1. Instrumentation & Measurement (ME342)
Chapter 2: Instrument Types and Performance Characteristics
Dr. Hani Muhsen

2. 2.2 Review of Instrument Types
Instruments can be subdivided into separate classes according to several criteria:
Active and passive instruments
Null-type and deflection type instruments
Analogue and digital instruments
Indicating instruments and instruments with a signal output
Smart and non-smart instruments

3. 2.2.1 Active and passive instruments
Passive instrument: its output is introduced entirely by the quantity being measured.
Active instrument: the measured quantity modulates the magnitude of some external power source.
Instrument
Active
Passive
Output source
External power source
The quantity being measured
Resolution
Can be increased by adjusting the external power source.
Defined by the nature of the instrument.
Example
Float-type petrol tank level indicator
Pressure-measuring device
Source
Common: Electrical but may be pneumatic or hydraulic form.
Mechanical or simple other form.
Construction
More Expensive
Simple (i.e. less expensive)

4. Example of A Passive instrument
Passive pressure gauge.
Energy expended in moving the pointer is derived entirely from change in measured pressure
No other energy inputs to system

5. Example of an Active instrument
Petrol-tank level indicator.
The change in petrol level moves a potentiometer arm, and the output signal consists of a proportion of the external voltage source.
The energy in the output signal comes from the external power source

6. Active Vs. Passive instruments
Active instruments use external power source in the electrical form (most common), pneumatic form, and hydraulic form.
In active instruments, adjustment of the magnitude of the external energy input allows greater control over measurement resolution (finer readings).
Passive instruments are of simple construction and therefore less expensive to manufacture.
Measurement resolution Vs. Cost.

7. 2.2.2 Null-type and deflection type instruments
Deflection-type instrument: The value of quantity being measured is displayed in terms of the change of the measured variable.
Null-type instrument: Measurement is made by applying an effect which opposes the measured quantity until reaching a datum level (null point). More accurate and they are used for calibration purposes. ( Example: Dead-weight gauge)
The measured quantity is displayed in terms of the amount of movement of a pointer.
Weights are put on top of the piston until the downward force balances the fluid pressure.

8. 2.2.2 Null-type and deflection type instruments
A dead weight tester uses known traceable weights to apply pressure to a fluid for checking the accuracy of readings from a pressure gauge.

9. 2.2.3 Analogue and digital instruments
Analogue instrument: gives an output that varies continuously as the quantity being measured changes. (the output can have an infinite number of values)
Digital instruments: have an output that varies in discrete steps and so can only have a finite number of values. (the output can only have a finite number of values)
On each revolution the cam opens and closes a switch and then the switching operations are counted by an electronic counter. (Can only detect a complete revolution.)
As the input value changes, the pointer moves with a smooth continuous motion.

10. Analogue vs. Digital instruments
Digital instruments are directly interfaced to automatic control computers.
Analogue instruments must be interfaced using an analogue-to-digital converter.
A/D converter: converts analogue output signal from instrument to an equivalent digital quantity that can be read into a computer.
A/D converter disadvantages:
Adds significant cost to system
Finite time involved in conversion process
Degrades the speed of operation and the accuracy of control system

11. 2.2.4 Indicating instruments and instruments with a signal output
Indicating instruments: give an audio or visual indication of the magnitude of the physical quantity measured. ( Examples: liquid-in-glass thermometer (Analogue) and Bathroom scale (analogue and digital)).
human intervention is required to read and record a measurement.
this process is prone to error in the case of analogue output displays.
Signal-type instruments: give an output in the form of a measurement signal whose magnitude is proportional to the measured quantity.
used commonly as part of automatic control systems.
the measurement signal involved is an electrical voltage, but it can be an electrical current, an optical or a pneumatic signal.

12. 2.2.4 Smart and non-smart instruments
The presence of the microprocessor is the determinant factor in classifying systems into smart or non-smart.
Example
Classify the instrument shown!
Active Passive
Null type Deflection Type
Analogue Digital
Indicating With a signal output
Smart Non-smart
Pendulum-scale mass measuring device.

13. 2.3 Static Characteristics of instruments
They are applicable when the input is not changing with time.
They are concerned only with the steady-state reading that the instrument settles down to.
The main key in considering the choice of instrument for any particular application.
Usually given in the data sheet for a particular instrument.

14. 2.3 Static Characteristics of instruments Definitions of Static Terms
Repeatability/Precision. The closeness of a group of measurements taken under the same conditions.
Drift. The variation in o/p not caused by a change in i/p. Due to random sources
Error. The difference between the result of the measurement and the true value.
Uncertainty. The range of values within which the true value lies (+/-x).
Accuracy. The sum of Uncertainty, Non-linearity,etc.
Resolution. The smallest increment of the measurand which can be detected.
Sensitivity or Responsivity. The Gain or Scale factor of the Instrumentation System.
Linearity. The degree to which the sensitivity remains constant, for all values of the measurand.

15. 2.3 Static Characteristics of instruments
Accuracy: a measure of how close the output reading of the instrument to the correct value.
In practice, we quote
inaccuracy: the extent to which a reading might be wrong.
given as a percentage of the full scale (f.s.) reading of an instrument.

