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SOP 34 

Standard Operating Procedure1

for the

Selection and Use of Sensitivity Weights and Tare Weights in Weighing Procedures

1 Introduction

Mass calibration procedures are based on comparing the unknown mass, X, to a standard 

mass, S, utilizing the balance as a comparator. This comparison relies on the accuracy of 

balance indications. When balance indications are not accurate enough for precision mass 

calibrations, and they drift with time, appropriate procedural adjustments are required. 

Drift can often be assumed to be linear over a short period. Concerns over balance 

inaccuracy and drift result in two possible causes of errors in mass determination 

procedures. Inaccuracy of the balance indications can be corrected by incorporating a 

sensitivity weight in the procedure that calibrates the range of use of the optical scale 

(mechanical balances) or of the digital indications (electronic balances). Errors due to drift 

can be minimized by using the correct comparison method, selecting a suitable sensitivity 

weight, and by consistent timing within the procedure. The proper selection of procedures 

(GMP 12), the adherence to those procedures, and equal time intervals between weighing 

operations will allow the measured difference between X and S to be corrected for 

inaccuracy of the balance indications and for balance drift.

Mass comparison procedures rely on the unknown and standard masses to be nominally 

equal. When the mass standards are not near to each other, tare weights need to be used to 

bring them closer together. Tare weights in this case function as additional mass standards 

and are essentially be treated as additional mass standards in summation. 

Purpose

The following practice will guide you through the process of selecting and using a 

correct sensitivity weight and/or tare weight(s) for mass determination procedures.

Prerequisites

Valid calibration certificates with appropriate values and sufficiently small 

uncertainties must be available for all the sensitivity weights and tare 

weights used in a calibration. All mass standards must have demonstrated 

metrological traceability to the international system of units (SI), which 

may be through a National Metrology Institute such as NIST.

Verify that weight-handling equipment is available and in good operational 

condition.

 1 This SOP was formerly Good Measurement Practice 14 (2003, 2012) and provides the procedure and general 

guidelines for the use of tare weights and sensitivity weights. 
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Verify that the operator is familiar with the design and the operation of the 

balances and familiar with weighing procedures.

Safety

Handling of large or small weights can represent a hazard to either the 

weights or personnel if the weights are dropped.

2 Methodology

Summary

A sensitivity weight is selected to calibrate the balance over the range to be used in 

the measurement procedure. Minimizing the difference in mass values between X

and S is critical when choosing an appropriate sensitivity weight. Therefore, tare 

weights may be necessary whenever the difference in mass values is significant. 

Minimizing the difference between X and S works to our benefit since the range of 

the measurements is minimized and reduces potential errors that can be introduced 

by nonlinearity or span inaccuracies of the balance as well as bringing the mass 

standards within range on mass comparators with limited weighing ranges. 

Apparatus

Clean forceps to handle the weights, or gloves to be worn if the weights are to be 

moved by hand.

Procedure for selection

Conduct preliminary measurements to determine the approximate mass 

value for the difference between the standard and the unknown (X – S).

Define the range of use for the balance to be used:

2.3.2.1 Equal arm – number of scale divisions

2.3.2.2 Mechanical – optical scale

2.3.2.3 Combination (Electro-mechanical) and Comparators – digital 

indications

2.3.2.4 Fully electronic – capacity

Determine the need for tare weights if the difference between X and S

exceeds the values shown in Table 1, considering the impact of potential 

errors based on comparison with calibration uncertainties.
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Table 1. Recommended maximum difference between X and S.

Balance (X – S)

Equal arm balance each other within one division on the 

scale

Mechanical

1/10 of the optical scale or the applicable 

tolerance depending on weight class and 

suitabilitya

Combination Electro-mechanical

and Comparator

Fully electronic

a

Evaluate the impact of potential errors on the uncertainty.

Select tare weights, if necessary, making sure that the difference between X

and S, with the appropriate tare weights, is generally less than the values 

shown in Table 1. If weights are of equal nominal value and all are within 

applicable tolerances, the need for tare weights is rare. Tare weights or 

multiple standards in summation are often required for unequal nominal 

values. 

Select a sensitivity weight within the ranges give according to Table 2.

Round the estimated mass of the sensitivity weight to the nearest convenient 

standard nominal mass (1-2-3-5).

