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GMP 10

Good Measurement Practice

for

Understanding Factors Affecting Weighing Operations

Good laboratory techniques and practices, as well as good standards and balances, are required to 

obtain good mass measurements. Errors can arise from a variety of sources, such as balance drift, 

air currents, lack of thermal equilibrium, magnetic or electrostatic fields, and manipulative skills 

while using the balance. The magnitude and significance of these errors depends upon the 

laboratory environment, the balance, and the accuracy required for the measurement. Different 

methods of weighing are used to eliminate or reduce the effects of sources of error. Sources of 

error and ways to eliminate the errors are discussed with the weighing procedures. The accuracy 

required for a measurement and the criticality of the measurement often influence the choice of 

the weighing method used in the measurement. Regardless of the procedure used, several practices 

should be followed to promote good measurements.

1 Environment and Preparation

First, the balance should be installed in a laboratory having good temperature and humidity control. 

The requirements for temperature stability increase as more precision is needed in the 

measurement. For high precision measurement, the temperature should not vary by more than 

± 1 °C in a 24 h period throughout the year and should vary by less than 0.5 °C during any one 

measurement series (typically less than 1 h). Automated systems will require tighter limits to 

environmental variations, possibly over a longer period. General laboratory measurements can 

probably withstand temperature variations of ± 2 °C per day. Varying temperatures result in 

balance drift and in unequal temperatures for the air, balance, and objects to be weighed. The 

relative humidity should be maintained between 40 % and 60 % which is the recommended limits 

provided by most balance manufacturers and by OIML R 111. If the relative humidity falls 

significantly below 40 %, electrostatic discharge may be generated both in and near the balance. 

This can result in erratic readings and make precision weighing impossible. If precision mass 

standards are cleaned, they should stabilize in the laboratory environment for at least seven to ten 

days. 

Thermal and environmental equilibrium are critical for mass calibrations. Therefore, each mass 

SOP provides instruction that the environment must be stable and the weights set in or near the 

balance for 24 hours prior to a calibration. The test objects, balances, reference standards, and air 

should all be in thermal equilibrium. When possible, the objects to be weighed and the reference 

standards should be placed either in the balance chamber or adjacent to the balance, so all can 

come to the same thermal equilibrium. If there is a lack of thermal equilibrium, convection currents 

will be generated when an object is on the balance pan and an erroneous reading may result. These 

types of errors are likely to go unnoticed when check standards are handled the same way, so care 

must be taken to reduce the likelihood of their occurrence. Tests have shown that these types of 

errors depend upon the temperature differences among the objects and the balance, and on the 

geometry of the objects being weighed. On 20 g standards of greatly differing geometry, studies 

have shown that errors as large as 0.2 mg can occur for a 4 °C temperature difference among the 

standards and the balance. Errors as large as 3 mg have also been observed at 1 kg loads when 
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standards were stored in a cabinet and unknown test items near the balance where a temperature 

gradient of 2 °C was present (when the expanded uncertainty was less than 0.1 mg). When weights 

are delivered to the laboratory, care must be taken to ensure adequate environmental equilibration, 

especially when outside conditions deviate from those in the laboratory by more than 10 °C. The 

presence of frost, ice, or condensation on the weights is a common occurrence and may not be 

readily visible. 

The balance must be installed in an area free from air currents. Air flow should be less than 2 m/s 

in mass calibration areas. Balances should be installed away from heating/cooling vents. It is not 

adequate to merely close vents when weighing because disrupting the temperature control system 

may result in temperature changes near the balances that exceed limits specified in the procedures 

and/or documentary standards.

2 Handling Weights

The masses of standard weights or objects weighed can be affected significantly by the way they 

are handled. Human contact can leave grease or oily films that affect the mass at the time of 

measurement and can even cause permanent damage due to corrosion.

Small weights should never be touched by hand, but handled using forceps, clean gloves, or 

swatches of cloth. In the latter case, the cloth must be lint free. Instructions for cleaning weights 

and for removing adhering foreign material are described in GMP 5.

Large weights of lower tolerance classes (NIST Class F) may be handled by bare hands. Large 

weights are a source of special problems. Fork lifts, portable cranes, hoists, or even overhead 

cranes may have to be used to move the weights within the laboratory and on or off the balances. 

Laboratory personnel must become expert in using such equipment, as necessary, to avoid damage 

to the laboratory facilities, to the balances used, and even to the weights themselves. Special 

hoist/crane hydraulics or multi-speed systems are available to gently set large weights in place on 

large comparators to avoid damage. The problem of temperature equilibrium for large weights is 

qualitatively the same as for small weights and needs consideration on an individual basis.

Large weights must be clean at the time of use, but cleaning may be a problem. Minimally, they 

should be inspected to ensure that foreign material is not present. Cleaning is addressed further in 

GMP 5.

