What this tool does
This tool, along with a pitot tube and a digital manometer, allows you to accurately measure the airflow within a duct of 4 inches (10.2 cm) or greater, such as that of a kitchen range hood or dryer vent.
It may be used with a standard steel pitot tube, such as a Dwyer series 160 (pictured below) or with an averaging flow sensor pitot tube, such as the Dwyer Series PAFS-1000.
Because ASHRAE 62.2 requires the measurement of existing and installed ventilation fans, the pitot tube is a useful device to have at hand. Additionally, for the sake of determining possible back drafting of vented combustion appliances, it is important to be able to measure the flow of clothes dryers, rather than estimating their airflow.
In order to find airflow from velocity pressure, calculations are required; this is what this Pitot Tube Airflow tool will do for you. First, it helps you calculate the air velocity based on pitot tube measurements and air density. Then it calculates the corresponding airflow in the duct by helping you determine the duct cross-sectional area.
Calculated Values
- Airflow rate.
- 90% Airflow rate – many pitot tube manufacturers recommend using only 90 percent of the measured airflow rate if only one pitot tube measurement is taken at the center of the duct. Note: This value is not calculated when the RED Calc Pro Pitot Tube Airflow tool is used for averaging pitot devices listed in the dropdown list.
Tips
- Clicking the label for any input or result will cause a popup help box to appear. This help box includes the allowed and normal values (for inputs).
- When you purchase a pitot tube, it is important to read and retain the instructions. When setting up your pitot tube, always follow the manufacturer’s recommendations.
- Some pitot tube manufacturers recommend multiplying the airflow result by 90 percent (0.9) if you take only one measurement with a single-port device at the center of the duct. The RED Calc Free Pitot Tube Airflow tool gives this adjusted airflow as its final result.
- When using a single-port pitot tube, It is more accurate to determine the airflow rates in a larger duct by taking readings at a number of places on a single cross-sectional plane of the duct, whether it be round or rectangular, according to industry-accepted procedures. The velocity pressures are then averaged and this averaged value is entered into the RED Pitot Tube tool to determine the airflow rate, rather than averaging the determined airflow rates at the traverse points. An averaging pitot tube performs this function for you when setup correctly. For information regarding traverse measurements in a duct, see ASHRAE Handbook of Fundamentals, 2017, 6.4 Measuring Flow in Ducts, page 37.18, or this resource from a manufacturer of instruments.
- When you connect your pitot tube to your manometer with the appropriate hoses, make sure you attach the hoses to your manometer so that you are subtracting the static pressure (the smaller pressure) from the total pressure (the larger pressure). If you are getting a negative number reading on your differential manometer, reverse the hose setup.
- A pitot tube of 8 to 12 inches (20 to 30 cm) in length is ideal for residential flow measurements.
Background
The pitot tube was invented by Henri Pitot, a French engineer, in the 18th century and later modified by another French scientist Henry Darcy. This ingenious device solved the problem of not being able to directly measure the velocity pressure of airflow. This is accomplished by subtracting the measured static pressure from the measured total pressure to find velocity (dynamic) pressure [total pressure – static pressure = velocity pressure]. The device is very useful for measuring airflow in a dryer duct or the duct of a ventilation fan.
The static or bursting pressure in a duct “pushes” against the walls of the duct, similar to what happens to the wall of a balloon when you blow it up. The velocity pressure is created by the movement of the air and its density. This is similar to what happens when you let the valve of a blown-up balloon go; the static pressure is converted to velocity pressure as the balloon flies through the air.
It is interesting to note that pitot tubes are a common protrusion from the fuselage of airplanes as an air-speed measurement device.
Best Practices
To ensure accurate velocity pressure readings, the tip of the pitot tube must be pointed directly toward the oncoming air stream.
When the pitot tube is correctly aligned, the velocity pressure reading will be at or near its maximum. Because accurate readings cannot be taken in a turbulent air stream, the pitot tube should be inserted at least 8-1/2 duct diameters downstream (after) and 1-1/2 duct diameters upstream (before) any elbows or other fittings/obstructions that cause turbulence. If this is not possible, do your best to insert the pitot tube in a smooth and straight duct section.
If circumstances do not permit you to average various readings from various locations in the duct (called a traverse), take one reading in the center of the duct and multiply the result by 0.9. This multiplication is done for you in the last result of the RED tool.
In order to make accurate readings, the pitot tube must be kept free of dust and debris.
Specific to RED Calc Free Version
Although this pitot tube airflow calculation tool allows you to enter “Pitot tube correction (K) factor”, we suggest you use this tool only for pitot tubes with a manufacturer’s listed K factor of one. The devices we know of include:
Dwyer single-port
160-8 ~ 5/16″ dia. (7.93 mm), 8-5/8″ (21.9 cm)*
160-12 ~ 5/16″ dia. (7.93 mm), 12-5/8″ (32.067 cm)*
160-18 ~ 5/16″ dia. (7.93 mm), 18-5/8″ (47.3 cm)*
166-6 ~ 1/8″ dia. (3.175 mm), 6″ (15.24 cm)*, 3″ (7.62 cm) tip
166-12 ~ 1/8″ dia. (3.175 mm), 12″ (30.48 cm)*, 3″ (7.62 cm) tip
167-6 ~ 1/8″ dia. (3.175 mm), 6″ (15.24 cm)*, 1-1/2″ (3.81 cm) tip
167-12 ~ 1/8″ dia. (3.175 mm), 12″ (30.48 cm)*, 1-1/2″ (3.81 cm) tip
* indicates insertion length
Specific to RED Calc Pro Version
Included Pitot Tube Models
The RED Calc Pro Pitot Tube Airflow tool uses specific pitot tube models which you are able to select from a dropdown list. We have checked manufacturer’s data to ensure that this tool will determine the proper airflow rate for the pitot tube you are using on the list, whether it is a single-port (one reading point at the center of the duct) or an averaging pitot tube, which take readings at more than one point in the duct.
Please let us know if you want us to consider a device that is currently not on in the dropdown list.
The single-port and averaging pitot tubes in the tool list are:
Dwyer single-port
160-8 ~ 5/16″ dia. (7.93 mm), 8-5/8″ (21.9 cm)*
160-12 ~ 5/16″ dia. (7.93 mm), 12-5/8″ (32.067 cm)*
160-18 ~ 5/16″ dia. (7.93 mm), 18-5/8″ (47.3 cm)*
160F ~19-9/32” (48.956 cm)
166-6 ~ 1/8″ dia. (3.175 mm), 6″ (15.24 cm)*, 3″ (7.62 cm) tip
166-12 ~ 1/8″ dia. (3.175 mm), 12″ (30.48 cm)*, 3″ (7.62 cm) tip
167-6 ~ 1/8″ dia. (3.175 mm), 6″ (15.24 cm)*, 1-1/2″ (3.81 cm) tip
167-12 ~ 1/8″ dia. (3.175 mm), 12″ (30.48 cm)*, 1-1/2″ (3.81 cm) tip
Dwyer averaging+
PAFS-1002 ~ 3-5/32″ (8.02 cm)
PAFS-1003 ~ 5-13/32″ (13.73 cm)
PAFS-1004 ~ 7-21/32″ (19.55 cm)
PAFS-1005 ~ 9-29/32″ (25.26 cm)
Kele averaging
FXP-4 ~ 4″ (10.16 cm)
FXP-5 ~ 5″ (12.7 cm)
FXP-6 ~ 6″ (15.24 cm)
FXP-7 ~ 7″ (17.78 cm)
FXP-8 ~ 8″ (20.32 cm)
FXP-10 ~ 10″ (25.4 cm)
FXP-12 ~ 12″ (30.48 cm)
KMC Controls averaging
SSS-1002 ~ 3-5/32″ (8.02 cm)
SSS-1003 ~ 5-13/32″ (13.73 cm)
SSS-1004 ~ 7-21/32″ (19.55 cm)
SSS-1005 ~ 9-29/32″ (25.26 cm)
SSS-1012 ~ 3-5/32″ (8.02 cm)
SSS-1013 ~ 5-13/32″ (13.73 cm)
SSS-1014 ~ 7-21/32″ (19.55 cm)
SSS-1015 ~ 9-29/32″ (25.26 cm)
* indicates insertion length
+ manufactured by KMC Controls
Disclaimer
We have done hours of research attempting to find reliable data and equations for the single-port and averaging pitot tubes included in the dropdown list for the Pro tool. There are a number of devices we rejected for our list because of lack of verifiable manufacturer test data. We have included links to the manufacturers airflow curves and equations in the list above. We cannot confirm or guarantee that this tool will give accurate results.