Tachometers are widely used to verify the rotational speed of motors, shafts, fans, pumps, spindles, conveyors and other moving equipment. Although the measurement appears simple, significant error can be introduced by poor target preparation, incorrect alignment, contact slippage, wrong pulse settings, unstable machine speed or an unsuitable test procedure.
Reducing tachometer error requires controlling both the measuring instrument and the way the test is performed. Optical, laser and contact tachometers have different error sources, so the correct technique depends on the measurement method.
Understand Where Tachometer Errors Come From
Tachometer error can originate from several places:
- The tachometer itself
- The reflective target or sensed feature
- Incorrect pulses-per-revolution configuration
- Measurement distance or angle
- Contact slippage
- Worn measuring wheels or contact tips
- Machine-speed fluctuation
- Vibration
- Operator technique
- Low-speed sampling limitations
- Environmental conditions
The first step in improving measurement quality is identifying which of these factors applies to the tachometer technology being used.
Reducing Errors in Optical and Laser Tachometers
1. Clean the Target Area Before Applying Reflective Tape
Oil, grease, dust and loose contamination can prevent reflective tape from adhering correctly or can reduce the contrast between the target and surrounding surface.
OMEGA's tachometer guidance instructs users to clean grease and dirt from the shaft before applying reflective tape. A clean target improves both adhesion and optical detection.
Before measurement:
- Stop the machine safely.
- Clean the target area.
- Allow the surface to dry.
- Apply the reflective target securely.
2. Use One Reflective Mark Per Revolution Unless the Instrument Is Configured Otherwise
With the simplest optical setup, one reflective target represents one revolution. If the sensor detects two reflective areas during each revolution, the displayed speed can be approximately twice the actual RPM.
Unintended reflections can come from:
- Polished metal
- Bright labels
- Painted markings
- Bolts or fasteners
- Multiple pieces of reflective tape
Use one clearly defined target and reduce surrounding reflections wherever practical.
3. Use Multiple Marks Carefully at Very Low RPM
At low rotational speed, only a small number of pulses occur during the instrument's sampling period. This can make readings update slowly or provide limited resolution.
Some tachometer manufacturers recommend applying several evenly spaced reflective marks at very low RPM. The displayed result must then be divided by the number of marks if the tachometer does not automatically compensate.
For example, if four equally spaced marks are used and the tachometer displays 120 RPM:
Actual RPM = 120 / 4 = 30 RPM
This method should only be used when the operator fully understands how the instrument interprets each pulse.
4. Aim the Sensor Directly and Consistently at the Target
The tachometer must reliably detect the reflective target each time it passes. Poor aiming can cause missed pulses and unstable readings.
Keep the instrument:
- Steady
- Within its specified sensing distance
- Aligned with the target path
- Clear of obstructions
Fluke's tachometer operating guidance specifically instructs the operator to aim at the reflective tape, hold the tachometer firmly and wait for the RPM reading to settle.
5. Stay Within the Specified Measurement Distance
Optical tachometers are designed to operate over a defined distance range. Measuring too far away can reduce reflected signal strength, while measuring outside the specified geometry can create unstable detection.
Distance limits vary between instruments. For example, current PCE tachometer specifications show optical measurement limits ranging from approximately 300 mm on some models to 50–500 mm on others.
Always use the distance specified for the actual tachometer being tested rather than assuming all laser tachometers have the same range.
6. Use a Target Large Enough for Reliable Detection
A very small reflective target can be difficult to detect when the shaft is vibrating, the measurement distance is long or the laser spot is relatively large.
The target should be large enough to create a strong and repeatable optical response without covering so much of the circumference that the pulse duration becomes excessive.
PCE instructions for one current handheld tachometer specify a 1 × 1 cm reflective target, while other tachometers use different target recommendations. Follow the instrument manual.
7. Control Strong Ambient Light and False Reflections
Direct sunlight, intense work lights and bright reflective machine surfaces can interfere with some optical systems.
If readings are unstable:
- Reduce direct light on the target if possible
- Change the aiming angle slightly
- Darken surrounding reflective surfaces
- Move closer while remaining within the specified range
- Confirm the tachometer's target-detection indicator operates once per revolution
Reducing Errors in Contact Tachometers
8. Prevent Slippage Between the Contact Adapter and Shaft
Contact tachometers depend on the mechanical adapter rotating at exactly the same speed as the measured shaft.
