Tachometers are instruments used to measure the rotational speed or RPM (revolutions per minute) of a rotating object, such as an engine, motor, or other machinery. They are widely used in automotive, aviation, marine, and industrial applications. Tachometers can be categorized into two main types: contact and non-contact tachometers.

1. Contact Tachometers:

   • Mechanical Tachometers: These tachometers use a mechanical connection, such as a cable or a magnetic pickup, to measure the rotational speed. They typically have a physical pointer or dial that indicates the RPM on a calibrated scale. Mechanical tachometers are often used in older vehicles or machinery.
   • Photoelectric Tachometers: These tachometers use a mechanical connection or reflective tape attached to the rotating object, and a photoelectric sensor detects the reflected light to measure the RPM. They provide accurate measurements and are commonly used in automotive and industrial applications.

2. Non-Contact Tachometers:

   • Optical Tachometers: These tachometers use optical sensors, such as a laser or infrared sensor, to measure the RPM without physical contact. They are aimed at a reflective surface on the rotating object, and the sensor detects the reflected light to determine the RPM. Optical tachometers are non-intrusive and suitable for measuring RPM from a distance.
   • Magnetic Tachometers: These tachometers use a magnetic sensor to detect the magnetic field generated by a rotating magnet or ferrous target on the object. They are often used in applications where a magnetic signal is readily available, such as inductive pickups in engines.

Key features and considerations when selecting a tachometer include:

1. Measurement Range: Tachometers have a specified RPM range that determines the minimum and maximum speeds they can accurately measure. Choose a tachometer with a range suitable for your specific application.

2. Display and Readability: Tachometers typically have a digital or analog display that shows the measured RPM. Consider the size, clarity, and readability of the display, especially in different lighting conditions or environments.

3. Accuracy and Resolution: The accuracy of a tachometer refers to how closely it measures the actual RPM. Higher accuracy ensures more precise readings. Resolution refers to the smallest increment at which the tachometer can display RPM. Look for tachometers with sufficient accuracy and resolution for your needs.

4. Additional Features: Some tachometers may offer additional features such as memory storage for multiple measurements, data logging, peak hold function, adjustable update rate, and compatibility with external accessories or data transfer.

5. Power Source: Tachometers can be battery-powered or powered by an external source. Consider the power source options that suit your application and ensure sufficient battery life or power supply availability.

6. Durability and Construction: Consider the durability and ruggedness of the tachometer, especially if it will be used in demanding or harsh environments. Look for tachometers with sturdy construction, protective features, and resistance to dust, water, or other contaminants.

When using a tachometer, follow the manufacturer's instructions for proper installation, calibration, and usage. Regular calibration and maintenance are recommended to ensure accurate and reliable RPM measurements.