Non contact digital tachometer problem

[Henry RancourtParticipant @henryrancourt22682]

I just got a non contact digital tachometer and tested it with a 900RPM ac motor with a 5/16 dia. spindle. I blackened the spindle with a magic marker and put a piece of the reflective tape, that came with the meter, on the spindle. I ran the motor at about 300 RPMs and got crazy results, just rounded, of 1000 to 3,000 RPMs. I thought maybe the black marker might a bit shiny and caused the bad results. So, I wrapped the spindle with black electrical tape…totally dull black, and got the same results. Has anyone tried this meter and what were the results?

[JasonBModerator @jasonb]

Not had problems with mine though tend to use it on larger diameters 25mm plus.

Will give it a go on something similar diameter but may not be until the weekend

[Wink HackmanParticipant @winkhackman25989]

Try a much thinner piece of the reflective tape, say 3mm width, stuck on the end of the shaft as a radial spoke. I’ve found that works reliably.

[duncan webster 1Participant @duncanwebster1]

I had a reflective sensor on a lathe spindle, cylinder with slots filled with blackboard paint as the target. Slightest film of oil stopped it working

[noel shelleyParticipant @noelshelley55608]

On small diameter targets strange results can be had due to multiple reflections. A small spot or stripe of white pain can work. Good luck. Noel.

[peak4Participant @peak4]

Try fitting a random pulley to test it with something of a larger diameter.

Bill

[Henry RancourtParticipant @henryrancourt22682]

I put a 1 1/4″ dia. chuck on the spindle and readings were still at least four times the spindle RPM. I even varied the sizes of the refective tape and no difference

[peak4Participant @peak4]

This is a photo off a random eBay ad.
It also shows m/min as well as rpm.
Is there some function, other than rpm, which is being measured and misleading you?

Bill

[JasonBModerator @jasonb]

Surface speed would give just over 3 times the rpm

[Michael GilliganParticipant @michaelgilligan61133]

FWIW

MichaelG.

 

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Edit: and something approximating a user manual [possibly from the ‘non-plus’ version:

https://www.circuitspecialists.com/content/105575/dt-2234c.pdf

[JasonB]

On JasonB Said:

Surface speed would give just over 3 times the rpm

That’s not metres per minute, but the last memory reading (triggered by the Mem button).

I’ve had one of these for a few years and found it mildly useful, but getting stable readings can be tricky. As others have said trying to get reading from small diameter shafts isn’t simple or reliable. As you would expect given the large ratio of reflective target to blank shaft. Putting on a larger diameter disc will help with that ratio and make readings more reliable.

[Michael GilliganParticipant @michaelgilligan61133]

Just to confirm Paul’s point

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The displayed abbreviation might be ambiguous to western eyes

MichaelG.

[paul1956Participant @paul1956]

My experience suggests that the ‘memory’ feature might not work very well. Although the instruction leaflet supplied is badly translated and difficult to make sense of.

[JasonBModerator @jasonb]

Here you go, no problem. 5/16″ drill shank, piece of tape about 4mm wide and it matches what teh lathe says give or take a rev or two.

[Henry RancourtParticipant @henryrancourt22682]

You got good results and didn’t even blacken the spindle. So I put a 3/16″ dia. dowel in my lathe chuck with a small piece of the reflective tape and with the lathe set at 2,400 the result was 2,600, but I had to rest the meter against the cross slide. The results varied holding it by hand. The problem measuring the motor maybe that the spindle has a flat and only about 1/4″, with no flat, sticks out of the housing face. So, even thought the spindle is blackened the edges of the flat edges may be causing the problem. So I wrapped the spindle with black tape….no edges, and the results are still way off for this 900 RPM motor.

[Grindstone CowboyParticipant @grindstonecowboy]

Has the alleged 900 rpm of the motor been reliably confirmed by any other method of measurement?

Rob

[HuubParticipant @huub]

I have such a meter and to get good and repeatable results the spot (mine has a spot, not a line) has to be smaller than the width of the reflective tape. Than it gives plausible results. I mainly use it to check once in a while the RPM of my dremel engraver, CNC router (ER11 collet) or the lathe spindle.

I have to remove the batteries after use otherwise they will be drained in 2 days.

