Specifications and Tests for Strain Gage Force Transducers

Transcription

1 ISA-S (R1995) Approved September 29, 1995 Standard Specifications and Tests for Strain Gage Force Transducers

2 ISA-S37.8 Specifications and Tests for Strain Gage Force Transducers ISBN Copyright 1995 by the Instrument Society of America. All rights reserved. Printed in the United States of America. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise), without the prior written permission of the publisher. ISA 67 Alexander Drive P.O. Box Research Triangle Park, North Carolina 27709

3 Preface This preface, as well as all footnotes and annexes, is included for informational purposes and is not part of ISA-S37.8. This Standard has been prepared as a part of the service of ISA, the international society for measurement and control, toward a goal of uniformity in the field of instrumentation. To be of real value, this document should not be static but should be subject to periodic review. Toward this end, the Society welcomes all comments and criticisms and asks that they be addressed to the Secretary, Standards and Practices Board; ISA; 67 Alexander Drive; P.O. Box 12277; Research Triangle Park, NC 27709; Telephone: (919) ; Fax: (919) ; standards@isa.org. The ISA Standards and Practices Department is aware of the growing need for attention to the metric system of units in general, and the International System of Units (SI) in particular, in the preparation of instrumentation standards, recommended practices, and technical reports. The Department is further aware of the benefits to USA users of ISA Standards of incorporating suitable references to the SI (and the metric system) in their business and professional dealings with other countries. Towards this end, this Department will endeavor to introduce SI and acceptable metric units in all new and revised standards to the greatest extent possible. The Metric Practice Guide, which has been published by the Institute of Electrical and Electronics Engineers as ANSI/IEEE Std , and future revisions, will be the reference guide for definitions, symbols, abbreviations, and conversion factors. It is the policy of ISA to encourage and welcome the participation of all concerned individuals and interests in the development of ISA standards, recommended practices, and technical reports. Participation in the ISA standards making process by an individual in no way constitutes endorsement by the employer of that individual, of the ISA, or of any of the standards which ISA develops. This Standard is intended as a guide for technical personnel at user facilities as well as by manufacturers' technical and sales personnel whose duties include specifying, calibrating, testing, or showing performance characteristics of strain-gage linear accelerometers. By basing users' specifications as well as technical advertising and reference literature on this Standard, or by referencing portions thereof, as applicable, a clear understanding of the users' needs or of the transducers' performance capabilities, and of the methods used for evaluating or proving performance, will be provided. Adhering to the specification outline, terminology and procedures shown will not only result in simple, but also complete specifications; it will also reduce design time, procurement lead time, and labor, as well as material costs. Of major importance will be the reduction of qualification tests resulting from use of a commonly accepted test procedure and uniform data presentation. The development of this Standard was initiated as the result of a survey conducted in December A total of 240 questionnaires was sent out to transducer users and manufacturers. A strong majority indicated in their replies a need for transducer standardization. As strain-gage force transducers were one of the types shown to be most in need of standardization, a Subcommittee, SP37.8, was formed. To provide a coordinated document, this committee was composed of representatives from government, user and manufacturer categories. This Standard was then processed over several mail-review and revision cycles until a consensus of reviewers was reached. ISA-S (R1995) 3

