Why Laser Tachometer RPM Readings Are Unstable Malaysia

Why Laser Tachometer RPM Readings Are Unstable Malaysia

Unstable laser tachometer RPM readings are a common challenge during industrial motor, pump, fan and rotating machinery inspections.

A maintenance technician may aim a digital laser tachometer at a rotating shaft and notice that the displayed speed changes continuously.

For example, the instrument may display:

1,450 RPM - 1,460 RPM - 1,520 RPM - 1,430 RPM

Such fluctuations can make it difficult to determine whether the machinery is operating correctly.

However, an unstable RPM reading does not automatically mean that the tachometer is defective.

The problem may be caused by optical signal interference, reflective target problems, measurement distance, actual machinery speed changes or incorrect measurement procedures.

The Benetech GM8905 Digital Laser Tachometer uses a non-contact optical method to measure rotational speed.

Understanding how optical tachometers detect rotation helps maintenance personnel identify the cause of fluctuating RPM readings.

This guide explains the most common reasons for unstable RPM measurements and provides a practical troubleshooting procedure for industrial applications in Malaysia.

1. What Is an Unstable RPM Reading?

An unstable RPM reading occurs when the displayed rotational speed changes repeatedly during measurement.

Some variation may be normal because rotating machinery does not always operate at a perfectly constant speed.

For example, a motor controlled by a VFD may experience small speed changes during load adjustments.

However, sudden or unusually large changes may indicate:

Poor optical pulse detection

Incorrect reflective target preparation

Unwanted reflections

Measurement distance problems

Actual shaft speed variation

Motor control fluctuations

Incorrect measurement location

The first step is to determine whether the displayed changes represent actual machinery behaviour or measurement problems.

2. How Does a Laser Tachometer Measure RPM?

A digital laser tachometer measures rotational speed by detecting repeated optical signals from a rotating target.

A suitable reflective marker is normally attached to the rotating component.

As the marker passes through the instrument's detection area, the tachometer detects an optical pulse.

For a conventional single-marker setup:

One detected pulse = One revolution

The instrument calculates RPM from the detected pulse frequency.

For example:

RPM = Pulse Frequency x 60

If the instrument detects 25 valid pulses per second:

RPM = 25 x 60 = 1,500 RPM

However, if the instrument detects missing or additional pulses, the displayed RPM may become unstable.

Reliable RPM measurement depends on consistent optical pulse detection.

3. Cause 1 - Weak Reflective Signal

One common cause of unstable RPM readings is a weak optical reflection.

The tachometer must detect a sufficiently clear signal from the rotating target.

A weak reflection may cause the instrument to miss some pulses.

Possible reasons include:

Reflective tape is dirty

Tape is damaged

Tape is poorly positioned

Target contrast is insufficient

The measurement angle is unsuitable

The optical target is partially obstructed

Recommended Solution

Stop and isolate the machine according to the approved safety procedure.

Inspect the reflective marker.

Replace damaged or contaminated tape where necessary.

Ensure the marker is securely attached to a suitable target.

Never touch or adjust reflective tape while machinery is rotating.

4. Cause 2 - Incorrect Measurement Distance

Every optical tachometer has a specified operating distance.

If the instrument is positioned outside its recommended distance, the reflected signal may become unreliable.

This can produce:

Intermittent RPM readings

No reading

Sudden displayed changes

Unstable pulse detection

Recommended Solution

Check the Benetech GM8905 manufacturer instructions for the specified measuring distance.

Position the instrument within the approved operating range.

Maintain a consistent measuring position.

Avoid moving the tachometer excessively during measurement.

Important: Do not move closer to hazardous rotating components simply to improve the optical signal.

If the target cannot be measured safely, use an approved alternative method.

5. Cause 3 - Multiple Reflective Surfaces

An optical tachometer may detect reflections from more than one feature on a rotating component.

For example, a rotating pulley may have:

A reflective tape marker

A polished metal surface

A shiny bolt head

A reflective label

Another optical target

If multiple features produce detectable pulses, the instrument may display an incorrect or unstable RPM.

Example

Actual Shaft Speed = 1,500 RPM

If two pulses are detected during each revolution, the tachometer may interpret the signal as:

Displayed Speed = 3,000 RPM

If the additional reflection is intermittent, the displayed reading may jump between different values.

Recommended Solution

Inspect the rotating target after stopping and isolating the equipment.

Use one suitable reflective marker for conventional single-pulse measurement.

Reduce unwanted reflections where practical and safe.

6. Cause 4 - Incorrect Reflective Tape Position

Reflective tape placement affects optical signal detection.

A marker positioned on an unsuitable surface may not pass consistently through the tachometer's detection area.

Possible problems include:

Marker positioned on a curved or uneven surface

Tape lifting during rotation

Marker partially hidden by equipment structure

Inconsistent optical angle

Target moving outside the detection area

Recommended Solution

Select a suitable approved measurement surface.

Ensure that the marker remains visible during each revolution.

Use an appropriate tape size and placement method according to the instrument and equipment instructions.

