PH4705 & ET4305: Measurements Measurement: assign numbers to property of object or event to describe it The absolute true value of a measurement can’t be known (our measurement method or system will affect the object or event). We measure the conventional true value accepting the there is some uncertainty. A measurement is a number with appropriate units A measuring system must be calibrated against some standard

PH4705 & ET4305: Measurements Units: a quantity in terms of which other quantities may be expressed (e.g. length in metres) Ideally units should be related to a standard which is permanent and unchangeable Many standards were originally material – properties of same physical object kept in one location Portable secondary standards copied from this were/are used for calibration, or a “recipe” for generating a standard given. All precision measurements are traceable back to secondary standard maintained in an official standards lab

PH4705 & ET4305: Measurements There are three independent fundamental qualities: mass, length and time* We use the SI –Systems International system of units this defines seven base units: Mass, Length, Time (independent base units) Ampere, Kelvin, Mole, Candela (derived base units) *ignoring relativity. Mass is still a material standard

PH4705 & ET4305: Measurements Errors: When we measure a quantity there are always errors, we aim to reduce them to an acceptable level. Two types: Intrinsic or systematic error associated with the measurement instrument or method. Extrinsic or Influence error arising from sources external to the instrument or generated by it

PH4705 & ET4305: Measurements Intrinsic errors are usually determinate, they can be compensated for –Dynamic error: the measuring instrument can’t respond fast enough to changes in the measurand –Law error: The instrument deviates from the stated (or assumed) relationship linking the measurand and the measured value (non linearity or zero errors are common) –Loading error: the instrument, when connected, causes a change in the measurand

PH4705 & ET4305: Measurements Extrinsic errors: Random, they cannot be assigned a definite value Every effort is made to minimise the effect of extrinsic errors through using the “right” instrument, selection of appropriate measurement technique, and estimate of expected results. Random errors are assumed to be Gaussian

PH4705 & ET4305: Measurements For Gaussian random errors 68.3% of reading lie within ±one standard deviation, σ, and 95.4% lie within ±2σ A common way of expressing the uncertainty is to quote the result as the mean value ±α, where α = 0.675σ Alternatively, the meaurement ± a value related to the % accuracy expressed in terms of least significant figures is given