IEC 61207-1:2010 pdf – Expression of performance of gas analyzers -Part 1: General.
NOTE 2 A primary standard is never ussd directly for measurement other than for comparison with other primary standards or reference standards
[IEC 60050-311, 311-04-02, ISO/IEC Guide 99, 5.4 modified]
direct (method of) measurement
method of measurement in which the value of a measurand is obtained directly, without the necessity for supplementary calculations based on a functional relationship between the measurand and other quantitIes actually measured
NOTE I Th. value I tie measurand is considered to be obtained directly even when tie scale of a measuring Instrument has values which are linked to corresponding values of the ineasurand by means of a table or a graph lIE VI
NOTE 2 The method of measurement remains direct even If It Is necessary to matie supplementary measurements to determin• th, values of influenc, quantifies in order to make corrections (lEVI
(IEC 60050-311, 311-02-011
NOTE 3 The definitIons of the metrologlcal characterIstics of the Instruments refer implicItly to their use in direct meSsurements
indirect (method of) measurement
method of measurement In which the value of a quantity Is obtained from measurements made by direct methods of measurement of other quantities linked to the measurand by a known relationship
(IEC 60050-311, 311-02-021
NOTE I In order to apøty an Indirect method of measurement a model Is needed wtiicll Is able to supply the relationship, and which is fully explicit between the measurand and th, parameters that are measured by direct measurement.
NOTE 2 Th, computations must be carried out on both valueS and uncerlainbes, and therefore require accepted rules for the propagation of the uncertainty as proHided by GUM.
(method of) measurement by repeated obs.rvatlons
method of measurement by which the result of the measurement IS assigned on the basis of a statistical analysis on the distribution of the data obtained by several observations repeated under nominally equal conditions
NOTE I On. should reeort to a statistIcal analysis when the instrumental uncertainty Is too small to ensure the measurement compatibility This may happen in two quite different sets of circumstances
a when the measurand is a Quantity subjected to intrinsic statistical fluctuatioM leg. in measurements involvIng decay) In this cas. the actual m,asurand is the statistical distribution of the stales of the measured quantity, to be described by its statistical parameters ( and standard deviation) Th. statistical analysis is carned Out on a populatIon of results of measurement, each wIth Its own value and uncertaInty, as each observation correctly describes one particular stat, of the measured quantity The situation may be considered a particular case of indirect measurement.
b) when the noise associated wIth the transmission of signals affects the reading-value more than In the operating conditions used for the calibration, contributing to the uncertainty of the measurement to en estent comparable with the instrumental uncertainty or higher (e.g. lithe field use of surveyor instruments). In this case, tile statistical analysis IS cameo out on a populatIon of reading-values with the purpose of separating the information on the m.asurand from the no.s• Th, situation may be considered as a new calasetion of the instrument for a set of operating conditions outsidC their rated range
NOTE 2 One cannot presume to obtain by means of repeated observation an uncertainly lower than the Instrumental uncertainty assigned by the calibratIon or the class of precision of the instrument. Indeed, if the results of the repeated measurements are compatible with each other within the instrumental uncertainty, the latter Is the valid datum for the uncertainty of the measurement and several observations do ml bring more Information than one, In the other hand, if they are not compatibl. within the instrumental uncertainty, the final result of the measurement should be espressed with a larger uncerlainty in order to make all results conipalibie as they should be by definition.