If the rubber tip slips, the tachometer usually reads lower than the true speed.
To reduce slip:
- Use the correct conical or cylindrical adapter
- Position it at the center of the shaft
- Apply enough pressure to maintain contact
- Avoid excessive pressure that could affect the machine or tachometer
- Replace polished, hardened or damaged rubber adapters
PCE operating instructions specifically direct the user to press the contact RPM adapter lightly against the center of the rotating shaft.
9. Measure at the Shaft Center
When using a conical contact attachment, placing the tip off-center can cause unstable motion, side loading and slip.
Align the tachometer as closely as possible with the axis of rotation. The instrument should not wobble while the measurement is being taken.
10. Use Consistent Contact Pressure
Too little pressure can cause slippage. Too much pressure can increase bearing load, deform soft surfaces or slow lightweight rotating equipment.
The correct pressure is the minimum required to maintain continuous, slip-free contact.
11. Inspect the Measuring Wheel
When a contact tachometer is used for surface-speed measurements, the wheel circumference becomes part of the measurement calculation.
Wear, contamination or deformation can change the effective diameter and create systematic error.
Inspect the wheel for:
- Flat spots
- Cracks
- Excessive wear
- Oil contamination
- Loose mounting
12. Prevent Surface-Wheel Slip
A measuring wheel must roll without sliding against the belt, roller, sheet or other moving surface.
Slippage can occur when:
- The surface is oily
- The wheel is worn
- The operator uses insufficient pressure
- The wheel is not aligned with travel direction
- The surface accelerates or vibrates
Keep the wheel aligned with the direction of movement and maintain stable contact.
13. Verify Pulses per Revolution
Installed tachometer systems often calculate RPM from a magnetic, inductive, Hall-effect or encoder-type pulse signal.
The system must know how many pulses correspond to one complete revolution.
The equation is:
RPM = (Frequency × 60) / Pulses per revolution
If the pulse setting is wrong, the displayed speed will be wrong by a predictable ratio.
For example, a 60-tooth gear generates 60 pulses per revolution. If the controller is incorrectly configured for 30 pulses per revolution, the displayed RPM will be twice the true value.
14. Make Sure the Machine Speed Is Actually Stable
Not every changing reading is a tachometer error. The rotating equipment itself may be changing speed.
Speed variation can occur because of:
- Variable-frequency drive regulation
- Changing mechanical load
- Belt slip
- Governor action
- Process disturbances
- Motor torque variation
Before blaming the instrument, determine whether the shaft speed is expected to remain constant.
15. Wait for the Reading to Settle
Handheld tachometers use sampling and averaging to calculate speed. The first displayed number may not be the most representative result.
After aiming or making contact, hold the instrument steady until the reading becomes stable. Fluke's tachometer instructions explicitly recommend holding the trigger until the RPM value settles before recording the result.
16. Repeat the Measurement
One measurement can be affected by operator movement or temporary machine variation.
For important tests:
- Take several readings.
- Remove and reposition the tachometer between trials if appropriate.
- Compare the results.
- Investigate any result that differs significantly from the others.
Repeated measurements help identify poor repeatability that may otherwise be mistaken for a true speed change.
17. Pay Special Attention to Low-RPM Measurements
Low rotational speeds are challenging because the instrument receives fewer measurement events in a given time.
At 600 RPM, one pulse-per-revolution produces 10 pulses each second. At 6 RPM, the same setup produces only one pulse every 10 seconds.
Low-speed measurement may therefore require:
- A longer observation period
- Period measurement rather than simple frequency counting
- Multiple targets per revolution
- A higher-resolution encoder
- A different measurement technology
18. Check the Instrument Range at High RPM
High-speed measurements can exceed the optical, contact or electronic frequency range of the tachometer.
Never assume that a device capable of displaying five digits can accurately measure every speed represented by those digits.
Check:
- Maximum optical RPM
- Maximum contact RPM
- Maximum sensor input frequency
- Accuracy at the upper end of the range
Some tachometers specify different accuracy values at very high RPM.
