Review on AGPtek® Professional Digital Tachometer Contact by Todd Mohr

The first one I got couldn't measure RPM. I pressed the test button and saw a very faint red laser dot on the target and the display read 0 RPM with the low battery icon, but the display was so dim that it had to be viewed from an extreme angle or with a flashlight . shine on it. Pressing the Mem button returns the display to normal brightness. I tried 3 different batteries up to 9.67v and also tried a 9.37v power supply. I even switched the power supply to 12V for a few seconds but it made no difference (probably unsure even for a few seconds but curiosity got the better of me). Several other one-star reviews have reported similar issues. They sent me a replacement free of charge before I returned the broken one and it arrived two days later and works perfectly. I used it to measure AC fan speed from 1130 to 1132 RPM and the reading was the same when I checked again the next day. The fan motor is said to run at 1075 RPM, so the tach can read fast, but that's enough to prove the motor isn't failing, and you can't expect high accuracy for $15. Be careful when installing the 9 volt battery as it can be pushed in with the wrong polarity far enough to touch the terminals. I did this on a broken device and I'm wondering if that's why it failed. There must be protection circuitry, but this circuitry can be of poor quality and fail. Again, no other reviewer I could find reported it working until they put the battery in the wrong way, so it's probably unrelated. Of course, if you tend to put the batteries in the wrong way around, it's a good idea to do so on purpose to test the protection circuitry while you're still in the return window. The case is comfortable to the touch and has a pocket. for the operating instructions, as well as a holder for a piece of reflective foil. The only thing missing is a pair of scissors. Other reviewers say that the tachometer burns batteries even when not in use, so I suggest leaving it without a battery.