Sunday, January 1, 2017

The proper way to express torque units is lbf·ft or N·m.



New Years Resolution #1.   We will make an effort to educate the masses on the proper way to express torque units.     

Looking at several scopes, manufacturer's specification sheets and other sources, as a metrology community, we do not always express units of torque properly.  

Torque =  Force * Length

This means N·m is acceptable, lbf·in is acceptable, and lbf·ft is acceptable.  

ft·lbf or in·lbf is is not the conventional method for expressing torque.   You will not find this as an accepted expression of torque in the ASTM torque standard (ASTM E2428) or any NIST handbook.   You will find several manufacturers expressing torque the improper way.    

The foot-pound force (symbol: ft. · lbf) is a unit of work or energy in the Engineering and Gravitational Systems in United States customary and imperial units of measure. It is the energy transferred on applying a force of one pound-force (lbf) through a linear displacement of one foot. 

pound-foot (lbf·ft) is a unit of torque or moment of force (a pseudovector). One pound-foot is the torque created by one pound force acting at a perpendicular distance of one foot from a pivot point.

Torque is derived from the SI units of Length, Mass and Time. The metre is the SI base unit of length. The kilogram is the SI base unit of mass.  The second is the SI base unit of time.  Torque is expressed in terms of SI base units as  m2 · kg · s−2.   

The improper use of torque units can easily be corrected.  When you see manufacturers using ft·lbf on a torque specification, let them know the proper way to express torque is Force times Length. 

So remember to express torque in lbf·ft, lbf·in, lbf·oz, or N·m.

Torque Conversions Per NIST sp811

It is important to note that N.I.S.T. does not currently offer torque calibration services.  A list of NMI's (National Metrology Institutes) offering torque can be found on the BIPM website here.



Morehouse currently has the second most accurate torque machine in the world.   This is the most accurate machine in North America. We can achieve measurements better than 0.0025 % of applied torque from 0.5 through 1475 lbf·ft.  Since NIST does not offer torque, Morehouse can provide torque measurements traceable to SI, through NPL for anyone in North America needing traceable torque measurements.    

More information on our torque calibration services can be found here.




written by Henry Zumbrun
www.mhforce.com

Tuesday, December 20, 2016

Recommendations for Specifying ASTM E74 Calibration Services

Recommendations for Specifying ASTM E74 Calibration Services

This document is intended to help properly specify what you may need for ISO/IEC 17025 accredited ASTM E74 calibrations to ensure you are getting exactly what you need:

Verification the Calibration Supplier Can meet the Requirement.

Reviewing many accredited laboratory Scope of Accreditation and certificates of calibration from the accredited laboratories, Morehouse has witnessed  many laboratories not following the guidelines set forth in the ASTM E74 standard. (blog about the top 3 common ASTM  E74 common calibration errors can be found here )  Also witnessed were secondary accredited laboratories claiming measurement uncertainties lower than the calibration service provider who calibrated their standard load cell and much more.   


Purchase Order (PO) Description Template:

1. The calibration laboratory shall be accredited in accordance with the:
requirements of ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories.

2. The calibration laboratory’s Scope of Accreditation to ISO/IEC 17025 shall:
encompass the calibration parameter of Force (e.g., ASTM E74,  Tension and/or Compression 100 to 1000K lbf with an CMC of XXX or better). Example below clearly shows the lab can perform calibration in accordance with ASTM E74, ISO 376 and other methods.   

Note:If the device is to be calibrated in both Ascending and Descending modes, ask the calibration provider if they have their standards calibrated in both modes.   We wrote a blog on ascending and descending measurement errors.  The blog can be found here.  Many scopes do not contain this information and the provider must be asked if they had their instruments calibrated in the descending mode.  



3. The accreditation of a testing or calibration laboratory shall be issued by an Accreditation Body (AB) that is:
an ILAC Mutual Recognition Arrangement (MRA) Signatory (e.g., A2LA, IAS, ANAB, PJLA) and that operates in accordance with ISO/IEC 17011, General requirements for accreditation bodies accrediting conformity assessment bodies.


Scope Example

4. The testing (or calibration) report or certificate must be endorsed by the:
Recognized Accreditation Body’s symbol (or must otherwise indicate accreditation status by a
recognized Accreditation Body).

