Distance Sensor HC-SR04: Incorrect measurement

[sunnyboy69]

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.

[bjacobse]

I have had similar issue and to bypass this, I did 3 last measurements needed to be equal, before the result was accepted.
Off cause this requires that your moving target was not too quick, which mine wasn't

[zboblamont]

@sunnyboy69 Getting false readings can be anything as it depends if it is a serial read from the device or simple timing of the echo, unstable voltage, or even an object caught in the path of quite a wide cone, but without further details it is impossible to identify. I've been caught before with a serial reading device, took a while before I realised....
An extension of what @bjacobse suggested is to repeat readings until you get two identical consecutive results which are <> known limits and not cuckoo results..