Resolution, Threshold, Range and span, Hysteresis, Dead band, Backlash, Drift, Impedance matching and loading

1. PHY 504(B): Instrumentation-II
Dr. Mrs. Pritee M. Raotole
KBC, North Maharashtra University, Jalgaon
MGSM’s ASC, College, Chopda
Department of Physics

2. Introduction to Instrumentation 1
 Resolution,
 Threshold,
 Range and span,
 Hysteresis,
 Dead band, Backlash,
 Drift,
Dr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce, College, Chopda

3. Resolution
• When an instrument is showing a particular output
reading, there is a lower limit on the magnitude of the
change in the input measured quantity that produces an
observable change in the instrument output.
• Thus, the resolution refers to the smallest change of
input for which there will be a change in output.
Dr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce, College, Chopda

4. Resolution
Dr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce, College, Chopda

5. Resolution
Dr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce, College, Chopda
• Using a car speedometer as an example, this has
subdivisions of typically 20 km/h. This means that when the
needle is between the scale markings, we cannot estimate
speed more accurately than to the nearest 5 km/h.
• This figure of 5 km/h thus represents the resolution of the
instrument.

6. Threshold
• The minimum value of input which is necessary to activate
an instrument to produce an output is termed its threshold
as shown in figure
Dr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce, College, Chopda

7. Threshold
• As an illustration, a car speedometer typically has a
threshold of about 15 km/h.
• This means that, if the vehicle starts from rest and
accelerates, no output reading is observed on the
speedometer until the speed reaches 15 km/h.
Dr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce, College, Chopda

8. Range and span
Dr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce, College, Chopda
• Range-
• Range is specified by the lower and upper limits in which it
designed to operate for measurement.
• Example- A thermometer has a scale from −40°C to 100°C.
• Thus the range varies from −40°C to 100°C.

9. Range and span
Dr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce, College, Chopda
• Span-
• It is defined as algebraic difference between upper and
lower range values is termed as span.
• (Range −40°C to 100°C) Thus its span is 140°C .
• (Range 300°C to 900°C) Thus its span is 600°C .
• (Range −0°C to 1000°C) Thus its span is 1000°C .

10. Hysteresis
• If the input measured quantity to the
instrument is steadily increased from a
negative value, the output reading varies
in the manner shown in curve (a).
• If the input variable is then steadily
decreased, the output varies in the
manner shown in curve (b).
• The non-coincidence between these
loading and unloading curves is known as
hysteresis.
• Two quantities are defined, maximum input
hysteresis and maximum output
hysteresis, as shown in Figure
Dr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce, College, Chopda

11. Hysteresis
• Hysteresis is most commonly found in instruments that contain springs, such as
the passive pressure gauge and the Prony brake (used for measuring torque).
• It is also evident when friction forces in a system have different magnitudes
depending on the direction of movement, such as in the pendulum-scale mass-
measuring device.
• Devices like the mechanical flyball (a device for measuring rotational velocity)
suffer hysteresis from both of the above sources because they have friction in
moving parts and also contain a spring.
• Hysteresis can also occur in instruments that contain electrical windings formed
round an iron core, due to magnetic hysteresis in the iron.
• This occurs in devices like the variable inductance displacement transducer, the
LVDT and the rotary differential transformer.
Dr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce, College, Chopda

12. Dead band
• Largest change in the measurand to which the
instrument does not respond.
• Fig shows output-input curve with hysteresis due to
Coulomb’s friction
Dr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce, College, Chopda

13. Dead band or Dead space
• Dead space is defined as the range of
different input values over which there is
no change in output value.
• Any instrument that exhibits hysteresis
also displays dead space, as marked on
Figure Some instruments that do not
suffer from any significant hysteresis can
still exhibit a dead space in their output
characteristics, however.
• Backlash in gears is a typical cause of
dead space, and results in the sort of
instrument output characteristic shown in
Figure
Dr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce, College, Chopda

14. Backlash
• Maximum distance or angle through which any part of the
mechanical system may be moved in one direction without
causing the motion of next part.
• Can be minimized if the components are made to very
close tolerances.
• Backlash is commonly experienced in gear sets used to
convert between translational and rotational motion
(which is a common technique used to measure
translational velocity).
Dr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce, College, Chopda

15. Backlash
Dr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce, College, Chopda
• It can be heard from the railway coupin actual practice
some backlash must be allowed to prevent jamming.
lings when a train reverses direction.

16. Drift
Dr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce, College, Chopda

17. Drift
Dr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce, College, Chopda

18. Impedance matching and loading
Dr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce, College, Chopda
What is Impedance?
• Impedance is defined as the combined effect of
capacitance, inductance, and resistance that a circuit
offers a signal at a given frequency.
• Impedance can also be defined as a measure of
resistance to electrical current flow when a voltage is
moved across it.
• Impedance is measured in ohms and is the ratio of
voltage to the flow of current allowed.

19. Impedance matching and loading
Dr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce, College, Chopda
Impedance loading
 Any measuring instrument extract some energy which
Changing the value of measured variable.
 Loading effect is the incapability of the system to faith
fully measure, record or control the input signal in
accurate form.

20. Impedance matching and loading
Dr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce, College, Chopda
For no impedance loading (Ideal case) EAB = E

21. Impedance matching and loading
Dr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce, College, Chopda

22. Impedance matching and loading
Dr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce, College, Chopda
Impedance Matching
In electronics, impedance matching is the practice of designing the
input impedance of an electrical load or the output impedance of its
corresponding signal source to maximize the power transfer or
minimize signal reflection from the load.

23. Impedance matching and loading
Dr. Mrs. Pritee M. Raotole, MGSM’s Arts Science and Commerce, College, Chopda
A source of electric power such as a generator, amplifier or radio
transmitter has a source impedance which is equivalent to an electrical
resistance in series with a reactance.
An electrical load, such as a light bulb, transmission
line or antenna similarly has an impedance which is equivalent to a
resistance in series with a reactance.
The maximum power theorem says that maximum power is transferred
from source to load when the load resistance equals the source
resistance and the load reactance equals the negative of the source
reactance.