Expressing fractions as decimals - Indicators on older or lower precision instruments

L

Lorrie

#1
I work in the scale industry. Instruments that we test generally have a digital readout, expressed in decimal units. Ocassionally, we will happen upon an older or lower precision instrument whose readout is expressed in fractions (for example, 1/4 lb increments). This could be a dial type indicator (like the scales at the fair where the guy guesses your weight) or a beam type scale such as you typically see in doctor's offices.

Is it proper with this type of indicator to write the test result using decimals? Our software doesn't like fractions, but I feel that writing "0.5" instead of "1/2" implies a level of precision that doesn't exist. Does 17025 address this issue?
 
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Tim Folkerts

Super Moderator
#2
Lorrie,

As a general suggestion, you might try expressing all your answers so that they don't imply any particular level of uncertainty, but rather have the answers explicitly indicate the uncertainty.

For example,
10.0 +/- 1 for a state fair scale
10.0 +/- 0.25 for a doctor's scale
10.0 +/- 0.01 for a laboratory scale
That way the uncertainty is clearly stated.


NIST has some free guidelines (and links to other ISO guidelines you can purchase) at http://physics.nist.gov/Pubs/guidelines/contents.html


Tim F
 
J
#3
Don't see a problem

Lorrie,
I don't see a problem with expressing these in decimal increments.
I do understand your concern since the level of precision implied can vary with the decimal .5, .25, .375, .0625 etc.

There should be an area to call out precision, or an area for notes/comments on the calibration form. You can address this issue in one of those areas.

Hope this helps.

James
 

Jim Wynne

Staff member
Admin
#4
Standard practice calls for reporting in the same units of measure as the specifications or graduations, but if the software won't allow it, you can add an explanation if there's a comments field.
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#5
A solution that may cause less confusion for customers could be to put both on the certificate, for example list the results in the fractions and put the decimal equivalent in parenthensis beside it.

Just a thought.

Hershal
 

Statistical Steven

Statistician
Staff member
Super Moderator
#6
Jim Wynne said:
Standard practice calls for reporting in the same units of measure as the specifications or graduations, but if the software won't allow it, you can add an explanation if there's a comments field.
In the FDA regulated industries, we have a significant figures document that spells out how numbers should be recorded. If you are just measuring and reporting something to the closest 0.25 pounds, then using 0.25, 0.50 and 0.75 are fine. If you need to report to the closest 0.1 pounds, then the old fashion scale would not be acceptable.
 

Jim Wynne

Staff member
Admin
#7
Statistical Steven said:
In the FDA regulated industries, we have a significant figures document that spells out how numbers should be recorded. If you are just measuring and reporting something to the closest 0.25 pounds, then using 0.25, 0.50 and 0.75 are fine. If you need to report to the closest 0.1 pounds, then the old fashion scale would not be acceptable.
As I understand the original question, it has nothing to do with precision, per se; it's a matter of UOM being in fractions and software that will only do decimals. I understand that a scale graduated in fractions isn't appropriate for some applications. There's nothing wrong with reporting 1/2 as .5 so long as there's something in the report that explains the issue.
 

Jim Wynne

Staff member
Admin
#8
Hershal said:
A solution that may cause less confusion for customers could be to put both on the certificate, for example list the results in the fractions and put the decimal equivalent in parenthensis beside it.

Just a thought.

Hershal
But the problem seems to be the software not being able to handle fractions. If there's an entry field that allows for a decimal entry of x places, but not a fraction value such as 3/4, then you're stuck with .750 and explaining that the device is graduated in fractions.
 
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