ASTM E898-88(2000)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: E 898 – 88 (Reapproved 2000)
Standard Method of Testing
Top-Loading, Direct-Reading Laboratory Scales and
Balances
This standard is issued under the fixed designation E898; 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.
INTRODUCTION
Thismethodisdesignedtotestcommonlyusedlaboratoryscalesthatreadtheentirerangeofweight
up to the capacity without manual operation. In essence, the entire reading range is on-scale and no
manipulation of weights, riders, or dials is required; except some scales with optical reading devices
may require the operation of a micrometer dial to interpolate the final one or two significant figures.
1. Scope Otherforce-measuringdevicesmaybetestedbythismethodas
long as a sample placed on a receiving platform produces an
1.1 This method covers the determination of characteristics
indicationthatissubstantiallyalinearfunctionoftheweightof
of top-loading, direct-reading laboratory scales and balances.
the sample.
Laboratory scales of the top-loading type may have capacities
1.3 This standard does not purport to address all of the
from a few grams up to several kilograms. Resolution may be
safety concerns, if any, associated with its use. It is the
from 1/1000 of capacity to 1/1000000 or more. This method
responsibility of the user of this standard to establish appro-
can be used for any of these instruments and will serve to
priate safety and health practices and determine the applica-
measure the most important characteristics that are of interest
bility of regulatory limitations prior to use.
to the user. The characteristics to be measured include the
following:
2. Summary of Method
1.1.1 warm-up,
2.1 Throughout this method, the instrument is used in the
1.1.2 off center errors,
manner for which it is intended. One or more weights are used
1.1.3 repeatability, reproducibility, and precision,
to test each of the characteristics, and the results are expressed
1.1.4 accuracy and linearity,
intermsoftheleastcountorultimatereadabilityofthedisplay.
1.1.5 hysteresis,
1.1.6 settling time,
3. Terminology
1.1.7 temperature effects,
3.1 Definitions of Terms Specific to This Standard:
1.1.8 vernier or micrometer calibration, and
3.1.1 accuracy—the degree of agreement of the measure-
1.1.9 resistance to external disturbances.
ment with the true value of the quantity measured.
1.2 Thetypesofscalesthatcanbetestedbythismethodare
3.1.2 capacity—the maximum weight load specified by the
of stabilized pan design wherein the sample pan does not tilt
manufacturer. In most instruments, the maximum possible
out of a horizontal plane when the sample is placed anywhere
reading will exceed the capacity by a small amount.
on the pan surface. The pan is located generally above the
3.1.3 full-scale calibration—the indicated reading when a
measuring mechanism with no vertical obstruction, except for
standardweightequaltothefullscaleindicationofthescaleis
draft shields. Readings of weight may be obtained from an
placed on the sample pan after the device has been correctly
optical scale, from a digital display, or from a mechanical dial.
zeroed. Usually some means is provided by the manufacturer
Weighing mechanisms may be of the deflecting type, using
to adjust the full scale indication to match the weight of the
gravityoraspringasthetransducer,ormaybeaforce-balance
standard.
system wherein an electromagnetic, pneumatic, hydraulic, or
3.1.4 linearity—the degree to which a graph of weight
other force is used to counterbalance the weight of the sample.
values indicated by a scale vs. the true values of the respective
test weights approximates a straight line. For a quantitative
statement of linearity errors, the concept of terminal-based
This method is under the jurisdiction ofASTM Committee E41 on Laboratory
Apparatus and is the direct responsibility of Subcommittee E41.06 on Weighing
Devices.
Current edition approved Sept. 30, 1988. Published November 1988. Originally ANSI/ISA S51.1 “Process Instrumentation Technology”. Available from
published as E898-82. Last previous edition E898–88. American National Standards Institute, 1430 Broadway, NY NY 10018.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E 898
non-linearityisrecommended,suchas,themaximumdeviation 5.3 Thermometer, room temperature, with a resolution of at
of the calibration curve (average of the readings at increasing least 1°C.
and decreasing test load, respectively) from a straight line
5.4 Stop-Watch, reading to ⁄5 s.
drawnthroughtheupperandlowerendpointsofthecalibration
curve.
6. Preparation
3.1.5 off-center errors—differences in indicated weight
6.1 Make sure that the scale and weights are clean.
when a sample weight is shifted to various positions on the
6.2 Place the standard weights near the instrument.
weighing area of the sample pan.
