Readability

The readability of a balance is the smallest difference between two measured values that can be read on the display. With a digital display this is the smallest numerical increment, also called a scale interval.



¹⁾ 1d = 1 digit = one numerical increment

TIP: “DeltaRange” and “DualRange” balances feature two different types of readability, which makes them an attractive alternative in terms of price.

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Accuracy

Qualitative name for the degree to which test results approximate the reference value, which can be the correct or expected value, depending on the definition or agreement [DIN²⁾ 55350-13].

Or in short: How close the balance display comes to the actual weight of a weighing sample.

Accuracy classes of test weights

Summary of various weight pieces in the same accuracy classes.

The recommendation of the weight class according to OIML³⁾ R111 ensures that the error limits with regard to the weight classification are observed and that the material and surface quality correspond to this Summary of various weight pieces in the same accuracy classes.

The recommendation of the weight class according to OIML R111 ensures that the error limits with regard to the weight classification are international recommendation. www.oiml.com

As part of control of inspection, measuring and test equipment, quality management standards require that balances be calibrated or adjusted at particular intervals with traceable weights. Certified weights with a corresponding accuracy class must be used for this purpose.

Sensitivity

Change in the output variable of a measuring instrument divided by the associated change in the input variable ([VIM] 5.10)⁴⁾.

For a balance, the change in the weighing value ΔW divided by the load variation Δm



The sensitivity is one of the most important specifications of a balance. The specified sensitivity of a balance is generally understood to be global sensitivity (slope), measured over the nominal range.

Sensitivity between weighing value W and load m, on the example of a balance with a nominal range of 1 kg. The middle line shows the characteristic curve of a balance with correct sensitivity (slope). The upper characterstic curve is too steep (sensitivity too high, exagerrated for reasons of illustration), while the lower curve is not steep enough (too little sensitivity).

⁴⁾ VIM International Vocabulary of Basic and General Terms in Metrology

Temperature coefficient of the sensitivity

Sensitivity is temperature-dependent. The degree of dependence is determined via the reversible deviation of the measured value owing to the influence of a temperature change in the surroundings. It is given by the temperature coefficient of the sensitivity (TC) and corresponds to the percentage deviation of the weight display (or sample weight) per degree Celsius. With an XP balance, for example, the temperature coefficient of the sensitivity is 0.0001%/°C.


This means that for a temperature change of 1 degree Celsius, the sensitivity changes by 0.0001 % or one millionth.

The temperature coefficient can be calculated as follows:



In this equation, ΔS is the change in sensitivity and ΔT the temperature change. The sensitivity change ΔS is equal to the result change ΔR divided by the weighing load m, or after taring by the sample weight. With this information the deviation of the measuring result at a specified temperature
change can be calculated by rearranging the above equation.

For the display value we can then obtain:



If you weigh a load (sample weight) of 100 g on the XP/XS analytical balance, and the ambient temperature in the laboratory has changed by 5 °C since the last calibration, this can lead to the following maximum result change ΔR (with the temperature coefficient of the XP of 0.0001 %/°C) in
the worst case scenario:

If, on the other hand, the load were only 100 mg, that is 1000 times less,



the maximum deviation would also be correspondingly less and amount solely to 0.5 μg.

FACT

Abbreviation for “Fully Automatic Calibration Technology” (“FACT”). Automatic calibration of the sensitivity, depending on the type and linearity of a balance. The calibration is triggered whenever a predetermined temperature change is exceeded.


During production, internal weights are traceably connected to international measuring standards by means of “primary calibration”. In this process, the mass of the internal weight is determined by placing a certified weight on the balance and storing the value in the balance.

proFACT

Abbreviation for “Professional Fully Automatic Calibration Technology” (“proFACT”). Professional automatic adjustment of sensitivity.

TIP: The Excellence and Excellence Plus family of semi-micro and analytical balances has two internal weights. This means that, during calibration, the balance tests not only the sensitivity but the non-linearity.

Linearity (Non-linearity)

The linearity expresses how well the balance is capable of following the linear relation between the load m and the displayed value W (sensitivity). Here, the characteristic weighing curve is imagined as a straight line between zero and maximum load (see Sensitivity).

On the other hand, the non-linearity defines the width of the band within which a positive or negative deviation of the measured value from the ideal characteristic curve
can occur.


For the METTLER TOLEDO Excellence Plus Analytical Balance XP205DR, for example, the deviation from the linear course of the characteristic curve is maximum ±0.15 mg over the entire weighing range of 200 g.

Repeatability

Repeatability is a measure of the ability of a balance to supply the same result in repetitive weighings with one and the same load under the same measurement conditions ([OIML⁵⁾ R 76 1] T.4.3).

The series of measurements must be carried out by the same operator, by the same weighing method, in the same location on the same pan support, in the same installation location, under constant ambient conditions, and without interruption.

The standard deviation of the measurement series is a suitable measurement for expressing the value of the repeatability.

Particularly with high resolution balances, the magnitude of the repeatability is a property that depends not only on the balance. Repeatability is also affected by the ambient conditions (drafts, temperature fluctuations, vibrations), by the weighing sample, and in part by the skill of the person performing the weighing.

