Definition of Terms

Readability of an instrument – indicates the closeness with which the scale of the instrument may be read

e.g. An instrument with a 12-in scale has a greater readability than an instrument with a 6-in scale given that they are both over the same range. Least count – smallest difference between 2wo indications that can be detected on the instrument scale

Sensitivity – ratio of the linear movement of the pointer on the instrument to the change in the measured variable causing this motion.

e.g. 1-mV recorder has 25 cm scale length

Sensitivity is 25cm/mV (assuming linear across scale)

 Hysteresis – instrument (or any piece of equipment exhibits this when there is a difference in the reading depending on the value of the measured quantity is approached from above or below e.g. aerospace value

Accuracy – deviation of reading from a known input. Generally expressed as percentage of full-scale reading. 

e.g. A 100-kPa pressure gage that has accuracy of 1% is accurate to within ± 1kPa over the entire range of the gage. Precision – ability of instrument to reproduce certain readings with a given accuracy. e.g. 
5 readings – 104, 103, 105, 103, and 105; accuracy is at best 5 volts, the precision is ± 1 percent
i.e. – mean is 104V and only deviates 1 Volt from this mean Calibration – involves a comparison of the particular instrument with either;
  • A primary standard
  • A secondary standard with a higher accuracy than the instrument to be calibrated
  • A known input source
Calibration establishes the accuracy of the instruments.
e.g. Voltage output from Wheatstone bridge circuit

Velocity U 

As known input use a pitot static tube. e.g. Heat transfer Lab

 Basic Concepts in Dynamic Measurements 

Static Measurements – performed when the quantity being measured is not changing with time.

Steady State – state that measurement systems reaches when it is no longer changing with time (i.e. transients have died out . e.g. – pump experiment)

Dynamic Analysis – e.g. Vibrations experiment

Natural Frequency – Frequency at which a system will have its maximum amplitude.

System Response 

Linear frequency response – system treats all frequencies the same within the range of applications

Amplitude response – ability of the system to react in a linear way to various input amplitudes. When this linear range is exceeded, system is said to be over driven.

Slew rate – maximum rate of change the system can handle when 

The ratio  has a value of 36.8%

Definition of Time constant is usually carried into all types of systems even those which do not exhibit the simple behavior of the above system.

i.e.

t - time it takes for the system to attain a state of 63.2% of the difference between initial and final states.

 Impedance Matching – When connections are made between electrical devices, proper care must be take to avoid impedance mismatching.

The bigger R the closer EAB approaches the internal voltage.

The impedance of the measuring device connected to the terminal should be lager compared with the internal impedance Ri.

 Distortion – general term that may be used to describe the variation of a signal from its true form. Depends on the measuring system. e.g.

Þ output from LP filter will be