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[JMW]

I produce a spreadsheet based on the calculation method to determine base density and alternative density which is downloaded from my website and has been downloaded from this forum by a goodly number of users.
The accuracy of the spreadsheet has been independently checked (though I include a note recommending users to test the accuracy before relying on it) and it is intended as a tool, not a substitute for the particular industry sectors own standard practice.
Occasionally I get some feedback (always welcome) and just recently I was contacted about an apparent error.
I always welcome such comments as it (a) lets me correct any errors found and lets me learn something new.
A user, following the recommendation to check the accuracy first, found that when he entered a hydrometer reading of 740.6 at 33degC the answer was 756.53kg/m3 (which, because he was using a hydrometer, he should round to 756.5kg/m3). He checked this against a program commonly used in the industry (bunkering fuel oils) and the answer it gave was 756.4kg/m3.
For the spreadsheet, that's a big error.
I checked in a program supplied to the industry by a leading test house (there are similar programs by the oil companies) and found I too got 756.4kg/3 in that program and another.
BUT:
When I looked up the result in table 53B and interpolated the results I got 756.5kg/m3 i.e. what the spreadsheet had said.
Back to the programs. I found that for any hydrometer reading from 740.3 to 740.7 the answer from programs was the same, 756.4kg/m3.

It appears that what these programs do is equivalent to rounding the hydrometer reading to the nearest 0.5 so 740.6 becomes 740.5 and if i use that value then the programs, the tables and the spreadsheet (afte rounding) all can agree on 756.4.

I checked with an industry expert and he says that he usually uses the industry leading program and says that for the industry, the results are "good enough".

Interesting discovery that while the tables can be interpolated to 0.1kg/m3 and hydrometer readings can be corrected with reference to the hydrometer calibration certificate and the reading itself interpolated, the programs, following industry established practise it would seem, use a fairly coarse rounding factor.

I wonder:
a) how many users are not aware of the aparrent rounding function nor of the resultant accuracy limitation?
if, despite the recommendations of ASTM D1298 about reading hydrometers and interpolating the results in tables how many industry sectors actually use simplified procedures with lower than potential accuracy
c) both the Manual of Petroleum Measurement Standards and the ASTM D1298 state that the calculation is now the standard and not the tables and hence, who still uses tables as their sector of the industry requires (or through preference) and how many people still use hydrometers rather than digital density meters?

For information, the latest version of the spreadsheet is attached.
This now resolves the Volume Correction Factor problem (which way to calculate?) by letting the user choose and it adopts nonclemature consistent with ASTM D1298.

PS keeps appearing when I type ( b ) sorry.

Attached Files

•  Density 12MF.xls   521.5KB   142 downloads

Edited by JMW, 24 January 2010 - 01:00 PM.