AB symbol with Morehouse Certificate #


5.  Specify the range is required by the customer:
(e.g. 1000 to 10,000 lbf)
Note: Some laboratories offer a multi range calibration for one load cell.  If required specify the range(s) on the PO.

6.  Specify if your load cell is required to be calibrated:
in tension, compression or both.

7.  Specify if the load cell is to be calibrated:
ascending, descending or both.

8.  Specify if:
a class AA or A loading range is required.  If Class AA if required, calibration can only be done by labs with Primary Standards.  See picture below for explanation



9. Specify calibration intervals:
Calibrations in accordance with ASTM E74-13 and later are found is section 11.2 
Decide if you want the calibration laboratory to enter the ASTM E74 due dates on the certificate and labels.  If the device does not meet the stability criteria set forth in ASTM, let the calibration laboratory know the time interval to ensure it meets the criteria found below.



Rationale:

Bullet Point #1 stipulates that the testing and calibration be accredited in accordance with ISO/IEC 17025.

Bullet Point #2 requires that the test or calibration services requested be listed on the service provider’s Scope of Accreditation.
Note: If this requirement is not stated, the laboratory could claim that it is ISO/IEC 17025 “compliant” (i.e., self-declared) even though the provider is not actually accredited for the test, calibration, or service requested. This is not to imply that a self-declared “accredited” organization is not technically competent or in compliance, but it should not be necessary for the customer to vet (assess) that organization if they are accredited by a recognized Accreditation Body.

Bullet Point #3 contains two important stipulations: [1] that the laboratory used be accredited to ISO/IEC
17025, and [2] that the Accreditation Body (AB) be under an ILAC Mutual Recognition Arrangement (MRA) signatory. Without this Bullet Point, the laboratory could be accredited by an AB that has inadequate or misleading credentials.
Note: There are ABs in the USA that claim to be in compliance with ISO/IEC 17011 [so-called “self-declared ISO/IEC 17011 compliant ABs”]. Some ABs in the United States that offer what equates to "mail-order" accreditation services and never visit the lab to ensure implementation of ISO/IEC 17025 or other requirements, or to verify its staff's technical competence. Holding an ILAC MRA signatory status indicates that the AB has been periodically peer-evaluated against the requirements of ISO/IEC 17011. This peer evaluation process ensures that ABs meet strict international requirements, thus affording a high level of confidence in their assessment process when accrediting laboratories.

Bullet Point #4 is required to ensure that the test or calibration performed was consistent with the providing organization’s accredited criteria.
Note: Many organizations offer multiple levels of service at varying costs. Furthermore, the lab may meet the requirements for the first three elements of the template, but deliver a service not covered under their accredited services.

Bullet Point #5,6,7and 8 are required to identify to the calibration service provider the required:
·         Force Range(s)
·         Mode(s)
·         Your planned use of the load cell
·         Loading Range Class
Many calibration service providers offer ASTM E 74 services, but not all of them can offer everything listed in Bullet Point 5 – 8, thus your requirements need to be communicated to the calibration service provider.

Bullet Point #9

ISO/IEC 17025 5.10.4.4 states:
A calibration certificate (or calibration label) shall not contain any recommendation on the calibration interval except where this has been agreed with the customer. This requirement may be superseded by legal regulations. 
ASTM E74 states:
11. Time Interval Between Calibrations and Stability
Criteria
11.1 All force-measuring instruments and systems shall meet the range, accuracy, resolution, and stability requirements of this standard, and shall be suitable for their intended use.
11.2 The calibration intervals for force-measuring instruments and systems used as secondary force standards or for the verification of force indication of testing machines shall be calibrated at intervals not exceeding two years after demonstration of stability supporting the adopted recalibration interval. New devices shall be calibrated at an interval not exceeding 1 year to determine stability per 11.2.1.
11.2.1 Force measuring instruments shall demonstrate changes in the calibration values over the range of use during
the recalibration interval of less than 0.032% of reading for force measuring instruments and systems used over the Class AA loading range and less than 0.16% of reading for those instruments and systems used over the Class A loading range. See Note 16.
11.2.2 Devices not meeting the stability criteria of 11.2.1 shall be recalibrated at intervals that shall ensure the stability criteria are not exceeded during the recalibration interval. See Note 16.