Table 2. Selection of Sensitivity Weight.

Balance Procedures Sensitivity Weight

(Criteria should be met by the selected sensitivity weight.)

Equal Arm SOP 3, 5, 6, 7, 8, 28 change turning points by about 20 %

Mechanical SOP 4, 5, 7, 28 

SOP 8

≥ 4 times (X – S)

a

; 

≤ 1⁄2 optical scale, usually ≈ 1⁄4 optical scale

Combination

Electro-mechanical 

and 

Comparator

SOP 4, 5, 7, 8, 28 ≥ 4 times (X – S); 

≤ 1⁄2 digital range

Fully Electronic

SOP 4, 5, 7, 28

SOP 8

≥ 4 times (X – S); 

≥ 2 times the applicable tolerance;

≤ 0.5 % capacity a

4 times the difference between X and S is required to avoid flagged errors in the output report 

when using the NIST Mass Code.
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A sensitivity weight is not required if using an electronic mass comparator 

or fully electronic balance that has been evaluated (with supporting data and 

documented analysis available), and which has ongoing periodic validation 

(e.g., prior to each use) to determine that the balance has sufficient accuracy, 

resolution, repeatability, and stability so that no advantage is gained by 

using a sensitivity weight. For example, any possible errors must be less 

than the last digit retained in the expanded uncertainty. Monitoring is 

required to verify metrological traceability. See calculations and 

uncertainties noted in Section 3. 

Using sensitivity weights

The sensitivity weight is incorporated into the mass procedures to ensure 

that the mass differences determined with the optical scale, or electronic 

range, have valid accuracy and traceability. The sensitivity weight 

calibrates the range of use of the balance used for making the mass 

determinations. Using a sensitivity weight provides us with a sensitivity 

value in terms of mass units per division. If the sensitivity is not constant 

with time, temperature and load, its variation must be included in the mass 

correction and in the uncertainty. What follows is a generic equation for the 

sensitivity correction factor. Equations are modified in each SOP when 

buoyancy corrections are performed. 

mass units sensitivity = = divisions deflection

Msw Eqn. (1)

where Msw represents the mass of the sensitivity weight.

3 Calculations

See each mass SOP for calculation of sensitivity as it is included in the procedure. 

Examples of sensitivity accuracy evaluation include the following:

SOP 8 – The error in sensitivity must be less than 2 percent of the balance 

reading. That is, the sensitivity factor portion of the mass calculation must 

be between 0.98 and 1.02 mass units per division when the sensitivity is 

equal to 1 (or 980 to 1020 if sensitivity is equal to 1000). 

Comparison SOPs – The potential systematic error due to sensitivity 

inaccuracies may be calculated by determining the average observed 

deflection of a sensitivity weight divided by the mass of the sensitivity 

weight and multiplied by the average or maximum difference between X

and S, from the following equation:

Observed deflection - Potential sensitivity error Average max sw

sw

M d

M = × Eqn. (2)
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The applicable tolerance may be substituted for the average maximum 

difference between X and S, d. 

4 Uncertainty

Sensitivity errors that may be incorporated in SOP 8 where sensitivity is assessed 

but not included in the calculations need to be evaluated and included as an 

uncorrected systematic error in the uncertainty (treated as a rectangular 

distribution), according to this approach and to instructions in SOP 8.

The uncertainty of the sensitivity weight may be treated in the same way as the 

difference between the observed deflection and mass of the sensitivity weight in 

the equation given in 3.1.2 to determine significance. 

Potential s ensitivity uncertainty Average max sw

sw

u d

M

= × Eqn. (3)

Again, the applicable tolerance may be substituted for the average maximum 

difference, d. The uncertainty of the sensitivity weight is generally relatively small 

and insignificant. However, it does no harm to incorporate it in uncertainty 

calculations when spreadsheets are set up to handle all of the data which will 

account for possible larger uncertainties on sensitivity weights.

The resulting systematic errors that are calculated in Section 3.1.2 that are evaluated 

and not corrected as a part of the procedure may be treated as a rectangular 

distribution in combination with all other sources of uncertainty. Note: An 

alternative equation such as C.6.4.2, from OIML R111 may be used as well.

Uncertainties associated with all tare weights are treated as if multiple standards 

are used in summation and evaluated accordingly (See SOP 29 for references to 

dependencies).