ASTM E1270-88(1997)
(Test Method)Standard Test Method for Equal Arm Balances
Standard Test Method for Equal Arm Balances
SCOPE
1.1 This test method can be used for testing equal-arm balances of any capacity and sensitivity. The testing procedure should enable the user to characterize his instrument sufficiently to determine whether or not it is suitable for the purpose for which it is to be used.
1.2 The characteristics to be examined include:
1.2.1 Sensitivity at all loads,
1.2.2 Lever arm ratio,
1.2.3 Damping ratio (for instruments without accessory dampers),
1.2.4 Period of oscillation,
1.2.5 Precision, and
1.2.6 Linearity and calibration of accessory devices that provide on-scale indication of weight.
1.3 This standard does not purport to address all of the safety concerns 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.
General Information
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Standards Content (Sample)
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
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Designation: E 1270 – 88 (Reapproved 1997)
AMERICAN SOCIETY FOR TESTING AND MATERIALS
100 Barr Harbor Dr., West Conshohocken, PA 19428
Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
Standard Test Method for
Equal Arm Balances
This standard is issued under the fixed designation E 1270; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
This test method is designed to test balances whose lever-arm ratio is substantially equal to unity.
Although largely superseded by new technologies, equal-arm balances retain a special niche for very
high precision weighing of larger samples (usually greater than 1 kg) as well as objects with large
buoyancy (such as gas bottles). Balances of this type can range from simple instruments of moderate
precision (1:10 000) to extremely high precision devices with precision of 1:10 000 000 or better. A
number of accessory devices may be included for assisting in the weighing process. These devices
may contribute to errors as well as can the basic lever mechanism. This method is designed to test the
entire instrument including the accessories.
1. Scope 3.1.1 capacity—maximum load recommended by the manu-
facturer. Usually, the capacity refers to the maximum load on
1.1 This test method can be used for testing equal-arm
each pan simultaneously.
balances of any capacity and sensitivity. The testing procedure
3.1.2 readability—value of the smallest unit of weight
should enable the user to characterize his instrument suffi-
which can be read. This may include the estimation of some
ciently to determine whether or not it is suitable for the purpose
fraction of a scale division or, in the case of a digital display,
for which it is to be used.
will represent the minimum value of the least significant digit.
1.2 The characteristics to be examined include:
3.1.3 sensitivity—smallest value of weight which will cause
1.2.1 Sensitivity at all loads,
a change of indication which can be determined by the user.
1.2.2 Lever arm ratio,
This may be independent of the readability because of the
1.2.3 Damping ratio (for instruments without accessory
choice of the reading device used. For example, a magnifying
dampers),
glass may be used in conjunction with a reading scale to
1.2.4 Period of oscillation,
observe a sensitivity not readily determined without the mag-
1.2.5 Precision, and
nifying glass.
1.2.6 Linearity and calibration of accessory devices that
3.1.4 precision—repeatability of the balance indication with
provide on-scale indication of weight.
the same load under essentially the same conditions. The more
1.3 This standard does not purport to address all of the
closely the measurements are grouped, the smaller the index of
safety concerns associated with its use. It is the responsibility
precision will be. The precision should be measured under
of the user of this standard to establish appropriate safety and
environmental conditions that represent the conditions under
health practices and determine the applicability of regulatory
which the balance is normally used.
limitations prior to use.
3.1.5 accuracy—degree of agreement of the measurement
2. Referenced Documents
with the true value of the magnitude of the quantity measured.
3.1.6 linearity—characteristic of a direct reading device. If
2.1 ASTM Standards:
a device is linear, calibration at 2 points (for example, 0 and
E 617 Specification for Laboratory Weights and Precision
full-scale) calibrates the device (for example, 2 points deter-
Mass Standards
mine a straight line); if a device is nonlinear, additional points
3. Terminology
are needed (perhaps a great many).
3.1.7 standard weight—any weight whose mass is given.
3.1 Definitions of Terms Specific to This Standard:
Since weights are not always available with documented
corrections, weights defined by class (see Specification E 617)
may be used if the class has sufficiently small tolerance limits
This test method is under the jurisdiction of ASTM Committee E-41 on
and there is an understanding that errors perceived as being
Laboratory Apparatus and is the direct responsibility of Subcommittee E41.06 on
Weighing Devices.
instrumental could be attributed to incorrectly adjusted
Current edition approved Sept. 30, 1988. Published November 1988.
weights.
