Weighing Guide

Safe Weighing Explained

BalancesScales

Safe Weighing RangeEnsures Accurate Results

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2 METTLER TOLEDO Weighing Guide

Content

1. Introduction 4

2. What is Acceptable Product Quality? 5

3. What is Measurement Uncertainty? 7

3.1 What are the absolute and the relative measurement uncertainties? 7

3.2 How does measurement uncertainty tie in with calibration? 8

4. What are Process Tolerances and How are They Specified? 10

5. Safe Weighing Range 12

5.1 Minimum Weight 12

5.2 Ensure accurate weight in non-ideal conditions 13

5.3 Lower and upper limit of the Safe Weighing Range 13

6. Establish the Safe Weighing Range of Your Balance or Scale in 3 Easy Steps 15

7. Tips & Tricks for Accurate Results from the Beginning – and over Time 16

7.1 Selection of the correct weighing device from the start – your specific recommendation 16

7.2 Calibration and testing improvements 16

7.3 Always weigh within the Safe Weighing Range – indication of warning limits 18

7.4 Improve repeatability and thus lower the minimum weight of your balance or scale 19

8. References 20

Cont

ent

33METTLER TOLEDO Weighing Gude

1. Introduction

Everyone knows that a weighing device has a maximum capacity limit where the device will stop indicating weight. If your kitchen scale has a 1 kg capacity, would you consider weighing a large watermelon on it? Of course not, but did you know that all weighing devices also have a minimum, or lower, limit? Any measurement below this minimum limit cannot be trusted to be accurate. For example, would you lend your kitchen scale to your pharmacist to weigh out your next prescription? Probably not, because these items are usually very light, resulting in an inaccurate weighing on the kitchen scale; thus a medication produced this way could be life-threatening. If you are going to buy a balance or scale, or you already have the device, it is important that you understand the “sweet spot” or range where the device gives the most accurate results. Since most weighing manufacturers do not declare this lower limit, it is important to understand the implications.

One method to determine this lower limit is to consult references such as weights and measures where they have attempted to approximate this value by multiplying the device’s readability by a fixed value. However, this method doesn’t take into account the device’s actual performance. The device could be better or much worse.

The purpose of this weighing guide is to explain the Safe Weighing Range which lies between a scientifically calculated lower limit, determined through calibration, and the maximum limit (capacity) of a balance or scale.


When considering acceptable product quality, questions such as: “Is it consistent?”, “Will it taste good?”, “Will it be safe?”, “Will it require rework to get it right?” or “Will it lead to Out of Specification results (OOS)?” should be answered.

Consider an analogy of making soup: Too much or too little salt significantly affects taste. But, how much is too little or too much? The answers to these questions are based on the local, or specific, expertise and quality requirements to produce an acceptable product (figure 1).

In production processes, the accuracy of the weighing device can essentially affect the quality of the end product, such as the amount of weighed-in salt in the soup in the example above.

The accuracy of an instrument is determined by the performance and quality of its measuring sensor and, in order to define the accuracy, we need to assess the uncertainty of the instrument in the environment where it is being used.

2. What is Acceptable Product Quality?

DesiredResult

Variation

Okay

OOS OOS

Okay

Your Product

Tastes Great

Amount of Salt

Tastes Okay

Tastes Okay

BlandTaste

Cannot be Eaten

Soup

Figure 1: Variability in quantity of production output and its relation to the process tolerance. Too much or too little salt significantly affects taste. The question which must be answered by defining process tolerances is: How much is too little or too much?

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Example: You weigh a sample on a precision balance and get a reading of 100.0 g. You prob-ably assume that this is the true amount of the material weighed, however, the amount weighed might differ slightly from the indication on the balance screen. This is due to the measurement uncertainty, which is applicable to every measuring process.

Figure 2: Measurement uncertainty in weighing. Being aware of the uncertainty related to the measurement is a fundamental concept. You should not really make any measurements unless you are aware of the measurement uncertainty.

What I read:100.0 g

What I get: Result range100.0 g ± 0.5 g

≤ ≤

Measurement Uncertainty


3. What is Measurement Uncertainty?

In simple terms, most metrologists agree that measurement uncertainty is the “doubt” in the measurement, which gives an indication of how good or how accu rate the measurement is. Every measurement is subject to some uncer tainty. Uncertainty comes from various sources, such as:

• The measurement device’s capability,• The item being measured,• Environmental conditions,• Standards used to calibrate and adjust the device,• Weighing methods or practices,• The influence of the operator.

