Interview Questions on Vibration Measurement

Check out this article on Vibration Measurement Interview Questions and Answers which are useful to crack your interview as an Engineer.

Vibration Measurement Questions

Vibration Measurement Questions

Why do we need vibration monitoring?

It is essential to monitor critical machines in the plants for increasing their efficiency and reliability. Hence real time vibration monitoring is the key to reduce frequent failures of machinery & keep high uptime.

What causes vibrations?

Unbalance of shaft, Bearing problem, Cracking of the rings, Fluid coupling problem, Shaft misalignment, Oil whirl and other dynamic instabilities

What is on line monitoring of vibrations?

Time Based Maintenance System (TBM) is called preventive maintenance. One can extend the life of the machines by monitoring these online in a cost effective way.

Vibration Monitoring and Analysis is the easiest way to keep machines healthy and efficient in the long run and increase the overall efficiency of the plant. It reduces the overall operating cost as well as the down time period.

Vibration sensors are used to predict faults in a running machine without dismantling it and give a clear indication of the severity by showing the amplitude of vibration.

Where do we need on-line vibration monitor?

In Industries rotating machines are divided according to their criticality into three categories :

  • First critical machine – Turbine and generator Secondary critical machines – ID fan, FD fan, PA fan, and boiler feed pump, cooling water pump, condensate extra pump, critical large HT motors of mills and other large motors.
  • Balance of plant machines – Coal handling plant crushers, cooling tower fans, raw water pumps and make up water pumps

This explains where Vibration Monitoring is required and how critical is each machine if there is shut down. For taking the machines for maintenance we need to know the healthy state of the machine without dismantling it. This is possible only by online monitoring.

What are the types of sensors for vibration monitoring?

The three principal vibration sensor types are displacement, velocity, and accelerometer.

The displacement transducer is an eddy current device, the velocity transducer is often a spring held magnet moving through a coil of wire or piezo velocity sensor, and the accelerometer is a piezoelectric device somewhat similar to ultrasonic transducers.

What is the benefit of vibration monitoring?

With vibration Monitoring system, we can prevent problems from arising and this saves a lot of time, money, and avoids frustration.

  •  To increase equipment protection
  •  To improve safety for personnel
  • To improve maintenance procedures
  • To detect problems early
  • To avoid catastrophic failures
  • To extend equipment life
  • To enhance operations

What is the benefits of Vibration Analysis & diagnostic Software?

Vibration analysis & diagnostic system that is applicable to a variety of rotating machinery, helps safe operation and to improve operational efficiency.

It precisely keeps track of and quickly feeds back conditions of rotating machinery which are the key production assets of plants.
It Helps customers improve productivity and reliability by optimizing plant operation.

  • Detects abnormal symptoms from vibration characteristics or subtle changes in vibration. Reduces risks of unplanned production shutdown by taking proactive approach.
  • Advanced diagnostics realize assumption of causes and areas of anomalies and detailed analysis. Help users practice optimum, efficient maintenance.

What is ‘vibration’?

Vibration is defined as the motion of the equipment or its part to and from its rest (static) condition.

Explain following with their sensitivity (output) and the measuring units:


A vibration measurement across the radius of a rotating shaft. It is measured in terms of Micron. The sensitivity of a radial vibration pickup (eddy probe) is 200mv DC/mill (refer to the drawings for the exact parameters).


It is defined as the rate of change of distance traveled by the equipment. Velocity measurement is generally used for measuring the equipment body vibration. The sensitivity of a velocity pickup is 500mv DC/inch/sec (refer to the manufacturer drawings for the exact parameters).


It is defined as the rate of change of velocity. Acceleration measurement is generally used for measuring the equipment body vibration. The sensitivity of a velocity pickup is 100mv DC/inch/sec2 (refer to the manufacturer drawings for the exact parameters).

What is the resistance of an ‘Eddy probe’?

An eddy probe resistance should be between 5-8 ohms.

Draw a sketch of an eddy probe calibration graph?

Eddy probe calibration graph : Check Here

What is gap voltage? Why and how is it set?

Gap voltage is feed back voltage derived by setting the standard gap between the eddy probe tip and the rotating shaft.

The gap voltage is the base DC voltage set to get the AC pulses peak per the radial vibration measured on the shaft.

The gap voltage is set at –8.00V DC i.e. equal to a gap of 40 mill (between the probe tip and the measuring surface). or at -10.00V DC as per the system standard.

Why an eddy probe and its probe driver is a matched pair?

Eddy probe output is always measured after the probe driver. The characteristic of the eddy probe slightly differs from one to another.

Always an eddy probe is installed on the shaft, after measuring and plotting an eddy probe calibration graph with its driver. The calibration is done by adjusting the sensitivity potentiometer on the eddy probe driver assigned for that particular eddy probe.

What do the AC and DC signal represent in radial vibration, where ‘AC is super imposed on DC’?

DC signal is the gap voltage set for –8.00V DC. DC signal is always present whether the unit is running or stopped.

How much is the radial vibration, if the signal measured on a DVM is 135.00mv AC?

The DVM measures the AC voltage in RMS (root mean square value).

AC peak to peak =135.00*2 root 2  =  181.837mv

Eddy probe sensitivity=200mv/mill or 200mv/25.4 Microns

Hence, the vibration in Micron =(381.837*25.4)/200   =  48.49 Micron

Why is vibration measurement very important on gas turbine and compressor?

