static characteristics of an instruments - basics of measurements definition & explanation important for selection of instrument & designing a new instruments YouTube channel : https://www.youtube.com/channel/UCgtlEPfuRGHbHLo46cxm7dQ

1. Static Characteristics
- Ameya P. Nijasure
napradeep@mes.ac.in

2. ➔ Criterias for which an instrument gives a meaningful
description.
➔ Static characteristic reflects into Quality of measurement.
➔ Criterias that are constant or very low with respect to time i.e
static.
Static Characteristics

3. ➔ Scale range of an instrument is defined as the difference between
largest & the smallest reading of an instruments.
➔ Scale range of an instrument is mentioned as Xmax to Xmin or
sometimes just Xmax
Scale span = Xmax - Xmin
E.g - thermometer having calibration -40℃ to 400℃ then Range is -40℃
to 400℃ or 400℃ only & Span is 400-(-40) = 440℃.
Scale Range & Scale Span

4. ➔ Readability of an instrument indicates closeness with which scale of
an analog instrument can be read.
➔ Only applicable for analog type of instruments.
➔ Scale readability depends on
a. Number of graduations
b. Spacing of graduations
c. Size of pointer
d. Parallax effect
e. Ability to read (Observer)
Readability

5. ➔ An environmental condition affects output of instrument is known as drift.
➔ Standard ambient conditions are defined (Specifications)
Drift
Types of drift
A. Zero Drift
Calibration gradually shifts due to
slipping, zero setting etc.
A. Span / Sensitivity Drift
Proportional change in characteristic
A. Zone drift
Occurs only over a portion of span

6. ➔ Repeatability describes the closeness of output readings, when the
same input is applied repetitively for short period of time with same
measurement condition, same instrument/observer, same location
and same conditions of use maintained throughout.
➔ Reproducibility describes the closeness of output readings, for
same input when there is change in method of measurement,
observer, location, conditions of use & time of measurement.
Repeatability & Reproducibility

7. ➔ Accuracy is closeness with which an instrument reading approaches
the true value.
➔ It is conformity to truth.
➔ Accuracy can be specified as
a. Point Accuracy
b. Accuracy as “Percentage of scale range”
c. Accuracy as “Percentage of True value”
➔ Precision is a measure of reproducibility of measurements.
➔ Degree of agreement within the group of measurements.
Accuracy & precision

8. ➔ If the instrument input is increased very gradually form zero there will
be some minimum value below which no output change can be
detected. This minimum value defines threshold of an instrument
➔ It is smallest measurable input.
➔ If input is increased from some arbitrary non zero input value, output
does not change at all until certain increment exceeds. This
increment is called resolution of an instrument.
➔ It is smallest measurable input change.
Threshold & Resolution

9. ➔ Dead time is defined as the time required by the measurement
system to begin to respond to a change in measurand.
➔ Dead zone is defined as largest change of input quantity for which
there is no output for an instrument.
Dead time & dead zone

10. ➔ Output is linearly proportional to input.
➔ Convenient to calculate output depending upon a constant factor.
➔ Simplifies design and analysis of control system.
Linearity

11. ➔ Hysteresis is non coincidence of loading & unloading curve.
➔ As energy in loading is not recoverable in unloading - no process is
reversible.
Hysteresis

12. ➔ The incapability of the system to faithfully measure, record, or control
the input signal (measurand) in undistorted form is called the loading
effect.
➔ Distortion due to reduction is magnitude, waveform distortion, phase
shift or combination of all.
Loading Effect

13. ➔ Static sensitivity of an instrument is a ration of the magnitude of
output quantity (response) to the magnitude of input (quantity being
measured)
➔ Unit depends on types of input & output
➔ Sensitivity of instrument should be high.
Static Sensitivity

14. ➔ Noise is a signal that does not convey any useful information.
➔ Can be external or generated internal in the system
➔ Signal to noise ratio : ratio of desired signal to unwanted noise
➔ Sources of noise
a. Generated noise - Internal to system
b. Conducted noise - Externally generated
c. Radiated noise - Electric/ magnetic disturbances
Noise

15. Thank you….!!
- Ameya P. Nijasure
napradeep@mes.ac.in