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8 Measurement Uncertainty
Example 1 The uncertainty of weighing is determined through the propagation of
uncertainties of all constituents that influence the measurement of mass. Most often,
among the factors that influence the uncertainty, one can list uncertainty of the balance
(calibration certificate of the balance), uncertainty of the weight used for intermediate
checking of the balance (calibration certificate of the weight); and uncertainty of
weighing in the laboratory (standard deviation of the control chart used for the longterm checking of the balance).
In the case of the analytical balance, with the accuracy of 0.0001 g, the expected
value of uncertainty can be assumed to be in the range 0.0003–0.0005 g.
It should, however, be highlighted that an uncertainty reflects the weight of an
‘ideal’ object—that is, the weight. In reality, objects weighted are not ‘ideal’ which
can significantly influence the weighing uncertainty.
Factors influencing the uncertainty when weighing real objects:
– Weighing of hygroscopicobject (sample);
– Weighing of object easy to collect electric charge;
– Weighing of object containing volatile components.
Table 8.3 shows the results of weighing two objects: a weight with the mass of
1.000 g and a soil sample (a sample of soil with the mass of 1 g has been placed in
the vessel). In the case of the measurement of the mass of the weight, the result is
the value read directly. In the case of the measurement of the mass of the soil, the
result is the value after the tare is subtracted (the mass of the empty vessel). For each
object, 12 subsequent weighings were performed.
BE AWARE! When measuring real objects (e.g., soil), the uncertainty (expressed
as a standard deviation) can come to a value much higher than the uncertainty obtained
during calibration with standard weights (as provided in the certificate of balance).
Example 2 The uncertainty of measuring a specified volume of liquid with measuring glassware is commonly taken from the calibration certificate of the pipette or
from the laboratory results of the measurement of mass of the pipetted liquid.
For example, for a pipette of the type A class, of 25.00 mL, the uncertainty
taken from the manufacture calibration certificate, is 0.03 mL. It should, however,
be mentioned, that the calibration is executed with an ‘ideal’ object—that is, the
distilled water with a specified temperature. In laboratory practice, pipetted liquids
are not ‘ideal,’ which can significantly influence the uncertainty of the measurement
result.
8 Measurement Uncertainty
Example 1 The uncertainty of weighing is determined through the propagation of
uncertainties of all constituents that influence the measurement of mass. Most often,
among the factors that influence the uncertainty, one can list uncertainty of the balance
(calibration certificate of the balance), uncertainty of the weight used for intermediate
checking of the balance (calibration certificate of the weight); and uncertainty of
weighing in the laboratory (standard deviation of the control chart used for the longterm checking of the balance).
In the case of the analytical balance, with the accuracy of 0.0001 g, the expected
value of uncertainty can be assumed to be in the range 0.0003–0.0005 g.
It should, however, be highlighted that an uncertainty reflects the weight of an
‘ideal’ object—that is, the weight. In reality, objects weighted are not ‘ideal’ which
can significantly influence the weighing uncertainty.
Factors influencing the uncertainty when weighing real objects:
– Weighing of hygroscopicobject (sample);
– Weighing of object easy to collect electric charge;
– Weighing of object containing volatile components.
Table 8.3 shows the results of weighing two objects: a weight with the mass of
1.000 g and a soil sample (a sample of soil with the mass of 1 g has been placed in
the vessel). In the case of the measurement of the mass of the weight, the result is
the value read directly. In the case of the measurement of the mass of the soil, the
result is the value after the tare is subtracted (the mass of the empty vessel). For each
object, 12 subsequent weighings were performed.
BE AWARE! When measuring real objects (e.g., soil), the uncertainty (expressed
as a standard deviation) can come to a value much higher than the uncertainty obtained
during calibration with standard weights (as provided in the certificate of balance).
Example 2 The uncertainty of measuring a specified volume of liquid with measuring glassware is commonly taken from the calibration certificate of the pipette or
from the laboratory results of the measurement of mass of the pipetted liquid.
For example, for a pipette of the type A class, of 25.00 mL, the uncertainty
taken from the manufacture calibration certificate, is 0.03 mL. It should, however,
be mentioned, that the calibration is executed with an ‘ideal’ object—that is, the
distilled water with a specified temperature. In laboratory practice, pipetted liquids
are not ‘ideal,’ which can significantly influence the uncertainty of the measurement
result.