The target must not interfere with machine balance, mechanical operation or safety protection.

7. Cause 5 - Actual Motor Speed Fluctuation

Not every unstable RPM reading is caused by an optical measurement problem.

The motor may genuinely be changing speed.

Possible causes include:

Variable mechanical load

VFD speed control changes

Process demand changes

Motor control behaviour

Intermittent mechanical loading

Equipment acceleration or deceleration

Example

A conveyor motor may operate at different speeds as the production system changes its speed reference.

A tachometer may correctly display these changes.

Recommended Solution

Record the operating conditions during measurement.

Where relevant, check:

VFD output frequency

Speed reference

Motor load

Production operating mode

Control system status

Compare RPM readings only under suitable and comparable operating conditions.

A fluctuating reading may be a valid measurement of changing shaft speed.

8. Cause 6 - Measuring During Startup or Shutdown

Rotating machinery normally changes speed during acceleration and deceleration.

If a tachometer is used while the machine is starting or stopping, the displayed RPM may change rapidly.

This behaviour is expected.

For example:

Operating StageExample RPMInitial Acceleration300Acceleration700Acceleration1,100Near Operating Speed1,400Stable Operation1,450

These values are illustrative.

Recommended Solution

For routine operating-speed verification, allow the machinery to reach a stable operating condition.

For acceleration or deceleration analysis, use a measurement method and instrument capability suitable for capturing changing speed.

A standard handheld RPM reading may not provide the time resolution required for detailed transient analysis.

9. Cause 7 - Incorrect Measurement Point

A machine may contain several rotating components operating at different speeds.

For example:

Motor shaft

Gearbox input shaft

Gearbox output shaft

Drive pulley

Driven pulley

A technician may accidentally measure a different component from the intended target.

The measured RPM may then appear inconsistent with the expected motor speed.

Recommended Solution

Identify the required measurement point before testing.

Confirm the drive arrangement and transmission ratio.

For gearbox-driven machinery, motor RPM and output RPM are normally different.

Always record which shaft or rotating component was measured.

10. Cause 8 - Optical Interference from the Environment

Industrial environments may contain conditions that affect optical measurement.

Potential sources of interference include:

Strong reflections from nearby metal surfaces

Moving reflective objects

Dust or contamination obstructing the optical path

Excessive ambient light under certain conditions

Vibrating measurement positions

Obstructed line of sight

Not every lighting condition will cause a problem, and the effect depends on the instrument's optical design.

Recommended Solution

Use a suitable measurement position.

Keep the optical path clear.

Avoid aiming at multiple reflective surfaces.

Maintain a stable instrument position.

Follow the manufacturer's operating recommendations.

11. Cause 9 - Instrument or Battery Problems

Unstable readings may occasionally be related to the tachometer itself.

Possible issues include:

Low battery condition

Dirty optical window

Physical damage

Instrument malfunction

Operation outside specified conditions

Recommended Solution

Inspect the instrument according to the manufacturer's instructions.

Check the battery condition.

Clean the optical window only using an approved method.

If the instrument continues to produce abnormal readings under controlled conditions, arrange suitable verification or servicing.

Do not assume that the instrument requires repair before checking the measurement setup.

12. Step-by-Step Troubleshooting for Unstable RPM Readings

The following troubleshooting process can help identify the source of fluctuating measurements.

Step 1 - Confirm the Machine Operating Condition

Check whether the machine is running at a stable speed.

Record relevant VFD settings or operating load.

Step 2 - Check the Measurement Point

Confirm that the correct shaft or pulley is being measured.

Step 3 - Verify the Optical Target

After stopping and isolating the equipment, inspect the reflective marker.

Ensure that the marker is suitable, secure and unobstructed.

Step 4 - Check for Additional Reflections

Identify shiny surfaces or multiple targets that may create unwanted optical pulses.

Step 5 - Verify Measurement Distance

Use the tachometer within the manufacturer's specified operating distance.

Step 6 - Maintain a Stable Measuring Position

Keep the instrument aimed consistently at the selected target.

Step 7 - Repeat the Measurement

Take multiple readings under comparable operating conditions.

Step 8 - Compare with an Appropriate Reference

Where necessary, compare the result with an appropriate calibrated reference instrument or approved measurement method.

Step 9 - Record the Findings

Document the original problem, corrective action and final readings.

Step 10 - Escalate Unresolved Problems

If the reading remains unstable, follow the facility's approved troubleshooting procedure.

Do not continue testing in unsafe conditions.

13. Unstable RPM Troubleshooting Table

SymptomPossible CauseRecommended ActionRPM jumps between high and low valuesIntermittent optical pulsesCheck target and reflectionsRPM occasionally doublesAdditional reflective pulseInspect target surfacesRPM disappears intermittentlyWeak optical signalCheck tape and measuring distanceRPM changes graduallyActual speed variationCheck VFD and operating conditionsRPM fluctuates during startupNormal accelerationMeasure after stabilizationRPM differs between shaftsGearbox or pulley ratioConfirm measurement pointRPM remains unstable after setup checksInstrument or machine issuePerform further verification

These are possible explanations rather than definitive diagnoses.

