Slide-2

Question 1

What constitutes an instrument?

Lengthy answer. try to recall max. possible.
See the slide again.

Answer 1

Basic and Auxiliary functional elements:

Physical world: quantities that are to be measured.

1. Transducer element - senses and converts the input into a more practicable form
2. Signal conditioning element- This element manipulates the transducer to output a suitable form
3. Data representation element - This element records, displays in graphical format and prints in a suitable format.

Feedback element: provides feedback to the controller based on measurement.

Microprocessor element: Works with signal conditioning element to process information

External Power : provides external power to energize the instrument

Calibration instrument : Used to calibrate the instrument before it is used.

Question 2

Transformer?

Answer 2

Transducer that converts an input of one physical form into another input of same form .

Question 3

Sensitivity or Gain

Answer 3

Ratio of change in output value to corresponding change in input values, in their respective units.

Question 4

Example of sensitivity?

Answer 4

1. Coil and magnet combination: converts current ( I - in amperes) to torque ( T - in Nm ) will have a senstivity Kt = T/I (Nm/A)
2. Control springs : Control torque T into angular deflection theta. K(theta) = theta/T
3. Pointer and scale: convert theta into deflection q in mm , Kd = q/theta (mm/deg)

Overall sensitivity = 123 = K = Kt * K(theta) * Kd = mm/A

Question 5

Signal conditioning element

Answer 5

1. Amplifier: Amplify the magnitude of the input signal without affecting the waveform
   eg. i) Mechanical amplifying elements: levers, gears, combinations
   ii) Hydraulic / Pneumatic elements: valves, Venturimeter, Orificemeter.
   Electrical: electrical amplifiers, ICs
2. Filters: These elements remove noise from the input signal . Mechanical - insulation, sound barrier
   Pneumatic: Small orifice or Venturi
   Electrical : RC circuits and other electrical circuits.
3. Analog to Digital / Dig to Analog converters
4. Differentiators/Integrators
5. Signal Compensation/ Linearization/ Zero settling/ Scale Adjustment
6. Signal averaging / Sampling
7. Signal modulations and demodulations.

Question 6

Classification of instruments:

Answer 6

Deflection type VS Null type
Analog vs Digital
Contacting vs non-contacting type
Dumb vs Intelligent

Question 7

Deflection type instruments:

Answer 7

the physical effect caused
by the measuring quantity produces an equivalent opposing effect in some part
of the instrument which in turn is closely
related to some variable like mechanical
deflection or displacement in the
instrument.
ADV: Good dynamic response
DISADV: They interfere with the state of measured quantity

Question 8

Null type:

Answer 8

the device generates an
equivalent opposing effect to nullify the
physical effect caused by the quantity
measured. The equivalent null causing
effect provides the measure of quantity.

ADV : i) Do not interfere with the state of measured
quantity

ii) Extremely accurate

DISADV: Slow dynamic response (could be improved
with automation)

Question 9

Analog type:

Answer 9

Analog type: The measured quantity
shows continuous variation with time.
Advantages:
 Cheap, easy to use and maintain
 Widely available

Question 10

Digital type:

Answer 10

Digital Type: The measured quantity is
discrete and vary in steps with time.
Advantages
 Easy to use with digital computers
 Accurate transmission, immune to noise

Question 11

Contacting type:

Answer 11

The instrument is in
contact with the measured medium.
eg: thermocouple, pitot tube in an airplane

Question 12

Non Contacting type

Answer 12

The instrument is not
in physical contact with the measured
medium.

eg: IR thermometer, Tachometer

Question 13

Dumb type

Answer 13

The instrument simply
measures, but does not process the
measured values.
eg: Bourdan tube pressure guage

Question 14

Intelligent type

Answer 14

The instrument measures and
conditions the measurement for further processing.
Advantages:
 Output is electrical, is digital or converted to digital using Analog to
Digital Converter (ADC), Easy to interface with computer
 Noise reduction, error estimation, self-calibration, gain adjustment,
etc.