Featured Video
This Week in Quality Digest Live
Metrology Features
James Rawstron
Jet wing surface scan reduced to under four minutes
Michael Huda
How do you control a color that absorbs so much light, spectrophotometers can’t measure it?
Matthew Martin
Traditional CMMs are being challenged by fast-emerging, highly advanced blue-light scanning
Dirk Dusharme @ Quality Digest
Christmas is creeping, should a four-year degree really be the minimum requirement, and what we learn from Disneyland
Dean Solberg
If you can think about it and model it, you can 3D-print it

More Features

Metrology News
Four times faster data acquisition, enabling precise measurement at the submicron level
Caters to the growing need for sophisticated scanning in an entry-level solution that is budget friendly
Topics for the Go Forth and Measure project are virtually unlimited
Ask questions, exchange ideas and best practices, share product tips, discuss challenges in quality improvement initiatives

More News

Henry Zumbrun

Metrology

May the Force Be With You

Recommended steps for calibrating instruments in accordance with ASTM E74-13a

Published: Thursday, July 7, 2016 - 15:36

There has been some misunderstanding about the intent of ASTM E74—“Standard practice of calibration of force-measuring instruments for verifying the force indication of testing machines.” When it was written back in 1974, the standard’s intent was to establish calibration traceability back to primary standards laboratories. This article offers some insight into the calibration procedure as outlined in ASTM E74-13a.

Anyone calibrating in accordance with the ASTM E74 standard should first purchase a full copy of the standard here. The major calibration steps include stabilization, preparation, the calibration procedure itself, and data analysis. Let’s look at each in turn.

Stabilization

There are two steps to the stabilization process:
1. Allow the unit under test (UUT) to come to room temperature. This is size-dependent and can take 24 to 48 hours.
2. Warm up the instrumentation. We recommend 20 to 30 minutes for electrical stabilization of the equipment.

Preparation

Select 10–11 test points. If the UUT will be used to calibrate other devices using both increasing and decreasing force, it must be calibrated in both of these modes. You want to choose 10 force points for decreasing and 11 force points for increasing, so the total number of force points selected will be at least 21. 

Note: Using an ascending calibration curve to calibrate a device in decreasing mode can result in errors of 0.05 percent and higher. Section 7, note 6 of ASTM E74-13a addresses descending loading.

There should be at least one calibration force for each 10-percent interval throughout the loading range. If the instrument is to be used below 10 percent of its capacity, a low force should be applied. For Class A devices this low force must be greater than the resolution of the device multiplied by 400; for Class AA devices the low force must be greater than the resolution of the device multiplied by 2000. (See figure 1.)

Note: This means that an applied force of 0 as the first test point is not allowable.

Figure 1: The various allowable errors at each level of the ASTM E74 Pyramid

Calibration procedure

There are eight steps to the calibration process:
1. Place the UUT in the test frame.
2. Exercise the UUT two to four times.
3. Apply the first series of forces (Run 1).
4. Rotate the UUT 120° if possible for Run 2.
5. Apply the second series of forces (Run 2).
6. If the UUT operates in both compression and tension, switch to the other mode after finishing Run 2 and repeat the steps above.
7. Rotate the UUT another 120° degrees if possible for Run 3.
8. Apply the third series of forces (Run 3).

For tension and compression calibration, intersperse the loadings. Be sure to re-exercise the UUT prior to any change in setup. More information can be found by reading section 7 of the ASTM E74-13a standard.

Note: I’ve written more about load cell reversibility here. The preliminary findings indicate that there’s not a statistically significant difference on a shear web-type load cell when using this method vs. compression and then tension loading.

Data analysis

ASTM E74 calibration data analysis should take into consideration the following (see figure 2):

Deflection calculation methods

There are two methods to calculate deflection:
1. Method A deflection readings are calculated as the difference between the deflection at the applied force and the initial deflection at zero force.
2. Method B deflection readings are calculated as the difference between readings at the applied force and the average or interpolated zero force readings before and after the applied force readings.

Criteria for using higher-degree curve fits

There are two criteria for using higher-degree curve fits:
1. The resolution must exceed 50,000 counts.
2. An “F distribution test” is used to determine the appropriate best degree of fit (instructions for this test can be found in Annex A1 of ASTM E74).

Criteria for lower load limit

There are three criteria for the lower load limit:
1. LLF = 2.4 * STD DEV. This corresponds to a 98.2-percent coverage factor, based on LLF or resolution, whichever is higher.
2. Class A = 400 times the LLF or resolution
3. Class AA = 2000 times the LLF or resolution

More information can be found by reading section 8 of the ASTM E74-13a standard.

Note: Per section 11.3, any instrument that is either modified or repaired should be recalibrated, and recalibration is required for a permanent zero shift exceeding 1.0 percent of full scale.

Figure 2: Points to consider for ASTM E74 calibration data analysis

Determining the calibration interval

Per section 11, secondary standards should be calibrated or verified annually to ensure that they don’t change more than 0.032 percent over the loading range. Instruments used as Class A devices (i.e., those typically used to calibrate testing machines) should be calibrated or verified annually to ensure that they don’t change more than 0.16 percent over the loading range. If the calibration is stable to within 0.16 percent over the loading range, then the calibration interval can be two years, as long as the UUT continues to meet the stability criteria.

ASTM E74 lower load limits matter

There are two rules to remember when calibrating to ASTM E74:

1. The Class A or Class AA loading range can’t be less than the first applied non-zero force point
Per Section 8.6 of ASTM E74-13a: “The loading range shall not include forces outside the range of forces applied during the calibration.

Per Section 7.2.1: “If the lower limit of the loading range of the device (see 8.6.1) is anticipated to be less than one-tenth of the maximum force applied during calibration, then forces should be applied at or below this lower limit.”

2. Zero cannot be a test point
Per Section 7.2.1 of ASTM E74-13a, In no case should the smallest force applied be below the lower limit of the instrument as defined by the values: 400 x resolution for Class A loading range and 2000 x resolution for Class AA loading range.

Calibration and measurement capability (CMC) spreadsheet for force

My organization, Morehouse Instrument Co., has developed a Calibration and Measurement Capability (CMC) Worksheet to help those trying to figure measurement uncertainty and calculate CMC (see figure 3). The worksheet is set up for labs following the ASTM E74-13a standard. With slight modification, the uncertainty sheet could be used for instruments calibrated in accordance with another test method or standard. Download the worksheet here.

Figure 3: Force CMC for ASTM E74 calibrations

Discuss

About The Author

Henry Zumbrun’s picture

Henry Zumbrun

Henry Zumbrun is president of Morehouse Instrument Co. where he has managed the force and torque calibration lab and services in the family-owned business since the 1990s. Morehouse helps labs lower their force measurement uncertainties and torque, resultin in more accurate measurements, which lowers costs, reduces risk, and increases quality. Morehouse designs and manufactures products in line with customer requirements, lean, Six Sigma, and best practices guidelines.