Optical and Photo Tachometers

Optical and Photo Tachometers are handheld, portable devices used to measure the rotational speed of various objects. These types of tachometers use a non-contact method to convert rotational motion into electric pulses that can be accurately measured. Many tachometers can also be used to measure the total number of optical pulses and be used as a timer.

Optical/Photo Tachometers are widely used in many industries to measure the rotational speed of shafts, motors, gears, conveyor belts, or other types of rotating systems. They are often used to monitor machine performance, conduct preventive maintenance inspections and maintain quality control of rotating equipment processes.


Operating Principle

Optical/Photo Tachometers operate by shining a light source, typically a LED light or Class 2 laser beam, against the rotating element. This light source creates a focused beam of light which will be reflected back off of a reflective object that is placed in its path. The optical sensor of the tachometer will be triggered as the light is reflected back towards the tachometer. By measuring the rate that the sensor receives the signal, a measurement of the rotational speed can be determined.


Optical/Photo Tachometer Advantages

Optical/Photo Tachometers have several advantages over contact tachometers that require physical contact with the rotating machinery. First, since the Optical/Photo Tachometers do not need to touch the rotating object, they will not induce a load on the shaft, which could cause measurement errors depending on the torque of the rotating system.

Most Optical/Photo Tachometers can measure the rotation of objects over 10 feet away and can be used in hard to reach locations where the rotating shaft is difficult to get to or where there are other industrial machines that could present a safety hazard while trying to physically measure the shaft. Optical/Photo Tachometers are also highly accurate, achieving accuracies of ± 0.01% of reading or better.


Use of Optical/Photo Tachometers

To use the Optical/Photo Tachometer, a reflective target must be placed on the rotating surface that is to be measured. Usually, that target is a small piece of reflective tape or drop of reflective paint. Multiple pieces of tape can be added to the shaft to assist in the measurement of slower speeds. You will just need to divide the tachometer speed displayed by the number of pieces of tape to achieve the correct rotational speed. For accurate measurements, the remaining surface should be non-reflective. A shiny surface could result in reflection errors.

Most optical tachometers will contain a supply of special reflective tape in their carrying case. This tape allows the light source to be reflected back consistently at any angle so that accurate measurements can be made while measuring the target at varying angles. The tachometer should only be used with the type of reflective tape provided with the device as the tape is designed specifically for the type of light source of the tachometer.

The light source should be aimed directly and steadily at the reflective tape. It is normal for there to be slight fluctuations in the readings due to variations of the aiming of the light source. It is usually not difficult to determine when the reading is ‘locked in’ to the correct reading. The distance from the tachometer to the reflective tape may also need to be varied slightly to obtain a stable reading.


RPM Measurements

The most common rotational speed measurement made with tachometers is Revolutions per Minute (RPM). RPM is the number of full rotations that the shaft will complete in one minute. RPM is also a measure of the frequency of the rotation. Since the frequency is a measure of cycles per second, the relationship between frequency and RPM can be calculated by using the equation: RPM = Frequency x 60. Conversely, the frequency can be determined by dividing the RPM value by 60.


Optical/Photo Tachometer Calibration

This relationship between frequency and RPM is used in the calibration of these types of tachometers. The calibration of Optical/Photo Tachometers is usually performed by using a LED circuit that is powered by a Function Generator that can generate a highly accurate frequency signal. The tachometer’s optical sensor is pointed at the LED and will count the pulses from the LED circuit and convert it into the appropriate measurement units. By using this system, calibration accuracies of less than ±10 ppm can be obtained.

can accurately calibrate all types of Optical/Photo Tachometers in our ISO/IEC 17025 accredited calibration laboratory. For more information on the calibration of Optical/Photo Tachometers, contact e2b calibration.

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