Thread: Photo tachometer

Looks pretty much the same as the one I have.

I bought mine nearly 10 years ago and only changed the original (panasonic) batteries it came with last month! I have used it a lot on my electric motor HP rig where it sits for hours with a piece of tape over the on button. That's the one thing that irks me about mine is I can't lock the ON button so either have to hold it down or use a piece of tape.

I also tested mine out against a couple from work and I found it agreed to the same digit up to about 1000 rpm and above that it was always within about 5 rpm up to about 10k rpm which was as far as I tested it..

One thing I found it could not measure and that was the speed of the micro fan in the PMS particle counter, I ended up doing that with an Arduino tacho/line marker connected to an oscilloscope because the signal was not strong enough to trigger a standard digital Arduino port. I could have got it to trigger but I would have had to fiddle a bit with the software and a bit of hardware.

I originally used it to to measure the RPM of chainsaws but it's a bit of a PITA for that so I bought a dedicated small engine SenDec tacho and mounted it permanently on the chainsaw wrap handle like this.
The blue gismo is RCV temp gauge.
Tempmon1.jpg

Bob, was that so you didn't over rev the motor or to know what rev range it cut best at? Is it the same with the temp gauge?
Thanks
Kryn

Originally Posted by KBs PensNmore

Bob, was that so you didn't over rev the motor or to know what rev range it cut best at? Is it the same with the temp gauge?
Thanks
Kryn

It was more because because of interest than anything else. The main thing is tuning. It's a milling saw so the engine is under huge loads at all times. The recommendation is the saw is run slightly rich (slightly reduced max RPM) so it needs a regular tuning check. Anyone that has tried holding a big chainsaw running at 12K rpm and a tacho and a screwdriver will appreciate its just easier to fix the tach to the saw and have one hand on the trigger and the other turning the screwdriver. My saws are usually fixed to the mill which when tuning and sharpening sit on a purpose built table with recesses in the top for the mill so it can sit snug in the recesses while being tuned.

It was also brand new milling saw with a muffler modification that I was not familiar with. It's very loud, so high end muffs are essential, but after a while you get a feel and an understanding of the sound when its working right and there's not much need to watch the tacho. The tacho helped get me up to speed with these things quicker than normal. The temp gauge is a just cos thing but I have learned a lot from it. For example when finishing a long milling cut most folks will tell you to let the saw idle and cool down for 20-30 secs before shut off but that does very little to cool the saw, instead what is needed is 2-3 minutes to really bring the temperature down

Lappa, what is the reflective tape used for that comes with the unit?.
Looks like it could be a handy device.

Originally Posted by artful bodger

Lappa, what is the reflective tape used for that comes with the unit?.
Looks like it could be a handy device.

Sorry to butt in here but you'll see why below.
The tape is for sticking to the rotating object to act as a reflector so the laser is bounced back to the unit to measure the revs.

I've had this unit for nearly 10 years so have nearly run out of the reflecting tape but meanwhile have discovered its not really necessary and sometimes does not work all that well e.g. smooth metallic surfaces like a new chuck.

Here are a few other solutions and tips that I have found that work - the reason they work is because the unit work on a change in reflectivity.

On rotating objects that have a dark or matt finish white electricians tape works pretty well. I've also use good old-fashioned typists "white out" and if its a very dark object that masking taped will do.

On really shiny objects a black mark sometimes works better. Ive also used black texta and black electrical tape with some success. Texta needs to be good and thick.

Sometimes the laser will reflect off patches on a rotating object that you are not aware off and output multiples of the true RPM. To check this, apply one reflecting or absorbing surface and measure the RPM. Then add another and the RPMS should double. If there are more unknown rejecting surfaces then it will be less than double.

Thanks for that Bob. I thought it must have been for something like that.
About 30 years ago I watched a bloke tune my chainsaw with a little electronic hand held tachometer that he simply held on to the saw while he adjusted the screws. It seemed to me this particular tachometer was working off vibrations and not a laser bouncing back off a reflective strip.
Has anyone heard or seen of this type of unit and better still know where to get one?.

Originally Posted by artful bodger

Thanks for that Bob. I thought it must have been for something like that.
About 30 years ago I watched a bloke tune my chainsaw with a little electronic hand held tachometer that he simply held on to the saw while he adjusted the screws. It seemed to me this particular tachometer was working off vibrations and not a laser bouncing back off a reflective strip.
Has anyone heard or seen of this type of unit and better still know where to get one?.

Those tachos work by counting the electomagnetic pulses generated by the motors ignition system and is exactly like the one in my chainsaw photo above.

Some (like mine above) have an antenna that are placed near or loosely wrap around a spark plug. You can see the white with black stripe antenna wire in the photo towards the bottom. This means you can operate the tacho some distance from the plug.

Others have no antenna and just have to be held close to the plugs or saw (since the whole saw receives the pulses).
I found this out when I bought one of those and mounted it on my Stihl 441 as shown here - unfortunately it did not work reliably as it had to be closer to the saw,.
closeup.jpg

I got my chains tachos (I have 3) from Baileys in the US see
Husqvarna Chainsaws, Outdoor Power Equipment and Tree Care Supplies from Bailey's

You can also buy cheaper ones on eBay - do a search within eBay for "small motor tachometer -laser"
If you don't put "-laser" in the search , the response will be swamped by the laser tachos.

Apart from the antenna there's two things worth knowing

1) Max RPM range
Some don't go high enough in RPM measurement for a chainsaw - i.e. designed for lawnmowers etc

2) Response time.
Some output the RPMs much quicker than others and you don't want to be waiting around for many seconds for the tacho to update after changing a carby setting while the chainsaw is at full RPM.
The faster the response (<1 s) the more expensive they usually are.
The Sendec unit on the big saw above cost over $100. It only has a 1.5s response but the main reason I bought it was it was designed to be attached to a motor so the whole unit is potted in epoxy. This means the battery is non-replacable so the unit only lasts for around 5 years. If you permanent mount a tacho on a motor and that tacho is not designed to cope with engine vibrating it will quickly render the the tacho inoperable.
I found this out on a Motor Scooter website where scooter dudes were fitting cheap tachos to their scooters and they only lasted for a few months.

If I was to get another I would get a "Tiny Tacho" they are available on eBay for ~$50 US and while they are not as rugged as the Sendec they do have 0.5s response time. The other one worth considering is the Sendec Max Tach but it costs a lot more.

These tachos are darned useful mounted on a saw for telling you if you have spark as they will show a signal when starting is there is a spark, so will also remind you if you have left the kill switch on!

Great!
Thanks for the informative reply Bob.
Not really looking at mounting it to any motor, just after something that I can see rev differences with rather than trying to hear with "old woodworkers ears" when adjusting the carbs on my old Moto Guzzi.
Actually imagine it would be a handy little unit for a number of reasons.

Originally Posted by artful bodger

Great!
Thanks for the informative reply Bob.
Not really looking at mounting it to any motor, just after something that I can see rev differences with rather than trying to hear with "old woodworkers ears" when adjusting the carbs on my old Moto Guzzi.
Actually imagine it would be a handy little unit for a number of reasons.

That's another thing to watch out for. Make sure they can handle multicyclinder engines - they don't all do that.