The following example shows a typical series of measurements, performed on a semi-micro balance with a readability of 0.01 mg.



Let us now determine the mean value and the repeatability of this series of measurements.

Mean value:

xi = i-th result of the series

N: Number of measurements (weighings), usually 10

The mean value is x = 27.514667 g.



The standard deviation s is used as a measure of the repeatability t. Consequently, the repeatability of the measurement series is s = 0.0095 mg.
The uncertainty in the measurement result is around two to three times the repeatability u ≈ 2s... 3s i.e. the true result x lies within the interval
x - u < x < x + u

In our series of measurements is u ≈ 2 s ≈ 2 x 0.01 mg = 0.02 mg, so that we can specify the weighing result by x ± u = 27.51467 g ± 0.02 mg
The very lowest measured value to be expected for this load with the balance used in the above series of measurements is thus 27.51465 g and the largest is 27.51469 g, which agrees well with the series of measurements.

⁵⁾ OIML International Organization of Legal Metrology

Traceability

The property of a measurement result, via an unbroken chain of comparative measurements with stated measurement uncertainties, relative to suitable nationally or internationally applicable standards ([VIM]⁶⁾ 6.10).

The normal weight pieces used for mass measurements are traced to the superordinate standards.

⁶⁾ VIM International Vocabulary of Basic and General Terms in Metrology

Leveling

Setup of a balance in its reference position (as a rule, horizontally), i.e. setting up its direction of action parallel to the perpendicular orientation of the balance. As a rule, this is the same as setting up the housing of the balance horizontally. The result is distorted by the cosine of the angle of inclination. Corrective measures: All balances offer the option of leveling by using adjustable feet.

TIP: The Excellence Plus balance has “LevelControl, a fully automatic level monitoring function. LevelControl automatically warns you and documents when the balance is not leveled, which increases measuring reliability and eliminates the risks inherent in visual control, such as when using a measuring cabinet.

Corner load

Deviation of the measurement value through off-center (eccentric) loading. The corner load increases with the weight of the load and its removal from the center of the pan support.


If the display remains consistent even when the same load is placed on different parts of the weighing pan, the balance does not have corner-load deviation. For this reason, with high-precision balances, it is important to make sure the weighing sample is always placed exactly in the middle.

The official designation for corner load is: “off-center loading error”.

Reproducibility

The degree of approximation between the measurement values of the same measured variable, even though the individual measurements are carried out under different conditions (which are specified) with regard to:

• the measuring process
• the observer
• the measuring device
• the measuring location
• the conditions of use
• the time

Accuracy

Qualitative term as a judgement regarding the systematic deviation of measurements. The closeness of agreement between the expected value (mean value) of a series of measuring values and the true value of the object being measured ([ISO⁷⁾ 5725] 3.7).

Remarks

The accuracy can be evaluated only when there are several measurement values, as well as a recognized correct reference value.

⁷⁾ ISO International Standards Organization

Precision

Qualitative term as a judgement regarding the mean variation of measurements.

The closeness of agreement between independent measurement values obtained under stipulated conditions ([ISO 5725] 3.12).

Precision depends only on the distribution of random errors and does not relate to the true value of the measurement variable (accuracy).

Example

The ability of a measuring instrument to supply measurement values that seldom deviate.

Remarks

Precision can be evaluated only when there are several measurement values.

Measurement uncertainty

A parameter, associated with the result of a measurement, that characterizes the dispersion of the values that could reasonably attributed to the measurement variable ([VIM]⁸⁾ 3.9).


This parameter, i.e. the measurement uncertainty, is generally expressed by the standard uncertainty u or the expanded measurement uncertainty U (confidence interval). GUM⁹⁾ contains instructions on determining measurement uncertainty. According to GUM, the measurement uncertainty is obtained by totaling the quadratic errors when these are not mutually influential.

Note

There are a variety of methods for calculating measurement uncertainty. In the pharmaceutical industry the reference content is often determined according to the U.S. Pharmacopeia. Otherwise, measurement uncertainty is often determined based on ISO¹⁰⁾ 17025. The latter corresponds to the GUM method.

TIP: In most countries, METTLER TOLEDO Service offers these measurement uncertainty calculations on-site, at the customer’s request.

Minimum initial weight value

The value below which the relative deviation of a measuring result is too large.

TIP: METTLER TOLEDO balances in the Excellence Plus line offer superior weighing technology for successfully weighing the very smallest of weighing samples.

Calibration

Determining the deviation between the measurement value and the true value of the measurement variable under specified measuring conditions.

TIP: METTLER TOLEDO Excellence and Excellence Plus balances document each error on the display or send it to an external software program or printer.

Adjustment

Determining the deviation between the measurement value and the true value of the measurement variable under specified measuring conditions.
A correction should then be made.

TIP: METTLER TOLEDO Excellence and Excellence Plus balances document each error by showing it on the display or sending it to an external software program or printer. For software, we recommend using “LabX balance” with integrated inspection, measuring and test equipment in accordance with METTLER TOLEDO Good Weighing Practice™.