lecture one Flashcards
what does metrology mean?
it’s the science of measurement.
why are instruments subdivided into different calcifications?
to broadly compare them in terms of cost accuracy and general applicability to various applications.
what are the first sub classifications of instruments?
Active instruments and passive instruments.
what are the differences between active and passive instruments ?
Passive instruments reflect a measurement of only the object being being measured while active instruments measure the quantity in relation to an external power source.
passive instruments have no energy source, active instruments have an energy source.
what is a potentiometer?
it is an instrument with three or more terminals with a slider or rotating contact that forms a potential divider.
if only 2 terminals are used it acts as a variable resistor.
what are the differences between active and passive instruments in terms of resolution and cost?
passive instruments can have their resolution improved by extending the pointer therefore increasing the range of the scale while active instruments have their resolution increased by increasing the magnitude of the source.
passive are less expensive, active more expensive.
what are the 2nd type of instruments?
deflection type instruments and null type instruments.
what are the differences between null type instruments and deflection?
deflection give the reading in terms of the deflection of the pointer while in null type you keep adding and subtracting weights to counterbalance the effects of the upward force and the reading is given in terms of the final weight.
what is calibration?
it is the comparison of values between a device undertest and a known value.
what are the differences between null and deflection instruments in term of accuracy?
deflection accuracy relies on calibration of spring which is much harder than the calibration of weights relied on by null type therefore null type is more accurate.
what are the 3rd types of instruments?
analog and digital.
differences between analog and digital?
analog gives a continuous stream of readings that are only limited by how much the eye can see and how fine the scale is while digital has an output that varies in discreet steps and so can only have finite steps.
analog readings need to converted into bits using A/D converter to use with computers while digital preforms all its calculations in bits already.
what are the downsides of the A/D?
adds a significant cost and adds a lot of time to convert data which is sometimes unfeasible because in some systems time is crucial.
what are the 4th types of instruments?
indicating instruments and instruments with signal output.
differences between indicating and signal output instruments?
indicating instruments only give a visual or audio indication of the magnitude of the physical quantity measured, these include all null type instruments and most passive ones it can have an analog or digital display while signal output instruments give a signal output that is proportional in magnitude to the measured quantity they commonly used in control systems and these signals are usually voltages.