NOTE 16—The above stability criteria provide minimum requirements for establishing calibration intervals for force-measuring Instruments and systems used as secondary force standards and those used for the verification of the force indication of testing machines. Users specifying percentage limit of errors other than Class AA or Class A should determine stability criteria appropriate to the instruments employed. For secondary force standards, it is recommended that cross-checking be performed at periodic intervals using other standards to help ensure that standards are performing as expected.




written by Phil Smith with edits from Henry Zumbrun and Dilip Shah
www.mhforce.com


Monday, December 12, 2016

Calibration Intervals - When is it time to recalibrate?



Phil Smith , email: psmith@mhforce.com


Since I became involved in the calibration industry in 1972, calibration interval has been a common topic of interest and confusion.  I was asked to write an article for the newsletter & blog, and after some thought, I determined there was no need.  Many people have published excellent content on the subject:
One of the best and most practical ways for a laboratory to justify calibration intervals is to maintain Shewhart control charts (SPC – Statistical Process Control charts). This is achieved by monitoring and charting key parameters of the equipment in between currently established calibration intervals for the equipment. Several established control limit criteria are then used to determine the stability of the artifact. This satisfies many requirements for ISO/IEC 17025:2005 (Dilip Shah’skill 5 birds with one stone” quotation – the “stone” being the SPC tools and techniques and the “birds” being the ISO/IEC requirements).

5.5.9 When, for whatever reason, equipment goes outside the direct control of the laboratory, the laboratory shall ensure that the function and calibration status of the equipment are checked and shown to be satisfactory before the equipment is returned to service.

5.5.10 When intermediate checks are needed to maintain confidence in the calibration status of the equipment, these checks shall be carried out according to a defined procedure.

5.6.3.3 Intermediate checks
Checks needed to maintain confidence in the calibration status of reference, primary, transfer or working standards and reference materials shall be carried out according to defined procedures and schedules.

5.9.1 The laboratory shall have quality control procedures for monitoring the validity of tests and calibrations undertaken. The resulting data shall be recorded in such a way that trends are detectable and, where practicable, statistical techniques shall be applied to the reviewing of the results. This monitoring shall be planned and reviewed and may include, but not be limited to, the following:
a) regular use of certified reference materials and/or internal quality control using secondary reference materials;

5.9.2 Quality control data shall be analyzed and, where they are found to be outside pre-defined criteria, planned action shall be taken to correct the problem and to prevent incorrect results from being reported.


Having the historical data in the SPC format helps to determine and provide justification if the calibration interval needs to be shortened or lengthened. It is also a preventive tool which will help identify issues before they actually become real problems (ISO/IEC 17025:2005 Clause 4.12).

Listed below are links to several articles on calibration intervals for your reference:

ILAC G24

HBM

NIST

NCSLI

Fluke

Quality Digest

Thursday, December 1, 2016

The 3 Key Changes to ISO 9001 Compliance and What this Means for Force and Torque Instrumentation






1.  Documented Procedures are No Longer Required.   - Documented procedures are helpful in regards to how to calibrate equipment and they may be useful to you.  However, you are no longer required to keep a physical manual on site.  There is going to be a lot more risk when equipment is not used properly or written procedures are not documented and followed.   There is an alternative to some of this with verifying the proper measurements via calibration, repeatability and reproducibility studies and proficiency testing. In addition, calibration data (records) will still be required (ISO9001 Clause 7.1.5.1 (b) are maintained to ensure their continuing fitness for their purpose.
The organization shall retain appropriate documented information as evidence for purpose of the monitoring and measurement resources.).


Image above is a normal distribution showing 50.1 % probability of of calling the measurement good, when it is not. The uncertainty of the measurement must be considered when evaluating risk.


2.   Preventative Action Clause No Longer Exists - Everything has been focused on risk-based thinking.  This requires the organization to start taking actions for the following:

Avoiding risk, both consumer and producer should be considered.   Measurement Uncertainty and the resolution of the Unit being tested should be considered when making statements of compliance. (ISO 9001 Clause 9.1.3 Analysis and Evaluation: The organization shall analyze and evaluate appropriate data and information arising from monitoring and measurement.
The results of analysis shall be used to evaluate;
a) conformity of products and services;
e) the effectiveness of actions taken to address risks and opportunities;

Also, see ISO 9001 Clause 10.2 Nonconformity and corrective action



Eliminating the source of risk - this is where SPC and good verification standards as shown above, may help.

