On Sat, 22 Jul 2017 19:44:26 -0500,
dpb wrote:

I got curious myself on what the numbers revealed and looked at the NIST
numbers again.

I computed an empirical cdf and compared it to normal...statistics from
the 20,036 observations are below:

[2 quoted lines suppressed]
s =
min: -50
max: 146
mean: -0.0788
std: 3.7681
median: 0
mode: 0
[1 quoted line suppressed]

I then compared to normal on the same plot and as outlined above
N(mean,std) is too long-tailed on both ends in comparison. It turns out
that N(mean,std/1.5) is pretty close on both tails to about the +/- 6 point.


Anyway, from the above it's simple enough to get some pretty good
estimates of what pump volume errors one might expect...the table below
is from the empirical cdf NIST data...

P error(in^3)/5Gal error(%)
0.001 -22 -1.82
0.005 -9 -0.78
0.010 -8 -0.69
0.025 -6 -0.52
0.050 -5 -0.43
0.250 -2 -0.17
0.500 0 0
0.750 2 0.17
0.900 4 0.34
0.950 5 0.43
0.975 6 0.52
0.990 7 0.60
0.995 10 0.86
0.999 22 1.82

From the above, one can conclude the pump metering error small for all
except the extreme outlier pumps.

I love that you are the only one quoting actual numbers and not pulling
them out of your butt to answer the question!

But your numbers confuse me because they seem to be in cubic inches.
You also mentioned that metric pumps are more accurate, but that's
impossible, simply because the pump is as accurate as the pump can get,
which, we can assume, is a mechanical thing (and not a metric thing).

All you're saying is that a liter is four times smaller than a gallon so
the error is four times less for a given liter versus a given gallon but
that's not saying it's more accurate. It's just saying the volume is less
so the resulting error is less.

Anyways, can you just summarize what the error is for a typical USA pump in
gallons?

For a typical 20-gallon fill, how many gallons off can reality be, plus or
minus from the indicated reading on the pumpmeter?