#Sf of louver per cfm airflow calculator how to
#Sf of louver per cfm airflow calculator series

In that case, your average air velocity will be artificially high, making your reading higher than what it is.Įach manufacturer publishes correction (Ak) factors that attempt to compensate for the open area of the register. Suppose you fail to include a correction factor for the increased air speed in your airflow calculation. So, your airflow calculation has to compensate for it. The phenomenon exists at the face of the register, the exact spot where you measure air velocity. This action increases airspeed and is called Vena Contracta. The louvers on the register face restrict airflow as it exits.

#Sf of louver per cfm airflow calculator how to

The grand mystery of traversing a supply register is how to compensate for its open area. This inconsistency will result in inaccurate readings. If you drift farther from the face of the register, velocity will decrease. As you move closer, velocity will increase. A consistent one-inch distance is usually recommended. Second, hold the anemometer an equal distance from the register throughout the test. Then rotate your device, so it faces directly into the airstream until it reads the highest velocity. To assure you're capturing the correct velocity, study the register to 'see' the air direction. If it is not, the velocity reading will be inaccurate. Angle and Distance are Criticalīoth types of instruments require you to use two test principles to obtain accurate average velocity readings.įirst, the measuring device must be held perpendicular (at 90 degrees) to the airflow blowing out of the supply register. Make sure you don't allow the probe to move out of the airstream. Measure the entire face of the register as you move the anemometer across it. You can program the anemometer to add all your velocity readings together and calculate the register's average velocity.

#Sf of louver per cfm airflow calculator series

Use this instrument to take and record a series of single-point velocity readings across the register face in a grid pattern. The instrument interprets velocity from the rate heat is removed from the heated wire. It has a heated wire embedded at the end of an extendable probe. The other instrument is a thermal or hot wire anemometer. At the end of the test, the anemometer will display the register's average velocity on its screen. You'll take an ongoing velocity reading over the 30 to 60-second test. Most rotating vanes include a calculating function that enables you to move it back and forth in a pattern over the register's face. The most common is a rotating vane anemometer that uses a propeller-style fan to measure average airspeed. There are two test instruments used in the field to measure the average velocity through a supply register. Finding and compensating for the open area of the supply register. The two challenges you'll face when traversing a supply register are:ġ.Accurately measuring average velocity from the register.Ģ. Airspeed in the United States is expressed in Feet Per Minute (fpm). Velocity is the average airspeed measured through the supply register.You need to make a correction factor to interpret the register open area. Area is the open area of the supply register in square feet.CFM is the volume of Cubic Feet of air passing through a supply register each Minute.Airflow TraverseĪn airflow traverse is the foundation of all airflow measurements. With practice, you can build skills and knowledge for yourself on how to take accurate, repeatable supply register readings in the field that you can trust. My interest is not to create an argument but to encourage you to gain the knowledge and experience you need. YouTube videos have left many good technicians confused about how to measure supply register airflow because several key principles are missing, and the test is usually oversimplified. Let's take a look at how you can take an accurate and repeatable supply register traverse measurement. But when space is tight or a hood doesn't fit on a register, you can use a traverse technique, and an instrument called an anemometer to measure airflow. Ideally, you can measure supply register airflow with a balancing hood.