Changing the likelihood or consequences of risk.

Making informed decision regarding elimination of risk versus managing the risk




3. Top Management is now held more responsible than before - The expectation is for Top Management to start risk-based thinking and may bear the responsibility for failures.    Remember that device that should have been calibrated and was not or its calibration interval was extended to save money? If there was no risk based evidence to support the decision, top management may be accountable. Use of Control Charts (SPC) is one of the tools to manage risk and calibration interval.   



Morehouse offers a SPC workshop that focuses on mitigating and managing risk in calibration.    Our full training schedule can be found here.



written by Henry Zumbrun with help from Dilip Shah
Dilip Shah teaches our SPC course
www.mhforce.com



Monday, November 7, 2016

Top 5 Common Force Measurement Errors


Top 5 Common Force Measurement Errors


At Morehouse we strive to replicate the loading conditions a customer may be using.  Some other labs, may not follow this process and the additional measurement uncertainty from not replicating use, may create significant error source.  Below are the top 5 common force measurement errors that may invalidate the calibration results.

#1 Pin Size is Critical with Tension Links
  • Errors of up to 20 times manufacturer's specification are possible, if the proper pin size is not used.
  • Sending the pin used with the instrument is recommended, to best replicate use during calibration.
  • If the pin is not available, Morehouse will list the pin size used to calibrate the device on the certificate of calibration.
  • If a pin of different size or material is used, it is quite possible that the instrument will not meet manufacturer's specification.

#2 Loading the Shoulder vs. Thread Loading
  • Load Cells will behave differently depending on whether they are shoulder loaded, or thread loaded.
  • Varying the thread depth on a shear-web type load cell can result in measurement errors of 0.5 %, or more.
  • Locking an integral threaded adapter into shear-web cells is the recommended practice to avoid these errors. If an integral adapter cannot be locked into place, we recommend the cell be shoulder loaded, and any adapters placed between the cell and force applied also be supplied for calibration. 
Note: Non shear-web type cells may have additional errors from shoulder loading. The end-user's adapters should be supplied for calibration, we have observed errors up to 5 % when different adapters are used during calibration.


#3 Load Cell Bolting, Thread Class, and Applied Torque
  • If a load cell is bolted to base or adapter of similar size, we recommend sending the cell attached to it's base for calibration.
  • If the load cell is attached to a larger machine or device, error sources from different classes of threads, materials, or bolting procedures (including torque applied) can be quite large. The bolts used to hold the cell in-place should be sent with the cell, and if possible, even a piece of similar material and flatness as the machine should be sent to best replicate use
  • Variability between bolting techniques, material hardness, and flatness can all contribute to significant error, which can be difficult to identify away from the point of use.


#4 Top Adapter Hardness and Flatness
  • Top adapters with different hardness values may affect the strain level in the load cell column or web, and result in different measurement outputs. We have observed errors of up to 0.15 % from varying just the material on top compression pads.
  • Adapters and bases that are not flat may produce additional errors.  We have conducted several tests, and found repeat-ability errors to be about three times higher when the surfaces interfacing with the load cell are not flat.
  • We highly recommend the end-user send us any top adapter which is used with the load cell.



#5 Not Following Published Standards 

  • Our last blog dealt with the top 3 ASTM E74 load cell calibration mistakes  and can be found here. There are several examples where calibration labs simply do not follow what is published in the standard.
  • We have observed several laboratories violating the ASTM E74 standard.  Specifically in not taking a non zero test point below 10 % and assigning a loading range below the first calibrated test point. Per Section 7.2.1 of ASTM E74-13a states  “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 & 2000 x resolution for Class AA loading range”    Per Section 8.6 of ASTM E74-13a  “The loading range shall not include forces outside the range of forces applied during the calibration”   This means, Zero cannot be the first test point.
  • There are other documents like ISO 17025 that are not followed.   We have seen numerous calibration reports mentioning trace-ability to N.I.S.T. without any consideration of measurement uncertainty.    "Traceable to N.I.S.T." is not correct.  The proper statement should be traceable to SI Units, through N.I.S.T.    This topic will be further expanded upon, in a future blog.    




