You will notice that the Testo 550 has both a positive and a negative port. The standard unit is hPa (hectopascals), but we can change it to inches of water column by turning the unit off and holding down the power button again. The unit should flash on the screen. If you hit the up arrow, you will cycle through a bunch of measurements, including mmHg (millimeters of mercury), mmH2O (millimeters of water column), millibars (mb), and PSI (pounds per square inch). You will eventually reach inH2O (inches of water column). Up top, there will also be some velocity units.

If you then hit “Mode,” you can select whether you’d like the manometer to turn off automatically or not. Then, you can hit “Mode” again and be able to get readings. To calibrate the manometer, you can zero it out by pressing the up arrow. ONLY zero it out in the room with good reference conditions.