Random values, unusable equipment

In my family there are three devices from different manufacturers that have proved to be relatively unproblematic. The range of variation is the systolic value at + -6 and diastolic at + -5 if you comply with all the requirements: relaxed, always the same seat and in particular arm position, pause between measurements, etc.
In comparison, this device provides only random values ​​and has apparently been difficult at all to find the pulse.
From 62 to 59-155 to 82 when blood pressure everything was there (20 measurements).
Conclusion: Totally Useless