Use an optical tachometer when a safe line of sight is available and a contact tachometer when a permitted mechanical coupling is the better measurement method. Both can report rotational speed, but their error sources and physical risks differ. The choice begins with guarding, access, surface condition and the RPM range.
The target surface determines the safer tachometer method
Optical tachometers require a detectable mark and a clear line of sight; contact tachometers require a suitable contact interface and create an additional mechanical hazard. Do not use reflective tape on equipment where loose material could enter a process. Qualified personnel should follow the machine guarding procedure.
How the two methods work
Optical tachometers detect repeated reflection or interruptions, often from a reference mark. Contact tachometers mechanically follow a rotating element through an appropriate attachment. An optical method may give an incorrect pulse count if several marks, reflections or background objects produce extra signals. A contact method can slip and alter the reading if attachment pressure or alignment is poor.
| Method | Main advantage | Main limitation |
|---|---|---|
| Optical | No mechanical connection to the shaft | Requires stable view and correct pulse interpretation |
| Contact | Can measure a specified accessible rotating surface | Requires approved, guarded access and suitable contact |
| Stroboscopic | Visualizes periodic motion without contact | Harmonic/aliasing ambiguity can mimic another speed |
How to select a method
- Confirm which rotating component must be measured and whether the plant allows access while it operates.
- Identify expected speed and select an instrument rated for the task.
- For optical measurements, check reference-mark quantity, reflectivity and possible background reflections.
- For contact measurements, follow the manufacturer-approved attachment and guarding procedure; never improvise a shaft connection.
- Take repeated readings at stable load, recording instrument mode and RPM units.
- Where a speed drives a maintenance or safety decision, compare against controller feedback or another verified technique.
Worked example: multiple targets
Hypothetical: a sensor counts 3,600 pulses per minute on a shaft with two equally spaced reference marks. The shaft speed would be 3,600 ÷ 2 = 1,800 RPM, assuming every mark produces exactly one pulse. Treating each pulse as one revolution would overstate speed by a factor of two. For another non-contact RPM method, see stroboscope RPM measurement guide.
Testo model for non-contact visual checks
The testo 477 LED stroboscope offers non-contact periodic-motion observation and supports external triggering. It is useful when optical access exists but attaching a sensor is undesirable. It does not make stroboscopic flashes an automatic, unambiguous tachometer signal; verify harmonics before reporting RPM.
Safety comes first
Do not remove guards, approach hazardous rotating elements or perform contact testing without the site’s qualified safety procedure. If no safe test location exists, use fixed machine instrumentation or arrange a controlled shutdown.