Tip Archive

The three basic units of measurement in reference to vibration are Mils, IPS, and gs.

• Mils is the unit of measure for displacement. Displacement is a measure of how far an object moves and is expressed in units of 1,000^th of an inch (mils). A Displacement sensor is used to gather vibration amplitude in Mils. Generally, it is measured from peak-to-peak to quantify the total travel and reported as "Peak to Peak" or "Double Amplitude." Since displacement measures physical travel it is better at measuring low speed vibrations that result in larger physical movements. Displacement is used for determining absolute shaft movements and is sometimes used in balancing. Due to its usage in the early days of vibration analysis, Mils is still a common unit for vibration analysis and balancing.

• Velocity is the measure of how fast an object is moving and is most often expressed in Inches Per Second (IPS). It can also be expressed in metric form such as mm/sec or cm/sec. A velocity sensor, or velocimeter is used to gather vibration amplitude in IPS. Generally, it is reported as the peak speed reached and labeled as "Peak." It is often a preferred unit since velocity is directly proportional to the energy produced by the vibration and responds to a wide range of frequencies.

• Acceleration is the measure of the rate of change of velocity and is expressed in units of gravitational force (gs). It can also be expressed in metric form such as mm/s/s or cm/s/s. An accelerometer is used to gather vibration amplitude in gs. It is generally reported as a Root Mean Square or "RMS" value. It is directly proportional to the forces being exerted by the vibration. Acceleration tends to be much higher at high frequencies for a given vibration and isn’t very sensitive to low frequency movements.

So which sensor should you use for balancing?

Generally, a velocity sensor is the most accurate sensor to use for balancing because it is proportional to the energy and this relationship isn’t frequency related. An energy produced by a reading of 1 IPS is the same at 750 RPM as it is at 1500 RPM and 3000 RPM. If you are collecting vibration amplitude in units of displacement and want to know the energy content, you must record the specific frequency. In other words, 5 mils at 1000 RPM is useful information but an overall measurement of 5 mils is useless for evaluating the energy produced by the vibration. Frequency must be known for both acceleration and displacement for meaningful vibration analysis.

Displacement sensors are most effective at lower frequencies of 10 Hz or less. For this reason they are not often selected for balancing high-speed turbines and even less for vibration survey information and analysis.

Accelerometers are best suited for higher frequencies but are not useful at frequencies below approximately 10 Hz.

If your application doesn’t specify a sensor, your best overall choice is a velocity sensor. Because it has a flat response over a wide frequency range, the velocity sensor is suited for both balancing and vibration surveys.

With the exception of the 1000 series balancers, all ACES Systems' analyzers will convert spectral units and display them in a format to match your documented vibration survey criteria. For instance, you may have a requirement to conduct a vibration survey and insure that no frequency exceeds 3 mils. If your vibration sensor is velocity, it collects data in engineering units of IPS. In order to verify compliance with the limitations of the survey, you must be able to convert the spectral units from IPS to Mils. A simple menu selection makes this possible in your ACES Systems' analyzer. For any job requiring a vibration sensor, always check the maintenance manual to see if the manufacture requires or recommends a specific sensor and follow those guidelines. If no recommendations or requirements are specified, define the purpose (balancing or analysis), for the sensor, review the performance requirements such as frequency response and environment, and then find a sensor that matches your needs.

Tip Archive