WHY UTM CANNOT BE USED FOR LOADCELL CALIBRATION

INTRODUCTION TO UNIAXIAL/UNIVERSAL TESTING MACHINES

A universal testing machine (UTM), is also generally known as

  • Universal tester
  • Materials testing machine
  • Materials test frame
  • Force testing Machine
  • Tensile tester,
  • Compression tester

A UTM is an innovative multi-purpose instrument that has a plethora of applications and is beneficial for QC departments and R&D labs. It is primarily utilized to test the tensile and compressive strength of materials. Previously a tensile testing machine was referred to as a Tensometer.

The term “uniaxial” means single axis or single direction testing and the term "universal" in the nomenclature denotes the ability of the equipment to perform various operations such as standard tensile and compression tests on materials, components, and structures.

There are several types of universal testing machines, some of them have been trimmed and marketed to specific sectors. Such developments have brought about specific names such as “texture analyzer” for food, “top load compression tester” for packaging and pipes, and “peel tester” for adhesives, tapes, and labels. 

In the event of an incorrect measurement, there is a probability that a flawed decision may be taken. The consequences of this could lead to massive recalls, product failures, humungous financial loss and even loss of life.

PROS & CONS OF ACCURATE FORCE TESTING

Accurate measurement of force by using load-cells / force transducers has become progressively imperative while designing safer buildings, evaluating the strength of materials, controlling production processes, thrust of jet engines, rockets, aircrafts, and gas turbines. As well as weighing of aircrafts on weigh bridges, controlling pressure in rolling mills, to engineering safety products.

Force transducers are also utilized in material testing machines, spring test stands, crimp force test stands, push-pull gauges and other custom built test rigs.

Highly precise force transducers are used as reference in calibration of universal testing machines (UTM) as per ISO 7500-1:2015, in secondary calibration machines for load-cell calibration or force proving instruments as per ISO 376:2011 or AST 374-18, etc.

Force measurement is an undeniable facet in many industries but the precision in testing could vary in parts per million (ppm) depending on the application.

Adopters are important while carrying out calibration and testing to reduce parasitic forces interference. Its good to recall at this stage, that Force is a vector quantity having both magnitude and direction unlike length and mass which are scalar quantities.

LOADCELL CALIBRATION STANDARDS

As per Standards ISO 376:2011 and ASTM E74-18

Uniform, valid, precise and internationally compatible accurate measurements and calibrations are beneficial in eliminating technical barriers, to enhance efficiency, quality and safety. Therefore it is advisable to inculcate National/International Standards of calibration.

The two important and universally acceptable standards for loadcell/force sensor calibration are

  1. ISO 376:2011
    1. Classifies Loadcells in Class 00, Class 0.5, Class 1 & Class2                                                
    2. Class 00 being the most accurate or having best overall uncertainty                                 
  2. ASTM E74-18
    1. Classifies Loadcells in Class AA, Class A                                                                     
    2. Class A being the most accurate or having best overall uncertainty

What does ASTM E74 say about force calibration machines?

As per ASTM E74-18, Standard practices for calibration and verification for Force-Measuring Instruments: Force measuring instruments used for the verification of force indication systems may be calibrated either by primary or secondary force standards (5.1of ASTM E74-18).

Primary force standards for calibration of Class AA and Class A force measuring device is as follows. A dead weight 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. (3.1.2 of ASTM E74-18)

Secondary force standards with Class AA reference force measuring device for calibration of class A force measuring device: An instrument or mechanism, the calibration of which has been established by comparison with primary force standards (3.1.3 of ASTM E74-18).

Secondary force standards may be either force measuring instruments used in conjunction with a machine or mechanism for applying force, or some form of mechanical or hydraulic mechanism to multiply a relatively small deadweight force.

Examples of the latter form include single-and multiple-lever systems in which force acting on a small piston transmits hydraulic pressure to a larger piston. (6.2 of ASTM E74-18) Force measuring instruments used as secondary force standards shall be calibrated by primary force standards and used only over the class AA verified range of forces. (6.2.1 of ASTM E74-18)

The masses of the weights shall be determined within 0.005% of their values by comparison with reference standards traceable to the International System of Units (SI) for mass. The local value of the acceleration due to gravity, calculated within 0.0001m/sec2 (10 milligals), may be obtained from the National Geodetic Information Centre, National Oceanic and Atmospheric Administration. (6.1.2 of ASTM E-74-18)

For force measuring instruments used as secondary force standards, the LLF (lower limit of Force) of the instrument shall not exceed 0.05% of the force. The Lower force limit of the force measuring instrument as expressed by Equation (7) (over the Class AA verified range of forces is therefore 2000 times the LLF, in force units, obtained from the calibration data. (8.6.3.1 of ASTME74-18)

Therefore, applied uncertainty for

Primary force standard: Dead weight force calibration machine is 0.005% of mass and for Local value of Gravity 0.0001m/sec2

Secondary standard: Secondary force calibration systems, LLF of the force measuring instrument shall not exceed 0.05% of the force.