4 The following individuals served on the 1975 SP37.8 committee: NAME COMPANY J. J. Elengo, Jr. Chairman Revere Corporation of America P. F. Fuselier Lawrence Radiation Laboratory R. E. Gorton Pratt & Whitney Aircraft H. E. Lockery Consulting Engineer H. W. Rosenburg Naval Weapons Center G. W. Godwin Howe Richardson Scale Company The following individuals served on the ISA Committee SP37, who reaffirmed ISA-S37.8 in 1995: NAME COMPANY E. Icayan, Chairman Westinghouse Hanford Co. J. Weiss Electric Power Research Inst. P. Bliss, Deceased Consultant M. Brigham The Supply System D. Hayes LA Dept. Water & Power M. Kopp Validyne Corp. C. Landis Weed Fiber Optics J. Miller Rosemount Inc. A. Mobley 3M Co. J. Mock Consultant D. Norton McDermott Energy Svces Inc. H. Norton Consultant M. Tavares Boeing Defense & Space Group R. Whittier Endevco J. Wilson Consultant This standard was reaffirmed by the ISA Standards and Practices Board on September 29, NAME COMPANY M. Widmeyer, Vice President Washington Public Power Supply System H. Baumann H.D. Baumann & Associates, Inc. D. Bishop Chevron USA Production Company P. Brett Honeywell, Inc. W. Calder III Foxboro Company H. Dammeyer Phoenix Industries, Inc. R. Dieck Pratt & Whitney H. Hopkins Utility Products of Arizona A. Iverson Lyondell Petrochemical Company K. Lindner Endress + Hauser GmbH + Company T. McAvinew Metro Wastewater Reclamation District A. McCauley, Jr. Chagrin Valley Controls, Inc. G. McFarland Honeywell Industrial Automation and Controls J. Mock Consultant E. Montgomery Fluor Daniel, Inc. D. Rapley Rapley Engineering Services 4 ISA-S (R1995)

5 NAME COMPANY R. Reimer Allen-Bradley Company R. Webb Pacific Gas & Electric Company W. Weidman Consultant J. Weiss Electric Power Research Institute J. Whetstone National Institute of Standards & Technology C. Williams Eastman Kodak Company G. Wood Graeme Wood Consulting M. Zielinski Fisher Rosemount ISA-S (R1995) 5

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7 Contents 1 Scope Purpose Drawing symbol Characteristics Design characteristics Performance characteristics Additional terminology Tabulated characteristics versus test requirements Individual acceptance tests and calibrations Basic equipment necessary to perform individual acceptance tests and calibrations of strain-gage force transducers Calibration and test procedures Qualification tests Steady state temperature effects Temperature gradient error Dynamic characteristics Life test Effects of other environments Storage life test Abnormal loading effects Test report forms Annex A References Figures Individual acceptance tests and calibrations Environmental test record ISA-S (R1995) 7

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9 1 Scope 1.1 This standard covers strain-gage force transducers, primarily those used in measurement systems. 1.2 Included among the specific versions of strain-gage force transducers, to which this Standard is applicable, are the following: Tension Transducers Compression Transducers Universal (Combination Compression and Tension) Transducers 1.3 Terminology used in this document is defined either herein or in ISA-S37.1, Electrical Transducer Nomenclature and Terminology. 2 Purpose This standard establishes the following for strain-gage force transducers. 2.1 Uniform general specifications for design and performance characteristics 2.2 Uniform acceptance and qualification test methods, including calibration techniques 2.3 Uniform presentation of test data 2.4 A drawing symbol for use in electrical schematics 3 Drawing symbol The drawing symbol for a strain-gage transducer is a square of dimensions 2x by 2x, with an added equilateral triangle, the base of which is the left side of the square. The triangle symbolizes the sensing element. The letter "F" in the triangle designates "force", and the additional sub-positioned letters denote the second modifier. ISA-S (R1995) 9

10 2X F C 2X C-Compression T - Tension U-Universal X The strain-gage bridge is symbolized by a small square, with diagonals x by x, centered in the large square. The diagonals of the small square are drawn perpendicular to the sides of the large square. Lines from each apex of the small square projected to the right side of the large square represent the electrical leads. 4 Characteristics 4.1 Design characteristics Basic mechanical design characteristics The following mechanical design characteristics shall be listed Type of force transducer Tension, compression, or universal Physical dimensions Outline drawing to be provided with dimensions in millimeters (inches) Force connection Force connections (both ends) shall be indicated on the outline drawing giving sufficient information regarding location, size, and tolerance of connection features, as well as any special considerations, to enable proper application of forces to the transducer Mountings and mounting dimensions Outline drawing shall indicate method of mounting with dimensions in millimeters (inches). 10 ISA-S (R1995)

