STATIC CHARACTERISTICS OF MEASURING INSTRUNMENTS

INTRODUCTION Static characteristics: are the definitions of a set of criteria that give a meaningful description of the quality and accuracy of the measuring system

RESOLUTION (SCALE VALUE) It is the smallest measurable input change. It also may be defined as the value of one division on the scale. SCALE DIVISION It is the distant between two adjacent scale marks in mms.

If Xmin = 0 then Range = Xmax SCALE RANGE If the maximum reading of the measuring system is Xmax and its minimum reading is Xmin, then Range = Xmin to Xmax   If Xmin = 0 then Range = Xmax

  a SPAN If the maximum reading of the measuring system is Xmax and its minimum reading is Xmin, then   Span =Xmax - Xmin

READABILITY It is the ease with which readings may be observed. This term is frequently used for analogue type instruments where the human observer noting the position of the pointer on the scale. It is obvious that as the scale division increases the readability improved.

Analog or Digital c) Analog quantitry SENSITIVITY It is the ratio of the change in output signal to the change in the input signal.  Sensitivity= change in output/change in input In case the output is a graduated scale, and pointer, then  Sensitivity = Scale division/Resolution

DEFLECTION FACTOR d)Digital quantity It is the inverse of the sensitivity   DRIFT Is the variation in the output of an instrument caused by no change in the input. It is commonly arise in electric devices due to the rise in its internal temperature.

FIRST STAGE SENSING-TRANSDUCING STAGE THRESHOLD   It is the smallest measurable input. If the instrument input is increased very gradually from zero there will be some minimum value below which no output change can be detected. This minimum value defines the threshold of the instrument.

SECOND or INTERMEDIATE STAGE SIGNAL-CONDITIONING STAGE HYSTRESiS Hysteresis is a phenomenon which depicts different output effects when loading and unloading. Hysteresis, in a system, arises due to magnetic Hysteresis, internal friction, free play or looseness in the mechanisms of the instruments.

THIRD OR TERMINATING or FINAL STAGE ACCURACY THIRD OR TERMINATING or FINAL STAGE It is the closeness of the instrument reading to the true value of the quantity being measured. Thus, accuracy of a measurement means conformity to truth.   The accuracy may be specified in terms of inaccuracy or limits of error

EXAMPLE OF MEASURING SYSTEM 3.12.1 Accuracy as “Percent of Scale Range “ Accuracy of measuring instruments are often guaranteed to a certain percent of its full scale deflection (f.s.d.). Specification of accuracy in this manner can be highly misleading. As the reading get smaller, the percent error will get bigger.   3.12.2 Accuracy as “Percent of Measured Value “ This to specify the accuracy in terms of the measured value, i.e. accurate to within ±percent of the measured value.. This statement means that as the readings get smaller so do the error.

PRECISSION (REPEATABILITY) It is the measure of the degree to which successive measurements differs from on to another.   Fig. 2.3 Gage for measuring pressure in automobile tires