STATIC AND DYNAMIC CHARACTERISTICS OF INSTRUMENTS

Do not change with time are known as static characteristic

Accuracy-Precision

sensitivity

Error.

reproducibility

Drift

Dead zone

No measurement is perfect accuracy and precision.

Error can be classified by:

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  1. Systematic or Cumulative Error

– Environmental error

– Loading error

– Instrument error

  1. Miscellaneous Type of Gross Error.
  1. Accidental or Random Error.

Presence of of certain system defects

– Human error – observe/ read

– Faulty component/ adjustment

– Improper application of the instrument

Accuracy

• How closely the instrument measure the actual value.

Precision

The ability of measurement to produce the sameoutput value on every time.

Accuracy = [(Result - Actual value) / Actual value] x 100

expressed as the ratio
of the output to the magnitude of the input signal.

Static sensitivity, K = (change of output signal / change in input signal)

the degree of closeness with which a given
value of a quantity is repeatedly measured within a close range.

defined as the variation of output for a given input caused due to change in the sensitivity of a instrument due to certain interfering inputs

can be classified into:

a. Zero drift.

b. Sensitivity drift.

the largest change in input quantity for which a noticeable change in the output quantity is observed from zero reading.

Selection of the instrument.

Instrument’s qualities, value guided.

Convenience aspects, value judged.

Cost, initial and cumulative total.

Accuracy & Precision Characteristic– Sensitivity, linearity, zero

Nature and type of data available– Analog, digital, continuous, etc

Further data computation

Dynamic response characteristic

Susceptibility to environmental disturbancs

Suitable for the given application– lab use, field use, or both

Adaptibility to different sizes of input– scale expansion, range changes

Safety in use

Maintanance, repair, steady delivery

Material durability & Non-Fouling design

• Initial cost– Installation, various attachment, accessories

• Maintainance, repair, calibration

• Running cost

• Expected lifetime

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Error = Vi - Va
Vi = indicated value
Va = actual value

Dynamic characteristic of instruments : .

Dynamic characteristic of measuring system: .applying a known and predetermined input signal to its primary element and the studying the output.

Types of dynamic input signal:

Dynamic characteristic can be classified into:

simple assumptions

The assumptions are:

The system is a single degree of freedom system.

Periodic signal with time period

Transient signal

Random signal

-Varying randomly with time, with no definite period and amplitude -may be continues, but not cyclic

Varying cyclically with time or repeating itself after a constant interval

-Varying non- cyclically with time
-Sometimes, the signal becomes zero after a certain period of time

Steady-state periodic.

Transient

The restoring element (spring) are linear.

Damping is viscous (consistent).

First order System

Second order system

There is zero mass in the first order system

The different of second order system with first order system is it consist of a mass

The elements are the linear spring and viscous damper.