🚀 go-pugleaf

In article <EK-dnV5FoYzmRFTXnZ2dnUVZ_jKdnZ2d@westnet.com.au>,
 Adam Nielsen <a.nielsen@remove.this.shikadi.net> wrote:

> > Note that for A >= 16, doubling A also doubles B'.
> > The best fit is
> >     B' = A/16 * 1.91, truncated to 1 decimal place.
> > 
> > 1.91/16 = 0.119375,
> > and your rational approximation for that is 
> >     1/10 + 1/52 = 31/260 = 0.1192307
> > 
> > Note that there are closer rational approximations, but any one that 
> > provides the correct B' values to 1 decimal place is close enough.
> 
> Ah, glad to see I was close!  Thanks again.  I've got all the current values
> sorted now, but I've discovered that the fan speeds seem to be reported on
> some sort of logarithmic scale:
> 
>    A  |  B
> ------+---------
>   128 | 8943 RPM / 943 RPM *
>   144 | 9727 RPM *
>   256 | 5493 RPM
>   512 | 2746 RPM
>  1285 | 1094 RPM
>  4096 | 343 RPM
>  8192 | 171 RPM
> 65535 | 21 RPM
> 
> I'm not 100% sure about the first two values (marked with *) because the
> values were so large I'm not sure the manufacturer's program was drawing them
> on the screen properly.
> 
> I *think* this is logarithmic - I can get an approximation of the curve using
> variations of -log(x), but I'm afraid that's as far I as can remember :-)  If
> it's possible, an approximation using "simpler" operations (+, -, *, /) would
> be even better from a runtime efficiency point of view!

A little hacking around in Excel shows that this is a log-log 
relationship.  In particular:
    log_2(B) = 20.44592638 - 1.002677224*log_2(A)
with a maximum absolute error of 0.010795444

With these coefficients, the B value for A=128 would be 11015.

> Thanks again,
> Adam.

HTH

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|  B  B   aa     rrr  b     |
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|  B  B  a  a   r     b  b  |    altum viditur.
|  BBB    aa a  r     bbb   |   
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