Technical terminology for microbalances

The readability of a readability balance is the smallest difference between the two weighing values ​​that can be read on the display. When using a digital display, this is the smallest numerical increment, also known as the "division value."

Note: The “DeltaRange” and “DualRange” balances have two different types of readability and are highly cost-effective.

The accuracy test results can be corrected or approximated to the extent of the reference value, according to the definition or protocol [DIN1] 55350-13]. Or in a nutshell: the display value of the balance is close to the actual mass of the sample.

The accuracy level of the test weight summarizes the different weights of the same accuracy level. The weight rating according to OIML2) R111 is recommended to ensure compliance with the error limits for the weight classification and to ensure that the surface quality is consistent with this international recommendation. As part of the control, weighing and testing equipment, quality management standards require the use of traceable weights to calibrate or adjust the balance in the appropriate cycle. Weights used for this purpose must be certified to have a corresponding level of accuracy.

The value of the output variable of the indication error weighing instrument is divided by the value of the relevant input variable ([VIM] 5.10)1). For the balance, the change value ΔW of the weighing value is divided by the load variable Δm

The indication error is one of the most important technical parameters of the balance. The indication error specified by the balance is generally understood to be the global indication error (slope) of the quantity weighed within the nominal range.

The temperature coefficient indication error of the indication error depends on the temperature. The degree of dependence is determined by the reversible deviation of the weighing value produced by the influence of temperature changes in the environment. This is derived from the temperature coefficient (TC) of the indication error, which is consistent with the percent deviation of display quality (or sample mass) per degree Celsius. For example, when using an XP balance, the temperature coefficient of the indication error is 0.0001% / ÌŠC. This means that when the temperature changes by 1 ÌŠC, the indication error changes by 0.0001% or one millionth. The temperature coefficient is calculated as follows:

In the equation, ΔS is an indication value change value, and ΔT is a temperature change value. The indication value change value ΔS is equal to the result change value ΔR divided by the load value m or the sample quality after the tare weight. Using this information, the deviation of the weighing result under a certain temperature change condition can be calculated by rearranging the above equation. For the displayed values, we can conclude:

If you load a 100g load value (sample quality) on an XP/XS analytical balance and the laboratory ambient temperature changes by 5 ̊C since the last calibration, the worst case can be obtained in the worst case The resulting change value ΔR (the temperature coefficient of XP is 0.0001% / ̊C):

If the loading value is only 100 mg, ie 1000 times smaller, the maximum deviation will also be reduced correspondingly to 0.5 μg.

FACT
Fully automatic calibration technology (FACT).
The indication error is automatically corrected according to the type and balance of the balance. The balance calibration will be triggered whenever the set temperature change value is exceeded.
During the production of the balance, the internal weight is traceably linked to the international weighing standard through “initial calibration”.
In this process, the quality of the internal weight is determined by placing a certified weight on the balance and storing the value in the balance.

proFACT
Professional-grade fully automatic calibration technology (proFACT). Professional automatic correction of indication error.
Tip: Beyond the series XP/XS series semi-micro and analytical balances have two built-in weights. This means that during calibration, the balance not only tests for indication errors, but also tests for non-linearity.

Linear (nonlinear)
Linear indicates the ability of the balance to follow a linear relationship between the load m and the display value W (indicator error). Here, assume that the ideal curve is a straight line between zero and the maximum load (see: indication error).
Conversely, the nonlinearity defines the width of the frequency band in which the weighing value is positively and negatively offset from the ideal curve.
For example, for the METTLER TOLEDO Excellence Series XP Analytical Balance XP205DR, the maximum deviation from the ideal curve linear progression is ±0.15 mg over the entire weighing range of 200 g.

Repeatability repeatability is the reference ([OIML1)R 76 1] T.4.3) of the balance providing the same ability to perform a load and repeated weighing of the same load under the same weighing conditions.
The same weighing method must be used by the same operator to perform a series of weighings without interruption in the same position on the same weighing pan, the same mounting position, and constant environmental conditions.
The standard deviation of a series of weighings is the weighing method that expresses repeatability. Especially when using high-resolution balances, repeatability depends not only on balance performance. Repeatability is also affected by environmental conditions (ventilation, temperature changes, vibration) and sample effects, as well as the skill of the person performing the weighing operation.
Let us now determine the average and repeatability of this series of weighings.

average value:

Xi = i-th result of this series of weighing
N: number of weighings, usually 10 average

This standard deviation is used as a repeatability reference t. Therefore, the repeatability of this series of measurements is s = 0.0095 mg.
The uncertainty of the weighing result is about two to three times the repeatability u ≈ 2s...3s, ie the actual result x is within the interval x - u < x < x + u. In our weighing, u ≈ 2 s ≈ 2 x 0.01mg = 0.02mg, so we can get the weighing result by x ± u = 27.51467g ± 0.02mg. Therefore, the minimum weight of this load predicted by the balance used in the above series weighing is 27.51465 g, and the maximum value is 27.51469 g, which is completely consistent with the weighing result of the series.

