Yup, U-tube manometer.

You need a length of clear (or at least translucent) plastic tubing. Bend it into a U shape and pour enough liquid into it to fill the U bend up, say 10" or 12". Just hanging there the liquid legs will be equal. If you put one end into the suction of your DC the leg closest will rise and the liquid on the other side will drop. Measure the height of the difference and that's the suction in inches of water column. You can add a drop of food color to make it easier to read. You can get fancy and mount it on a board with a ruler.

Measuring the airflow is more difficult.

 

To measure air flow accurately you will want to use an anemometer, and I believe the difference will be pretty subtle so you will need a pretty precise instrument to measure the delta. I bought one for about $100 to test my system when I upgraded to a cyclone, and it is adequate but not world class. Assuming you don't want to spend that much to test results of a filter upgrade, you might ask around to see who might loan you one (any friends in the HVAC biz) or you may be able to rent one as well.

 

Also note that as soon as you start using your DC unit, regardless of it being a bag or a Wynn or some other brand, it will begin to suck less and less over time as dust gets caught in the filtering media (as it is supposed to do). That decrease in suction is related to efficiency of the filter. All filters, if ran long enough, will eventually become more and more efficient and they will reach 99.99% efficiency shortly before they hit 100% and become blocked.

I suggest you study this page on Filter Efficiency from the Wynn Environmental website.

That said, even a professional manufacturer has some issues with the ability to accurately measure dust filtering capacities with regards to a national standard. There are no clear cut answers here, though one can look at the trend lines and draw some general conclusions about all filters and their performance over time.

Happy reading.

 

When I had a day job, we had to measure some air flows daily on ducted systems for hygiene purposes. We used a Magnehelic gauge set up with a pitot tube. This stuff isn't expensive, as industrial testing equipment goes, but it is fairly specialized making it not that useful for anything else. The anemometer or the manometer will give you some idea, but not with precision. Like Paul said, the change may be very subtle. You can also get a Magnehelic off e-bay for about $20 and use it like a manometer, I have one on my system to gauge when the filter needs cleaning. You might also want to check your motor amps to make sure you aren't flowing so much air it overloads the motor.

 

Check out how Matthias did it on his video

 

Installing different filter will increase (or decrease) flow rate (cfs). It will not change suction (static pressure) per se. The latter is the property of the fan and motor and is easy to measure by sucking water into a vertical hose and measuring how high it will go. It could be between 10-15" for stationary DC to more than 70 " for shop vacuum.
Measuring airflow is more complicated and would require either flow meter or some kind of homemade contraption such as manometer, Venturi vain etc. If it is just simple qualitative comparison between two filters you cold, for example, move the end of the hose towards a pile of dust noticing at what distance the dust will get picked up. Then repeat with the other filter. It will tell you nothing about actual cfs, but at least you'll see whether one setup has greater flow.

If you can channel the exhaust through a house you could do this:
http://www.physics.org/interact/physics-to-go/bernoulli-balls/index.html
and measure/compare the elevation of the ball. Note that flow/height relationship is not linear.

 

To make valid comparisons, the end of the tube where you want to measure depression must remain exactltly at the same place;
so it would be advisable to have a small section of rigid tube to avoid it being bend by the air flow and having it clamped during any measure and while you change any other parameter of the system.
Any filter must be in the same state of cleanliness.

Ideally it must be conducted at the same temperature and with the same atmospheric pressure, so preferably the same day in a few hours.
Otherwise you will compare apples and pears.
The video of Matthias wandel shows quasi immediate comparisons where the tube of the probe is not moved..

 

Ok, let me change the question. How do I determine (cheaply) if the additional filter area of a canister filter is improving the amount of "material" (chips and dust) being collected at the end of my flexible hose?

 

0. Have a pile of dust.
1. clean the filter
2. weigth your empty bucket;
3. start your system, wait something like 30" to have a stable regime;
4. collect dust during a well specified time (the longer the best).
5. stop the system.
6. weigth the bucket with dust. Substract the empty weigth; note the result.
The higher the weigth of the dust in comparison to the weight of the empty bucket, the more it is precise. Use a scale sensitve enough. Don't use a scale with a 250 Lb range to weigth 2Lb.
7. emty the bucket on the dust pile
- repeat from 1. X times and determine the average value.

change the filter model and repeat the procedure to determine the average value with the new filter model.

 

Also, weigh the filter before and after (to include the fine dust). While it is finer, it is also denser by volume than the chips/dust and that translates to weight.

Limit your piping/hose to a specific length and shape(bends are bad). BTW, a larger/longer/greater filter will allow/pickup dust farther from the machine because of a decrease in the back pressure caused by the filter media.