Extech 461891 Contact Tachometer High Precision.

Contact Tachometer
Surface wheels measure contact surface speeds (10 to 9,999 rpm) of rotating objects. The memory button holds last reading and recalls min/max readings display. Complete with wheels for linear surface speed (0.2 to 6560 feet/minute; 0.05 to 1999.9 meters/minute) or RPM measurements.

Applications
Motor speed, moving fans, pumps, and gears, propeller devices, and many other general maintenance, production, and automotive uses.

What is a Tachometer?
A tachometer is a device that measures the angular speed of a rotating shaft in terms of revolutions per minute (RPM). Also, a tachometer can measure linear speed in terms of feet per minute (feet/minute) or meters per minute.

How do Contact Tachometers Operate?
The sensor tip is placed in direct contact with the rotating object. After 2 seconds an accurate reading is displayed. Linear surface speed can also be measured by placing the edge of the wheel on the moving object.

How do Photo (Non-contact) Tachometers Operate?
First, a small piece of reflective tape (approximately 1/4-inch) is mounted on the object to be measured. Second, the light beam or laser is pointed at the revolving tape. The reflection from the tape provides a reading to be displayed and updated once per second. For the most accurate measurements, the distance between the tachometer and the reflective tape should be within 6 inches for a light beam and up to 6.5 feet for a laser. Fluorescent or bright ambient lights may interfere with the measurement.

Which Extech Tachometer Model Is the One for You?
A non-contact (photo) tachometer was designed for applications where it is not feasible to have the tachometer come in contact with the rotating device. This is the case in high-speed applications and/or where the object to be measured is not easily reached. A contact tachometer is also available for applications where direct contact of the tachometer with the object is preferred. You can also select a combination (contact/non-contact) tachometer for the maximum in flexibility.