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In a driving school, every learner-driver is told that the car's engine should preferably run within a certain rev range. 
So that the driver could see whether he or she is keeping within this range, there is also a display for the revs on the dashboard in addition to the speed display. This RPM display constantly shows the current reading from the vehicle's RPM meter. For the vehicle repair and maintenance purposes, a mobile vehicle independent tachometer is used and not the permanently installed one. A simple mobile tachometer for internal combustion engines can, for example, measure the speed of common internal combustion engines on the ignition cable. To do this, the measuring terminal is placed on the ignition cable and records the ignition pulses. From this, the device then determines the engine speed in revolutions per minute.
The tachometers are used not only on vehicles, but also on many machines and technical systems in order to obtain the indications of malfunctions or to optimize the speeds and increase efficiency. For measuring the speed of motors, gears, shafts, rollers, centrifuges, fans, pumps and turbines, there is a wide range of devices with different equipment. In the following text, we will briefly discuss the selection criteria and the most common measuring principles and equipment for a mobile tachometer.
Selection criteria for a tachometer
The measured variables are among the most important criteria when looking for a mobile tachometer. If only the rotational speed is to be measured, all the measuring principles can be used. However, if surface speeds or line lengths on rollers or conveyor belts are also to be measured directly with the tachometer, the tachometers with mechanical contact are primarily suitable. With them, the number of rotations of the measuring wheel can be output as speed, e.g. in meters per minute, and as length in meters via the measuring wheels with a defined circumference.
The measuring range required for the machines and systems to be tested must be taken into account. While a mechanical speed measuring device can usually only detect rotational movements up to about 20,000 revolutions per minute, the optical devices often have a measuring range of up to 100,000 revolutions per minute.
The accessibility of the components and the possible measuring distance must be taken into account when selecting the device. A mechanical tachometer with a contact sensor must be placed directly on the measuring point. In the devices that measure via optical reflection, the distances of 50 mm to 500 mm or even up to 1,500 mm are possible.
Combination devices allow the use of two measuring principles and thus a wider range of applications. There is a selection of devices with contact sensors and reflection measurement and some that allow both contact measurement and stroboscopic measurement.
Memory and data transfer functions are important in a tachometer if extreme values are to be detected or if the measurement data are to be evaluated later. Many tachometers store the minimum and maximum speed of a measurement series. Some have memory locations for the extreme values of several measurement series, and certain types of tachometers also calculate average values. If the tachometer has an RS232 interface, the measurement data can be transferred directly to a computer where it can be stored and further processed.
Additional measurement parameters such as surface temperature or vibration values make it easier to find the problem areas during troubleshooting. During maintenance and servicing rounds, it makes sense to use the tachometer that can measure not only rotation-dependent values, especially in the difficult-to-reach areas. Stroboscopes are generally suitable not only for measuring rotational speeds, but also for visual inspection of moving components and materials.
In the tachometer that records the rotational movement through mechanical contact, the sensor is also rotated. For the measurement, the appropriate measuring tip or a measuring wheel is first placed on the rotary sensor. Then the sensor is placed on the rotating component and rotated by it. In the case of shafts, discs or wheels, the sensor must be placed directly at the point around which they rotate in order to measure the speed. To measure the surface or line speed of belt drives and conveyor belts, the sensor with the measuring wheel is placed in the running direction of the driven belt. However, a prerequisite for the contact measurement is always the accessibility of the parts to be measured.
A speed measuring device that emits radiation and evaluates the radiation reflected by the 
object under examination only requires visual contact with the measuring point. In order to make a measurement, a part of the rotating component must reflect well. Special reflective tape or paint is often applied as close as possible to the outer diameter of the object. The non-reflective area of the component must be significantly larger than the reflective area. During the measuring process, this mark is aimed at and the tachometer uses a light-sensitive sensor to record the time intervals at which the emitted light is reflected back. Many devices allow optical control of the targeted point with a visible laser aiming point.
A stroboscope emits short flashes of light in rapid succession. The single flash lights up the illuminated objects only for a very short time. If many short flashes of light follow each other very quickly, then for the viewer it looks like a steady illumination. If the moving objects are viewed with these timed flashes of light, also the irregularities, that are not recognizable in the truly continuous illumination, are noticeable. The user of the stroboscope must therefore also use his visual ability and correctly interpret what he perceives visually. Such a stroboscope can also be used as a speed measuring device by adapting the frequency of the light flashes to the speed of the rotating component. Similar to the RPM meter with optical reflection, an optical mark is required for viewing. Where no such mark is present, e.g. in the form of a unique notch, groove or hole, a mark can be made with chalk or pen. This mark can also be used again for later measurements. If the frequency of the light flashes corresponds to the speed, the same image with the marking always appears in rapid succession and it appears as if the rotating object is standing still. If the speed meter has adjustable multipliers and phase shifts, it is easy to check whether the set speed is correct.
Modern stroboscopes are available with both xenon lights and LEDs. The units with LEDs require little maintenance and often offer additional adjustment options. Often, the light intensity can be lowered from 100 per cent to 30 per cent via a rotary control, so that you can adapt to the local lighting situation. Some units even allow separate adjustment of the flash duration in the range from one microsecond to 1,000 microseconds.
To ensure high measurement accuracy, almost all the tachometers can be calibrated. If an ISO calibration certificate is required, the tachometer is tested in the calibration laboratory under the specified conditions and an up-to-date certificate is issued based on the results. This applies both to new devices and to recalibrations, which are to be carried out annually or as specified in the user's operational ISO manual. If it is determined during a recalibration that the tachometer does not comply with the product-specific error limits, it can be checked whether an adjustment or repair is possible and reasonable.