[SillyOldDufferModerator @sillyoldduffer]

I approach this kind of problem by addressing the 3-Ms:

• Machine – faulty,  maladjusted, calibration, bits missing etc.   As these meters are cheap, maybe the laser or sensor is misaligned or dirty.  Likely to be confused by a flat battery.   Some way of confirming the machine is operating correctly is needed.
• Material – shiny, poor contrast, diameter too small,   not round.   The machine works by pulsing the laser and counting how many are reflected.  RPM is calculated from the ratio pulsesReceived : pulsesSent, so anything that disturbs pulsesReceived will mess the meter up.
• Man!  –  He’s the biggest problem in my workshop.  I fail to understand instructions, or don’t read them at all, tend to impatience, and have wobbly hands.   Wobble may explain why Jason’s tachometer works and Henry’s doesn’t.

So:

• Good batteries
• Double check the instructions, and be clear how the thing is supposed to operate.   Manuals are often poor.
• Make sure the machine is working correctly by aiming it at a much larger target.  I suggest a black cardboard disc, end-on, about 80mm diameter.  Pin or glue it to a length of dowel in the lathe.  Get up close.  A tachometer that can’t read reliably read a large target is faulty – send it back.
• Excessive reflections can be seen by measuring in the dark.  If they appear, improve the matt-black.  If the measurement improves, the problem may be poor contrast due to ambient light desensitising the sensor.  TV remotes are fitted with plastic filters to reduce this.  Probably unnecessary with a laser, but is anything missing?
• Calibration is tricky without a working tachometer, so good the lathe has one.  Don’t expect high accuracy, but they should be in the same ball-park.   Try different speeds: if the tachometer disagrees with the lathe, it’s probably faulty.
• On a smaller diameter,
  • experiment in the dark, and make sure the target it’s round.  A flat won’t return a clean echo.
  • Eliminate wobble with a support.  If a support fixes it, use one.   Wobble causes much less trouble on a large diameter target than a small one because the pulsesReceived : pulsesSent ratio is less disturbed.
  • Experiment to find the smallest diameter that can be read reliably.  If unreasonably big, the tachometer is probably faulty

Back to, the 900rpm motor, in days of yore, motor speed was established with a stroboscope.  Often just a neon lamp flashing at a black and white sectored disc .  A neon powered by the mains flashes at 50Hz, so confirming 900rpm needs sectors to match:

• 900rpm = 15rps
• 15 rps is 1/3rd 50Hz, so the number of sectors needed is a factor of 3.  I’d try 30.
• If the motor is spinning near 900rpm, the disc should appear stationary

Neon lamps are hard to come by today, and electronic stroboscopes are pricey, though ebay often has them secondhand.  £50?  Easy to make with an Arduino if you have the skills, otherwise read on.

Audio. A gear or make a stiff-ish disc about 80mm dia, 1mm thick plastic would do, or aluminium, and notch the diameter such that a playing card or similar placed on the spinning disc will vibrate.    At 900 rpm,  4 notches should buzz at 60Hz, 8 at 120Hz, 16 at 240Hz, 32 at 480Hz etc.    If you have a:

• musical ear, 240Hz is near a B, or the note can be compared high/low with a musical instrument.
• smart phone, download a spectrum analyser
• multimeter with frequency capability, measure directly.
• computer with microphone, download Audicity or oscilloscope

Fairly obviously, the best answer is a tachometer than works!  By the by, 900rpm suggests a 60Hz motor, if so running it on 50Hz will get less, about 750rpm if my maths is right.  Which it often isn’t.

Dave

[noel shelleyParticipant @noelshelley55608]

900rpm is that not a 6 pole motor at 50Hz ?  Noel.

[Henry RancourtParticipant @henryrancourt22682]

The motor tag shows 115V 60Hz 1.0A 900RPM and based on an online pole calculator (https://repaircompanion.com/industrial/motor-poles), where input is RPM and Hz, it has 8 poles.

[JuddyParticipant @juddy]

The tool is 4M Fishbone/Ishikawa, the 4th being method and often the root cause

[southernchapParticipant @southernchap]

IIRC, one important thing to consider is the ambient lighting.

Certain lighting can cause problems with these tachos.  I can’t remember which but it might be fluorescent lighting that causes issues.

Maybe try it with the lights off. 😉

[Author]

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