not to derail the discussion again. But, if you're just looking for a good, cheap tach a few of us at the field downloaded the $3 'Head Speed Tachometer' by id-mobile for android from the google store.
We tested it on two phones side by side with a western robotics chinook optical tach.
none of the three devices were more than 10-20 RPM off from each other.
the nice thing about the phone tach is it logs the HS over time. So, I can put the phone on the ground, hover, change HS by IU switch, load the head, etc. Then land and look at the logs to see what the headspeeds were. Or, if I'm just looking for a quick baseline, or trying to set the curve on my idleups, i can spool up at 0Â° with the phone on the ground and simply watch the HS change on the phone as I change my curve settings. No need for it to be a two man operation like with the optical tach.
if it ain't broke, break it.