ASTM D6027-96e1
(Test Method)Standard Test Method for Calibrating Linear Displacement Transducers for Geotechnical Purposes
Standard Test Method for Calibrating Linear Displacement Transducers for Geotechnical Purposes
SCOPE
1.1 This test method outlines the procedure for calibrating linear displacement transducers (LDTs) and their readout systems for geotechnical purposes. It covers any transducer used to measure displacement which gives an electrical output that is linearly proportional to displacement. This includes linear variable displacement transducers (LVDT's), linear displacement transducers (LDTs) and linear strain transducers (LST's).
1.2 This calibration procedure is used to determine the relationship between output of the transducer and its readout system and change in length. This relationship is used to convert readings from the transducer readout system into engineering units.
1.3 This calibration procedure also is used to determine the accuracy of the transducer and its readout system over the range of its use to compare with the manufacturer's specifications for the instrument and the suitability of the instrument for a specific application.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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e1
Designation:D6027–96
Standard Practice for
Calibrating Linear Displacement Transducers for
Geotechnical Purposes
This standard is issued under the fixed designation D6027; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
e NOTE—This standard was corrected editorially in January 2000.
1. Scope 3.2.1 calibrated range, n—distance for which the trans-
ducer is calibrated.
1.1 This practice outlines the procedure for calibrating
3.2.2 core, n—central rod that moves in and out of the
linear displacement transducers (LDTs) and their readout
transducer body.
systems for geotechnical purposes. It covers any transducer
3.2.3 linear displacement transducer (LDT) or linear vari-
used to measure displacement which gives an electrical output
able displacement transducer (LVDT), n—an electrical trans-
that is linearly proportional to displacement. This includes
ducer which converts linear displacement to electrical output.
linear variable displacement transducers (LVDTs), linear dis-
ALVDTconsistsofaLVDTbodyandaLVDTcorewhichcan
placement transducers (LDTs) and linear strain transducers
be removed. A LDT holds the core within the sensor body.
(LSTs).
3.2.4 null position, n—the core position within the sensor
1.2 This calibration procedure is used to determine the
body at which the transducer voltage output is zero (some
relationship between output of the transducer and its readout
transducers may not have a null position).
system and change in length. This relationship is used to
3.2.5 percent error, n—the difference between a measure-
convert readings from the transducer readout system into
ment of a reference standard and the actual length of the
engineering units.
reference standard divided by the reference standard and the
1.3 This calibration procedure also is used to determine the
result converted to percent.
accuracy of the transducer and its readout system over the
3.2.6 power supply, n—a voltage source with output equal
range of its use to compare with the manufacturer’s specifica-
to that required by the sensor.
tionsfortheinstrumentandthesuitabilityoftheinstrumentfor
3.2.6.1 Discussion—Some LVDTs use ac voltage while
a specific application.
others use dc. The LVDTs and LDTs may be damaged if
1.4 This standard does not purport to address all of the
connected to the incorrect power supply.
safety concerns, if any, associated with its use. It is the
3.2.7 readout system, n—electronic equipment that accepts
responsibility of the user of this standard to establish appro-
output from the signal conditioner for the transducer and
priate safety and health practices and determine the applica-
provides a visual display or digital record of the transducer
bility of regulatory limitations prior to use.
output.
2. Referenced Documents
3.2.8 repeatability voltage error, n—the difference in volt-
age output for successive measurements of the same reference
2.1 ASTM Standards:
standard.
D653 Terminology Relating to Soil, Rock, and Contained
3.2.9 signal conditioner, n—electronic equipment that
Fluids
makestheoutputofthetransducercompatiblewiththereadout
3. Terminology
system. The signal conditioner may also filter the transducer
output to remove noise.
3.1 Definitions—Definitions of terms used in this practice
3.2.10 total linear range (TLR), n—total distance that the
are in accordance with Terminology D653.
core may move from the position of maximum voltage output
3.2 Definitions of Terms Specific to This Standard:
through the null position to the position of minimum voltage
output with a linear relationship between displacement and
This practice is under the jurisdiction of ASTM Committee D-18 on Soil and
voltage.
Rock and is the direct responsibility of Subcommittee D18.95 on Information
3.2.11 traceability certificate, n—a certificate of inspection
Retrieval and Data Automation.
certifying that the transducer meets indicated specifications for
Current edition approved Oct. 10, 1996. Published June 1997.
Annual Book of ASTM Standards, Vol 04.08. itsparticulargradeormodelandwhoseaccuracyistraceableto
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D6027
the National Institute of Standards and Technology or to 6.5.1 Micrometer Fixture, a precision instrument for linear
another international standard. measurementcapableofobtainingreadingsoverthetotallinear
range of the LDT. The spindle must be nonrotating and spring
4. Summary of Practice loaded. The micrometer fixture is to be calibrated annually by
the manufacturer or other qualified personnel.
