quantity that, in a direct measurement, does not affect the quantity that is actually measured, but affects the relation between the indication and the measurement result
Example 1 Frequency in the direct measurement with an ammeter of the constant amplitude of an alternating current.
Example 2 Amount-of-substance concentration of bilirubin in a direct measurement of hemoglobin amount-of-substance concentration in human blood plasma.
Example 3 Temperature of a micrometer used for measurement of length of a rod, but not the temperature of the rod itself, which can enter into the definition of the measurand.
Example 4 Background pressure in the ion source of a mass spectrometer during a measurement of amount-of-substance fraction.
Note 1 An indirect measurement involves a combination of direct measurements, each of which may be affected by influence quantities.
Note 2 In the GUM2, the concept 'influence quantity' is defined as in the 2nd edition of the VIM, covering not only the quantities affecting the measuring system, as in the definition above, but also those quantities that affect the quantities actually measured. Also, in the GUM this concept is not restricted to direct measurements.
[OIML V2-200:2012, 2.52]
2Guide to the expression of uncertainty in measurement, see OIML publication G 1-100.
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