6.3 Place the thermometer on the bench in such a position
3.1.6 hysteresis—difference in weight values indicated at a
that it can be read without being touched.
given test load depending on whether the test load was arrived
6.4 Allow the instrument and the weights to sit undisturbed
at by an increase or a decrease from the previous load on the
for at least 2 h with the balance turned off. Monitor the
scale.
temperatureduringthistimetomakesurethatthereisnomore
3.1.7 repeatability—closenessofagreementoftheindicated
than approximately 2°C variation over the last hour before
values for successive weighings of the same load, under
beginning the test.
essentially the same conditions, approaching from the same
6.5 Read the manufacturer’s instructions carefully. During
direction (such as, disregarding hysteresis).
each step of the test procedure, the instrument should be used
3.1.8 reproducibility—closeness of agreement of the indi-
in the manner recommended by the manufacturer. Know the
cated values when weighings of the same load are made over
location of any switches, dials, or buttons as well as their
a period of time under essentially the same conditions but not
functions.
limited to the same direction of approach (such as, hysteresis
errors are included).
7. Test Procedure
3.1.9 precision—the smallest amount of weight difference
7.1 Warm-up Test:
between closely similar loads that a balance is capable of
7.1.1 If it is required in the normal operation of the scale to
detecting. The limiting factor is either the size of the digital
turn it “on” as an operation separate from weighing, perform
stepoftheindicatorreadoutortherepeatabilityoftheindicated
that operation simultaneously with the starting of the stop-
values.
watch.
3.1.10 standard deviation—used as a quantitative figure of
7.1.2 If a zeroing operation is required, do it promptly.
merit when making statements on the repeatability, reproduc-
Record the temperature.
ibility or precision of a balance.
3.1.11 readability—the value of the smallest unit of weight 7.1.3 At the end of 1 min, read and record the indication
with the pan empty.
that can be read without estimation. In the case of digital
instruments, the readability is the smallest increment of the 7.1.4 At the center of the sample pan place a standard
least significant digit (for example, 1, 2 or 5). Optical scales weight nearly equal to but not exceeding 98% of the capacity
may have a vernier or micrometer for subdividing the smallest of the scale. If the scale allows no weight readings above the
scale division. In that case, the smallest graduation of the
stated nominal capacity, then this test should be performed
vernier or micrometer represents the readability. with standard weights equal to 90% of capacity. When the
3.1.12 standard weight—any weight whose mass is given.
indication is steady, record the indication and remove the
Since weights are not always available with documented weight from the pan.
corrections, weights defined by class may be used if the class
7.1.5 At the end of 5 min, repeat steps 7.1.3 and 7.1.4
chosen has sufficiently small limits and there is an understand- without rezeroing.
ing that errors perceived as being instrumental in nature could
7.1.6 At the end of 30 min, repeat again.
be attributed to incorrectly adjusted weights.
7.1.7 At the end of 1 h, repeat again. Record the tempera-
ture.
4. Significance and Use
7.1.8 Compute for each measurement as follows:
4.1 Thismethodwillenabletheusertodevelopinformation
k 5 W/~I 2 I ! (1)
t w o
concerning the precision and accuracy of weighing instru-
ments. In addition, results obtained using this method will
permit the most advantageous use of the instrument. Weak-
where:
nesses as well as strengths of the instrument should become I = indication with the standard weight on the pan,
w
I = indication with pan empty,
apparent. It is not the intent of this method to compare similar
o
W = known or assumed value of the standard weight, and
instruments of different manufacture, but to enable the user to
k = calibration factor for time t.
choose a suitable instrument. t
7.1.9 Plot the values of k against the time (1 min, 5 min, 30
t
5. Apparatus
min, and 60 min). The time at which k apparently no longer
t
5.1 Manufacturer’s Manual. drifts in one direction can be assumed to be the warm-up time
required.
5.2 Standard Weights—A set of weights up to the capacity
of the scale with sufficient subdivisions of weight so that 7.1.10 If there is a user-adjustable full-scale calibration
incrementsofabout10%ofthecapacityuptothecapacitycan procedure recommended by the manufacturer, this adjustment
be tested. should be made after the warm-up time determined in 7.1.9.
...