Published July 11, 2026 at 09:44
At 07:15 on a Tuesday morning, maintenance technician Petter stands by the water pump that supplies pressure to the district heating network. The pump has a slight noise anomaly — a low vibration signal that operators heard during yesterday’s shift. The motor is a 4 kW Siemens drive motor for a centrifugal pump. According to the maintenance schedule, Petter is to perform a routine check: RPM measurement + bearing housing temperature.
He aims his RPM10 instrument at the motor’s visible shaft flag (a 5 mm piece of reflective tape he placed there 2 years ago) from a distance of 60 cm. The laser beam hits the reflective strip. The instrument displays 1,448 rpm. Motor spec: 1,465 rpm at 4 kW load ±5 rpm. The motor is running 15 rpm too slow — NOT within tolerance.
He switches the RPM10 to IR thermometer mode. He aims it at the motor’s bearing housing, drive side. The instrument shows 62 °C. Motor spec: max 55 °C at full operation. The bearing housing is 7 °C over spec.
Any one of these two deviations could be dismissed as an instrument error or measurement error. Both occurring simultaneously are, however, a clear sign: bearing clearance has developed. Petter notes the findings, plans replacement within 2 weeks, and avoids the catastrophic failure that would otherwise have occurred within 3-6 months — and which would cost 40,000-80,000 kr in downtime + repair.
This is the combined diagnostic methodology behind the RPM10’s design — RPM + temperature simultaneously.
Why RPM alone is not enough
Motors are delivered with a nominal speed specification that depends on load. At idle, the speed is almost synchronous with the grid frequency. Under load, it decreases slightly (slip). At 100% load, the slip is typically 2-4%.
So a 4-pole motor marked “1,500 rpm synchronous” has:
- 1,500 rpm at idle.
- 1,490-1,480 rpm at half-load.
- 1,470-1,460 rpm at full-load.
In case of problems:
- Increased load (increasing wear): RPM drops further. A bearing with a worn inner race gives 10-30 rpm extra slip.
- Low voltage supply: the motor loses torque, RPM drops below nominal value.
- Belt fluctuation: variable load on pump/compressor, RPM oscillates.
But RPM alone can be misinterpreted: on a cold day, water pumps have higher viscous resistance in the liquid → lower RPM. A compressor with less demand has less load → higher RPM. Without context, RPM figures are not diagnostic.
Why temperature alone is not enough
Surface temperature on the motor housing varies with:
- Ambient air temperature.
- Cooling from adjacent equipment or drafts.
- Load.
- Operating time (the motor needs 20-40 min warm-up to reach steady-state).
- The motor’s internal cooling (fan condition, air grille blockage).
A 55 °C reading can mean: normal operation, a normally functioning motor on a hot summer day OR bearing overheating with 20% more heat generation on a cold winter morning. Without context, the temperature is difficult to interpret.
RPM + temperature = diagnostic signal strength
The combination of both parameters under the same operating conditions provides dramatically improved diagnostic strength. Practical patterns:
- Normal RPM + normal temp: Healthy motor. No action required.
- Normal RPM + high temp: External cooling problems. Fan blockage, blocked air grille. Cleaning helps.
- Low RPM + normal temp: Electrical supply problems (low voltage, imbalance). Electrician work required.
- Low RPM + high temp: Mechanical resistance – INTERNAL. Ball bearing wear. Replacement within 2-4 weeks.
- Normal RPM + very high temp: Electrical insulation breakdown. Winding fault. Motor replacement soon.
No single measurement provides this analytical strength.
Optical non-contact RPM measurement
The RPM10’s primary RPM mode is optical, non-contact. Here is how it works:
- The instrument sends a visible red laser beam towards the rotating object.
- On the object, there must be a reflective mark — a piece of reflective tape, a white painted area, or a color marking.
- Each time the mark passes the laser beam, the light is strongly reflected back.
- The instrument’s photo-detector senses the reflection pulses.
- The instrument counts the pulses over time and calculates RPM.
Advantages of the optical method:
- No mechanical contact = no load on the motor = no braking of the measured object.
- Can measure from 2 m distance.
- Works at high speeds (up to 100,000+ rpm).