These are just some examples of measurement error.  Force calibration requires attention to details, such as alignment, adapters, pin sizes, thread engagement, and wiring. Morehouse offers a force calibration workshop designed to make the participant a better calibration technician by providing the knowledge to obtain more accurate force measurements, along with tools to create a full measurement uncertainty budget as required by ISO/IEC 17025 and ANSI Z540.3.   To become or have someone become a better calibration technician, click here.



written by Henry Zumbrun
www.mhforce.com

Tuesday, November 1, 2016

Top 3 ASTM E74 Load Cell Calibrations Mistakes

We are writing this post to clear some common ASTM E74 misconceptions.   We have written two early blogs on the ASTM E74 standard ASTM E74 Simplified, Calculating CMC using ASTM E74 standard and have a slide share presentation detailing the ASTM E74 standard. 

This blog post is to help clear the three top misconceptions commonly observed in industry.


Misconception #1: Zero can be used as the first calibrated test point. 

This is not true in anyway possible.    In the ASTM E74-13a standard the following sections point to this not being allowed.


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 of ASTM E74-13a states  “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 & 2000 x resolution for Class AA loading range” 

Do Not assign a Class A or Class AA range below the first non-zero force point.   Note:  We have observed numerous labs violating this rule!  If your verified range of forces is less than the first non zero test point, your calibration provider is not following ASTM E74.






Misconception #2: Designation of loading ranges.  

Some labs think a Class AA verified range of forces can be used to assign a Class AA range.  This is not true.  

A force measuring device with a Class AA range cannot assign another Class AA range; A force measuring device with a Class A  range cannot assign another Class A range.       

Do Not Assign a Class AA verified range of forces, unless you are calibrating with primary standards accurate to better than 0.005 %


Do Not Assign a Class verified range of forces, unless you are calibrating the device using a secondary standard that was calibrated directly by primary standards.




Misconception #3:  A Calibration interval of one year is required for all force measuring devices not meeting the stability criteria set forth in ASTM E74.

(Note:  The maximum calibration interval is two years and this includes any force device)  

Calibration Intervals Per ASTM E74-13a:

New Devices are calibrated at a one year interval per Section 11.2.1  "New devices shall be calibrated at an interval not exceeding 1 year to determine stability" 

"The calibration intervals for force-measuring instruments and systems used as secondary force standards or for the verification of force indication of testing machines shall be calibrated at intervals not exceeding two years after demonstration of stability supporting the adopted recalibration interval" 

If the force measuring instrument does not demonstrate changes in the calibration values over the range used during the calibration of more 0.032 % of reading for Class AA verified range of forces or 0.16 % of reading for a Class A range, A two year calibration interval can be assigned.


Per Sections 11.2.2 Devices not meeting the stability criteria of 11.2.1 shall be recalibrated at intervals that shall ensure the stability criteria are not exceeded during the recalibration interval.   This means the user needs to shorten the calibration interval to ensure the device will meet the stability.   This could mean 16 months or it could mean 10 days.  It all depends on the quality of the instrument.

For Class AA force measuring devices ASTM recommends in note 16 - " For secondary force standards, it is recommended that cross-checking be performed at periodic intervals using other standards to help ensure that standards are performing as expected."   If you cannot cross check your instrumentation, it may be best practice to have the force measuring instrument calibrated on a periodic basis.

This  post covers the basics.  Anyone calibrating in accordance with the ASTM E74 standard should  purchase a full copy of the standard here http://www.astm.org/Standards/E74.htm
Morehouse Developed a Calibration and Measurement Capability worksheet for instruments calibrated in accordance with the ASTM E74 standard.   This sheet can be downloaded at  http://www.mhforce.com/s/CMC-CALCULATIONS-FOR-FORCE-MEASUREMENTS.xlsx


Morehouse offers a force calibration workshop designed to make the participant a better calibration technician by providing the knowledge to obtain more accurate force measurements, along with tools to create a full measurement uncertainty budget as required by ISO/IEC 17025 and ANSI Z540.3.   To become or have someone become a better calibration technician, click here.



written by Henry Zumbrun
www.mhforce.com