Originally Posted by Walt Strong
What is API 670 definition of "true peak"?
A definition per se doesn't exist within API 670.
Quoting from API 670 5th Edition, Annex E, specifically
E.2.5 Several important additional factors shall also be considered:
a) The detection circuitry in the monitor shall be consistent with the displayed units. If peak is displayed, a peak circuit detector shall be used in the monitor circuitry. Confusion occurs when an rms detector is used in the monitor and its output is scaled by 1.414 to display as peak units. This conversion is only valid for purely sinusoidal signals, which is rarely the situation except during calibration. An instrument displaying peak as 1.414 × rms may yield significantly lower values than one with a true peak detector when observing the same vibration signal. Many portable instruments use this approach, which can create confusion when comparing readings. To avoid confusion, it is recommended that peak measurements derived from rms be referred to as “derived peak” to distinguish them from “true peak” measurements.