Compensation for Interfering and Modifying InputsMethod of inherent insensitivityMethod of high gain feedbackSlide 4Method of calculated output correctionMethod of Opposing InputsMethod of filteringReporting Instrument ErrorNIST 1297 -cont-Slide 10Traceability of measurand and uncertainty1/31/01 BAE 5413 1Compensation for Interfering and Modifying Inputs•Methods of compensation–Inherent insensitivity•eg. Shielding instrument from EMI•Construction of strain gauges from invar–High Gain feedback–Calculated output corrections–Filtering–Opposing inputs1/31/01 BAE 5413 2Method of inherent insensitivity•Minimize Fi, Fmi, Fmd•Selectivity - ability to of an instrument to select the input of interest and reject interference signalsFdFmiFiIiImIdOFmdInterfering inputModifying inputDesired inputOutputModifiedInterferingOutputModifiedDesiredOutput1/31/01 BAE 5413 3Method of high gain feedback•System with modifying inputFdImIdOFmdModifying inputDesired inputOutputModifiedDesiredOutput)(mddIFIO 1/31/01 BAE 5413 4Method of high gain feedback•Feedback loop added•Kfb >>1•O is no longer a function of Fd(Im)FdImIdOFmdModifying inputDesired inputOutputModifiedDesiredOutputKfbKfbdmdfbfbmddmdfbfbmddIIFKKIFIIFKKIFIO )()()(1)(1/31/01 BAE 5413 5Method of calculated output correction•Computer based systems allow correction to be made easily•Requires that Ii be measured alsoIiIdInterfering inputDesired inputOOutputFiFd-FiCalculationInstrumentsumsum1/31/01 BAE 5413 6Method of Opposing Inputs•Input information is used in some manner to form a correcting signal within the instrumentIiIdInterfering inputDesired inputOOutputFiFd-FiInstrumentsum1/31/01 BAE 5413 7Method of filtering•Removes interfering signal–Frequency of desired signal must be different from frequency of interferenceFdFmiFiIiImIdFmdInterfering inputat Frequency fiModifying inputDesired inputModifiedInterferingOutputModifiedDesiredOutputOOutputBand reject filterrejecting fi1/31/01 BAE 5413 8Reporting Instrument Error•NIST 1297–Standardizes the reporting of uncertainty–Coordinated with CIPM/ISO–Based on representing each component of uncertainty (ui) in a measurement by an estimated standard deviation associated with that component. ui = standard uncertainty–Divides types of uncertainty into two categories•Type A - can be evaluated directly as a standard deviation•Type B - must be estimated in non-statistical fashion, but is expressed as a standard deviation–Assumes a correction factor is applied to remove systematic error (bias is removed by calibration)–Defines combined uncertainty (uc) as the RMS of the component uncertainties:  iicuu21/31/01 BAE 5413 9NIST 1297 -cont-–Note:and– –Defines expanded uncertainty, U, as the range within which the “value of the measurand”(Y) can confidently be expected to lie. And expanded uncertainty can be calculated as a product of coverage factor (k), and combined uncertainty (uc)–Y- measurand, y - measurement resultiiixxuuiiiicxxuu2ckuU ckuyUyY 1/31/01 BAE 5413 10NIST 1297 -cont-–Recommends use of a coverage factor of 2 (This is equivalent to 2 standard deviations or 95% if normal) or an explanation why.1/31/01 BAE 5413 11Traceability of measurand and uncertainty •A traceable measurand is not needed for determination of uncertainty.–Correction of systematic error or bias is assumed–Standard deviation of y given a single Y allows u(y) to be determinedInstrumentY