Lecture 2
EE305 Instrumentation and Measurement
Teaching Assistant Šejla Džakmić
Chapter 2
Instrument Types and Performance Characteristics
Active Analog Smart
Null-type Indicating
Passive Digital Non-smart
Deflection- With signal
type outut
Instrument Types and Performance Characteristics
Active & Passive
Active
Measured quantity modulates the magnitude
of external power source
Float-type petrol tank
Change in petrol level moves potentiometer
arm
Output as a proportion of external voltage
applied across two ends of potentiometer
Measurement resolution
Balance between cost and resolution
Instrument Types and Performance Characteristics
Active & Passive
Passive
Output produced entirely by
measured quantity
Passive pressure gauge
Presure of the fluid translated onto
movement of pointer
Simple construction
The energy expended in moving the
pointer is derived from the change in
meassured pressure
No other energy inputs
Simple construction
Less expensive
Instrument Types and Performance Characteristics
Null-type & Deflection type
Null-type Deflection type
Requires adjustment until a Output measurement in terms of
appropriate level is reached deflection of pointer against a scale
Dead- weight gauge Passive pressure gauge
Weights added on top of the piston Accuracy: linearity and calibration of the
until it reaches null point spring
Accuracy: calibration of the weights More convenient
More accurate
Instrument Types and Performance Characteristics
Analog & Digital
Analog Digital
Continuously varying output as the Output varies in discrete steps
measured quantity changes
Finite number of values
Infinite number of values
Revolution counter
Pressure gauge
Limited with scale and eye
discriminaton
Instrument Types and Performance Characteristics
Indicating instruments Instruments with signal output
Merely give magnitude of measured Give output in form of measurement
physical quantity signal whose magnitude is propotional
measured quantity
Include null-type and most passive
ones Part of automatic control systems
Analog and digital outputs Recorded measurements for later usage
Liquid-in-glass thermometer
Require human intervention to read
and record measurement
Smart vs. Non-smart
Incorporated in microprocessors or
not
Characteristcs of instruments
Static Characteristics
(Thermometer example)
Parameters which determine the choice of
instrument for a particular application
The set of criteria defined for the instruments,
which are used to measure the quantities which
are slowly varying with time or mostly constant,
i.e., do not vary with time, is called ‘static
characteristics’.
Accuracy, Precision
Linearity
Range, Span
Threshold
Resolution, etc...
Used under specified standard calibration
Static Characteristics of Instruments
Accuracy vs. Inaccuracy
Accuracy - a measure of how close the output reading is to the correct value.
Inaccuracy (measurement uncertainty) – extend to which reading might be
wrong
Often quoted as a percentage of the full scale reading
Choose instrument with apropriate range
Example:
A pressure gauge with a measurement range of 0-10 bar has a quoted inaccuracy of
± 1.0% 𝑓. 𝑠. (𝑜𝑓 𝑓𝑢𝑙𝑙 𝑠𝑐𝑎𝑙𝑒 𝑟𝑒𝑎𝑑𝑖𝑛𝑔).
a) What is the maximum measurement error expected for this instrument?
b) What is the likely measurement error expressed as a percentage of the outut
reading if this pressure gauge is measuring a pressure of 1 bar?
Static Characteristics of Instruments
Precision/ Repeatability/ Reproducibility
Degree of freedom from random
errors
Confused with accuracy
High precision – low accuracy; low
precision – high accuracy
Repeatability: closeness to output
when input is repeated (same
conditions: e.g. instrument,
observer, location)
Reproducibility: repeatability if
conditions vary
Static Characteristics of Instruments
Tolerance
Maximum error expected in some value
Maximum deviation of a manufactured component from specified value
Example:
A packet of resistors bought in an electronics component shop gives the
nominal resistance value as 1000 Ω and the manufacturing tolerance as
± 5%. 𝐼𝑓 𝑜𝑛𝑒 𝑟𝑒𝑠𝑖𝑠𝑡𝑜𝑟 𝑖𝑠 𝑐ℎ𝑜𝑠𝑒𝑛 𝑎𝑡 𝑟𝑎𝑛𝑑𝑜𝑚 𝑓𝑟𝑜𝑚 𝑡ℎ𝑒𝑝𝑎𝑐𝑘𝑒𝑡, 𝑤ℎ𝑎𝑡 𝑖𝑠 𝑡ℎ𝑒
𝑚𝑖𝑛𝑖𝑚𝑢𝑚 𝑎𝑛𝑑 𝑚𝑎𝑥𝑖𝑚𝑢𝑚 𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑣𝑎𝑙𝑢𝑒 𝑡ℎ𝑎𝑡 𝑡ℎ𝑖𝑠 𝑝𝑎𝑟𝑡𝑖𝑐𝑢𝑙𝑎𝑟 𝑟𝑒𝑠𝑖𝑠𝑡𝑜𝑟 𝑖𝑠
𝑙𝑖𝑘𝑒𝑙𝑦 𝑡𝑜 ℎ𝑎𝑣𝑒?
