Ihave noticed mine is unreliable when it is windy.

i use flat black on my test props or a sharpie on the smaller one's. then have a large white poster board in front.
this in bright light give the best reading when testing engines.

<span style="font-family: Arial"><span style="font-size: larger">Saito 91
15% nitro
Fox RC long plug

Master Airscrew K series old style 14x6 peak RPM 9720
Master Airscrew K series new style 15x6 peak RPM 9300
Evolution15x6peak RPM 8300

Surprising that the drop in RPM from MA14x6 to MA15x6 was only about 400 RPM. A testament to the efficiency of the design. Also nice that the MA15x6 allows the engine to rev in its optimal power band between 8500 and 9500 RPM.</span></span>

OS 91 Surpass I
5% nitro / 15% synth ML70
OS #8
50' alt

APC 14x6 9600