4.1 A displacement transducer is mounted in a manner to
permit it to be subjected to a precise, known displacement.
7. Hazards
4.2 Displacement is applied in steps over the full range of
7.1 Safety Hazards:
the transducer and readings taken from the readout device.
7.1.1 This practice involves electrical equipment. Verify
4.3 The slope of the best-fit straight line relating sensor
that all electrical wiring is connected properly and that the
readout data to displacement is determined by linear regres-
power supply and signal conditioner are grounded properly to
sion.
prevent electrical shock to the operator. Take necessary pre-
4.4 The percent error of the transducer readout system is
cautions to avoid exposure to power signals.
calculated and compared with the requirements for the specific
7.2 Safety Precautions:
use of the sensor.
7.2.1 Examine the sensor body for burrs or sharp edges, or
both. Remove any protrusions that might cause harm.
5. Significance and Use
7.2.2 The transducer can be permanently damaged if incor-
5.1 The LDTs play an important role in geotechnical appli-
rectlyconnectedtothepowersupplyorifconnectedtoapower
cations to measure change in dimensions of specimens.
supply with the wrong excitation level.
5.2 TheLDTsmustbecalibratedforuseinthelaboratoryto
7.2.3 Follow the manufacturer’s recommendations with re-
ensure reliable conversions of the sensor’s electrical output to
gard to safety.
engineering units.
7.3 Technical Precautions:
5.3 TheLDTsshouldbecalibratedbeforeinitialuse,atleast
7.3.1 The core and body of the LDT are a matched set. For
annually thereafter, after any change in the electronic configu-
best performance, do not interchange cores with other LDT
ration that employs the sensor, after any significant change in
bodies.
test conditions using the transducer that differ from conditions
7.3.2 Replacethecoreandbodyifeithershowsanysignsof
during the last calibration, and after any physical action on the
dents, bending, or other defects that may affect performance of
transducer that might affect its response.
the device.
5.4 LDTs generally have a working range within which
7.3.3 Store the body and core in a protective case when not
voltage output is linearly proportional to displacement of the
in use.
transducer. This procedure is applicable to the linear range of
7.3.4 Do not exceed the allowable input voltage of the
the transducer. Recommended practice is to use the LDT only
sensor as specified by the manufacturer.
within its linear working range.
7.3.5 Do not connect a voltage source to the output leads of
the sensor.
6. Apparatus
7.3.6 Do not over tighten the sensor within the mounting
block.
6.1 Linear Displacement Transducer, to be calibrated.
6.2 Power Supply with Output, equal to that required by the 7.3.7 The behavior of some transducers may be affected by
metallicholders.Ifpossible,theworkingholdershouldbeused
sensor.
during calibration.
NOTE 1—Some LVDTs use ac voltage while others use dc.The LVDTs
and LDTs may be damaged if connected to the incorrect power supply.
8. Calibration and Standardization
6.3 Signal Conditioning, Readout Equipment, and Related
8.1 If using Test Method A, verify that the gage blocks are
Cables and Fittings.
of sufficient precision and bias and in a clean, unscratched
6.4 Test Method A—Precision Gage Block Calibration:
condition.
6.4.1 Precision Gage Blocks, a set of precision reference
8.2 If using Test Method B, verify that the micrometer
blocks traceable to the National Institute for Standards and
fixture is in good working order and of sufficient precision and
Technology.Agageblocksetshouldcontainsizesnecessaryto
bias.
perform satisfactorily the calibration procedures as outlined in
9. Procedure
Section 9 over the total linear range of the transducer.
6.4.2 Comparator Stand, consisting of a base of warp-free
9.1 Perform this calibration in an environment as close to
stabilityandgroundtoaguaranteedflatness,asupportcolumn, that in which the sensor will be used as possible. The LDT,
and an adjustable arm onto which the sensor mounting block
calibration gage blocks, micrometer fixture, and comparator
canbesecurelyattached.Alternatively,mountthesensorinthe stand should be in the environment in which they are to be
configurationitwillbeusedinsuchawaythatgageblockscan
calibrated for at least 1 h prior to calibration to stabilize
be inserted to displace the core for calibration purposes. temperature effects.
6.4.3 Sensor Mounting Block, a device used to attach the
9.2 Verify that the power supply is adjusted to supply the
sensortothecomparatorstand.Alternatively,mountthesensor recommended voltage to the sensor.
to the test equipment in which the transducer is to be used.
9.3 With equipment turned off, connect all power supply,
6.5 Test Method B—Micrometer Fixture Calibration: signal conditioning, and recording equipment exactly as it will
D6027
beusedinservice.Allowallelectronicstowarmupforatleast 9.8.12 Continue to add gage blocks at the selected displace-
30 min before beginning any readings. mentincrementsandrecordreadingsinFig.1untilthecorehas
been displaced through its total linear range.
9.4 Record type and serial number of the sensor to be
calibrated. If it has no serial number, record the
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