If you just measure something without knowing the related measurement uncertainty of the measuring device, then you have no means to justify whether the device is accurate enough to meet the requirement “fit for purpose”.

3.1 What are absolute and relative measurement uncertainties?

Results are generally written in the form of e.g. 100 g ± 1.0 g. The range after the ± is the uncer-tainty of the measurement, or more specifically, the absolute uncertainty. This absolute measure-

ment uncertainty has always the same unit as the measured value.

The relative measurement uncertainty puts this range in relation to the measured value and therefore allows comparisons. It’s the ratio of the absolute uncertainty and the quantity itself, usually expressed as a percentage.

Measurement UncertaintyMeasurement uncertainty assumes that no measurement is perfect but is distorted by a combination of random and systematic errors:• Measuring instrument uncertainty,• Uncertainty of the calibration standards (e.g. test weights) and• Uncertainty due to the measurement process (sample preparation, sample filling, …).

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Figure 3: Relative vs. absolute uncertainty

The two circles in figure 3 have exactly the same fuzzy edge (9 units of blur) but the larger circle looks sharper. Why? The diameter of the larger circle is about 90 units and on the same scale the smaller circle has a diameter of about 30 units. The absolute uncertainty is the same for each cir-cle, about 9 units. However, the relative uncertainty is about 10 % for the large circle and about 30 % for the small one. This example shows why relative uncertainty is a better indicator of the uncertainty of the measured quantity than absolute uncertainty.Generally speaking, the greater the load on a balance or scale, the larger the absolute measure-ment uncertainty. Therefore the measurement uncertainty of weighing systems can be approxi-mated by a positive sloped straight blue line (figure 4).

Looking at the relative measurement uncertainty, we can see that the smaller the load is, the larger the relative measurement uncertainty becomes. At a certain point, the relative measure-ment uncertainty becomes so high that the weighing result can no longer be trusted (fig. 4, illustrated by the green curve).

Figure 4: Absolute and relative measurement uncertainty of a balance or scale. The straight line (blue line, left axis) is the absolute measurement uncertainty in grams and the hyperbolic curve (green line, right axis) represents the relative measurement uncertainty. The hyperbole is derived as follows: From the linear curve read the absolute measurement uncertainty at a given weight, divide it by this weight and express this as a percentage (see table, calculation example). Doing this for all the points of the linear curve, you will obtain the green hyperbole. The hyperbole illustrates that in the lower measuring range, the relative measurement uncertain-ty becomes so high that the weighing result can no longer be trusted.

Calculation example

Weight (g)

Absolute Measurement Uncertainty* (g)

Relative Measurement Uncertainty (%)

0.5 0.082 16.40

1 0.082 8.20

4 0.082 2.05

10 0.083 0.83

50 0.088 0.18

100 0.094 0.09

* according to calibration

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(g)

20

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Attachment to Calibration Certificate: NA0001‐057‐120112‐ACC

Software Version: 5.0 © METTLER TOLEDO

Page 1 of 4 Report Version: 1.0 Form Number: VF9ACC

The document was provided electronically. The original is stored in a secure database.

GWP® Certificate

As Found The weighing device meets the process requirements.

AsLeft The weighing device meets the process requirements.

Tests Performed:

Process Requirements Weighing Tolerance: 0.1% | Smallest Net Weight: 0.050 g | Safety Factor: 2

Safe Weighing Range While the values in this graph reflect the actual calibration results, the measurement uncertainty curves are simply a visual representation. This graph reflects As Left testing, unless only As Found was performed.