11 Location of electrical connection Indicate location and orientation of electrical connector or connecting wiring Overload rating, ultimate Specify percentage of rated force that will not result in structural failure at environmental extremes Mounting torque Allowable mounting torque shall be specified if it will tend to effect transducer performance Weight The weight of the transducer shall be specified in kilograms (pounds mass) Identification The following characteristics shall be given and preferably permanently affixed to the outside of the transducer case and indicated on the outline drawing: a) Nomenclature of transducer (per ISA-S37.1, Section 3) b) Manufacturer's name, part number, and serial number c) Range d) Excitation e) Identification of electrical connections f) Bridge identification, if more than one bridge provided. Listing of the following characteristics is optional: a) Sensitivity (usually millivolts per volt at full scale load) b) Customer's specification and/or part number c) Maximum allowable force that will not influence prescribed performance d) Maximum operating temperature range Temperature range, safe Temperature range of environment in which the transducer may be used and which will not cause permanent calibration shift or permanent change in any of its characteristics shall be listed Supplemental mechanical design characteristics Listing of the following mechanical design characteristics is optional: a) Case Material b) Surface Finish c) Type of Strain-Gage Used Metallic; bonded or unbonded, wire or foil; Semiconductor; bonded or unbonded d) Location of Strain-Gage Mounted directly on force sensing element or mounted on auxiliary member activated by force sensing element ISA-S (R1995) 11

12 e) Number of Active Strain-Gage Bridge Arms (elements) f) One, Two-arm active, Four-arm bridge g) Number of Strain-Gage Bridges h) Mounting Surface Requirements Basic electrical design characteristics The following electrical design characteristics shall be listed. They are applicable at "ambient conditions" as specified in Excitation* Expressed as " volts dc" or " volts rms at Hz," or, expressed as " ma dc" or " ma rms at Hz." Preferred values of voltage 5, 10, 15, 20, and 28 volts Maximum excitation* Expressed as " volts dc" or " volts rms at Hz," or, expressed as " ma dc" or " ma rms at Hz," and defined as the maximum value of excitation voltage that will not permanently damage the transducer Input impedance* Expressed as " ± ohms at ± Hz" and " C ( F)." If impedance is resistive, specify "dc." NOTE Output terminals are to be open-circuited for this measurement Output impedance* Expressed as " ± ohms at ± Hz" and " C( F)." If impedance is resistive, specify "dc." NOTE If input terminals are to be short-circuited for this measurement, so specify Electrical connections Whether the electrical termination is by means of a connector or a cable, the pin designation or wire color code shall conform to the following: *Defined in ISA-S ISA-S (R1995)

13 : W= WHITE 3/C -OUTPUT R=RED 1/A +INPUT W-R = WHITE-RED W-G = WHITE-GREEN G=GREEN W-B = WHITE-BLACK 5/E 6/F 2/B +OUTPUT 8/H B=BLACK 4/D -INPUT W-Y = WHITE-YELLOW 7/G 9/I SHEILD (FLOATING) CONNECTOR Primary wiring terminals1/a,2/b,3/c,4/d Auxiliary wiring terminals 5/E, 6/F, 7/G, 8/H ( Optional) NOTES 1. The output polarities indicated on the above wiring diagram apply when an increasing force (compression or tension) is applied to the transducer. For universal force transducers, the indicated polarities apply when the tension force is applied to the transducer; a compression force will produce a negative output. 2. For shielded transducers, pins 5, 7, and 9 shall be shield terminals for 4, 6, and 8 wire systems, respectively. 3. Type connection: Solder or weld Insulation resistance Expressed as " megohms at volts dc at C( F) between all terminals or leads connected in parallel, and the transducer case." Supplemental electrical design characteristics Listing of the following design characteristics is optional Shunt calibration resistor(s) Expressed as " ± ohms for % ± % of full scale output at C( F)." NOTE The terminals across which the resistor(s) is (are) to be placed shall be specified if the resistor(s) is (are) listed. ISA-S (R1995) 13

