Displacement: Mils True Pk-Pk to Mils RMS?

Hi there! How do you convert displacement units of mills TRUE pk-pk into mils pk-pk and mils RMS?  I am reviewing vibration settings a contractor recommended for our 2000 HP vertical pumps and notice our alarms differ from their recommendation.  We currently have them set for 9 mills RMS.  I don't know if that's too high, and I can only find severity charts for mils pk-pk and not TRUE pk-pk.


Original Post

This is interesting! 

My experience is limited so what I can say is that I am keen to learn about the reference on which your settings (9 mils rms) are based. This way of expressing vibration amplitude is not common in my small world.

I imagine this machine might be a tall low speed (possibly cooling water) pump.

Regards- Ali M. Al-Shurafa 

Luze, here are several resources for conversion of units for a specific frequency, I would assume the TRUE pk-pk is the same as pk-pk. Your RMS alarm value is .707 or 70% of peak





It depends what you're doing: 

  • Peak is the actual level of the digital samples in Logic. 
  • True Peak is the level of the analog signal that will be rebuilt if you convert Logic's digital output to analog.


Vibration severity charts conforming to ISO are based on overall vibration level in velocity mm/sec RMS. The conversion to in/sec RMS is easy. For converting from 6 mils RMS displacement to velocity in/sec RMS or (mm/sec), you have to assume that most of the vibration level is dominated by a single frequency (usually 1xSS for vertical motor-pumps). Take the pump speed (Hertz or rpm) and calculate/convert displacement to velocity to compare to the ISO severity guide. The Hydraulic Institute has velocity limits, but it generally does not apply to top (NDE) of vertical motor. There are a lot of fine points about True P-P, True Peak, Derived Peak, and RMS with much of the confusion from GE-Bently instrumentation and the differences from old analog circuits and new digital signal processing.

Online conversion calculators:

Search: convert vibration displacement to velocity



Hi, thank you all very much for responding! It was my first time posting to a blog.  After reviewing your comments, I did some additional research and found that “Peak” or “Peak-to-Peak” is a Calculated Peak not a True Peak.  GM’s vibration standard makes a reference to this on page 11 in the link below.

 The TRUE pk-to-pk value is often higher than the derived peak-to-peak value, and the conversion factors used to convert pk-to-pk into RMS cannot be used the same way for True pk-to-pk. It really depends on whether the motion is harmonic or non-harmonic. I tested this by evaluating an old displacement measurement.

 I believe the True pk-pk shows the real severity in comparison to the derived pk-pk value. Unfortunately, most severity charts and industry standards use derived mils pk-to-pk or micrometer pk-to-pk units for displacement.  I have not found severity charts that use True pk-pk units.

 I prefer to reference industry standard severity charts so staff can see where I’m getting my alarm values.  Therefore, this means I have to setup our displacement alarms in mils pk-to-pk units, or convert a severity chart to mils RMS.

 Ali, (In regards to the 9 mils RMS)

After reviewing ISO 20816-5:2018, I found that our pump’s displacement alarms should fall within 270-310 micrometers pk-pk. This equals 10-12 mils pk-pk or 3.7-4.3 mils RMS. These values are within the contractor’s recommended True pk-pk values.  Therefore, the 9 mils RMS alarm we currently have is too high.

Thank you all!

GM Vibration Standard (page 11) https://www.maintenance.org/fi...7/GMVibeStandard.pdf

 Overall Vibration Measurements and True Peak (See pages 3 and 4)  http://citeseerx.ist.psu.edu/v...ep=rep1&type=pdf



Now, I understand what you mean.

I guess despite the accuracy of the amplitude conversion from mil pp to mil rms, your way of expressing amplitudes is against the common convention. Perhaps, it could lead to confusions and mistakes.

I guess the majority of vibration practitioners and the members here in the forum would advise using mil p-p for shaft vibrations or very low-frequency structural vibrations.

On the accuracy aspect, as mentioned by others earlier,  the conversion factor (0.707) cannot hold correct if the vibration signal is complex meaning it contains two components or more. The conversion between mil pp and mil rms is true only if the waveform is a purely sinusoidal signal with a constant frequency. Maybe this is your case but I doubt that.

Regards- Ali M. Al-Shurafa

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