19. Reduce the Effect of Vibration
Vibration can cause optical targets to move out of the sensing beam and can make contact measurement unstable.
For optical testing:
- Increase target size within manufacturer recommendations
- Use a stable measuring position
- Mount the tachometer on a support if necessary
For permanently installed sensors, verify that the sensor bracket is rigid and that the sensing gap does not change excessively during machine operation.
20. Set the Correct Sensor Gap
Magnetic, inductive and Hall-effect speed sensors operate correctly only within their specified sensing gap.
If the sensor is too far from the target, pulses can be missed. If it is too close, mechanical contact may occur due to shaft movement or vibration.
Measure and adjust the gap according to the sensor manufacturer's requirements.
21. Confirm the Correct Number of Teeth, Magnets or Blades
Speed can be calculated from repeating machine features such as:
- Gear teeth
- Fan blades
- Magnets
- Slots
- Encoder lines
The actual count must match the configured count. Incorrect assumptions about the number of detectable features are a common source of ratio errors.
22. Keep the Tachometer Calibrated
A tachometer used for quality control, commissioning or formal testing should be calibrated at an interval appropriate to its use, stability and quality-system requirements.
Calibration can be performed against a known rotational-speed reference or, for electronic pulse-input instruments, against a traceable frequency source.
Where required, the calibration should be traceable through an appropriate accredited calibration system and should cover the speed range actually used in practice.
23. Perform Intermediate Verification Checks
Periodic verification between formal calibrations can help identify damage or drift.
A verification check can compare the tachometer with:
- A known reference tachometer
- A stable motor with a verified speed reference
- A rotational calibrator
- A pulse or frequency generator
The purpose is not to replace formal calibration, but to detect obvious problems before critical measurements are made.
24. Check Battery and Instrument Condition
Weak batteries, dirty optical windows, damaged contact adapters and loose connectors can all reduce reliability.
Before important testing:
- Check battery condition
- Clean optical windows
- Inspect reflective tape
- Inspect mechanical adapters
- Check measuring wheels
- Inspect cables and connectors
- Confirm the correct measuring mode
25. Stay Within the Environmental Operating Range
Tachometers have specified operating temperature and humidity ranges. Extreme conditions can affect electronics, batteries, optical detection and mechanical accessories.
For example, several current handheld PCE tachometers specify operation between approximately 0 °C and 50 °C. The correct range should always be checked for the actual instrument.
26. Avoid Changing the Measurement Geometry Between Tests
Repeatability improves when measurements are taken from the same position, distance and angle each time.
For routine machine inspections, consider marking a standard tachometer target location and recording:
- Target position
- Measurement distance
- Instrument model
- Operating condition
- Expected RPM range
This makes trend comparisons more meaningful.
27. Consider Measurement Uncertainty, Not Only Instrument Accuracy
The tachometer's stated accuracy is only one contributor to overall measurement uncertainty.
Other contributors can include:
- Resolution
- Calibration uncertainty
- Repeatability
- Machine speed fluctuation
- Target geometry
- Pulse-count uncertainty
- Contact slip
- Wheel dimension uncertainty
- Operator technique
For formal testing, these factors should be considered when determining whether the measurement is suitable for the required tolerance.
28. Cross-Check With a Second Method When the Result Is Critical
If an unexpected speed measurement could lead to an important maintenance or process decision, confirm it with another method where practical.
Examples include:
- Optical tachometer vs. contact tachometer
- Handheld tachometer vs. drive-frequency calculation
- Tachometer vs. encoder output
- Optical tachometer vs. stroboscope
Agreement between independent methods increases confidence in the result.
Common Tachometer Errors and Corrections
| Observed Problem | Likely Cause | Corrective Action |
|---|---|---|
| Reading is approximately twice actual speed | Two optical targets or wrong pulse count | Use one target or correct pulses per revolution |
| Reading is approximately half actual speed | Missed pulses or incorrect pulse configuration | Improve target detection and verify configuration |
| Optical reading jumps randomly | False reflections, vibration or poor alignment | Improve target contrast, stabilize instrument and check distance |
| Optical tachometer shows no reading | Weak reflection or excessive distance | Clean target, improve alignment and move within range |
| Contact RPM reads low | Adapter slipping | Improve alignment and contact pressure; inspect adapter |
| Surface speed reads incorrectly | Wheel slip or worn wheel | Inspect wheel and improve contact |
| Low RPM is slow to update | Too few pulses during sample period | Use longer measurement interval or more pulses per revolution |
| Installed sensor intermittently loses speed | Incorrect sensing gap or vibration | Adjust gap and strengthen mounting |
| Repeated tests disagree | Different measurement geometry or unstable machine speed | Standardize test position and confirm process stability |
Recommended Tachometer Testing Procedure
A repeatable testing procedure can significantly reduce operator-dependent error.