14 4.2 Performance characteristics The pertinent performance characteristics of strain-gage force transducers shall be tabulated in the order shown. Unless otherwise specified, they apply at the following ambient conditions: Temperature 23 ± 2 C (73.4 F ± 3.6 F); Relative Humidity 90% maximum; Barometric Pressure 98 ± 10 kpa (29 ± 3 inches of Hg) Range* Usually expressed as " to newtons (pounds force) compression or tension" or " to newtons (pounds force) compression and to newtons (pounds force) tension." NOTE If and are used to specify performance characteristics, the tolerance in may be omitted. Alternately, the following may be specified: End points* Expressed as " ± mv and ± mv open circuit per volt (ma) excitation," or " ± mv and ± mv open circuit at volts (ma) excitation Full Scale Output (FSO)* Expressed as " ± mv open circuit per volt (ma) excitation," or " ± mv open circuit at volts (ma) excitation." Zero-measurand output Expressed as "± % of full scale output." Determined at full rated excitation, with zero measurand applied to the force transducer Zero drift Expressed as "± % of full scale output over a period of (specify time) with no load applied." Sensitivity drift Expressed as "± % of full scale output over a period of (specify time) with newtons (pounds force) applied." Linearity* Expressed as " linearity within ± % of full scale output in [specify direction(s) of loading]." NOTE The type of linearity specified shall be one of the types defined in ISA-S37.1; namely, end point, independent, least squares, terminal, or theoretical slope Hysteresis Expressed as " % of full scale output upon application of ascending and descending forces including rated force." Alternately, and may be combined as follows. *Defined in ISA-S ISA-S (R1995)

15 4.2.9 Hysteresis and linearity Expressed as "combined hysteresis and linearity within ± % of full scale output upon application of ascending and descending forces including rated force. " Repeatability* Expressed as "within % of full scale output over a period of (specify time) and with cycles of load application." Alternately 4.2.7, 4.2.8, and may be combined as follows Static error band Expressed as "± % of full scale output as referred to straight line," (see 4.2.7). NOTE The static error band includes errors due to linearity, hysteresis, and repeatability Creep at load Expressed as "± % of full scale output with the transducer subjected to rated force for a period of ( specify time)." Creep recovery Expressed as "± % of full scale output measured at no load and over a period of (specify time) immediately following removal of rated force, that force having been applied for an identical period of time as specified in " Warm-up period* Expressed as " minutes for subsequent drifts in sensitivity of zero-measurand balance not to exceed % of full scale output." Static spring constant Expressed in newtons per meter or (pounds force per inch), see Equivalent dynamic masses Expressed in kilograms (pounds mass), for both ends of transducer, see Internal mechanical damping Expressed in newtons per meter/second relative velocity (pounds force per inch/second relative velocity), between ends at a frequency of Hz and a dynamic load of ± newtons (pounds force) Overloading rating, safe Expressed as "application of newtons (pounds force) for minutes will not cause permanent changes in transducer performance beyond specified static error band." Rated force Expressed as " newtons (pounds force) either compression or tension." This is the maximum axial force the transducer is designed to measure within its specifications. ISA-S (R1995) 15

16 Thermal sensitivity shift* Expressed as "± % of sensitivity per C( F) temperature change over temperature range from to C ( F) Thermal zero shift* Expressed as "± % of full scale output per C ( F) temperature change over temperature range from to C ( F)." Temperature error band* Expressed as "output values are within ± % of full scale output from the straight line establishing static error band (as defined in ) over temperature range from to C ( F)." Temperature gradient error* Expressed as "less than ± % of full scale output while at zero load and subjected to a step function temperature change from to C ( F) lasting for minutes and applied to (specify particular part) of the transducer." Cycling life Expressed as "± full scale cycles over which transducer shall operate without change in characteristics beyond its specified tolerances." Other environmental conditions Other pertinent environmental conditions that shall not change transducer performance beyond specified limits shall be listed. The following are examples: a) Shock Triaxial b) High Level Acoustic Excitation c) Humidity d) Salt Spray e) Electromagnetic Radiation f) Magnetic Fields g) Nuclear Radiation Storage life Expressed as "Transducer can be exposed to specified environmental storage condition for (days, months, years) without changing the following performance characteristics beyond their specified tolerances." NOTE Environmental storage conditions shall be described in detail. Pertinent performance characteristics (examples: sensitivity zero drift) shall be specified. *Defined in ISA-S ISA-S (R1995)