Annual Book of ASTM Standards, Vol 14.02.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E 1270
3.1.8 off-center errors—differences in indicated weight 7.6 Read the manufacturers instructions carefully. During
when a sample is shifted to various positions on the weighing each step of the test procedure, the instrument should be used
area of the weighing pan. No separate test is described. in the manner recommended by the manufacturer.
3.1.9 full-scale calibration of an accessory device—
8. Procedure
indicated reading at equilibrium of an accessory device when a
standard weight equal to the full-scale range of the device 8.1 Sensitivity—The sensitivity can be measured at a num-
isplaced on the sample pan. Usually, some means is provided ber of different loads from zero to the capacity to provide a
by the manufacturer to adjust the full-scale to match the weight sensitivity versus load curve, or, it can be measured at the load
of the standard. of particular interest. This test applies to balances which have
a null position indicator. Balances which are direct reading in
4. Summary of Test Method
the on-scale range must be calibrated according to 8.8.4, 8.8.5,
4.1 Throughout this test method, the instrument is to be
8.8.6 or 8.8.7.
used in the manner for which it is intended by the manufac- 8.1.1 Place nominally equal weights on each pan for the
turer. All measurements are made with weights whose values
selected load.
are sufficiently well known for the purpose of the user. The 8.1.2 Observe the indication. If necessary, place small
nominal value of the weights used will be determined by the
weights on the appropriate sample pan to obtain an indication
capacity and rated sensitivity of the balance as well as by the near zero.
resolution and range of the accessory reading devices.
8.1.3 Place a small weight on the left pan sufficient to
change the indication about ⁄2scale of the on-scale range.
5. Significance and Use
Record the indication as d .
5.1 This test method should enable the user of the balance to
8.1.4 Remove the small weight and place it on the right pan
interpret data determined thereon in terms of accuracy and
and record the new indication as d (remember that for
precision. It should be helpful in using a particular instrument
indicator scales graduated either side of center zero, indications
to best advantage. Weaknesses as well as strengths should
to the left are recorded as negative values).
become apparent. It is not the intention of this test method to
8.1.5 Compute the sensitivity as follows:
compare similar instruments of different manufacture but
S 5 2 3 W/~d 2 d ! (1)
1 2
rather to assist in choosing an instrument which will meet the
needs of the user.
where:
S 5 sensitivity in mass units/scale division, and
6. Apparatus
W 5 mass of small test weight.
6.1 Standard Weights—Individual or summations of Example: d 5 5.5 div.
1 1 3
d 5 −5.3 div.
weights equal to approximately ⁄4, ⁄2, ⁄4 and the total capacity.
6.2 Tare Weights—Weights of the same denominations as W 5 10 mg
S 5 2 3 10/(5.5 − (−5.3)) 5 1.85 mg/div.
the standard weights but not necessarily calibrated.
6.3 Calibrating Weights—Balances equipped with acces- 8.2 Sensitivity as a Function of Load—Balance designs vary
but in the case of high precision balances, the manufacturer
sory devices such as sliding beam weights, chainweights,
optical scales or electrical transducers require small standard usually tries to provide a nearly level sensitivity at all loads.
This is accomplished by the position of the plane determined
weights equal to the full-scale reading as well as smaller
weights suitable for calibrating intermediate points between the by the terminal pivots in relation to the central pivot. If this
plane is lower than the central pivot, the sensitivity will
zero and full-scale points of the devices. Summations of small
standards can be used for this purpose. decrease with increasing load. Conversely, if the plane is
higher than the central pivot, the sensitivity will increase with
6.4 Stop Watch:
6.5 A room-temperature thermometer with a resolution of at increasing load and can reach a state of instability if the center
least 1°C. of gravity goes above the center pivot. Placing all of the pivots
in the same plane provides a nearly level sensitivity limited by
7. Preparation of Apparatus
the elastic properties of the weighbeam. To measure the
7.1 Place the instrument in the location at which it is to be relationship of sensitivity to load, repeat 8.1 at various loads
tested. If electrically operated, plug in the line cord to the type from zero to the capacity and plot sensitivity as a function of
of socket recommended by the manufacturer. load.
