Orifice plates have long been used for measuring and metering airflow. Fixed orifices are used to measure airflow in laboratories and for balancing duct runs in industrial applications. Orifice plates are sized for a particular amount of airflow to pass through, given the upstream static pressure.
To be useful as a device for measuring airflow, an orifice plate is mounted in a tube with two pressure taps, one on the upstream side and one on the downstream side. This device is an airflow meter. The pressure tap nearest the fan is called the high pressure side, while the other tap is on the low pressure side. The difference between these two pressures is called the differential pressure and is a function of the amount of air mass flowing through the orifice. Usually, we don’t worry about air density (air mass per unit volume) for typical ambient temperatures and pressures encountered in duct leakage measurements. The error in measurement is relatively small for temperatures from 50 to 120 degrees F, and elevations up to 1200 feet.
If you are leak testing a system in high elevations such as Denver, or performing a leak test with unusually high or low air temperatures, a density correction should be made. Oriflow can supply data for your Oriflow orifice so that it is corrected for your particular environment. Oriflow has a duct test summary sheet that will automatically make those corrections for you as well as calculate duct surface area and other parameters.