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Does anyone knows how to calibrate AJ-SR04M using the Arduino? I am working on AUV. underwater reading from current code; it is giving continuous 20cm while moving inches. http://shop2.hanjindata1.cafe24.com/product/aj-sr04m-ultrasonic-ranging-module/206/

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    what is the speed of sound in air? ...... what is the speed of sound in water?
    – jsotola
    Mar 5, 2019 at 18:28
  • have you tested the sensor in air?
    – jsotola
    Mar 5, 2019 at 18:28
  • Tested on both. Air was giving me correct results as by using scale but underwater around20cm only despite displacement. Mar 5, 2019 at 19:10
  • the short answer is, that the minimum measurable distance in water with this sensor is 1 m as sempaiscuba explains in his answer
    – Juraj
    Mar 5, 2019 at 19:39
  • this sensor do not work underwater is only waterproof.
    – catalin
    Dec 9, 2020 at 22:13

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If I understand your question correctly, your problem is probably that the working range of the AJ-SR04M sensor is 20cm - 500cm.

The sensor uses the same transducer to create the sound pulse and to listen for the echo. It needs time to switch from one mode to the other, which explains why your sensor can't measure distances of less than 20cm.


The speed-of-sound in water is about 5x the speed of sound in air. More specifically, at 20 degrees Celsius the speed-of-sound in air is approximately 343 meters per second, while the speed-of-sound in water is approximately 1482 meters per second.

This will cause the sensor to 'under-report' the distance in water. The minimum distance will also be approximately 5x the minimum distance in air.


As an aside, you will also need to make allowance for the differences in the characteristic impedance and absorption between sound and water.

It's worth reading the paper Underwater Sound by P. Vigoureux (Proceedings of the Royal Society of London. Series B, Biological Sciences, Vol. 152, No. 946) for a discussion of all the factors involved.

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  • and sound travels much faster in water
    – Juraj
    Mar 5, 2019 at 19:23
  • @Juraj Yes, I was trying to edit the answer to include accurate numbers & a reference. Not easy on a mobile phone, and even less easy now that the Android SE app is no longer working. Mar 5, 2019 at 19:29

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