I experimented with several HC-SR04 using a very simple scetch with Arduino 1.8.5, reading the measurements on the serial monitor. For a static setup I fastened the HC-SR04 in a vice and had it point against a white wall, inside a dust free room. No matter which distance between the wall and the sensor adjusted, the readings kept on being more or less constant BUT all of a sudden for a few readings the values decreased to almost zero and then continued with the expected measurement.
Does anybody have an explaination for this phenomena?
Thanks for your assistance in advance.