7.2 Place the standard weights near (or within) the instru- 8.3 Lever Arm Ratio—Equal arm balances are not usually
ment. used as direct-reading instruments. Rather, they are used as
7.3 Place the thermometer on the bench in position so that it comparators using standard weights for reference. For preci-
may be read without being touched. sion measurements such as weight calibration, the measuring
7.4 Make sure that the instrument and test weights are clean. technique eliminates errors due to the inequality of arm-
7.5 Allow the instrument and weights to sit undisturbed lengths. For relative measurements such as quantitative chemi-
sufficiently long to reach temperature equilibrium with the cal analysis, if the inequality is considered to be in a constant
surrounding area. In the case of a large, high precision ratio, the results of a number of weighings on the same balance
instrument in a controlled environment, it may be necessary to will have a common multiplier (L /L ) and the resulting
1 2
allow 24 h for such equilibrium. computations representing, perhaps, fractional components of a
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E 1270
compound will be mathematically correct. If there is a need to 8.5 Period of Oscillation—The time required to make one
determine an absolute mass value from a single direct mea- full oscillation is an indicator of the time required to make a
surement, the lever ratio must be determined. measurement either for a damped or undamped balance. The
8.3.1 Observe the rest point with empty weigh pans. period is a function of the magnitude of the moving mass and
of the sensitivity of the balance. For a given arm length,
8.3.2 Place approximately equal weights on each pan whose
value is near the capacity of the balance. balances of high sensitivity have longer periods.
8.3.3 Observe the new rest point. 8.5.1 For the convenience of the user, high sensitivity
8.3.4 Transpose the weights to the opposite pans and ob- balances may have means for magnifying the indication thus
serve the rest point. allowing the sensitivity to be lowered and the period shortened.
8.3.5 Measure the sensitivity at this load from 8.1. However, such an approach must be used with care since such
magnification means smaller angles of deflection are measured
8.3.6 Compute the lever ratio as follows:
and the balance becomes more sensitive to the tilting which
M
r 5 (2)
might occur on a bench or floor of insufficient rigidity.
L
M 1 S ~d 2 ~d 1 d !/2!
1 1 2
8.5.2 Place weights of equal value on the pans at or near the
where:
load of interest. Release the beam and start the stop watch as
r 5 lever ratio,
L the direction of the indicator changes. Count several turning
S 5 sensitivity in (mass units)/(scale division),
points and stop the watch after n periods of oscillation.
d 5 rest point of empty pans in 8.3.1 (scale divi-
Calculate the period, p:
sions),
p 5 t/n (5)
d 5 rest point from 8.3.3,
d 5 rest point from 8.3.4, and
where:
M 5 mass of test weights (the value on each pan).
t 5 total elapsed time, and
Example: 5
n 5 number of turning points.
M 5 100 g (on each pan)
8.6 Precision—The term 8precision’ in weighing usually
S 5 1.85 mg/div. 5 0.00185 g/div.
means repeatability. In quantitative terms, it refers to expected
d 5 + 1.5 div.
uncertainty of a single reading. The usual method for deter-
d 5 + 8.5 div.
mining the precision is to compare the results of a series of
d 5 −2.5 div.
100 measurements by some statistical treatment and to compute
r 5
L
some value which gives the user an estimate of the potential
100 1 0.00185~1.5 2 ~8.5 2 2.5!/2!
r 5 1.0000278.
L
uncertainty of a single reading. A common technique is to
8.3.7 A ratio greater than 1 indicates that the left lever is
compute the standard deviation (s) of a series of observations.
longer and if a sample is placed on the left pan and standard
The larger the number of observations the better; but 10 is
weights on the right, the “true’’ weight is:
usually enough. Assuming a normal distribution of data, 3s will
W 5 W /r (3)
represent with a high degree of certainty the maximum
T I L
anticipated error of a single measurement. One convenient
where:
measurement model is a series of double substitutions.
W 5 indicated weight.
I
8.6.1 Place a weight, 8A’, considered to be the standard, on
8.4 Damping Ratio—An undamped balance will oscillate
the left pan and a tare weight of the same nominal value on the
around a rest point with decreasing amplitude of oscillation due
right pan. Observe the balance indication (A ).
to air damping on the weight pans and to friction in the bearing
8.6.2 Remove the standard from the left pan and place a test
system. The ratio of the amplitude of one oscillation to that of
weight 8B’ on the left pan. The tare weight remains on the right
the next may be a measure of several characteristics of the
pan. Observe the balance indication (B ).
balance. Since these cannot easily be separated, this measure-
8.6.3 Add a small weight (S) to the left pan chosen so that
ment is not especially useful since pivot conditions can be
the change in indication will be approximately equal to the
better measured as part of a measurement of precision.
...
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