HOW TO MEASURING THE VIBRATION AMPLITUDE

Amplitude of the vibration level can be measure with four ways. The value of amplitude can be measure with peak to peak, zero to peak, RMS and average. Peak to peak is the distance from the top of the positive peak to the bottom of the negative peak. This method of measurement is most often used when referring tp displacement amplitude.

The value of zero to peak is the measurement from to zero line to the top of the positive peak or the bottom of the negative peak. This type of the measurement is most often used to describe the vibration level from a velocity transducer or accelerometer.

The Root Mean Square (RMS) is the true measurement of the power under the curve. The RMS value is 0.707 x peak only applies to pure sine wave. The true RMS value can be calculated by the square root of the sum of the squares of a given number of points under the curve.  The formula for true RMS they are:

RMS = square {(P₁² + P₂² + P₃² _+… P_n²)/n}

For measurement the value of the true RMS, the measurement for signals that it contain pulses must consider the value of the crest factor and duty cycle. The crest factor (CF) is the ratio of the peak value to the RMS value with the DC component removed. These formulas can be write:

CF = (P-DC)/RMS

For accurate measurement of pulse, the value of the crest factor is 7 normally can be used. The duty cycle is the ratio of the pulse width (PW) to pulse recurrence frequency (PRF). The formula for duty cycle they are:

duty cycle = PW/PRF

There are also several forms of pseudo are used in some equipments. This value of the pseudo RMS can be formula with

RMS = 0.707 x peak

RMS = square (AC² + DC² )

Analog meters is the equipments that can be use as to measure average amplitude. There are various the value of constants are the used to calculate peak, peak to peak, zero to peak, or RMS. Most measurements that are not true RMS measurements are either overstated or understated. When describing the vibration level of a machine, the RMS value should be used if possible. However some cases require peak to peak measurements, for example when measuring mils of displacement. Other case require zero to peak displacement measurement such as high places on the roll.

 The average values are measured by analog meters. The value average is the converted to peak by multiplying a constant of 1.57. These calculated values are accurate only when measuring pure sinusoids. The following constants may be helpful. However they apply to pure sine waves only. The more of the signal deviates from a true sine wave, the more a error is introduced.

Average = 0,637 x peak

Average = 0,90 x RMS

Peak to peak = 2 x peak

Peak = 1,414 x RMS

Peak = 1,57 x average

RMS = 0,707 x peak

RMS = 1,11 x average