As Found As Left

CH4000-005-082514-ACC

Mettler Toledo

1900 Polaris Pkwy

Columbus, OH 43240

1-800-METTLER

Calibration Certificate ID

Accuracy Calibration Certificate

Test

11

1

111111 1

Customer

Company:

Address:

City:

Zip / Postal:

State / Province:

Contact:

Instrument Type:

Asset Number:

Terminal Model:

Terminal Serial No.:

Customer Equipment No.:

Weighing Device

Mettler Toledo

XP205DR

1234+

I

NA

Manufacturer:

Model:

Serial No.:

Class:

Multiple Interval

fadsfsdf; 1; 1Location:

Range Max Capacity Readability (d) Verification Scale Interval (e)

1 81 g 0.00001 g 0.001 g

2 220 g 0.0001 g 0.001 g

Procedure

EURAMET cg-18 v. 3.0

1234

Calibration Procedure:

METTLER TOLEDO Work Instruction:

This calibration certificate contains measurements for As Found and As Left calibrations.

The sensitivity of the weighing instrument was adjusted before As Left calibration.

25-Aug-2014

-

Date of Calibration:

Next Calibration Date:

Service Technician:

Martin Huber

Software Version: 1.3.0.14

Report Version: 1

Form Number: 1234

© METTLER TOLEDO

The document was provided electronically. The

original is stored in a secure database.

Page 1 of 4

3.2 How does measurement uncertainty tie in with calibration?

The measurement uncertainty of a weighing device can be determined through a calibration by a qualified technician – the result is issued in an appropriate calibration certificate.

Calibration is the comparison of measurement values delivered by a device against weights with known reference values. The information derived from the calibration enables the uncertainty of any measurement result to be calculated [1].

Definition of Calibration“Operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication.” Source: International Vocabulary of Metrology, produced by the Joint Committee for Guides in Metrology (JCGM) [2].

Figure 5: Example of a calibration certificate.The performance of a balance or scale in terms of measurement uncertainty is determined bycalibrating the instrument in its working environment by a certified service technician.

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4. What are Process Tolerances and How are They Specified?

Process tolerances can come from a variety of sources, such as: • Authorities with regulations: OIML (legal for trade), USP (pharma convention, consumer safety),• Manufacturer’s specifications, e.g. Mettler-Toledo GmbH,• Application specific methods, e.g. NCC (Net Content Control),• Your customers’ expectations,• Own / QC department’s process requirements.

For an acceptable and accurate result, measurement uncertainty must always be less than the predefined process tolerance (figure 6).

What is the largest weighing variation you can accept for a specific process? Which weighing accuracy do you need to guarantee the quality of the end product? What is the accuracy requirement for your processes where variations from this value can no longer be tolerated?

To answer these questions, process tolerances must be specified.

Figure 6: What does accuracy in weighing mean for your processes? A result is accurate and acceptable if the measurement uncertainty is smaller than the pre-defined weighing process tolerances.

Table 1: Weight indications, acceptable variations and weighing process tolerances.Examples of different weighing process tolerances for the same quantity.

WeightIndication

Acceptable Variation

Weighing Process Tolerance

100.0 g ± 0.1 g 0.1 %

100.0 g ± 0.5 g 0.5 %

100.0 g ± 1.0 g 1 %

100.0 g ± 2.0 g 2 %

100.0 g ± 5.0 g 5 %

What I read:100.0 g

What I get: Result range100.0 g ± 0.5 g

What I can accept:100.0 g ± 1 %

≤ ≤ ≤ ≤

Measurement Uncertainty

Process Tolerance


Table 1 shows the quantitative method to derive a process tolerance, figure 7 and table 2 below describe a qualitative approach to defining a process tolerance. Choosing a quantitative toler-ance provides a starting point for maintaining and tightening the tolerance to as a means to improve measurement and product quality.

≤ 0.1 %

0.1 %

0.2 %

0.5 %

1 %

2 %

5-10 %

Low Medium High

highe

r qua

lity im

pacts

= tighte

r proc

ess to

leran

ces

Quality Impact

Process Tolerance

Figure 7: Qualitative approach to define weighing process tolerances based on the expected level of quality. Process tolerances must be defined tighter as a function of higher level quality impacts on the final product.

Table 2: Weighing process tolerances.Qualitative approach to determining weighing process tolerances when a weighing tolerance in % is undefined.