17 Abnormal loading effects (Refer to Figure 1.) Concentric angular load effect Expressed as "± % of full scale output difference from true output (axially loaded output multiplied by cosine of angle) resulting from a load applied concentric with the primary axis at the point of application and at degrees angle with respect to the primary axis." Eccentric angular load effect Expressed as "± % of full scale output difference from true output multiplied by cosine of angle) resulting from a load applied eccentric with the primary axis and at a degree angle with respect to the primary axis." Eccentric load effect Expressed as "± % of full scale output difference from axially loaded output resulting from a load parallel to but displaced mm (in.) from concentricity with the primary axis." 4.3 Additional terminology ambient pressure effects: The change in sensitivity and the change in zero-measurand output due to subjecting the transducer to a specified ambient pressure change creep at load: The change in output occurring with time under rated load and with all environmental conditions and other variables remaining constant creep recovery: The change in zero-measurand output occurring with time after removal of rated load, which had been applied for an identical period of time as employed in evaluating Creep at Load. ISA-S (R1995) 17

18 18 ISA-S (R1995) Figure 1 B Error = L La B L a = L cos B L s = L sin B B L L ECCENTRICITY Fig. 1a Fig. 1b Fig. 1c a 1/cosB % Rated X100 Error = a Output a 2/cosB X100 a L B % Rated Output Error = L a 3 a ECCENTRICITY X100 a = Rated output at rated axial loading. 1, 2, 3 = Output under any unfavorable rated loading conditions. = Load L a = Axial Load L s = Side Load Rated Output

19 4.4 Tabulated Characteristics Versus Test Requirements Design Characteristic Verified During Characteristics Paragraph Basic Supp. Acceptance Qual. Type of Force Transducer X Physical Dimensions X Force Connection X Mounting Dimensions X Electrical Connection Location X Overload Rating, Ultimate X Mounting Force or Torque X Weight X Identification X Temperature Range X Case Material X Surface Finish X Type of Strain-Gage Used X Location of Strain-Gage X Number of Active Strain-Gage X Elements Number of Strain-Gage Bridges X Mounting Surface Requirements X Excitation X Maximum Excitation X Input Impedance X Output Impedance X Electrical Connections X Insulation Resistance X Shunt Calibration Resistor X Range X End Point X Full Scale Output X Zero-Measurand Output X Zero-Drift X Sensitivity Drift X Linearity X Hysteresis X Hysteresis and Linearity X Repeatability X Static Error Band X Creep X Creep Recovery X Warm-up Period X Static Spring Constant X 6.3 Equivalent Dynamic Masses X 6.3 Internal Mechanical Damping X 6.3 Overload Rating X Rated Force X Thermal Sensitivity Shift X 6.1 Thermal Zero Shift X 6.1 Temperature Error Band X 6.1 Temperature Gradient Error X 6.2 Cycling Life X 6.4 Other Environmental Conditions X 6.5 Storage Life X 6.6 Abnormal Loading Effects X 6.7 ISA-S (R1995) 19

