Useful Mobile Apps

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Video tachometer

When the frame rate of a video matches the speed of a particular part, the effect is the same as using a strobe light. In the video, that part freezes.


Since the frame rate of a video can be adjusted, this effect can be used to determine the rotational speed of a part.

The smartphone must support slow-motion video recording (filming at a high frame rate). This is the case for all smartphones starting with iPhone 5S. The range is as follows:

• 6 - 240 Hz (360 - 14400 RPM), for all phones from iPhone 6 onward

• 6 - 120 Hz (360 - 7200 RPM), for iPhone 5S

To get a clear image, good lighting is important. The clarity of the image is controlled by the exposure of the video (exposure time and light sensitivity (ISO)). If the exposure time is reduced, the image will increase in clarity, but brightness will lower. To increase the brightness, you need to raise the camera's light sensitivity (ISO), but then the amount of noise increases. You also need to manually adjust the focus of the camera.

Videos can be captured using the iOS "Screen Recording" function.

It is possible to choose a video format that has the best quality, but has a significantly lower maximum frame rate. The difference between the formats is clearly visible only when using the digital zoom. Also, when using a format with better quality, the Rolling Shutter effect is stronger. Therefore, the best quality format can be used at speeds less than about 1000 rpm.

Rolling Shutter effect.

You can turn on hints for easier observation of changes in rotation speed. Hints show how to change the frame rate (decrease or increase) depending on the direction of rotation of the mark. The hints are selected manually, but then you do not need to remember this information. The hints are relevant for frequencies close to the rotation frequency. If the direction of rotation of the impeller is known, hints can be selected in advance. The arrow showing the direction of rotation of the impeller must correspond to the "+" button.

The video tachometer measures the rotational speed in exactly the same way a stroboscopic tachometer would.

Measurement of rotational speed by video tachometer

To determine the rotational speed (e.g., to determine the fan rotation speed), a plane perpendicular to the axis of rotation should be visible, and only one mark should be on this plane. For example:


In the app settings, you can adjust the speed at which the frame rate changes when you press and hold the button.

The observed pattern will change. For example, if the actual speed of the fan is 2660 RPM, and you began measuring the frame frequency at 11000 RPM, then as you reduce the frame frequency, the following still images will appear:

Four or three marks can be seen at other frame frequency; for example, three fixed marks can be seen at a frame frequency of 3990 rpm (real RPMs multiplied by 1.5), four marks at a frame frequency of 3546 rpm (real RPMs multiplied by 1.333.) and five marks at a frame frequency of 3325 rpm (real RPMs multiplied by 1.25).

The wrong conclusion can be made when defining the RPMs by increasing the occurrence of the frame frequency. For example, at a frame frequency of 887 rpm (real RPMs divided by 3), one mark is visible.

This frequency can be misleading, since after multiplication or division of this reading by 2, one may obtain the same pattern as what is observed at the correct frequency (in this case, at 2660 RPM). THEREFORE, YOU SHOULD NOT START WITH A SMALL FRAME FREQUENCY AND THEN INCREASE IT.

Calculation of the rotation speed when the standard method has caused difficulties

When the approximate rotation speed is not known and the standard method has caused difficulties, the rotation speed can be calculated using a simple formula. Let us take an example where the actual rotational speed is 6100 RPM. When you decrease the frame frequency from 2000 RPM, you should determine the frequency at which it is possible to observe a single fixed mark for the first time:

This occurs at a frequency of 1525 RPM (F1). With a further reduction of the frequency, the reading at which one fixed mark can be observed will be at 1220 RPM (F2). And the next time it happens, it will be at 1017 RPM (F3). A calculation is then performed by using two adjacent measurements:

RPM = (F1 • F2) / (F1 – F2)
1525 • 1220 / (1525 – 1220) =
= 6100 RPM


RPM = (F2 • F3) / (F2 – F3)
1220 • 1017 / (1220 – 1017) =
= 6112 RPM

Privacy policy

1. To display the strobe effect on the smartphone screen, the application needs access to the camera. Data received from the camera is not transmitted anywhere.

2. The app DOES NOT collect any data at all, and especially confidential ones. The app doesn't send messages even if the app crashes. Therefore, if you do not inform me about frequent app crashes by email, I won't know about it and I won't fix the bug. The app may only need an Internet connection when making in-app purchases to unlock locked features.