Instrumentation and measurement

1. (i) LINEAR RELATIONSHIP: The relationship b/w a physical parameter and the resulting
electrical signal must be linear.
(ii) SENSITIVITY: This is defined as the electrical O/P per unit change in the physical
parameter(For example V/ ⁰C for a temperature sensor). High sensitivity is generally
desirable for a transducer.
(iii) DYNAMIC RANGER: The operating range of the transducer should be wide, to permit its
use under a wide-range of measurement conditions.
(iv) REPEATABILITY: The input/output relationship for a transducer should be printable over
a long period of time. This ensures reliability of operation.
(v) PHYSICAL SIZE: The transducers must have minimal weight and volume, so that its
presence in the measurement system does not disturb the existing conditions.
PARAMETERS

2. TRANSDUCERS
• ACTIVE TRANSDUCERS
These are self generating
transducers. They does not need
external source of electricity.
Example:- Thermocouple
piezoelectric transducers
• PASSIVE TRANSDUCERS
These need an external source of
energy.
Example:-RTD, Thermistors etc.

3. TEMPERATURE TRANSDUCERS
Resistance Temperature Detector (RTD) or Resistance Thermometer
• Consists of metal conductor
• Works on the principle that: The resistance of metal changes with the change in
temperature.
• RT=Ro (1+αΔT)
• α has a positive value which means that there is an increase in resistance with the
increase in temperature.
• Positive α usually has low value which means low sensitivity i.e. a small change in
resistance corresponding to large change in temperature.
• Change in resistance is recorded with the help of Wheatstone Bridge.

4. TEMPERATURE TRANSDUCERS
Resistance Temperature Detector (RTD) or Resistance Thermometer
Applications of RTD's include
• Stoves and grills
• Textile production
• Plastics processing
• Petrochemical processing
• Micro electronics
• Air, gas and liquid temperature measurement
• Temperature control relays directly sense a motor's temperature using a resistance
thermal device (RTD) probe which is embedded in the motor winding. When
the nominal temperature of the probe is reached, its resistance increases rapidly. This
increase is then detected by a threshold circuit, which opens the relay contacts.

8. TEMPERATURE TRANSDUCERS
Thermistors
• Semiconductor temperature transducer (STT).
• Also known as thermal resistors.
• Works on the principle that when temperature rises, there is a decrease in
resistance in case of semiconductors.
• Modern thermistors are manufactured from oxides of metals like manganese,
nickel, cobalt etc.
• Temperature range is -65°C to 200°C.
• Used for lower ranges as compared to RTD.
• More sensitive as compared to RTD.

9. TEMPERATURE TRANSDUCERS
Thermistors

10. TEMPERATURE TRANSDUCERS
Applications of Thermistors
Microwave
Automotive
Cars, trucks, and buses all use thermistors. They are used to determine the
temperature of oil and coolants. This is how you are able to know if your car is
overheating or not. Thermistors in cars do not prevent or regulate. Instead, they
are used to gather information. This allows a driver to fix their car or truck before
something serious happens
Digital Thermometers
monitor the temperature of an incubator.
Food Handling and Processing industry,

11. TEMPERATURE TRANSDUCERS
Thermocouple
SEEBECK EFFECT: When two different metals are joined together forming two
junctions and the temperature is kept different at both the junctions, a current
flows through the loop.

12. TEMPERATURE TRANSDUCERS
Applications of Thermocouple
Temperature measurement of multimeter
Oven
Furnaces
Gas turbine temperature measurement

13. TEMPERATURE TRANSDUCERS
Thermocouple
The valence electrons in the warmer part of metal are solely responsible for that
and the reason behind this is thermal energy. Also because of the kinetic energy of
these electrons, these valence electrons migrate more rapidly towards the other
(colder) end as compare to the colder part electrons migrate towards warmer part.

14. PIEZOELECTRIC TRANSDUCER
Piezoelectric Effect
• When pressure is applied against the opposite planes of a slice of certain crystals,
charges are accumulated on the opposite planes which can be measured using a
voltmeter.
• Inverse is also true.
• Rochelle salt: good piezoelectric activity but mechanically weak.
Tourmaline: good mechanical strength but poor piezoelectric activity.
Quartz crystal: a compromise between the other two.

15. PIEZOELECTRIC TRANSDUCER
Applications:
• Buzzers
• Kitchen lighter
• Automatic doors
• Speakers
• Microphones
• Ultrasonic sensor
• Sound producing toys

16. MAGNETIC FLOW METER
Magnets
Electrodes
V