Traceability The weighing result ([VIM]1) 6.10) obtained by weighing a series of uninterrupted comparison chains with specified weighing uncertainty, can be traced back to international or national applicable standards. Common weights for quality weighing can be traced back to the higher standard.

The adjustment level sets the balance at the reference position of the balance (conventional method: horizontal), ie the direction of travel is set parallel to the vertical direction of the balance. As a rule, this is the same as the horizontal method of setting the balance housing. As a result, distortion occurs due to the cosine of the tilt angle. Corrective Action: All balances offer the option of leveling by using adjustable feet.
Tip: The Excellence XP range has a Level Control system. When the balance is not level, the LevelControl will automatically issue an alarm and record it, which will help to improve the reliability of weighing and eliminate the inherent risks of visual control (eg when using weighing instruments).

Offset
1. Deviation of the weighing value obtained by off-center (eccentric) loading. When the load is loaded and unloaded from the center of the pan, the increase in the eccentric load is related to the quality of the load.
If the same load is placed on different parts of the weighing pan, the displayed values ​​remain the same, indicating that the balance is biased. For this reason, when using a high-precision balance, make sure that the sample is always exactly in the middle position.

Reproducibility The degree of approximation between the weighing values ​​of the same weighing variable, even when weighed separately under different conditions (specified) according to the following:
称 Weighing process  Observer  Weighing instrument  Weighing position  Conditions of use  Time

Accuracy is a qualitative term used to assess the systematic bias of a symmetric quantity result. The closeness of the consistency between the predicted value of a series of weighing values ​​and the actual value of the weighing object ([ISO1) 5725] 3.7).
Note Accuracy can only be evaluated when there are multiple weighing values ​​and an approved correct reference value.

Accuracy is a qualitative term used to assess the mean deviation of the results of a symmetric quantity.
The degree of closeness between the independent weighing values ​​obtained under specified conditions ([ISO1) 5725] 3.12).
Accuracy depends only on the distribution of random errors, independent of the actual value (accuracy) of the weighing variable.
The ability of the weighing instrument to provide the weighing value is rarely biased.
Note Accuracy can only be evaluated when there are multiple weighing values.

Measurement uncertainty A parameter that is related to the weighing result and that characterizes the dispersion of the weighing value that can reasonably be attributed to the weighing variable ([VIM]1) 3.9).
This parameter, ie the weighing uncertainty, is usually represented by the standard uncertainty u or the extended weighing uncertainty U (confidence interval). GUM2) contains a description of the method for determining the uncertainty of weighing. According to the GUM description, when the quadratic errors do not affect each other, the weighing uncertainty is obtained by calculating the sum of the quadratic errors.
Note There are several ways to calculate the weighing uncertainty. For the pharmaceutical industry, it is usually determined in accordance with the United States Pharmacopoeia. Otherwise, the weighing uncertainty is usually determined in accordance with ISO 3) 17025. The latter conforms to the GUM method.
Note: In most countries, the METTLER TOLEDO Service Center conducts weighing uncertainty calibration services according to customer needs.

If the minimum weight is below this value, the relative deviation of the weighing result will be too large.
Tip: The METTLER TOLEDO Excellence XP Balance offers the most advanced weighing technology for successful weighing of small samples.

Calibration Determines the deviation between the weighing value and the true value under the specified weighing conditions.
Tip: The METTLER TOLEDO Excellence Series XP and XS balances record and display individual errors on the display or send them to an external software program or printer.

Correction Determines the deviation between the weighing value and the actual value under the specified weighing conditions and corrects it.
Tip: The METTLER TOLEDO Excellence Series XP/XS balance records individual errors by displaying an error on the display or by sending it to an external software program or printer. Regarding the software, we recommend using the "LabX balance software" that complies with the METTLER TOLEDO Good Weighing PracticeTM () and integrates inspection, weighing and testing.

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