Gas turbines and compressors are high speed rotating equipment. On equipment when vibration exceeds the manufacturer limits can cause sever damage to both itself and to its associated components/parts.

Hence it is important to measure and monitor vibration on the running equipment.

Generally a high vibration pre-alarm and shutdown limits are set as per the manufactures recommendations.

What type of vibration measuring instrument is used on a gas turbine? Where are the vibrations measuring points?

Since a gas turbine is hot engine, its bearing vibration measurement cannot be done through an eddy probe system.

Generally a velocity or an acceleration pick-up is used for measuring and monitoring the vibration on the body of the gas turbine. The vibration pick-ups are generally installed on the turbine’s CT/GP and PT points.

What is the type of vibration pick-up (contact or non-contact type) commonly installed on a gas turbine?

On gas turbine vibration measurement, the ‘contact type’ peck-up such as a velocity or acceleration pick-up is used.

What type of vibration measuring instrument is used on the compressors? Where is the vibration measuring points?

On a gas compressor shaft/bearing, measurement is carried out using a eddy probe displacement system.

On each gas compressor, on both forward and aft radial bearings (journal bearing) two eddy probes are installed in the ‘x’ and ‘y’ positions.

What is the type of vibration pick-up (contact type or non-contact type) commonly installed on a gas compressor?

A ‘non contact type’- eddy probe is installed on gas compressors.

What is ‘x or y function’ and, ‘x and y function’ selection on a radial vibration monitor?

X or Y function:

in this mode, the unit shutsdown when either ‘X’ or  ‘Y’ probe detects a high vibration exceeding the setpoint.

X and Y function:

in this mode, the unit shutsdown only when ‘X’ and ‘Y’ probes detects a high vibration exceeding the setpoint.

What is a ‘Probe driver’? What does is it do? What is the other name for a probe driver?

Probe driver is an amplifier installed in the field closer to the sensor. It transmits the field vibration signal to the remote control panel.

The other name for the probe driver is ‘proximeter’. Radial, velocity and accelerometer probe drivers are of different types.

What are the three wires used on a probe driver?

The three wires terminated on the probe driver are: -24V DC (power supply), common and output signal.

What is a ‘charge amplifier’? Where is it used?

Charge Amplifier is a probe driver installed for the accelerometer pick-up.

What is an ‘axial displacement’? How is it measured?

Axial displacement is the movement of the rotary shaft in the axial direction to and fro. Every centrifugal shaft is permitted to have a fixed axial movement designed by the manufacturer, this is to allow the compressor to take the load on the thrust collar.

Axial displacement is measure using an eddy probe system. It is measured using the proportional change in the DC voltage (gap voltage) caused by the movement of the shaft from its center position to either side (+or-).

What is the unit measurement of axial displacement?

Axial displacement is measured in terms of “Microns”

What is the term ‘float/end float’ in a compressor?

It is the free axial movement of the compressor shaft. End float is the maximum possible movement of the shaft. It is a small amount approximately 250 Microns (refer to the manufacturer drawings for drawings for the exact parameters).

What are the following assessments/recommendations of a vibration analysis? How are they corrected?


misalignment may cause two time RPM vibration. This is corrected by re-aligning the compressor shaft one to another.


unbalance in the compressor shaft may cause one time RPM vibration. This is corrected by balancing the compressor shaft on a balancing wheel.


multiple RPM vibration is symptom of damaged bearings. This is corrected by a total overhaul of the compressor or replacement of parts.

(The above are only a simple clue and not a standard confirmed judgment)

What precautions are to be taken when installing a vibration measuring instrument?

Vibration measuring instrument such as a eddy probe, velocity and acceleration pick-ups, eddy probe drivers, charge amplifiers are sensitive instruments. The following care has to be taken while installing in the field.

-Do not drop the instrument, this may result in the loss of their characteristic.

-Fixing and tightening to be done as per the manufacturers recommended torque.

-Proper care of the signal cable insulation.

-Prevent any loose connections. …Etc.

What are the reasons for a vibration ‘spike signal’?

The following may cause a ‘spike signal’ in the vibration measurement.

-Loss of signal cable insulation.

-Signal cable passing next to high voltage lines.

-Improper earthing facility.

-Grounding of the wires.

-Loose mounting of the instrument field components…Etc.

List the manufacturers names of vibration measuring instruments?

The common vibration measuring instrument manufacturers are:

Bently nevada

Forbes Marshall Shinkawa


Bell and Howell

Which of the above manufacturer’s product is more reliable and why?

With past experience, it is observed that the ‘Bently nevada’ vibration measuring system is more reliable due to its component layout on the control panel, accuracy and high gauge signal cable system in the field.

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14 thoughts on “Interview Questions on Vibration Measurement”

  1. Anyone please tell me how to get 181.837mv

    Regarding this Question

    * How much is the radial vibration, if the signal measured on a DVM is 135.00mv AC?

    AC peak to peak =135.00*2 root 2 = 181.837mv

    In my side I get 135*2√2=381.837 then why mentioned 135.00*2 root 2 = 181.837mv

    If there is any wrong please contact me…

  2. Thank you for sharing your knowledge on vibration analysis. More on the fundamental bases is required for beginners like me.

  3. I see you are mixing units here. Sometimes using mil, others using micron. mil = 1/1000″. micron is 1 millionth of a metre.
    mil is from the imperial system of measure, micron is from the SI (metric) system of measure.


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