Your Rating: Effects on Quality, Cost, Safety, Efficiency and Waste

Quality Impact Process Tolerance [%]

Super critical components: high effect and/or cost Acute ≤ 0.1

Expensive, very critical, components or processes Crucial 0.1

Key components or processes Very high 0.2

Above average quality & processes High 0.5

Average requirements & processes Medium 1

Inexpensive base materials Low 2

Low cost materials and products with low effectivity Negligible 5 – 10

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Your Rating: Effects on Quality, Cost, Safety, Efficiency and Waste

Quality Impact Process Tolerance [%]

Super critical components: high effect and/or cost Acute ≤ 0.1

Expensive, very critical, components or processes Crucial 0.1

Key components or processes Very high 0.2

Above average quality & processes High 0.5

Average requirements & processes Medium 1

Inexpensive base materials Low 2

Low cost materials and products with low effectivity Negligible 5 – 10

5. Safe Weighing Range

5.1 Minimum Weight

The lower accuracy limit of a balance or scale, the so-called minimum weight is determined dependent upon your process tolerance limit (figure 8).

The information in a calibration certificate enables calculation of the uncertainty of any weighing made following the calibration, and can be used to quantify the point at which the relative measurement uncertainty is so high that no measurements below this point can be trusted to be accurate.

Minimum Weight“Value of the load below which the weighing results may be subject to an excessive relative error.” Source: EN 45501 standard; T 3.1.2 [3]

NOTE: The minimum weight varies for different balance types and even varies between balances of the same type, when they are placed in different weighing locations or conditions. This is why the minimum weight should be determined experimentally (or by testing) in-situ. Manufacturer’s specifications are only an indication or guideline.

The accuracy limit of the instrument, the so-called minimum weight, is the intersection point between the required weighing process tolerance and the relative measurement uncertainty.

If conditions remain stable and the performance is not affected in any way or by wear and tear, weighing above the minimum weight is safe.

Figure 8: Measurement uncertainty, minimum weight and Safe Weighing Range of a weighing instrument.As a calculation example here in red 1. for a weighing process tolerance of 1 % (relative measurement uncertainty of > 1 %) the minimum weight is 9 g (at the time and circumstances the calibration took place). Below this accura-cy limit (of a net weight of 9 g) the measurement is not “safe” (indicated as red range).

1. Required weighing process tolerance (%)2. Minimum weight determined: 9 g

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2.1.


However, as you can imagine, weighing usually occurs under less than ideal conditions, affected by many factors.

5.2 Ensuring accurate weight in non-ideal conditions

A safety factor should be applied to allow for fluctuations in the minimum weight due to environ-mental influences and variations in operators, samples, and statistical factors. It ensures that the risk of returning OOS results is reduced to a minimum.

The safety factor is simply a multiplication factor applied to the minimum weight. Table 3 gives some recommendations in determining appropriate safety factors for each of your weighing processes.

The rule of thumb is to begin with a safety factor of “2” for typical laboratory environments and “3” or more for typical manufacturing environments.

Your Considerations for Safe Weighing Safety FactorNo considerations for variations in the device, operators or environment. High potential for Out of Specification (OOS) results.

1

Devices are installed in an ideal environment. Not recommended due to statistical variations. Potential for OOS results.

1.5

“Laboratory conditions”, insignificant environmental influences, one or two operators. 2

“Production conditions” – accounting for one or two low-magnitude influences such as tem-perature variation or low-frequency vibration, several operators.

3

Increasing levels of safety consideration accounting for many low-magnitude environmental influences, several operators, heavy usage or accumulated debris, varying tare containers. Portable scales.

4–10

Note: Higher magnitude variations in environmental conditions (temperature, vibration, wind) must be eliminated. In these cases, increasing the safety factor will not bring the desired results.

Table 3: Safety factor guidelines.Determination of an appropriate safety factor depending on various considerations and environmental conditions in-fluencing the performance of the balance or scale. A well-chosen safety factor will ensure that measurements remain consistent under less-than-ideal conditions.

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Smallest Net WeightThe Smallest Net Weight is the smallest amount required to be weighed-in according to the SOP. It’s the smallest weight the user plans to weigh on that specific device.

Figure 9: Safe Weighing Range.There are three areas in the devices weighing range:• RED AREA: Lowest weighing range, below the minimum weight of the balance, indicates that the results

are inaccurate.• YELLOW AREA: This range indicates that the weighing accuracy is still met but there is a potential for inaccuracies

caused by other influence factors such as the environment, object to be weighed, operator, etc. In other words it may not be safe to weigh in this yellow zone.

