New ASTM E74-18 Changes - Intro
It’s been five years since ASTM E74 was last updated. The last version of the standard was ASTM 13a, and I believe we may have an ASTM E74-18a. However, the new standard has been released and this blog is going to detail some of the major changes between ASTM E74-13a and ASTM E74-18.
A list of changes
Section 3 Changes
3.1.2 primary force standard—a deadweight force applied directly without intervening mechanisms such as levers, hydraulic multipliers, or the like, whose mass has been determined by comparison with reference standards traceable to the International System of Units (SI) of mass
- A great change that needed to happen as the previous version was referencing national standards of mass and all measurements are traceable to SI and not NIST or traceable through SI units through an NMI.
3.2.6 verified range of forces—in the case of force-measuring instruments, the range of indicated forces for which the force-measuring instrument gives results within the permissible variations specified.
- The first instance of the change away from the terminology of “loading range” the standard now uses a verified range of forces throughout the document. Other changes are “devices” are now referenced as force-measuring instruments.
Section 6 Changes
6.1 Primary Force Standards—Weights used as primary force standards shall be made of rolled, forged, or cast metal. Adjustment cavities shall be closed by threaded plugs or suitable seals. External surfaces of weights shall have a finish (Roughness Average or Ra) of 3.2 µm (125 µin.) or less as specified in ASME B46.1.
- Previously the standard referenced a surface finish of 125 or less as specified by ANSI B46.1. Not really much of a change but it is definitely better guidance on the appropriate finish for weights.
Throughout the document, the term Lower Limit Factor (LLF) has become a lower limit factor (LLF)
Section 7 Changes
This section is about Calibration and has what may be the most noticeable changes
7.1.2 added, “While it is the responsibility of the calibration laboratory to calibrate the force-measuring instrument in accordance with the requirements of these practices, it is the responsibility of the user to determine the measurement uncertainty of the instrument in service.”
- The wording is specific as several were using the ASTM lower limit factor as the uncertainty. This dates back to ASTM E74-06 when the actual word Uncertainty was used instead of the lower limit factor (LLF). Morehouse has some guidance on calculating the measurement uncertainty that is available online here.
7.4.1 “……For any force-measuring instrument, the errors observed at corresponding forces taken first by increasing the force to any given test force and then by decreasing the force to that test force may not agree. Force-measuring instruments are usually used under increasing forces, but if a force-measuring instrument is to be used under decreasing force, it shall be calibrated under decreasing forces as well as under increasing force. Use the procedures for calibration and analysis of data given in Sections 7 and 8 except where otherwise noted. When a force-measuring instrument is calibrated with both increasing and decreasing forces, the same force values should be applied for the increasing and decreasing directions of force application, but separate calibration equations should be developed.”
- Major change as the former note that stated should be calibrated is now a shall. Morehouse has run tests and shown the errors between the ascending curve and descending curve to be in the magnitude of 0.05 % on very good force-measuring instruments. Simply put, the device should be calibrated in the manner it is being used and if you need to use the force-measuring device for decreasing loading or to determine hysteresis, etc.., then it needs to be calibrated in that manner.
7.5.1 “In a compression calibration, position the force-measuring instrument to a 0-degree reference position and then rotate to positions of approximately 120 degrees and 240 degrees. An exception is made for force-measuring instruments that cannot be rotated by 120 degrees such as some proving rings, force dynamometers, and Brinell Hardness Test Calibrators. For these types of force-measuring instruments, position the force-measuring instrument at 0 degrees and then rotate to positions of approximately 60 degrees and 300 degrees, keeping its force axis on the center force axis of the machine. This exception is made to minimize parallax error.”
- Major change as before the text allowed rotation by one-third, one-quarter, or one-half turn. In some cases, this would allow calibration laboratories to avoid sections of the force-measuring instrument that may have had high deviations. Now the standard is clear as to what needs to be done to test for reproducibility. Section 7.5.2 is basically the same change but for tension.
7.5.3 is a subtle change allowing compression or tension to be used as the first part of the calibration.
Section 8 Changes
8.2.3 “8.2.3 For force-measuring instruments calibrated for use over the following verified ranges of forces, the creep recovery error limits of the output at the applied force are: Class AA: 0.020 % Class A: 0.050 %”
- Not a major change just wording. The requirements stay the same
8.3.1 “For high-resolution force-measuring instruments (see 7.1.3), the procedure of Annex A1 shall be used to obtain the maximum degree of the best fit polynomial equation statistically supported by the calibration data set.”
- Major Change as the Previous version used the wording should and now using Annex A1 is required.
8.6 the old graph is now gone
8.6.2 “The verified range of forces shall not include forces outside the range of forces applied during the calibration. If the lower force limit is less than the lowest non-zero calibration force applied, then the lower force limit of the verified range of forces is equal to the lowest calibration force applied”
- The standard is just restating that the range of forces cannot be less than the first non-zero force point. If the calibration provider starts with 0 and then the next calibrated point is 10,000 lbf, the Class A verified range of forces cannot be less than 10,000 lbf.
Section 9
9.4.1 “For force-measuring instruments with such compensation calibrated for use over the following verified ranges of forces, the temperature error limits as a percent of reading are (See Note 11): Class AA: 0.010 % Class A: 0.050 %”
-Major change as if the manufacturer does not compensate the device properly, then they may not be suitable for use.
9.4.2 “If a force-measuring instrument is used at temperatures other than the temperature at which it was calibrated, it is the user’s responsibility to ensure that the performance of the force-measuring instrument does not exceed the limits of paragraphs 9.4.1, or if such limits would be exceeded, that the force-measuring instrument is calibrated at the expected temperature of use, or over a range of the expected temperatures of use and corrected accordingly.”
-Major change as this is a new section that puts the risk on the end-user to correct accordingly. This is a major buyer beware on some devices that claim to be temperature compensated but are not.
Section 10 Force-Multiplying System
10.1.1 has some major changes referencing NIST Handbook 44
Lots of additional changes in this section that one should purchase the standard to read.
Section 13 Report
Significant changes
13.1.7 Listing of the calibration forces applied and the corresponding deflections at each rotational position, including the initial and return zero forces and measured deflections
-Major Change in accordance with 7.5.1 and 7.5.2 requires the rotational position to now be listed on the certificate
13.1.11 The result of the creep recovery test, when performed,
13.1.12 The excitation voltage and waveform used for calibration when known,
- Both of these are new and when performed and when known is the requirement.
There are more changes than what is listed here. Anyone wanting to be compliant with the ASTM E74-18 standard can purchase a full copy here . If you are accredited and calibrate to the ASTM E74 standard, then it is a requirement that the latest standard be purchased.
References
[1] ASTM E74-13a is titled Standard Practice of Calibration of Force-Measuring Instruments for Verifying the Force Indication of Testing Machines
[2] ASTM E74-18 Standard Practices for Calibration and Verification for Force-Measuring Instruments
ASTM E74-18 Changes - Outro
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