- Identify the tachometer type and correct measuring mode.
- Confirm that the expected RPM is within range.
- Inspect instrument condition and calibration status.
- Prepare the reflective target or contact point.
- Verify pulses per revolution where applicable.
- Run the machine under the required operating condition.
- Allow machine speed to stabilize.
- Position the tachometer at the correct distance and angle or establish slip-free contact.
- Wait for the display to settle.
- Record several readings.
- Compare repeatability.
- Cross-check unexpected results if necessary.
- Record the test conditions with the result.
What Should Be Recorded With the Measurement?
For maintenance or quality records, consider recording:
- Machine identification
- Measurement location
- Operating condition
- Tachometer model and serial number
- Calibration status
- Measurement method
- Reflective targets or pulses per revolution
- Measured RPM
- Repeated readings
- Ambient conditions if relevant
- Operator
- Date and time
This information makes later comparisons and troubleshooting much easier.
Reducing Errors When Using a Stroboscope
Stroboscopic speed measurement has a unique error risk: the object can appear stationary when the flash frequency is a multiple or fraction of the actual rotational speed.
To reduce this risk:
- Approach synchronization from different flash frequencies
- Double or halve the flash rate and observe whether the image remains consistent
- Confirm the result with a tachometer when uncertain
- Use a clearly identifiable feature on the rotating component
Do not assume that the first stationary image represents the true rotational speed.
Safety During Tachometer Testing
Measurement accuracy is never more important than operator safety.
For rotating machinery:
- Do not bypass guards unnecessarily
- Do not place hands near exposed rotating components
- Do not wear loose clothing or jewelry near rotating equipment
- Stop and isolate the machine before applying reflective tape when required
- Use non-contact measurement when direct contact creates unnecessary risk
- Follow site lockout and machine-safety procedures
Frequently Asked Questions
Why does my laser tachometer reading keep changing?
Possible causes include poor reflective tape, excessive distance, changing measurement angle, vibration, strong ambient reflections or actual machine-speed variation.
Why does my tachometer show twice the expected RPM?
The instrument may be detecting two targets per revolution, or the pulses-per-revolution configuration may be incorrect.
Why does a contact tachometer read lower than an optical tachometer?
Slippage between the contact attachment and shaft is a common cause. Worn adapters, poor alignment or insufficient pressure can contribute.
Why are low-RPM measurements difficult?
At low speed, fewer measurement pulses occur during each sampling interval. The tachometer may therefore need more time to calculate a stable result.
Can I use more than one reflective mark?
Yes, especially for very low-speed measurements, but the number of marks must be included in the calculation or instrument configuration. Otherwise the displayed RPM will be multiplied by the number of marks.
Does measurement distance affect a laser tachometer?
Yes. Optical tachometers have a specified sensing-distance range. Measurements outside that range can become unreliable.
How often should a tachometer be calibrated?
The appropriate interval depends on instrument stability, frequency of use, environmental conditions, previous calibration history and the requirements of the user's quality system.
Summary
Tachometer measurement errors are usually reduced by controlling the measurement method as carefully as the instrument itself.
For optical and laser tachometers, use a clean, clearly defined target, avoid false reflections, maintain the correct measurement distance and keep the instrument steady. For contact tachometers, prevent slippage, align the adapter with the shaft and inspect contact tips and measuring wheels for wear.
For installed speed sensors, verify the pulse count, target geometry and sensing gap. At low RPM, allow more measurement time or use additional pulses per revolution when appropriate.
Finally, maintain calibration, repeat important measurements and document test conditions. A consistent procedure is one of the most effective ways to improve repeatability and reduce uncertainty in tachometer testing.