• GREEN AREA: Indicates the Safe Weighing Range in which it is safe to weigh, even if conditions vary and may influence the performance of the instrument.

2.0

1.8

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.00 10 20 30 40 50 60 70 80 90 100

Weighing Range (g)

Safe Weighing RangeSafety range, ensuring accurate weight results in non-ideal conditions (Safety Factor)Smallest Net Weight

1. Required weighing process tolerance: 1 %2. Accuracy limit of the balance, minimum weight determined: 9 g

1.

2.

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5.3 Lower and upper limit of the Safe Weighing Range

In order to remain stable even under varying conditions, the minimum weight is multiplied by an appropriate safety factor, depending on the influences of the changing environment on the accuracy of the balance or scale. This is the lower end of the Safe Weighing Range. This limit must always be smaller than the Smallest Net Weight required (by the SOP) to ensure accu-rate and trusted results.

The upper limit of the Safe Weighing Range is determined by the capacity of the balance or scale. Your largest sample, including tare, must always be smaller than this upper limit.


6. Establish the Safe Weighing Range of Your Balance or Scale in 3 Easy Steps

A weighing result is reliable and accurate when the following requirements are fulfilled:• The relative measurement uncertainty is smaller than the process tolerance.• The minimum weight of the balance is smaller than the Smallest Net Weight to be weighed.• The result is within the Safe Weighing Range (= above minimum weight x safety factor and within the capacity of the weighing device).

A calibration of the balance or scale, the subsequent interpretation of its data and relating these to required process tolerances determines the minimum weight under ideal conditions. To take into account that conditions might not always remain constant, a safety factor is applied to ensure the lower accuracy limit. The upper limit of the Safe Weighing Range is determined by the balance capacity.

Required input data are a calibration and the resulting measurement uncertainty, your process tolerance, a safety factor depending on various considerations and environmental conditions influencing the perfor mance of the instrument and the balance capacity.

The lower limit of the Safe Weighing Range is determined by the minimum weight of the balance multiplied by the safety factor; the upper limit is determined by the balance capacity.

Process quality requirements are met if the Smallest Net Weight lies within the Safe Weighing Range.

Step Safe Weighing Contribution Limits 1 Minimum

weight of the balance

Accuracy limit of the balance or scale determined by a calibration (incl. relative measurement uncertainty) and process tolerances

Theoretical smallest possible amount which can be weighed. Below this limit, weighing is not safe; above this limit weighing is only safe in ideal conditions.

2 Safety factor Safety factor x minimum weight = lower end of the Safe Weighing Range

Below this limit, weighing might not be safe due to less-than-ideal conditions. Above this limit, weighing is safe.

3 Capacity Upper end of the Safe Weighing Range

Limit for the largest sample (incl. tare) to be weighed - maximum gross weight.

Table 4: Establish the Safe Weighing Range of your balance or scale in 3 easy steps.Weighing in the safe weighing range ensures results are valid, reliable and right first time.

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7. Tips & Tricks for Accurate Results from the Beginning – and over Time

7.1. Selection of the correct weighing device from the start – your specific recommendation

Quality management of equipment and machinery includes design qualification (DQ), installa-tion qualification (IQ), operational qualification (OQ), and performance qualification (PQ).

Good Weighing Practice™ (GWP®)1 summarizes your weighing needs in one single document.This helps you to ensure that the equipment you choose will be “fit-for-purpose”. The document is usually integrated into design qualification (DQ) and is based on the following elements that have been introduced and explained during the course of this paper:

• Maximum amount weighed,• Smallest sample weighed,• Weighing tolerance (limits of the maximum permissible error),• Standards or regulations that must be adhered to.

7.2. Calibration and testing improvements

GWP® Verification evaluates the accuracy of your equipment on site and gives information based on a risk assessment to maintain optimum performance over time. It provides all information for regular calibration and testing. With a combination of calibration and a risk assessment it allows you to establish an optimal testing plan for maximum security at the lowest costs.

Figure 10: GWP® Recommendation service. The free GWP Recommendation service supports you in selecting the right weighing instruments by matching its metrological performance to your measurement needs. Each GWP Recommendation report includes minimum weight and safety factor calculations to confirm that you have selected the right instrument for your process.