14. How to Distinguish Measurement Error from Actual Speed Variation

A useful troubleshooting approach is to compare RPM readings with other operating information.

Situation A - RPM Changes with VFD Frequency

If the VFD frequency changes and measured RPM changes accordingly, the variation may represent actual motor speed control.

Situation B - RPM Jumps Without Operating Changes

If the displayed RPM suddenly doubles or drops while operating conditions remain unchanged, investigate optical pulse detection.

Situation C - RPM Fluctuates with Mechanical Load

If the machine experiences changing loads, actual shaft speed may vary depending on motor and drive characteristics.

Situation D - RPM Remains Unstable After Target Correction

If the optical setup has been verified, consider additional instrument checks or independent speed verification.

Important: A tachometer reading alone may not establish the exact cause of a speed fluctuation.

15. Preventing Unstable RPM Readings During Factory Maintenance

Maintenance teams can improve measurement consistency by establishing a standard procedure.

Recommended practices include:

Use the correct measurement point.

Prepare reflective targets safely.

Maintain suitable measurement distance.

Avoid unwanted optical reflections.

Record VFD frequency and operating conditions.

Allow machinery to stabilize when appropriate.

Take repeated measurements.

Compare results with suitable references.

Maintain the tachometer according to manufacturer instructions.

Document unusual readings for follow-up inspection.

A consistent procedure improves the usefulness of RPM records across different technicians and inspection dates.

16. Industrial Applications in Malaysia

Manufacturing Factories

Maintenance teams may encounter unstable RPM readings when inspecting motors, production equipment and mechanical drives.

Industrial Pump Systems

Pump motor speed may vary with VFD control or process operating conditions.

HVAC and Ventilation Equipment

Fan RPM measurements may fluctuate because of variable-speed operation or optical measurement issues.

Conveyor Systems

Conveyor motor and pulley speeds may change according to production line control settings.

Mechanical Maintenance Contractors

Portable tachometers are useful for suitable on-site rotational speed verification.

Engineering and Technical Services

RPM troubleshooting can support machinery commissioning and maintenance inspections.

17. Frequently Asked Questions

Why does my laser tachometer RPM keep changing?

Possible causes include weak reflective signals, unwanted reflections, incorrect measuring distance or actual machinery speed variation.

Does an unstable RPM reading mean the tachometer is faulty?

Not necessarily. Measurement setup and machine operating conditions should be checked first.

Why does my tachometer sometimes show double RPM?

Additional optical pulses from multiple reflective targets or unwanted reflections may cause a doubled reading.

Can VFD operation cause fluctuating RPM?

Yes. VFD-controlled motors may change speed according to the speed reference, control method and operating conditions.

Can dirty reflective tape affect RPM readings?

Yes. Contamination may reduce optical signal quality and cause missed pulses.

Should I measure RPM during motor startup?

Startup RPM changes naturally. For routine steady-state verification, measure after the motor reaches its intended operating condition.

Can the Benetech GM8905 measure unstable machinery speed?

The GM8905 can display rotational speed measurements, but the suitability of the instrument for rapidly changing speed depends on its measurement characteristics and the application.

What should I do if the reading remains unstable?

Verify the target, operating conditions and instrument setup. If necessary, use an appropriate reference method or arrange instrument verification.

Where can I enquire about the Benetech GM8905 in Malaysia?

Contact MTM Precision Sdn Bhd for product information, pricing and availability enquiries.

18. Buy Benetech GM8905 Digital Laser Tachometer in Malaysia

The Benetech GM8905 Digital Laser Tachometer is suitable for industrial users who need portable non-contact RPM measurement for motors, pumps, fans, conveyors and rotating machinery.

MTM Precision Sdn Bhd supplies industrial testing and measurement instruments for factories, engineering contractors, maintenance teams and technical service providers throughout Malaysia.

We welcome enquiries involving:

Laser tachometer RPM measurement

Unstable RPM troubleshooting

Industrial motor speed inspection

Pump and fan RPM verification

Conveyor and gearbox speed measurement

Factory preventive maintenance instruments

Customers from Kuala Lumpur, Selangor, Johor, Penang, Melaka, Negeri Sembilan, Perak, Pahang, Kelantan, Terengganu, Kedah, Perlis, Sabah and Sarawak are welcome to contact our team.

For current pricing, stock availability or product enquiries, please contact MTM Precision.

Contact MTM Precision Sdn Bhd

MTM Precision Sdn Bhd

Showroom & Service Centre:

No. 29-1 & 29-2, Jalan Bandar 18, Pusat Bandar Puchong, 47160 Puchong, Selangor, Malaysia.

Tel: 03-8080 7172

WhatsApp: +6016-660 7346

Email: mtmpre@yahoo.com

Website: www.mtmpre.com.my

Business Hours: Monday to Friday, 9:00 AM - 6:00 PM

Contact MTM Precision for Benetech GM8905 Digital Laser Tachometer quotations and RPM troubleshooting enquiries in Malaysia.