20 5 Individual acceptance tests and calibrations 5.1 Basic equipment necessary to perform individual acceptance tests and calibrations of strain-gage force transducers The basic equipment for acceptance tests and calibration consists of a force calibrator, a source of electrical excitation for the strain-gages, and a device which measures the electrical output of the transducer. The errors or uncertainties of the measuring system comprising these three components should be less than one-third of the permissible tolerance of the transducer performance characteristic under evaluation. Traceability to the National Bureau of Standards should be established Force calibrator The maximum inaccuracy of the force calibrator, for ranges of 50,000 newtons and less, should be not more than one-fifth the permissible tolerance of the transducer performance characteristic under evaluation. Force ranges in excess of 50,000 newtons require force calibrator maximum inaccuracy not more than one-half the permissible tolerance of the transducer performance characteristic under evaluation. Range of the instrument supplying or monitoring the calibration force should be selected to provide the necessary accuracy to 125 percent of the full scale range of the transducer. The force calibrator may be either continuously variable over the range of the instrument, or may vary in discrete steps provided that the steps can be programmed in such a manner that the transition from one force to the next during calibration is accomplished without creating a hysteresis error in the measurement due to over-shoot. DEAD WEIGHT CALIBRATOR Typical ranges (tension or compression) newtons Max. Error ± 0.01% of Test Load newtons Max. Error ± 0.01% of Test Load 0-20,000 newtons Max. Error ± 0.01% of Test Load 0-50,000 newtons Max. Error ± 0.01% of Test Load 0-500,000 newtons Max. Error ± 0.01% of Test Load PROVING RING CALIBRATOR Typical ranges (tension or compression) newtons Max. Error ± 0.1% Full Scale newtons Max. Error ± 0.1% Full Scale newtons Max. Error ± 0.1% Full Scale newtons Max. Error ± 0.1% Full Scale 0-20,000 newtons Max. Error ± 0.1% Full Scale 0-50,000 newtons Max. Error ± 0.1% Full Scale 20 ISA-S (R1995)

21 HIGH ACCURACY REFERENCE FORCE TRANSDUCER Typical ranges (tension or compression) newtons Max. Error ± 0.1% Full Scale newtons Max. Error ± 0.1% Full Scale newtons Max. Error ± 0.1% Full Scale 0-10,000 newtons Max. Error ± 0.1% Full Scale 0-50,000 newtons Max. Error ± 0.1% Full Scale 0-100,000 newtons Max. Error ± 0.1% Full Scale Stable source of electrical excitation of accurately-known amplitude For dc excitation, commonly used sources are line powered, electronically regulated, power supplies. For ac excitation, commonly used sources are the power line in association with a step-down transformer or an oscillator. Where radiometric measurement techniques are employed, input power shall be within both instrument and force transducer specified range Readout instrument Examples of suitable devices are MANUALLY BALANCED POTENTIOMETER Typical ranges 0 to 16 millivolts, Maximum error ± 0.015% of reading or ± 1 microvolt, whichever is greater. 0 to 160 millivolts, Maximum error ± 0.015% of reading or ± 3 microvolts, whichever is greater. 0 to 1.6 volts, Maximum error ± 0.015% of reading or ± 30 microvolts, whichever is greater. DIGITAL ELECTRONIC VOLTMETER WITH PREAMPLIFIER Typical ranges Sensitivity Maximum error 0 to 10 millivolts 1 microvolt ± 0.02% of reading, or ± 2 microvolts, whichever is greater. 0 to 100 millivolts 10 microvolts ± 0.01% of reading, or ± 10 microvolts, whichever is greater. NOTE The input impedance of the readout instrument should be as high as possible. Unless otherwise stated, adjustments and compensation of the transducer apply to open circuit conditions on the output terminals. 5.2 Calibration and test procedures Results obtained during the calibration and test procedures should be recorded on data sheets like the sample data sheets in Section 7. These procedures shall be performed under ambient conditions as defined in 4.2. ISA-S (R1995) 21

22 5.2.1 The transducer is inspected visually for mechanical defects, poor finish, and improper identification markings The transducer shall be connected to the force calibrator with axial alignment as specified by the manufacturer. The excitation source and readout instrument shall be connected to the transducer and turned on. Adequate warm-up time for test equipment shall be allowed before tests are conducted. The force calibrator and connecting hardware shall have passed a prior test for proper operation. It may be desirable prior to calibration to exercise the force transducer by applying rated load and returning to zero load, if so, the number of cycles and time duration should be noted on the data sheet Two or more complete calibration cycles (dependent on desired statistical confidence levels) are run consecutively, including five to ten points in both ascending and descending directions. Excitation amplitude shall be monitored as required. From the data obtained during these tests, the following characteristics should be determined: a) End points b) Full scale output c) Zero balance d) Linearity e) Hysteresis f) (or Hysteresis and linearity) g) Repeatability h) (or Static error band) Repeated calibration cycles over a specified period of time after warmup, establish the following characteristics for that period of time: a) Zero drift b) Sensitivity drift NOTE They may be abbreviated cycles with fewer data points than required in Application of rated force to the transducer during a specified short period of time and measurement of changes in output at constant excitation during this time should establish a) Creep at load NOTE See Figure 2. Rate of application of force to be as high as possible without resonant excitation of transducer By measuring zero-measurand output and sensitivity over a period of time (one hour should suffice), starting with the application of excitation to the transducer, the following characteristic should be determined: a) Warmup period NOTE It is desirable to test for these effects separately, establishing the warmup change of zero-measurand output first. 22 ISA-S (R1995)