1 The registered trademark GWP is a trademark of Mettler-Toledo in the European Community, USA, China and 11 further countries.

1 / 2

GWP® RecommendationCompany Nick PharmaContact 1Department / Position TestBuilding xxStreet xxxZip code / City 1 1

Process Requirements Value UnitMaximum weight 200 gSmallest net weight 60 mgWeighing tolerance 0.10 %Safety factor required 2Standards and regulations USP <41>Legal for trade Yes

Specified BalanceBalance XPE205DRCapacity 220 g

Range Range 10 g - 81 g

Range 281 g - 220 g

Readability 0.01 mg 0.1 mgVerification scale interval 1 mg 1 mg

Automatic internal adjustment Yes

EvaluationTypical minimum weight1)2)

for 0.10% weighing tolerance 14 mg

OIML minimum capacity 1 mgSmallest net weight 60 mgSafety factor calculated3) 4.29Safety factor required 2

The specified device meets the process requirements.

This evaluation is based on typical specifications published. The actual accuracy and minimum weight of this device should bedetermined at its location with a GWP® Verification.


Process Requirements Value UnitMaximum weight 200 gSmallest net weight 50 mgWeighing accuracy 0.10 %Safety factor required 2Standards and Regulations USP <41>

Device InformationBalance XPE205Manufacturer METTLER TOLEDOSerial number xxxxxxID of balance DEFCapacity 220 gOperated readability 0,01 mgAutomatic internal adjustment Yes

ResultsCalibration certificate ID xxxxxxMinimum weight determinedfor 0.10% weighing tolerance

15

Smallest Net Weight required 60 mgSafety factor determined 4000 gSafety factor required 2

The device meets accuracy requirements.The device meets safety requirements.

MTLABTEC-101-20140316-463 1 / 3

GWP® VerificationCompany InTest Ltd.Contact Mary SmithDepartment / Position Quality AssuranceBuilding GD 623Street 77 West StreetZip code / City 112233 Citytown

Date Sunday, 16. March 2014

Device Information

GWP® Verification No. MTLABTEC-101-20140316-463

Balance XPE205Manufacturer METTLER TOLEDOID of balance 123475Maximum capacity 220 gReadability (d) 0.01 mgBuilt-in adjustment mechanism yesHazardous approval noWet application (≥ IP 65) no

User Requirements Value UnitMaximum weight 200 gSmallest net weight 50 mgWeighing accuracy 0.10 %Safety factor required 2Standards and Regulations USPExpansion Factor k 2

AchievementsMinimum weight certificate USP A77788/1Minimum weight achieved 18 mgWeighing accuracy 0.10 %Safety factor achieved 2.78

The device meets accuracy requirements.The device meets safety factor requirements.

Risk AssessmentWeighing accuracy 0.10%Impact of wrong measurements on business process highImpact of wrong measurements on human, animals and the environment mediumWrong measurements can be discovered very easily noHarsh environment and/or extensive use no

MTLABTEC-101-20140316-463 2 / 3

Recommended Performance Verification Activities

Service contractPreventive Maintenance andCalibration annually

TestsCalibration (by service) twice a yearMinimum Weight Certificate twice a yearLinearity (by service) twice a yearEccentricity (by user) quarterlyRepeatability (by user) monthlySensitivity (by user) weeklyInternal Adjustment (by balance) daily

WeightsWeight 1 200 g Class F2, F1 or E2Weight 2 10 g Class F2, F1 or E2For correct handling of weights we recommend the use of appropriate tweezers and/orgloves.

Recalibration interval of weights1) annually

Test tolerances2)

SensitivityWeight 1 Warning limit 50 mg

Control limit 100 mgRepeatability (Number of measurements (minimum): 10)Weight 2 Warning limit 0.013 mg

Control limit 0.025 mgEccentricityWeight 1 Warning limit 50 mg

Control limit 100 mg

Performance verificationGWPExcellence™The routine testing functionalities of the Excellence balances ensure that your balance isalways tested correctly and on time.

