All the things you've done should have helped some, except the duct boosters. They can't add much flow to the system unless the return ducts were increased proportionally (or similarly boosted). You get increased flow by increased pressure or larger ducting. Keeping upstairs doors closed will help by slowing the 'fall' of cooler air downstairs. Try leaving fan "ON" for a 24 hr period to see if your house will equalize better.
You can force the system to concentrate on the upstairs by reducing both the supply and return vents in the main floor and basement. You may have to do some makeshift blocking (reduction
) of the returns with plastic sheeting and 'no residue' duct tape. If successful, I believe magnetic covers are available.
AFA blocking above the upstairs ducts, by all means. Save yourself some expense (and trouble, that much foam is hard to scaffold in a vertical shaft) and make 2x4 blocking (essentially fire blocking) to go above the vents. If you have typical framing 16 inches on center, a 14 1/2 inch piece will span stud-to-stud. Use a pair of pocket screws on each end to secure the blocking in place. Use fire caulk/foam for any gaps.
The fan in an air handler is typically stated to have a flow rate at a given static pressure (e.g. page 7 of the Lennox CBX25UH air handler specifications here). In my experience, almost all pressures in HVACR are stated in terms of gauge pressure; as you suspected, 0.5 in w.c. is a gauge pressure (generally noted as in w.g.). Some fan flow rates are stated in terms of total static pressure, where the evaporator, heater core, filter, duct friction loss, diffuser pressure drop, etc. contribute and must be taken into account when sizing the blower package.
The higher the external static pressure, the lower the air flow rate--the system pressure is a measure of resistance to the flow created by the fan. For external static pressure, the major contributors are friction loss in the duct work and at the diffuser. CaptiveAire (an exhaust hood manufacturer) has a good explanation of total, static, and velocity pressure here. The ASHRAE fundamentals handbook also has an extensive section on duct work, but unfortunately is not freely available online--you may be able to find a copy at your local library or an older version at a discount book store (the duct chapter has not changed much for this purpose).
The fan creates a low pressure region on one side (return) and a high pressure region on the other (supply). If you have a very large static pressure (i.e. high friction), you will have a low resulting velocity pressure and low resulting air flow rate. To simplify design, manufacturers state the fan's capacity to overcome a given static pressure with a resultant flow rate. Unless your static pressure is so large that the fan stalls or operates outside of its stable region (see this page on fan curves), you will not damage the air handler, but you will reduce the air flow rate and waste energy.
Assuming you are using a device which can measure the static and dynamic/velocity pressure (e.g. pitot tube), you will ideally have a constant total pressure at all parts of the system (this concept is part of Bernoulli's principle, pressure drop due to friction does represent energy loss and is added to one side of the equation when comparing two states). In your scenario, the diffuser represents an obstruction to the flow, so you will have a higher static pressure and lower velocity pressure before the diffuser. The region outside of the diffuser is at a lower pressure relative to the duct, so the energy represented by the static pressure component becomes part of the velocity component, resulting in a greater fluid velocity. If you were to close all of the diffusers, you would create a very large static pressure on the supply side of the system and would lower the flow rate. You can measure pressure at any point in the system and will get the same total pressure (minus energy loss due to friction). Immediately at the supply side of the air handler, the energy (and pressure) is highest, near the diffuser, you have experienced some energy loss due to friction.
For diagnostic purposes, it can be useful to measure pressure along the supply side to determine if you have excess friction loss or, more commonly, duct leakage.
Best Answer
If you are measuring the pressure inside the duct, before the diffuser, the pressure should be detectable. The velocity pressure is given by Pv = pV^2/2 where Pv is velocity pressure, p is the density and V is the gas velocity.
Taking a wild guess at a velocity of 5 m/s, gas density of 1.29 kg/m3, then Pv = 16 Pa
The static pressure should be equal to the pressure loss across the diffuser, which may not be very large, but is probably in the range of 5 - 10 Pa.
I would suspect your problem is one or both of two:
The BMP180 is a barometric pressure sensor, and is designed to measure pressures on the order of 900 - 1000 mbar, you are asking it to detect a change of about 0.25 mbar, which is very much at the limit of its accuracy (regardless of how it's advertised). Moreover, the electronics to which it is connected will add error to its measurement as well. Static and total pressures in a duct are typically measured using a differential manometer, comparing the pressure inside the duct to the pressure outside the duct, because this is a much smaller number and allows a much more sensitive instrument. Velocity pressure is usually measured by comparing the static pressure and the total pressure inside the duct.
Placing a flat pressure sensor (presumably mounted on a circuit board) into an airstream will likely not yield the stagnation pressure of the gas in the airstream at the aperture. The sensor and the circuit board will so dramatically disturb the gas flow as to change the direction and likely create a point of 0 velocity at the pressure sensor. Total pressure measurements are taken using pitot tubes which are carefully designed so that they will allow the gas to flow around them with minimal disturbance and collect the stagnation pressure at the aperture. Once again, I think the BMP180 is the wrong instrument.
If you aren't too concerned about accuracy, you can make a makeshift pitot tube out of a piece of copper tubing bent so that the tip will point into the gas stream. The more aerodynamic the tip, the better. I wouldn't use this for spec'ing a $100,000 fan, but for fun around the house, it might be enlightening.
As far as the manometer, you may try using a tube, folded in a U-shape and taped to a ruler marked in millimeters, and filled halfway with water. This will still just barely register at the small pressures you're looking at. YOu can experiment with changing the angle of it to make it more sensitive. The greater the slope, the more the water will move in response to a given pressure.