23 NOTE: CREEP MAY ALSO TAKE THIS FORM t 1 t 2' 2 t 3' 3 t 0' 0 TIME t 6' 6 t 7' 7 t 4 t 5' 5 t t = Load Application. Should be carefully considered in 1 0 comparative creep measurement. t 2 t 1 = Should be as short as possible. (Suggested 5-10 s.) t t = Creep measurement period. Suggested 3 min., for 3 2 short term and 30 min., for long term. 2 3 X 100 = Creep in % rated output. t 4 t 3 = Load release peroid. (Should equal t 1 - t 0 ) t t = Should be as short as possible. ( Suggested 5-10 s.) 5 4 t 6 t 5 = Creep recovery period. Suggested 3 min. for short term and 30 min. for long term. 5 6 X 100 = Creep recovery in % rated output. t = Time at which zero return is measured X 100 = Zero return in % rated output. Figure 2 ISA-S (R1995) 23

24 5.2.7 After application of the specified overload a specified number of times (and in the specified duration for compression or tension), at least one complete calibration cycle shall be performed to establish that the performance characteristics of the transducer are still within specifications. a) Overload rating, safe Measure the insulation resistance between all terminals, or leads connected in parallel, and the case of the transducer with a megohmmeter or similar acceptable device, using a potential of 50 volts, unless otherwise specified. Insulation resistance should be measured at room temperature. a) Insulation resistance Wheatstone bridge (for dc) or impedance bridge shall be used to measure a) Input impedance b) Output impedance Qualification tests 6.1 Steady state temperature effects The transducer shall be placed in a suitable temperature chamber. After allowing adequate stabilization time at a specified temperature, one or more calibration cycles shall be performed within the chamber. This procedure shall be repeated at an adequate number of temperatures within the operating temperature range of the transducer. These tests should establish the following characteristics: a) Thermal sensitivity shift b) Thermal zero shift c) Temperature error band Temperature gradient error The force transducer, at "room temperature," shall be subjected to a thermal transient by immersion in a fluid, which is kept at a specified temperature above or below room conditions." With no force applied, the output is observed over a specified period of time. NOTE The type of fluid and method of application shall be specified. These tests should establish a) Temperature transient error ISA-S (R1995)

25 6.3 Dynamic characteristics The dynamic response of an installed force transducer depends on the stiffness and mass distribution throughout the entire system in which the force transducer is a part. In many cases, especially those which have many springs and masses, the dynamic response of the installed force transducer may best be determined experimentally by applying a suitable time-varying force to the complete system and measuring the output vs. time response of the transducer. Alternatively, if the distribution of stiffness and mass throughout the system is known quantitatively, the response of the system to a time-varying force may be computed analytically. This approach, however, is apt to be very cumbersome unless (1) attention is limited to frequencies from zero to a little above the lowest natural frequency and (2) the stiffness and mass system is relatively simple. The dynamic characteristics of the force transducer itself may be determined over a wide frequency range with suitable test fixtures and instrumentation either by applying sinusoidal forces or step function forces. Generally, such a determination yields very complex results. Over a limited low frequency range, however, the dynamic response of a force transducer may be defined adequately in terms of a simple equivalent model as shown in Figure 3. c M M 1 2 k k c = Static Spring Constant = Damping Force Parameter M 1 = Effective Mass of "Base" of Transducer* M 2 = Effective Mass of "Top" of Transducer* * The "base" and "top" of the transducer are defined in theoutlinedrawing. Figure 3 M 1 and M 2 may be determined experimentally or analytically so that the calculated natural frequency of the M 1 -k-m 2 model agrees with the actual first mode natural frequency of the transducer. The effective damping parameter generally varies with the force amplitude and possibly with frequency; consequently, the value of c must be determined for the particular load and frequency values of interest. ISA-S (R1995) 25

