It is really quite easy to create a pressure manometer. This can be some clear plastic tubing, some water, a bit of isopropyl alcohol (10%, reduces water surface tension) and a drop of food coloring.
A U tube, with vertical elements, can't measure air pressure difference with enough resolution. This is why many water manometers have one part of the "U" going up at a 45 degree angle. Remember that the pressure difference is 1/2 the water height between the two ends. The unit even has a specific designation: "inches of water" pressure.
Or just remember that there are 250 Pascals of pressure per inch of water.
Years ago (actually decades), I built a quick water manometer that was accurate to about 5 Pascals. I actually stapled the clear plastic tubing to a wall and one part of the U tube slowly went up the wall at a very gradual slope. Had to make measurements in a structure to serially document how build practices can be done to reduce infiltration.
Started with a conventionally built structure, but without drywall. Then sealed sill plate (silicone), then sealed window edges with silicone, then sealed outlets - on and on.
Took a picture of this after each intervention to show contractor (DoD) how small changes can make a big difference.
Just place one end of the air filled U tube in the house (pressurized or evacuated) and the other outside.
You can buy electronic manometers (really just pressure transducers), but a simple water filled manometer is cheap, easy to read and incredibly intuitive.
One advantage of a furnace blower is that they don't load down at the low speeds. You can easily measure the CFM measurement with a hand held device such as this:
Pyle Digital Anemometer Thermometer for Air Velocity Air Flow New | eBay
Steve