1b. Introduction – Classification of Instrument

EP 320Process Instrumentation and Instrumental Analysis

January April, 2015

1. Introduction Classification of instrument.

Teaching Plan: EP320(W1 to W4)

Homework exercise

Textbook Q1.6

A thermocouple having a sensitivity of 4.8 mV/oC hasbeen used for the measurement of temperature. Itsoutput is connected to a moving coil milivoltmeterwhich has sensitivity of 1o/mV.

If the length of the pointer of the instrument is 30mm, determine the overall sensitivity oftemperature-sensing system in mm/oC.

[10 Marks]

Answer: 2.51 /

Solution: Textbook Q1.6Determine the overall sensitivity of temperature-sensing system in mm/oC.

A thermocouple having a sensitivity of 4.8 mV/oC hasbeen used for the measurement of temperature.

Its output is connected to a moving coil milivoltmeterwhich has sensitivity of 1o/mV.

length of the pointer of the instrument is 30 mm,

T

Transducer element Signal conditioning element

Data presentationelement

thermocouple Transducer = ./

Moving coil = /

Mili-volt meterV L max = 30 mm

Temperature, oC Voltage angle Pointer & scale, mm

Determine the overall sensitivity of temperature-sensing system in mm/oC.

T

Transducer element Signal conditioning element

Data presentationelement

thermocouple Transducer = ./

Moving coil = /

Mili-volt meterV L max = 30 mm

Temperature, oC Voltage Angle, o Pointer & scale, mm

Thus, max = 180o

the transfer function gain for milivoltmeter is calculated as;

=

=30

180= 0.5236 /

Finally, the overall sensitivity mm/oC is;

= = 4.8 1 0.5236 = 2.51 /

Assume semi-sphere shape meter has been used:

Solution: Textbook Q1.6

Extended exercise: Textbook Q1.6

Repeat Q1.6

Determine the unknown temperature when the output of millivolt meter is 15.482 mm

T = ? oC

Temperature measurement device

sensitivity = . / L = 15.482 mm

Temperature, oC Pointer & scale, mm

=

=

=15.482

2.51= 6.168

Instrument Parameters

1. Accuracy

2. Range

3. Span

4. Precision

5. Reproducibility

6. Sensitivity

7. Offset

8. Drift

9. Hysteresis

10. Resolution

11. Repeatability

12. Linearity

Classification of instrument

Instrument can be classified according to themode of operation, energy conversion, outputsignal, and etc.

In general is it classified as:1. Deflection and Null Types

2. Manually Operated and Automatic Types

3. Analog and Digital Types

4. Self-generating and Power-Operated Types

5. Contacting and Non-Contacting Types

6. Dumb and Intelligent Types

Operational mode of instrument

Deflection and Null Types

An iterative balancing operation using some type ofcomparator to achieve balance, and a null deflectionat parity.

Deflection Type

Null Type

Balance Input(known)

Balance Input(unknown)

Deflection and Null Types

Deflection instrument:

A measuring device whose output

deflects proportional to the

magnitude of the measurand.

Manually Operated and Automatic Types

Manual Operated Type Any instrument thatrequires the service of human.

Automatic Operated Type Any instrument thatincorporated the auxiliary device to remove manualhuman service.

Automatic type is preferred because :1. Fast dynamic response

2. Lower operational cost

Manually Operated Type example

Manually Operated Type example

Manually Operated Type example

Manually Operated Type example

Manually Operated Type example

=

Manually Operated Type example

Automatic Operated Type example

auxiliary device 0+

-

Desire Input(set-point)

Manually Operated Type: Null-bridge resistance thermometer

Manually Operated Type: Null-bridge resistance thermometer

T

Transducer element

Signal conditioning element

thermocouple Transducer

Signal ConditioningV

Temperature, oC Voltage

Data presentationelement

Scale

Actual Value

Bridge Circuit

Slider

Manual Null-detection

Automatic Operated Type: Null-bridge resistance thermometer

TTransducer element

Signal conditioning element

thermocouple Transducer

Signal ConditioningV

Temperature, oC Voltage

Data presentationelement

Scale

Actual Value

Bridge Circuit

SliderAmplifierRev. Motor

Relay

+-

controller

Desire value

Automatic Operated Type: Null-bridge resistance thermometer

Analog and Digital Types

Analog instrument present the physical variable in continuous/stepless variations with respect to time.

Digital instrument provide a signal that has been sampled from continuous data.

Analog Signal

Analog and Digital Types

Analog Signal

Digital Signal

Analog and Digital Types

Self-generating and Power-Operated Types

Photovoltaic cell (self-generating type)

Energy require by self-generating instrument are met entirely from the input signal.

Contacting and Non-Contacting Types

Variable reluctant tachometer (Proximity type)

Non-contacting instrument desire input even though they are not in-close contact with the measuring medium.

Dumb and Intelligent Types

Conventional Types - input variable is measured and display, but the data is processed by the observer.

Intelligent Type incorporated AI to process data by using microprocessor. The purpose are as follows:

1. Noise reduction

2. Auto calibration

3. Drift correction

4. Gain adjustment, etc.

Computer Based Measurement

To be continuedInstrument characteristic (Static & Dynamic)