Cleanliness of weight handling materials is just as critical as their use. Forceps, gloves, and other 

lifting materials are prone to contamination and must be regularly cleaned. Care must be taken 

when using gloves to ensure that heat and moisture are not transferred to the weights, that other 

laboratory materials (balances, pens, computers) are not handled that will cause contamination, 

and that gloves are not used simply to keep hands clean!

3 Balance and Weighing Operations

Analytical balances are high precision instruments and should be operated carefully. Excessive 

shock can damage a balance. Avoid shock loading the balance. Gently place all weights in the 

center of the weighing pan/platform. The dials on mechanical balances should be turned slowly 

and gently. Careful balance operation will improve the repeatability of measurements.
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Mechanical analytical balances are provided with partial and full release positions. The partial 

release position is used when determining if an unknown load will provide an on-scale reading. 

The balance beam has limited movement in this position. The partial release position provides 

some protection to the balance when the dialed-in weights are not close to the actual mass placed 

on the pan. It is good practice to arrest the pan each time a dial is being changed to protect the 

balance from shock loading. It is acceptable to change the dial representing the smallest built-in 

weights when in the partial release position because the small weight changes should not result in 

shock loading of the balance.

When releasing the pan to either the full or partial release position, the action should be done 

slowly and carefully. The objective is to minimize disturbances to the balance as much as possible.

Similarly, all loads should be placed on the balance pan carefully and centered on the pan.

When a mechanical balance is released, the beam goes through a series of oscillations. The 

frequency of these oscillations diminishes as time passes until they are almost imperceptible to the 

naked eye. At this point, optimal stabilization is achieved. This stabilization of the balance 

typically lasts for a very short period, after which the balance reading will usually drift. A similar 

situation occurs when a mass is placed on the pan of an electronic balance. Therefore, readings 

should be taken at the precise moment of achieving balance stability. This interval between the 

releasing of a pan on a mechanical balance, or the placing of a mass on an electronic balance, and 

the reading of the observation, varies from balance to balance. Stabilization time differs for all 

balances, even among those of the same type and model. Knowledge of the operating 

characteristics of each instrument is critical to correctly establish this time interval. Although 

manufacturers will usually state this value, it is necessary for the metrologist to verify its reliability. 

Many electronic balances have a stability signal incorporated into the design, but stability also 

must be verified. All measurements in a calibration should be performed at the same time interval, 

and within the shortest time possible.

All balances should be exercised before taking readings. A load equal to the load to be measured 

should be placed on the balance, a reading taken, and the pan arrested, if appropriate, or the weight 

removed from electronic balances. This operation should be repeated several times before readings 

are taken for measurement purposes. Once the balance has been "warmed-up", better repeatability 

will be achieved. Balances can be very accurate even when used without being exercised first, but 

improved results can be obtained by going through a "warm-up" procedure. If the larger variation 

present in initial weighings on a balance that has not been exercised is not significant to the 

measurement, the warm-up procedure may be minimized. The warm-up description provided here 

is not related to ensuring an instrument is plugged in and turned on, although following 

manufacturers recommendations regarding electronic warm-up periods are also important.

To determine the repeatability of measurements when a balance has not been exercised versus its 

performance after being exercised, and to determine how much warm-up time or exercising is 

required, begin measurements using the process that will be used starting with the unexercised 

balance and record the readings. Repeat a series of measurements until you have obtained several 

measurements after the balance has been exercised. This process can be repeated over several days 

using the same objects and the results compared. The readings obtained when using an unexercised 

balance are likely to show a slightly larger variation than those obtained after the balance has been 
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exercised. Balance drift is likely to be larger initially and then reach a steady state when the balance

has been exercised.

4 Comparison (Substitution) Weighing - Mechanical Balance

For mass calibrations, the unknown object must be compared to, or substituted with, a mass 

standard with known calibration values. Comparison weighing (also called substitution weighing) 

eliminates the errors of the built-in weights, reduces disturbances during the measurement because 

dial settings are not changed during the measurement, and can minimize the effects of drift by 

selecting the appropriate weighing design and appropriate sensitivity weight(s). Comparing the 

unknown, X, to a standard, S, eliminates the built-in weights (and associated calibrations) from the 

measurement: thus, the built-in weights act only as counterweights; they do not affect the 

difference measured between X and S. Consequently, the dial settings must not be changed during 

a comparison measurement; otherwise the built-in weights would be part of the measurement.

When comparison measurements are made on a single pan mechanical balance, all readings are 

taken from the optical scale. The unknown and the standard must have nearly the same mass so 

that the difference between them can be measured on the optical scale. If the masses of the 

unknown and the standard are significantly different, small mass standards are used as tare weights 

with either the unknown or the reference standard or both to obtain an observed mass difference 

that is significantly less than one-fourth the range of the optical scale.

