How to measure RPM: tachometers, strobes and reflective tape

Safety first

Spinning machinery can catch and pull people in. The Health and Safety Executive (HSE) says never wear dangling chains, loose clothing, rings or loose long hair near moving parts, never remove guards, and switch off and isolate a machine before adjusting it. Fit the tape with the machine stopped and isolated, keep your hands clear while it runs, and never let children operate machines.

There is also an optical hazard. Wikipedia’s article on the wagon-wheel effect notes that under flickering light, a spinning lathe can look slow or stopped when it is not. Never judge whether something has stopped by how it looks under a strobe or flickering light.

How to measure RPM with an optical tachometer

An optical tachometer shines light at the shaft and counts the pulses reflected from one piece of tape. National Instruments’ guide to rotational speed describes this setup: reflective tape gives one pulse per revolution, it suits high speeds, and it is useful where you cannot drill or modify the shaft.

1. Stop and isolate the machine.
2. Fit one small square of reflective tape to the shaft, pulley or blade. Clean the surface first.
3. Restart the machine and aim the tachometer at the tape from the distance its manual gives.
4. Hold steady until the reading settles.

NI also notes that for low speeds, under about 100 RPM, an encoder that gives many pulses per turn is usually the better tool, because one pulse per turn arrives too slowly.

How to measure RPM with a strobe

Monarch Instrument, which makes stroboscopes, explains the method. A strobe flashes at a set rate, and when the flash rate matches the speed, the mark appears frozen in one place.

The catch is that the mark also looks frozen at other speeds. If the strobe flashes at twice the real speed, you see two marks. At half or a third of the real speed, you see a single mark again, because the part turns two or three times between flashes. Monarch’s advice:

• Start at the highest flash rate and work down.
• The first single, still image you reach is the true speed.
• Check by halving the flash rate. You should still see one single image.

Monarch also says an optical tachometer is easiest when you can stop the machine to fit tape, and a strobe is the answer when you cannot shut it down. Strobes become very hard to use at very slow speeds.

Why cameras get spinning things wrong

Any camera takes separate frames, so it can fall for the same trap as a strobe. Wikipedia describes the wagon-wheel effect as temporal aliasing: when the rate at which spokes or blades pass is close to the frame rate, or a multiple of it, they can look slow, stopped or even backwards. That is why a fan on video can seem to crawl.

Where TachCraft helps, and where it doesn’t

TachCraft: Optical Tachometer is a Zarkon app on Google Play. It uses the phone camera as an optical tachometer: you tape a mark on the part, point the camera at it and read the RPM. On phones with manual camera control, it staggers its frame timing to avoid the wagon-wheel trap where a fast part is mistaken for a slow one, and it shows a plus or minus figure and a “sure” bar so you can see how settled the reading is. It also has a torch strobe, but phone torches top out at about 25 flashes a second, so faster parts only freeze every few turns. The app checks how steady the torch is and shows a flashing light warning.

It works best for hobby checks: a workshop fan, a drill, a model motor or a lathe spindle in your own shed. It is not a calibrated instrument. Phones and cameras differ, and lighting, distance and vibration all affect the reading. For anything where the number matters, such as setting machinery to a specification, safety cut outs, or work you will sign off, use a calibrated tachometer or stroboscope. For a car or motorbike engine, use the vehicle’s rev counter or ask a mechanic with the right diagnostic tool. Don’t lean into an engine bay with a phone while the engine is running.

Questions people ask

Sources

1. Using A Stroboscope To Measure RPMMonarch Instrument · monarchinstrument.com
2. Rotational SpeedNational Instruments · ni.com
3. Wagon-wheel effectWikipedia · en.wikipedia.org
4. Introduction to machinery safetyHealth and Safety Executive · hse.gov.uk