Pressing the Treysit sirometer against the case of a lawn mower motor will tell you its RPM and frequency.
The Treysit sirometer is a device that measures the speed of a gasoline-powered or electric motor in revolutions per minute, as well as its frequency in cycles. It features a sturdy Bakelite plastic cap that holds two simple analog meters, one for RPM and one for frequency. You can use this device to check the efficiency of riding or push lawn mower motors, power tool motors, boating motors and even motorcycle engines by pressing it firmly against the exterior case or housing of a running motor.
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Place the sirometer firmly against the case of any running motor.
Turn the inner dial of the sirometer to the left with one hand while maintaining pressure on the device with your other hand until an inch of wire protrudes from the indented slot that is located to the left of the dial.
Watch to see when the protruding wire vibrates cleanly and evenly, in resonance with the motor.
Read the top gauge and note the RPM reading. Read the bottom gauge and note the frequency reading.
Practice using the Treysit sirometer by measuring an engine for which you already know the correct RPM reading, such as the motor of a new power tool that has rating information on its housing or owner's manual. You will be able to see how to determine that the wire is vibrating at the correct degree of resonance by checking to see when the top meter matches the number of RPM listed on your motor.
Lubricate the sirometer if it becomes difficult to release the wire by pulling out the entire wire from the reel around the dial and putting a few drops of machine oil inside the hole in the dial from which the wire is released.