As part of a comparison measurement, the mass value of a scale division is determined by 

performing a sensitivity determination. The small mass standard used as part of the weighing 

design to calibrate the optical scale is called the sensitivity weight and should have a mass between 

1/5 and 1/2 the range of the optical scale, inclusive. Additionally, the mass of the sensitivity weight 

should have a mass that is at least twice the observed mass difference between the standard and 

the unknown. Since the maximum size of the sensitivity is limited to one-half the range of the 

optical scale, it may be necessary to carry tare weights with the standard and the unknown to ensure 

that the observed difference between them is less than one-half the mass of the sensitivity weight. 

The value of the sensitivity weight should include the correction plus its nominal value. For high 

precision calibrations, the inclusion of the air buoyancy correction for the sensitivity weight is 

critical and the sensitivity weight usually needs to be four times the difference between the 

standard and unknown (with any associated tare weights). See SOP 34 for detailed guidance on 

the selection of sensitivity weights.

The first readings for the standard and the unknown in a comparison on a single pan balance should 

fall in the first quarter of the optical scale, but well ahead of zero, so the balance drift will not 

result in negative values for any readings. Although negative numbers may be used in calculations, 

they are avoided to simplify calculations and reduce calculation errors. Because the sensitivity 

weight may have a mass as large as one-half the range of the optical scale and the measured 

difference between the standard and the unknown may be as large as one-fourth the range of the 

optical scale, it is necessary to obtain the first two readings in the first quarter of the optical scale 

so all readings will remain on-scale for the measurement. In this way it is not necessary to change 

the dial settings to measure the difference between the standard and the unknown.
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5 Comparison Weighing - Electronic Balance

Measurements made on a full electronic balance are simplified because there are no built-in 

weights to consider. Although many electronic balances are equipped with a built-in calibration 

weight, the internal weights are not involved in the comparison (substitution) weighing.

The principles for comparison weighing on a full electronic balance are the same as when using a 

single pan mechanical balance. The balance indications are used to measure the mass difference 

between the standard and the unknown, and a sensitivity weight is used to establish the mass value 

for a digital division on the balance. Since there are no built-in weights in the full electronic 

balance, the entire range of the digital indications can be considered for "optical scale" of the 

balance.

For comparison weighing the standard and the unknown should be equally nominal or "nearly the 

same mass." Since a full electronic balance has a much larger range for indicating mass values, 

the masses do not have to be as close together as when a mechanical balance is being used. When 

using an electronic balance, the difference in mass between the standard and unknown should be 

less than 0.05 % of the balance capacity. Tare weights that are known standards should be used if 

the masses are not sufficiently close together. The sensitivity weight used to determine the mass 

per digital division should have a mass that is at least 10 to 20 times the mass difference between 

the standard and the unknown but not exceeding 1 % of the balance capacity. For high precision 

weighing, air buoyancy corrections must be made for all objects used in the intercomparison. See 

SOP 34 for detailed guidance on the selection of sensitivity weights.

6 Magnetic and Electrostatic Fields

Care must be taken when weighing magnets or objects having electrostatic charges. A magnetic 

field will likely affect results of a mass measurement. A magnet is attracted to ferromagnetic 

materials in the balance and balance chamber. The magnetic field may also affect the magnetic 

field generated in an electronic balance that utilizes the principle of magnetic force restoration as 

its method of measurement.

Weights made of ferromagnetic material can become magnetized during manufacture or during 

use if they are placed in a strong magnetic field. Magnetized weights can result in measurement 

errors that may go undetected. If a measurement problem is suspected, the weights should be 

checked for magnetism and may have to be rejected if excessively magnetized. 

If magnets or magnetized material must be weighed, the material should be placed in a 

magnetically shielded container to prevent the magnetic field from generating measurement errors. 

If balance design and conditions permit, an alternative is to position the magnetized material a 

relatively large distance from the balance pan using a non-ferromagnetic object on the pan to serve 

as a platform for the magnetic. Since the strength of the magnetic field drops off at a rate of the 

cube of the distance from the magnetic, it may be possible to effectively isolate the magnet from 

other ferromagnetic material in the balance.

Electrostatic fields can also cause errors in measurements. If there is a static charge in a mechanical 

balance, the balance may give erratic readings and lack repeatability. If the object being weighed 

has a significant electrostatic charge, it may result in measurement errors and may leave a static 
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charge on the balance. Electrostatic charges are of concern when plastic containers are placed on 

the balance.

Care should be taken to remove electrostatic charges from objects being weighed by grounding 

the objects, if necessary, before placing them on the balance. To prevent the build-up of static 

electricity in a balance, the relative humidity in the laboratory should be maintained between 40 % 

and 60 % which are also the general manufacturer’s specified limits for precision balance 

operations. The water vapor in the air will serve to drain off electrostatic charges from the balance.

Balances utilizing the magnetic force restoration principle for weighing should be checked to 

verify that the magnetic field generated by the magnetic cell in the balance does not exist around 

the balance pan. If the shielding of the magnetic cell is inadequate, measurement errors may occur 

when weighing ferromagnetic objects or when the balance is placed on a surface comprised of 

ferromagnetic material.