The Difference Between RMS, Peak and Peak to Peak Amplitudes
Again, all data is collected as an "RMS" value ("Root Mean Square"). That is an ISO convention and applies no matter what manufacturer's collector you may have.
The "RMS" value is calculated by simply multiplying the peak amplitude (shown in the graphic below) by 0.707:
RMS = Peak x 0.707
    But is that technically correct ?
    • Technically, the RMS value of a pure sinusoid is equal to the area under the half-wave.
    • That corresponds to (peak x 0.707) on a pure sinusoid.
    • Pure sinusoids are rarely encountered in the world of mechanical vibration.
    • Due to that, the trend amplitude value is calculated by simply multiplying the peak amplitude by 0.707.
But some analysts prefer to display the amplitudes as a 'peak' amplitude (usually because the vibration severity charts they are using are in peak amplitude values). So how is that value calculated ?
  • To calculate the 'Peak' amplitude, we divide the RMS value by 0.707.
  • For those of you paying close attention, you may have noticed that, from start to finish, the way a peak amplitude is calculated is:
    • Measure the peak amplitude.
    • Multiply by 0.707 to obtain the RMS value.
    • Divide by 0.707 to obtain the Peak value.
In some cases - specifically the use of displacement amplitudes where the analyst wants the total amount of bearing movement back and forth - the 'Peak to Peak' value is desired by the analyst. In that case, the peak value is simply multiplied by 2. Velocity and acceleration units are not typically displayed as peak to peak since the important piece of information is what the maximum (peak) value is.
So let's summarize:
  • The highest (peak) value of collected data is multiplied by 0.707 to obtain the ISO standard of RMS amplitude. The majority of programs world-wide use this value.
  • For those programs that wish to display the data as a 'peak' value, the RMS value is then divided by 0.707 to obtain the peak amplitude.
  • For those situations where peak to peak amplitudes are desired, the peak amplitude is simply multiplied by 2.
Summary: It doesn't matter which value you use - RMS or Peak - so long as you are CONSISTENT (another very important convention) !! Amplitude is, after all, simply a number from which we make certain generalizations about the machine condition. There is no exact, precise number above which disaster awaits and below which you are safe. 
Be aware of the conversion if comparing values between programs that use RMS and Peak values. This applies to spectrum plots as well.
Now let's look at the vibration sensors - transducers.