Can't gainsay the negative reviews, as mine worked as advertised. We monitor cooling fan RPM, with them. It works best when held about 6" to 8" from the reflector tape and at right angles to the tape. We noticed that bright, ambient light reflected from a shiny fan blade will false-trigger the tachometer. Turn the lights down. Otherwise, the tachometer updates every couple of seconds, or so. Very nice, reliable unit, when used as described.