26 6.4 Life test After applying the specified number of full range excursions of force, at least one complete calibration cycle shall be performed to establish minimum value of a) Cycling life Effects of other environments Expose transducer to other specified environmental conditions followed in each case by one complete calibration cycle to test ability of transducer to perform satisfactorily after such exposure. 6.6 Storage life test After storing the transducer under specified conditions for the specified period of time, two complete calibration cycles shall be performed to establish a) Storage life Abnormal loading effects For determination of the effects of concentric angular loading (and side loading), insert wedge blocks above and below the force transducers as illustrated in Figure 1a. The angle subtended by the two larger surface areas (B) of each block shall be equivalent to the angle of interest or shall result in the side load of interest Measure zero-measured output Apply rated load and read output as soon as load has stabilized Remove load and record zero-measurand output after output has stabilized For determination of the effects of eccentric angular loading, remove the upper wedge block (Figure 1b) and repeat steps through If eccentricities other than that obtained in the foregoing are desired, a flat load button should be used and the amount of eccentricity should be adjusted through placement of the force transducer under a convex loading ram surface For determination of the effects of eccentric loading, remove the lower wedge block, use a flat load button, and adjust eccentricity through placement of the force transducer under the convex loading ram surface (see Figure 1c). Repeat steps through The effects of the various types of loading related to axial loading conditions can be determined in accordance with the expressions included in Figure ISA-S (R1995)

27 7 Test report forms 7.1 The test report forms listed are recommended for use during the testing of strain-gage force transducers. 7.2 When using the forms, all pertinent information shall be inserted in its proper place. On some forms, blank space has been provided for additional tests. Where the test is prolonged, more than one form may be required. 7.3 Individual Acceptance Tests and Calibrations (Figure 4) used during acceptance testing of Section 5 may also be used during qualification testing of Section 6. ISA-S (R1995) 27

28 Figure 4 Individual acceptance tests and calibrations 28 ISA-S (R1995)

29 7.4 "Environmental Test Record" (Figure 5) used to record thermal sensitivity shift, thermal zero shift, temperature error band, temperature transit error, and other environmental tests. Figure 5 Environmental test record ISA-S (R1995) 29

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31 Annex A References ISA ISA-S ISA-S (R 1995) Available from: Electrical Transducer Nomenclature and Terminology Specifications and Tests for Strain Gage Pressure Transducers ISA 67 Alexander Drive P.O. Box Research Triangle Park, NC Tel. (919) MISCELLANEOUS Handbook of Transducers for Electronic Measuring Systems, Norton, Harry N., Prentice-Hall, Inc. (1969). MIL-E-5272C (ASG) Environmental Testing, Aeronautical and Associated Equipment, General Specifications for (1970). MIL-E-5400P, (ASG) Electronic Equipment, Aircraft, General Specifications for (1974). MIL-STD-810C, Environmental Test Methods for Aero-Space and Ground Equipment (1975). Standard Load Cell Terminology and Definitions, developed by the Industrial Instrument Section of the Scientific Apparatus Makers Association and published by the Scale Manufacturer's Association (1962). ISA-S (R1995) 31

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34 Developing and promulgating technically sound consensus standards, recommended practices, and technical reports is one of ISA's primary goals. To achieve this goal the Standards and Practices Department relies on the technical expertise and efforts of volunteer committee members, chairmen, and reviewers. ISA is an American National Standards Institute (ANSI) accredited organization. ISA administers United States Technical Advisory Groups (USTAGs) and provides secretariat support for International Electrotechnical Commission (IEC) and International Organization for Standardization (ISO) committees that develop process measurement and control standards. To obtain additional information on the Society's standards program, please write: ISA Attn: Standards Department 67 Alexander Drive P.O. Box Research Triangle Park, NC ISBN: