Support Hotline (781) 890-7440
Tech support: Ext. 200; Email: support@extech.com
Repair/Returns: Ext. 210; Email: repair@extech.com
Website: www.extech.com
CFM Measurements
Measure the area of the duct using the diagrams below for rectangular and circular ducts
(If the duct measurements are made in inches, divide the inches by 144 to get the area in
square feet). Plug the area value (in square feet) in the cubic equations below. Note that
the air velocity must be plugged into the cubic equations also.
Height r
Width
Cubic Equations: CFM (ft
3
/min) = Air Velocity (ft/min) x Area (ft
2
)
CMM (m
3
/min) = Air Velocity (m/sec) x Area (m
2
) x 60
Maintenance
Battery Replacement
The 45158 has a low battery indicator (battery symbol). Important: Turn the meter off
before opening the battery compartment. Using a coin, turn the battery compartment cover
CLOCKWISE to remove it. Once opened, observe the position of the battery, placing the
new one in the same position. Secure the battery compartment cover and dispose of the
lithium battery in accordance with local, state, or national disposal codes.
Impeller Replacement
• Remove the set screw next to the impeller assembly (on the rear of the meter).
• Twist the impeller assembly counter-clockwise to the OPEN position and remove it.
• Install impeller by inserting & twisting the new impeller assembly clockwise
• Tighten the set screw.
Calibration and Repair Services
Extech offers complete repair and calibration services for all of the products we sell. For
periodic calibration, NIST certification or repair of any Extech product, call customer
service for details on services available. Extech recommends that calibration be performed
on an annual basis to insure calibration integrity.
For rectangular
ducts, multiply the
Height x Width
For circular ducts measure
the radius (r), square the
radius and multiply the
radius by
π (3.14)
A =
πr
2
(or A = 3.14 x r x r)
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