may and I will think on.
Conclusion.
I have been trying to stop myself thinking about and carrying out experimental work on the Thien baffle because I thought I had moved on from my previous existence and would rather smooth wood these days. However I cannot resist it and when I have completed some of the jobs on my list, a daughter needs her shoe storage cabinet after all is said and done, then I intend to examine rectangular inputs; outlet pipe EXTERNAL diameter; height of outlet pipe and the depth of the baffle under the end of the outlet tube. I can really only afford to do this for one diameter of Top Hat, probably about 20" and as my Clubs' system uses 4" diameter spiral steel tube and a 1 ½ KW blower I am also stuck with these fixed factors. It would be great if someone else could repeat these tests with different separator, blowers and/or tubing diameters.

I have posted a proposed standardised testing method elsewhere on this blog and will confirm this and the results obtained on this blog if I may do so.

[guy48065]

Fascinating ideas and observations Dave.  I hadn't given much thought to how debris gets through the slot.  I guess I just assumed it swirled down like any horizontal-flow separator and eventually finds its way through... and then is trapped (the real advantage to the baffle).  IF your vortex idea is true that means there is a definite mechanism in play forcing the dust down.

I do think you may be correct that smoothing the chamber cross-section to resemble a donut with one square corner at the slot location could increase the efficiency of the "axial vortex" and increase downforce on the dust stream.  Such a shape would then resemble a pump volute in cross section (just more proof that everything useful in physics has already been done).  The one remaining "dead zone" would then be the only area of low pressure that would attract the debris out of suspension--and right into the slot.
I would also wonder if the addition of a bellmouth would allow the air entering the fan to have "more reach" and would support this vortex better than the ragged turbulent flow into a simple straight inlet.

Fairing the chamber and the fan inlet could be a mighty one-two punch that promotes quick debris removal from the air stream.  Maybe.

[alan m]

some good theory about it bernard

my understanding (from nothing more than blogs,forums etc) is that the smaller the hole in a venturi the greater the pressue difference and therefor pulling power. is that correct.

if it is then can i deduce that if you put a series of holes all around the perimeter of the seperater instead of the slot you would get greater pulling force and posibly more seperation
it would probablyh clog up more on larger debris but might work on finer dusts


gus
are you saying that putting a radiused curve to the top corner of the cross section.
it might help. add it to the list of things to try out.
i would be worried that it might add too much downward force to the otherwise almost horizontal spinning air mass
this might force more air down into the slot and increase air speed below the baffle plate therefor reducing efficience

[BernardNaish]

Alan, You are correct regarding the size of the orifice determining the vacuum achieved. Phils' original work found that the 1.125" slot was optimal both for maximal separation and for ensuring nothing was trapped in the slot. On occasion he has also suggested that the slot be increased to 1.25" wide when shavings were being snagged. I feel that a row of holes would clog with shavings however we could have one Thien separator for these large shavings followed by another designed more for getting the fine dust out of the air. I suspect the circulating air flow "sees" the narrow slot as a single orifice. It would be worth seeing if the quarter inch right angle presented by the hardboard usually used  is optimal or would it work better if it were beveled? I suspect it is best left as a right angle but would love to know.

I am hoping that a member with a Thien and a manometer will try measuring the pressure across the baffle!

Thanks for your reply.

[alan m]

iv been meaning to do some scientific testing on my system

how would you test above and below
iv got a 55 gallon barrel.
if i mounted a tank union pluming fitting to the side of the barrel and another to the top of the seperater
where would you put it on the seperater

[WayTooLate]

I have been tied up with a number of other projects and have been following threads 'from a distance'... 

I have a few quick suggestions about taking some measurements on a Separator. 
There are three empirical tests that will judge the actual performance of your Separator.
1. Manometer.  Attach a manometer across the ducts leading into and out of your Separator.  The actual number is not as important as reducing the amount of vacuum across the Separator.  The greater the number, the greater the load to your DC.  Power lost in the Separator is power lost sucking dust. 
2. Weight Measurements.  Using a scale that measures in 0.1grams, weigh a small board.  Also, weigh your final filter.  Note these weights.  Saw or sand (or ?) the  board so that a significant portion of it has disappeared into your DC (not on the floor).  Re-weigh the board and filter.  The differences show your practical efficiency.  I don't care if there is dust inside my ducts, or even in the bottom of my filter can.  I don't want it to get to my filter.  Efficiencies will vary greatly between fine sanding MDF and planing doug-fir.  The important number is the improvement. 
3. Electrical.  Put an ammeter on your DC's motor.  This is counter-intuitive.  If you plug your DC inlet, it whines like you whipped your dog, but the current drops because there is no air to move.  Remove the inlet duct and it may make less air noise but the current goes up because it is doing more work.  Similarly, plugging the outlet stops the air flow and the work the blower can do.  Note the readings with zero and 'full' air flow.  (Blowers are purposely designed with restrictions so the motor stays within its safe operating range.)  Your attached Separator will show a current drop due to its restriction to air flow.  Making a more efficient Separator means the motor can pull more air through and hence work harder. 

By paying attention to these empirical numbers, I have been able to judge what improvements have made a difference... 

If we make these tests 'Before and After', we can judge our relative improvements! 

Hope this helps!
- Jim