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Durometer Calibration for the Oil, Gas and Coatings Industry


Durometer calibration for the oil, gas and the coatings industry should be performed by an ISO/IEC 17025 accredited lab


When selecting a supplier to calibrate your durometer, be sure the lab you select can supply you with accurate and reliable results per ASTM D2240. The technical competence of a laboratory depends on many factors.
  • The right equipment- appropriately calibrated and maintained and traceable to NIST.
  • Qualified experienced technicians
  • Adequate quality assurance and control procedures
  • Accurate recording and reporting of all data required
Accreditation by an 3rd party organization that evaluates the laboratory, equipment, and staff against established standards (such as ISO/IEC 17025, ANSI Z540) is recommend by PTC® Instruments. The accrediting body requires the calibration chain to trace back to NIST. In doing so the lab must calculate the actual uncertainties as each link in the chain increases the uncertainty of the measurement. The processes used and competency of people performing the calibration are also key elements of ISO/IEC 17025, and is a critical difference in this standard say as compared to ISO 9000 type approval.

Durometer calibration is covered by ASTM D2240 Section 7. Your calibration report should include indenter geometry, gage display at 0 and 100, force curve measurements and gage linearity from 10 to 90. If your certification does not include all aspects of Section 7 your durometer may be provide inaccurate readings. All durometer calibration reports should include as received and as left readings. Calibration reports with no as received readings are of little use to the customer if the unit was out of tolerance when received.

The Problem with NIST Traceable Numbers

What is traceability? According to NIST, "[it] requires the establishment of an unbroken chain of comparisons to stated references." NIST stands by its own measurements and can provide standard reference materials and calibrations that we can be assured conform to other NMIs around the world. Since it's not possible to have NIST calibrate every instrument, traceability allows us to propagate measurement standards.

So we create a chain that can be traced back to the reference. If Cal Lab A has a thermometer calibrated by NIST, then Cal Lab B can have their thermometer calibrated by Cal Lab A, and be NIST-traceable. Cal Lab B can then calibrate thermometers for Lab C, and so forth.

What is important to understand, however, is that as you move farther and farther from that original calibration of Cal Lab A to the national standard reference, the uncertainties increase. Each link has its own uncertainties. Accuracy is really about uncertainties, and each instrument-that which is calibrated and that which is the reference- has discrete uncertainties. NIST-traceability cannot equate to accuracy in calibration; it is, rather, a starting place.

Keep in mind that there is no auditing process for using the term "NIST-traceability." Any calibration lab can claim it, and it may well be true. But how many calibration labs down the chain of traceability are you? If your device is calibrated to a reference that has degraded in its accuracy, you may not know it until you have a catastrophic failure of a product or process.

ISO 9001 VS. ISO/IEC 17025

Laboratories can be audited and certified to the international management systems standard ISO 9001, which is used to evaluate their systems for managing the quality of their products. ISO 9001 is not appropriate for evaluating technical competence of a lab calibrating Durometers. The evaluation of a supplier against ISO 9001 does not assure you or your customers that the test, inspection, and calibration data are accurate and reliable. There are differences in the purpose, criteria, and emphasis of the ISO 9001 quality system standard and those of the accreditation standard ISO/IEC 17025. For laboratories concerned with demonstrating technical competence, ISO/IEC17025 is the appropriate standard for laboratories calibrating durometers for the Oil and Gas Industry.

Durometer calibration reports on Durometers manufactured by PTC® must include the following or are not recognized as valid calibration by PTC® Instruments or PTC Metrology™.

1.Indenter Extension: The indenter extension is 0.098 0.002 in.). (2.5±0.04mm) Out of Tolerance Indenter extension can cause out of tolerance readings.

2.Indenter Geometry: Verified by optical comparator at a minimum of 20X power. 100X power is recommended by PTC Metrology. ASTM Type D Indenter 30º ±1/2º 0.004r ± .0005 inch r (0.100±0.012mm R). A flat or misshapen indenter that is non-conforming will cause inaccurate readings.
ASTM Type A Indenter 35º ±1/4º 0.031 ±0.001 inch. A worn or dimensionally incorrect indenter will cause inaccurate readings.
(Imported Durometers that are not marked with ASTM D2240 may have indenters that are non-conforming.)

3.Indenter Display: The indicator displays a value equal to the indenter travel measured within -0.0 +1.0 durometer units measured at 0.
Display at 0 -0.0 +1.0 Durometer units.
Display at 100 ± 0.50 Durometer units measured at 100. Any out of tolerance readings on the display (Durometer Gauge) at either 0 or 100 will cause inaccurate readings.

4. Linearity The durometer shall be checked for linearity at the 10, 20, 30, 40, 50, 60, 70, 80, and 90. In most cases a non-linear durometer cannot be repaired.

5. Spring Force Calibration The durometer spring shall be calibrated at displayed readings of 10, 20, 30, 40, 50, 60, 70, 80, and 90. An out of tolerance spring force measurement will CAUSE inaccurate readings.

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