16. Comments on the example
The max. measurement error is a constant value independent of the measured value.
If measured quantity is significantly less than ( f.s.) reading possible measurement error is amplified.
Using an instrument of a wide range (i.e ) bar to measure variables with expected low values (i.e. 0-1 bar) is a design flaw.
1.0% x 10 bar = 0.1 bar
The max. measurement error is a constant value independent of the measured value
 max possible error = 0.1 bar
 10% of 1 bar

17. 2.3 Static Characteristics of instruments
Precision. describes an instrument’s degree of freedom from random errors. If an instrument is said to be a high-precision instrument, then the spread of readings will be very small. (It is a measure of how close the reading are to each other!)
Repeatability. describes the closeness of output readings when the same input is applied repetitively over a short period of time under the same conditions. (Reproducibility!)

18. 2.3 Static Characteristics of instruments
Tolerance. the maximum error that is to be expected in some value. Its not strictly static characteristic of an instrument, but some instruments are sometimes quoted as a tolerance value to describe the maximum deviation of a manufactured component from some specified value

19. 2.3 Static Characteristics of instruments
Range or Span. the minimum and maximum values of a quantity that the instrument is designed to measure.
Linearity. the system is linear if its output reading is linearly proportional to the quantity being measured.
Nonlinearity. max. deviation of any of the output readings from straight line (as a percentage of f.s. value).
(Ans.) Max. and Min. lengths are 5.1 and 4.9 mm, respectively.
(Ans.) Range = 2.5 cm

20. 2.3 Static Characteristics of instruments
The normal procedure is to find a good fit straight line passing through the Xs. (best fit line)
This can be done by applying a mathematical least-squares line-fitting technique.
The Slope of this line represents the sensitivity!

21. 2.3 Static Characteristics of instruments
Sensitivity: the ratio of the change in the instrument output to the change in the measured value.
Sensitivity is the slope of the straight line in figure 2.6 (best fit line)
S= Output change/ Input change
For example if a pressure of 2 bar produces a deflection of 10 degrees S = 5 degrees/bar

22. 2.3 Static Characteristics of instruments

23. 2.3 Static Characteristics of instruments
Threshold. minimum level before the change in the instrument output reading is of a large enough magnitude to be detectable. ( Given either as absolute values or % of f.s. value)
Example. If a car starts from rest and accelerates, no output reading is
observed on the speedometer until the speed reaches 15 km/h.

24. 2.3 Static Characteristics of instruments
Resolution: The smallest increment of the measurand that can be detected. (observable change in the output).
In indicating instruments it depends on how finely the output scale is divided into subdivisions.
Example. A car speedometer typically has subdivisions of 20km/hr (we can’t estimate speed more accurately than nearest 5km/hr).
 resolution is 5km/hr

25. 2.3 Static Characteristics of instruments Sensitivity to Disturbance
All calibrations and specifications of an instrument are only valid under controlled conditions of temperature, pressure, and so on. These standard ambient conditions are usually defined in the instrument specification.
Environmental changes affect instruments with:
Zero drift (bias) : the zero reading is modified by change in ambient conditions. This causes a constant error that exists over the full range of measurement of the instrument.
- e.g. bathroom scale
- The typical unit by which such zero drift is measured is volts/°C (zero drift coefficient). The Change of the zero measurement due to the ambient changes not due to the change of the Measurand
- There can be several zero drift coefficients (temp, pressure, etc.)

26. 2.3 Static Characteristics of instruments Sensitivity to Disturbance
Sensitivity drift (scale factor drift). Drift for a unit change as ambient conditions change.
Example. temperature changes: for instance, the modulus of elasticity of a spring is temperature dependent.

28. 2.3 Static Characteristics of instruments

29. 2.3 Static Characteristics of instruments

30. 2.3 Static Characteristics of instruments
Hysteresis Effects. The relationship between the input and the output depends on whether the input is increasing or decreasing.

31. 2.3 Static Characteristics of instruments
Commonly found in:
Instruments that contain springs (such as passive pressure gauge).
Systems with friction forces depending on direction of movement.
Instruments that contain electrical windings around an iron core (magnetic hysteresis, LVDT).

32. Hysteresis https://www.youtube.com/watch?v=rtVofRyo1fA

33. 2.3 Static Characteristics of instruments
Dead space. The range of input values over which there is no change in output value. Any instrument that exhibits hysteresis also displays dead space.

34. 2.3 Static Characteristics of instruments
backlash, is clearance or lost motion in a mechanism caused by gaps between the parts. It can be defined as "the maximum distance or angle through which any part of a mechanical system may be moved in one direction without applying appreciable force or motion to the next part in mechanical sequence

35. 2.4 Dynamic Characteristics of Instruments
Static characteristics of instruments are concerned only with the steady-state reading that the instrument settles down to.
Dynamic characteristics of instruments describe its behavior between the time a measured quantity changes value and the time when the instrument output reaches the steady state value.
In any linear, time-invarient measuring system, the following relation can be written between input and output for time (t)> 0:
where qi is the measured quantity, q0 is the output reading, and a0… an , b0… bn are constants.

36. 2.4 Dynamic Characteristics of Instruments
If we limit consideration to that of step changes in the measured quantity only then equation (2.1) reduces to: