How to measure duct static pressure?

Now if u want to external statics pressure for fan selection than u have to find the losses in duct for this u can refer to carrier design manual book .for this u take the various sizes of duct and there pressure looses including the various fittings which can also be found out by using ashrae duct fitting data base program.

If your question is regarding the actual static pressure than we employ 

Static Pressure Testing, Step By StepIt typically should take less than five minutes to measure a residential system’s static pressure. Here are sample instructions for a furnace and an external coil:

STEP 1: Locate the appropriate locations to drill the test ports on the supply side (+) between the furnace and the coil, and on the return side (-) between the filter and the furnace. Center the test ports for neat appearance. Stay away from any coils, cap tubes, condensate pans, or circuit boards to avoid damage. Always look before you drill.

STEP 2: Drill test ports using a 3/8” drill bit with a metal piercing tip. A bullet-tip drill makes a clean round port. Make sure to use your drill bit sheath (described above) to help prevent you from drilling into the coil. If there’s duct liner inside, be sure to penetrate it to assure a good reading.

STEP 3: Push one end of the tubing onto a static pressure tip. Place the other end of the hose on the HIGH (+) pressure port of the gauge. If required, make sure the gauge is level and zero it out by adjusting the screw on the face with a small screwdriver. Digital gauges each zero differently, so check owner’s manual.

STEP 4: Read the supply or positive (+) static pressure by inserting the static pressure tip into the test port with the tip facing into the airflow. The magnet on the tip will hold it in place while the value is read and recorded. This measurement is the pressure the fan is “seeing” on the supply side of the system.

STEP 5: Read the return or negative (-) static pressure by moving the tube from the HIGH to the LOW-pressure port on the gauge. Insert the static pressure tip into the test port on the return side with the tip facing the airflow. Read and record the negative static pressure. Don’t forget to insert port plugs into the test ports when you’re done testing.

STEP 6: Calculate the system’s Total External Static Pressure by adding the two values. Since the negative and positive signs identify the type of pressure measured, you can ignore them when adding the two values together.

For example:The supply static pressure reading is (+) .26 w.c.The return static pressure reading is (-) .21” w.c.The total system static pressure is .47” w.c.

.26” + .21” = .47” w.c