Aluminum Calibration Foils for Bursting Strength Tester

Question :

Should we use Aluminum Foils for “Calibrating” a Bursting Strength Tester ?

Answer : NO.

Explanation :

Let us first see what the International Standards say about Aluminum Foils.

TAPPI standards (all BS Testers are made according to these standards) say :

“The pressure indicating device shall be calibrated by means of a dead -weight tester of the

piston type.”

and

“Aluminum foils of various thickness may be available for use as t est pieces of known burst

value. Such devices are a useful means of checking the overall function of an instrument, but

since the behavior of foil under stress is different from paper, they should not be used as

calibration standards.”

Now let us understand CALIBRATION.

http://en.wikipedia.org/wiki/Calibration is a link to general calibration information

The 3rd

paragraph of http://en.wikipedia.org/wiki/Calibration#Basic_calibration_process states :

“The next step is defining the calibration process. The selection of a standard or standards is

the most visible part of the calibration process. Ideally, the standard has less than 1/4 of the

measurement uncertainty of the device being calibrated. When this goal is met, the

accumulated measurement uncertainty of all of the standards involved is considered to be

insignificant when the final measurement is also made with the 4:1 ratio. This ratio was

probably first formalized in Handbook 52 that accompanied MIL-STD-45662A, an early US

Department of Defense metrology program specification. It was 10:1 from its inception in the

1950s until the 1970s, when advancing technology made 10:1 impossible for most electronic

measurements.”

Uncertainty means Accuracy in this case.

In simple English, it means to say that – “the accuracy of the calibrating device should be

greater than the device to be calibrated. Earlier it used to be 10 times. Now it is 4 times.”

In our context :

For all practical purposes we say that the calibrating device (Calibrator) should have an

accuracy “greater or atleast equal” to the device being calibrated (Calibratee)

How can you use an Aluminum Foil (eg. 10 ± 0.4 kg/cm2) having an accuracy (uncertainty) as

poor as 0.4 Kg/cm2 be used to calibrate a BS Tester which has a resolution of 0.2 – 0.5 Kg/cm2

in case of Analog Machines and as high as 0.01 Kg/cm2 in case of Digital Machines.

Isn’t it like trying to calibrate a Micro -Second Stop Watch with a Wall Clock ?

Traceability :

Finally we come to the point of evaluating, how accurate is the Foil reading ?

How was the reading derived (obviously by testing it on another BS Tester) ?

How accurate the “other” BS Tester was ?

And how was the “other” BS Tester calibrated ?

If you use a foil which was tested (calibrated) by a machine, which was also calibrated by

another foil, then this cycle is endless. What it the traceability ?

At what level does the Foil have traceability to National Physical Standards (NPL or NABL) or

International Physical Standards (NIST, etc)

If there is no traceability, it is similar to an illiterate person signing on a Graduation Certificate

of a University.

Hope my explanation above was helpful.

Please feel free to contact us should you need any further information / clarification.

Please also download our latest catalog from

http://www.packtest.com/downloads/catalog2009hi.pdf

Rohit Chawla

Rohit Instruments & Testing Services

K-1, 23/2, G.I.D.C. Ranoli

Vadodara - 391350

Gujarat. INDIA.

Phone : +91 - 265 – 2308600 Fax : +91 - 265 – 2308600 Mobile : +91 - 98240 69001

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