SUSHMA FORCE CALIBRATION SYSTEM - PRIMARY
SUSHMA FORCE CALIBRATION SYSTEM - SECONDARY

What does ISO 376 say about force calibration machines?

As per the standard ISO 376 Calibration of force-proving Instruments which are used for the verification of uniaxial testing machines:

The applied force uncertainty required for calibration of different class force proving instrument are as follows:

Class of Accuracy

   Uncertainty of applied calibration force required with k=2

00

± 0.01%

0.5

± 0.02%

1

± 0.05%

2

± 0.1%

 

HOW TO CALIBRATE OR VERIFY STATIC UTMS

UTMs are calibrated or more technically “verified” as per standard ISO 7500-1:2018. Here the UTM is verified using an accurate Loadcell and then the UTM is classified as per the below table

As per the standard ISO 7500-1:2018 calibration and verification of static uniaxial testing machines”

Class of the Machine

        Maximum permissible values %

       Class of Force proving                instrument to be                used for calibration as per ISO 376

Relative error of

        Accuracy    (q)

    Repeatability (b)

Reversibility*

(v)

     Zero (f0)

Relative     resolution ‘a’

0.5

± 0.5

0.5

± 0.75

   ± 0.05

0.25

0.5

1

± 1.0

1.0

± 1.5

   ± 0.10

0.5

1

2

± 2.0

2.0

± 2.0

   ± 0.2

1.0

2

3

± 3.0

3.0

± 3.0

  ± 0.3

1.5

2

Specification of uniaxial Testing machine (UTM) if, used as a force calibration machine for calibration of force proving instrument.

Can UTM be Used for calibration of loadcells?

A universal testing machine (UTM) is used to test the mechanical properties (Tension, compression, etc.,) of a given test specimen by exerting Tensile, compressive or transverse stresses. UTM has been named so because of the wide range of tests it can perform on various kinds of materials.

Principle of operation of these machines is generally by hydraulic transmission of load from the test specimen to a separately housed force indicator. The pressure developed by the hydraulic machine is converted into force values on the indicator. Equally most UTM also are driven by motor for more precise applications. These days, almost all universal testing machines use built-in force transducer having specified range and accuracy depending on its capacity.

As per the standard ISO 376, calibrated Force proving instruments are utilized for verification of uniaxial testing machines that means they are more accurate than the UTMs.

Hence, it may not be feasible to use the UTMs for calibration of the Force measuring instruments, as the measurement uncertainty level of UTMs is lesser due to their inherent qualities. It is like calibrating a master pressure gauge using ordinary pressure gauge. This is one of the disadvantages of a universal testing machine.

There are many classes of accuracy in universal testing machine (UTM) as mentioned in the table above. The best measurement uncertainty that can be achieved even by using 0.5 class of accuracy UTM cannot be less than 0.25% for the entire range of calibration. 

The applied uncertainty of force itself using a 0.5 class force transducer is not less than 0.15%. Even if class 00 Force transducer with applied uncertainty of 0.08% is used for calibration it is practically impossible to achieve measurement uncertainty of < 0.1 to 0.12%.

Even to calibrate low quality force proving instrument of class 2 the applied uncertainty requirement as per ISO 376 is 0.1% Therefore, the most accurate 0.5 class UTM if used, one cannot achieve 0.1% or better applied uncertainty for calibration of a force transducer.

ASTM E74 -18 specifies for secondary standard, Force transducer of Class AA of 0.05% to be used as reference to calibrate Class A Force transducer.

Hence, even using an UTM of class 0.5 for calibration of Class 2 Force transducer is ruled out either as per the Standard ISO 376 or as per ASTM E74-18 let alone other better class of force transducers.

If the UTM has built in Force transducer with a fixed capacity, calibration of different force transducers may not be feasible because of the limitation in its measurement uncertainty range to cater to minimum and maximum range of the unit under calibration.

Alternatively, additional reference force transducer with suitable range, (min. and max.) capacity is used and UTM is used only as a force generating system with proper mounting adopters between the reference and the test transducers, to avoid parasitic forces during application of force, it may be feasible to calibrate roughly the force transducer.

With all the best engineering efforts a new UTM if at all is developed with a highly accurate force sensor, even then the best measurement uncertainty achievable is +/- 0.05% for class A or +/-0.1% for class 2. Let alone other higher classes.

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