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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- Systematic or Cumulative Error
– Environmental error
– Loading error
– Instrument error
- Miscellaneous Type of Gross Error.
- 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.