On Sun, 23 Jul 2017 11:21:22 -0500, dpb wrote:

On 07/22/2017 11:39 PM, wrote:
On Sun, 23 Jul 2017 02:40:10 -0000 (UTC), Mad Roger
wrote:

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:

,,,

min: -50
max: 146
mean: -0.0788
std: 3.7681
median: 0
mode: 0
...

For a typical 20-gallon fill, how many gallons off can reality be, plus or
minus from the indicated reading on the pumpmeter?
The poorest pump checked in that data would be +/- 44.6 oz per 20
gallon tank - the average about +/- 12 ounces.
...

Actually, the extrema limits in the sample are much worse: -50 and +146
in the 5 gal test collection. I simply computed the table for typical
exceedance limits; didn't include the endmost points there. The 0.999
point, for example, is the int(20036*0.999)-th observation or 20015 so
there were 21 more observations above that value. The last two,
however, were really, really outliers that skew things quite a lot. The
last five observations were

NIST(end-5:end,
ans =
28 1
29 1
30 4
56 1
127 3
146 1


It's quite an oddity that there were 3 observations at 127; illustrating
again that "random data aren't" or more correctly that one can always
find patterns visually even in random data.

Of course, if the pump is putting out 5.6 gal/5 gal reading, you'll only
have "pumped" 20*5/5.6 -- ~17.8 gal when the tank is full already...

Hmmm...I had noticed the outlier on the positive end and looked at it;
wonder what the LH tail looks like now...

NIST(1:5,
ans =
-50 1
-45 1
-36 1
-35 4
-32 1


Pretty similar pattern, just not as extreme. There are two outliers
separated quite a bit from the bulk of the rest of the observations, but
they're only 10 cu in differential to nearest whereas there's 70+ on the
other end.

Interesting that from a customer viewpoint you're just about as likely
to get more than you're paying for as under and on the extreme ends by a
lot more than by what you get shorted.

I didn't try to find what Canadian limits are -- I presume they must be
somewhat more stringent in order to match, more or less, the size of the
measurement interval? You know?
I do not know the requirements or test results today, but I DO know
back when we had mechanical meters our pumps (at the stations where I
worked)were never out by more than a couple oz in the 5 gallon
calibration, and the new electronic metering pumps (deployed when we
switched from imperial Gallons to Liters) were "significantly more
accurate" in metering. The accuracy changed a bit with delivery speed
on the mechanical pumps - can't remember if the change was that the
read higher or lower with reduced pump speed, but the variance was
quite low. The electronic meters were supposedly less susceptible to
volume arrors based on fuel velocity.

The last years of my automotive career were not involved with gasoline
retailing in any way - from a short time after the switch to Metric.