Figure 11: Example of a GWP® Verification certificate and its most important contents. The GWP Verification service calibrates your weighing equipment, assesses risks, recommends measures to improve accuracy and ensures compliance with process and regulatory requirements. GWP Verification can be issued for any weighing device independent of model or manufacturer. It provides the all the necessary information to maintain lifelong accuracy of your balances and scales; audit-proof documentation is included.

The “Smallest Net Weight” is the smallest weight the user plans to weigh on that balance or scale (the user dictates this value). GWP Verification tests and reports if the device meets process requirements or if this Smallest Net Weight is within the Safe Weighing Range. This is done by a comparison of safety factor determined (acc. to the Smallest Net Weight required) and safety factor required/defined.

METTLER TOLEDO’s globally-standardized calibration certificate, the Accuracy Calibration Certifi-cate (ACC) and the GWP Annex, include traditional calibration testing and assessments, plus:

• Proof of required accuracy and fit for purpose – for all measurements in day-to-day use,• Fast interpretation – with clear pass/fail notifications and graphical representations,• Assured compliance – with all standards applied within the weighing industry,• Enhanced documentation – which now includes measurement uncertainty,• A clear statement on pass or fail.

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Figure 12: Accuracy calibration certificate from METTLER TOLEDO. The Accuracy Calibration Certificate (ACC) consists of a core section documenting calibration results and measure-ment uncertainty. Complementing annexes are optional and contain “Pass” or “Fail” statements dependent on the specific tolerances in question. The key benefit comes with the GWP Annex. While a traditional calibration informshow the instrument is behaving in relation to certain standards, the GWP Annex answers the question: “Does it behave well enough to fit my process?” and the resulting “Pass” will indicate that the Smallest Net Weight is in the Safe Weighing Range.

Figure 13: Stay within the Safe Weighing Range.The pre-programmed warning function assists dosing processes. As soon as the weight is above the warning limit, the figures on the balance turn from red to black and indicate that Safe Weighing Range requirements are met.

7.3. Always weigh within the Safe Weighing Range – indication of warning limits

METTLER TOLEDO balances with colour touch display – like the XPR, XPE, MS-TS and ML-T lines – support you to stay within the Safe Weighing Range: The determined minimum weight, multiplied with the safety factor, can be pre-programmed in the balance2, so that the balance can give an indication if the lower end of the Safe Weighing Range is not adhered to. During weighing-in, the weight value remains red until the net sample is above this pre-programmed warning limit. It is a clear visual indicator of when process tolerances and safety considerations have been met.

2 Dependent on the balance model either by the user or by a service technician.


The highest quality label in weighing is the GWP Approved label, which confirms that the balance has undergone a risk-based assessment (GWP Verification), in which the Safe Weighing Range has been defined and confirmed according to your tolerances and specifications.

In METTLER TOLEDO’s’ XPR balances, a GWP Approved function can be activated, which ensures that you always weigh within the Safe Weighing Range. The on-screen icon is your reassurance of accurate results.

7.4. Improve balance repeatability and thus lower the minimum weight on your balance

To improve the repeatability of your balance and thus lower the minimum weight value possible, please pay attention to the following:

• Switch on the balance at least half an hour before weighing,• Avoid drafts and direct sunlight,• Place the balance on a stable weighing table,• Level the balance carefully,• Adjust the balance,• Use tweezers for weighing,• Always close the weighing chamber with a draft shield,• Use an additional inner draft shield if available,• Use de-ionizers where appropriate.

Figure 14: GWP Approved functionality of METTLER TOLEDO XPR balances.The GWP Approved label is shown on the weighing display screen of an XPR balance. It gives you the confidence, the traceability and the proof that – at the time of the measurement – the balance was performing correctly, warning limits are not violated and the result is trustworthy.

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8. References

[1] Guidelines on the Calibration of Non-Automatic Weighing Instruments, EURAMET/cg-18/v.02, January 2009. https://www.euramet.org/Media/docs/Publications/calguides/previous_versions/EURAMET-cg-18-02_Non-Automatic_Weighing_Instruments.pdf

[2] International Vocabulary of Metrology – Basic and General Concepts and Associated Terms (VIM 3rd edition) JCGM 200:2012http://www.bipm.org/utils/common/documents/jcgm/JCGM_200_2012.pdf

[3] EN 45501:2015 Metrological aspects of non-automatic weighing instruments

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