2002-07-17 If the frequency is not steady, but still cyclic (repeating), then the definition of the hertz based Rowlett is still valid. The increase and decrease of the frequency would be treated like an acceleration and would be measured in hertz per second. (Hz/s). And if there is a change in acceleration, then you have jerk, and that would be measured in hertz per second squared (Hz/s^2).
But, in the case of drift in frequency, if the drift is slight, one can still view it as being steady with the average being the centre frequency. When you car is idling at say about 750 r/min (or about 12.5 Hz), it is not constant. The idle speed is an approximate average. And that is how it would be viewed whether one measures it in hertz or in revolutions per second. John ----- Original Message ----- From: "Gene Mechtly" <[EMAIL PROTECTED]> To: "U.S. Metric Association" <[EMAIL PROTECTED]> Cc: "U.S. Metric Association" <[EMAIL PROTECTED]> Sent: Wednesday, 2002-07-17 20:16 Subject: [USMA:21204] What can we replace rpm with? > Carter, > > By the rowlett definition, Hz is restricted to a "periodic" cycle (with > a (at least implied) fixed frequency of rotation. > > In many cases of rotating shafts, the rate of rotation is a variable, > not periodic. In these cases, the rowlett definition must be rejected. > > In contrast, the BIPM Brochure does not require cycles measured by the > Hz to be constant. Thus, the Hz is acceptable for a variable rate of > shaft rotation, but I would prefer rev/s for simple cases of rotating > shafts, or rad/s for more complicated cases of rotations where the > calculus and differential equations are the appropriate tools. For > example, analysis of a rigid body "tumbling" about all three axes. > > Gene. > ......................................... > On Wed, 17 Jul 2002, Carter, Baron wrote: > > > If we accept the following definition of Hz (from > > http://www.unc.edu/~rowlett/units/dictH.html) then surely Hz would be the > > obvious choice in replacing rpm? > > > > "the SI unit of frequency, equal to one cycle per second. The hertz is used > > to measure the rates of events that happen periodically in a fixed and > > definite cycle; .............." >
