A photo tachometer is a thing that I don't truly, absolutely
need, but it's something I've wanted to have for a long time. Sometimes, I'd just like to know the actual speed of a piece of rotating workshop machinery for no other reason than 'just because'. So, when I found this item on Amazon for a very reasonable price ($14.95 CDN + shipping), I couldn't resist. I ordered one up and it arrived yesterday, one day earlier than Amazon said it would. Here's a view of the thing.
It comes without a battery. A single 9V rectangular battery is all that's needed. The strips down in front are adhesive reflective tape strips. There's about a 24-inch supply there.
I read the little manual, stuck a piece of tape on the spindle of my wood lathe and tried the device out. It works as advertised; I'm delighted with it.
Setting Up For A Measurement
Any rotating device to be tested must have a target adhered to it that the instrument can get a light reflection from -- that's where the reflective tape comes in. Stick a bit of the tape onto your rotating shaft or whatever, like so.
The pressure-sensitive tape adheres quite well to clean, smooth surfaces.
Set the shaft to rotating, aim the tachometer at the target and press and hold the TEST button and you'll begin to get your reading, like so.
A highly visible red light beam floods the target area, making it very easy to see that you're holding the tachometer correctly aimed. When the instrument is getting a signal, there's a 'MONITOR INDICATOR' symbol that comes on in the display directly above the least significant digit. (It looks like three sets of parentheses around a dot.) If you're not seeing the MONITOR INDICATOR in the display, you're not getting a reading.
And that's about it. As long as you keep the TEST button depressed, and you have a MONITOR INDICATOR in the display, you'll continue to get a live reading of the target shaft's rotational speed.
Following a reading, the MEM button lets you retrieve the lowest, highest and last speeds that the instrument observed. I'm not sure how useful that information is, but it's there for whatever it's worth.
An Alternative To The Reflective Tape
Don't worry about running out of tape; Liquid Paper or Wite-Out work fine. This splotch of Wite-Out on a table saw motor's shaft yielded an acceptable target for the tachometer to read from.
If you get an erratic reading, or a reading that's not credible, wrap the shaft to be tested with black friction tape and apply a Wite-Out splotch to the tape, like so.
Black electrical tape is too glossy to work reliably.
An Operational Test Method
There's a quick-and-dirty way to check the tachometer's functionality that doesn't involve any machinery. Just aim the unit at a nearby conventionally-ballasted fluorescent light, and you should get a reading of 7,200 rpm, or very close to it.
Our eyes' persistence-of-vision doesn't allow us to see it, but fluorescent lights actually extinguish and relight 120 times per second. Multiply 120 times per second by 60 seconds per minute, and you get 7,200, so 7,200 rpm is what the tachometer 'observes' when it's given a fluorescent light's output for an input. Fluorescent lights with solid-state ballasts operate at a different rate entirely. I tried getting a reading off of one, and obtained nothing sensible.
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Further To Wite-Out Splotch Markers --
SUNDAY, May 28, 2017
I tried the Wite-Out splotch method on a lawnmower engine's flywheel and it worked fine. Here's a view of the splotch peeking out from under the engine's shroud.
The contrast between the white splotch and the metallic substrate is sufficient for the instrument to obtain a reading.
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