- No wear on the instrument.
Limitations:
- Requires a reflective mark on the object (must be pre-installed or applied).
- At very low RPM (below 5-10 rpm), single-pulse detection is unreliable.
- Outdoors in strong sunlight, the pulses can be drowned out by ambient light.
Contact mode for when optical does not work
The RPM10 also has a CONTACT mode for when the optical method does not work:
- A rubber or steel wheel adapter is screwed onto the front of the instrument.
- The adapter is pressed against the motor’s shaft or a rotating wheel.
- The adapter rolls as the object rotates; an internal encoder counts revolutions.
The contact method works when:
- No reflective mark can be installed.
- The object is too dark or too shiny for reliable optical detection.
- Measurement underwater or in lighting conditions where optics do not work.
Disadvantage: contact = mechanical load on the motor. On small motors, this can brake them and give erroneously low RPM. Only for medium to large motors.
Surface temperature measurement
The IR thermometer in the RPM10 measures surface temperature from 60-120 cm distance. Uses standard NIR detector (8-14 μm band) with:
- Adjustable emissivity 0.30-1.00.
- D:S ratio typically 8:1 (so at 80 cm distance, the measurement spot is 10 cm in diameter — for larger motors, this is sufficient to measure the bearing housing without additional components).
- Temperature range: -20 to +200 °C typically (enough for most motor bearing housings).
- Accuracy: ±2 °C or ±2 %.
For diagnostic work: measure from 60 cm distance on the bearing housing surfaces. Note values at both ends (drive side and shaft side). An abnormal difference between the two (5 °C or more) indicates uneven load distribution.
Diagnostic applications
Motor maintenance (electric motors): PDM (predictive maintenance) on 3-phase motors in industry.
Pump maintenance: Centrifugal pumps for water, oil, chemicals. Combined RPM + bearing temperature monitoring.
Fan and compressor maintenance: Industrial fans, HVAC fans, compressors.
Auto shop work: Engine RPM during diagnosis, catalytic converter temp, brake disc temp after test drive.
Electrician work on generators: Diesel generators and backup equipment — RPM verification and load operation follow-up.
CNC machine spindle maintenance: Spindle RPM verification after calibration. Bearing temperature during operation.
Machining workshop: Verification of programmed spindle speed against actual value.
Conveyor system maintenance: Roller conveyors, drive motors, wheel bearings on forklifts and cranes.
What lies outside the RPM10’s capability
- Vibration measurement: Motor diagnosis with vibration spectral analysis (FFT) requires dedicated vibration meters (typically 15,000-40,000 kr).
- Current measurement: For electric motor diagnosis, a separate clamp meter is needed.
- Insulation resistance: Insulation testing requires a megger.
- Frequency analysis of RPM fluctuations: RPM10 shows average value, not transient analysis.
- High speeds above 100,000 rpm: For centrifuges, turbomachines, specialized instruments are needed.
Practical workflow time
A full motor diagnosis with the RPM10 takes typically:
- Positioning at the motor: 30 seconds.
- RPM measurement: 15 seconds.
- Switch mode to IR-temp: 5 seconds.
- Temp measurement on two bearing housings: 30 seconds.
- Note-taking: 20 seconds.
- Total: approx. 1.5 minutes per motor.
For a facility with 50 motors on a maintenance round, this is 75 minutes — fast enough for daily or weekly inspection tasks without burdening the entire workday.
What you get for the money
Extech RPM10 combined optical/contact tachometer and IR thermometer, optical non-contact RPM measurement from 5-100,000+ rpm with laser up to 2 m measurement distance, contact speed measurement with rubber adapter included, IR thermometer with D:S 8:1 and emissivity adjustment 0.30-1.00, temperature range -20 to +200 °C, LCD display with automatic orientation, double-injected ergonomic housing for field use. 5,537 kr.
Standard instrument for maintenance technicians with both industrial motor care and workshop diagnostic work. The combination of RPM + surface temperature provides significantly better diagnosis than individual parameters and enables early warning for bearing wear before catastrophic failures. The premium of ~2,000 kr over a standalone optical tachometer is justified for most maintenance departments by the time savings (one instrument, one operator round) and the better diagnostic strength.