Static Characteristics of Instruments
Range; Span; Linearity
Range – the limits between which the input
can vary i.e. the values between the lower
limit and the upper one, which an instrument
can measure
Span – the algebraic difference between the
Min and Max range value
Linearity - (nonlinearity) – maximum
deviation of any output readings from the
straight (fitted) line
The voltmeter range A = 0 – 10. The span = 10
The voltmeter range B = 0 – 5. The span = 5
Static Characteristics of Instruments
Sensitivity of Measurement
A measure of the change in instrument output that occurs when quantity
being measured changes by given amount
The ratio of change in output to the corresponding change in the input
Example:
If given resistance values were easured at a range of
temperatures, determine the sensitivity of
instrument.
7/30=0.233 Ω/֯C
Static Characteristics of Instruments
Threshold; Resolution; Sensitivity to disturbance
Threshold – min. detectable input (at start). E.g. car speedometer
( 15km/hr)
No input is observed before threshold is reached
Resolution - Minimum input produces detectable change in
output. E.g. if car speedometer subdivision is 20 km/hr we can
estimate changes up to 5km.hr roughly ( 5km/hr is the resolution)
Standard ambient conditions are usually defined (e.g.
temperature)
Sensitivity to disturbance - the magnitude of change in
characteristics of instrument due to condition change
Zero drift (bias) and Sensitivity drift
Sensitivity to disturbance
Zero drift and Sensitvity drift
Zero drift describes the effect where zero reading of an
insturment is modified by a change in ambient
condition.
Constant error over full range of instrument
I.e.: bathromm scale
Sensitivity drift defines the amount by which an
insturment‘s sensitivity of measurement varies as
ambient condition changes
Sensitivity to disturbance
Zero drift and Sensitvity drift
Example:
Static Characteristics of Instruments
Hysteresis Effects
Time-based deendence of system‘s output
on the present and past inputs
If a given input alternately increases and
decreases, a typical mark of hysteresis is
that the output forms a loop as in the
figure
When two sets of readings are taken one
for increasing values of input and then for
decreasing values of the same input, the
output reading may differ
Non-coincidence between loading and
unloading curves
i.e.: Thermostat, Air condition
Dead space – range of different input
values over which there is no change in
output
Dynamic Characteristics of Instruments
Represents behaviour during the measured quantity changes until it reaches
steady response
The performance of the instrument when the input variable is changing
rapidly with time
Input/output relation in linear time-invariant systems (t>0) is as follows:
are constants (physical system parameters)
- For step change in the input, the equation reduces to:
Zero – Order Instrument
Output follows the state of input (without any lag or phase change)
The system ouput corresponds to the input signal instantly
all coefficients : a1 , a2,….etc, except a0 are zeros then:
e.g. Potentiometer
The output voltage
changes instantaneusly
as slider is displaced
First – Order Instrument
all coefficients except a0 , a1 are zeros then:
E.g. Thermocouple
Slowly indicating
measured quantity
Second – Order Instruments
all coefficients except a0 , a1, a2 are zeros then:
Static sensitivity
Undamped natural frequency
Damping ratio
i.e.: Mass – spring system
Calibration
Instruments gradually diverge ( wear, dirt, dust, chemicals,…etc)
Varies with type of instrument, frequency of use, severity of
conditions
Instrument characteristics drifted from standart specification
Recalibrate back to standard secifications
Calibrated against standard instrument (usually kept for this
purpose).. Over whole measuring range.
Null-type with high accuracy is commonly used (less operator
interaction)
