EP 320Process Instrumentation and Instrumental Analysis

January April, 2015

1a. Introduction - Basic concepts in instrumentation.

Teaching Plan: EP320(W1 to W4)

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Typical application

Measurement of system parameters

Control of a certain operation/process

Simulation of system condition

Experimental design studies

To perform various manipulation

Testing materials, maintenance of standard and specification

Verification of physical phenomenal scientific theories

Quality control in industry

Functional Elements

In general:

1. Basic functional Elements Transducer element

Signal conditioning or intermediate modifying element

Data presentation element

2. Auxiliary Functional Elements Calibration element

External Power element

Feedback Element

Microprocessor element

In general:

1. Basic functional Elements Transducer element

Signal conditioning or intermediate modifying element

Data presentation element

Basic functional Elements

Transducer Element

It sense the desired input in one physical form and convert it to an output in other physical form.

Example:

Output

Displacement

Voltage

Current

Resistance

Pressure

Liquid movement

Inductance change

Input

Pressure

Acceleration

Temperature

Flowrate

Force

Humidity

Blood flow

Example 1 Manometer

Input: Pressure

Output: Movement of a liquid column

Operation The impressed pressure is balanced by the pressure generated by a column of liquid.

Example 2: Thermocouple

Input: Temperature

Output: Voltage

Operation An emf is generated across the junctions of two dissimilar metals when that junction is heated.

Junction

Small voltage

Example 3 Electrodynamic generatorInput: Motion

Output: Voltage

Operation Relative motion of a coil with respect to a magnetic field generates a voltage

Signal Conditioning element

The output from transducer is TOO SMALL to acquire and tend to be influenced by unwanted noise.

Type of Signal conditioning

1. Amplification

2. Signal filtration

Note: Other signal conditioning operators are such as:

Signal compensation/ linearization, differentiation/integration, Analog-to-Digital conversion, averaging/sampling, and etc.

Type of Signal conditioning

1. Amplification Mechanical such as levers, gear or combination of both.

Hydraulic/Pneumatic valve or orifice (constrictions items)etc .

Optical lenses, mirror or combination of both

Electrical transistor circuits or integrated circuits.

2. Filtration Mechanical mechanical element to protect from external

factor. i.e. Water bath

Pneumatic to avoid fluctuation during measurement. i.esmall orifice

Electrical remove stray signal due to magnetic and electrical fields. i.e. R-C curcuits

Data Presentation Element

This element gather the output of the signal conditioning element and PRESENT to be read or seen by operator.

Criteria of the Data Presentation Element:

Fast response

Impose little drag on the system

Small inertia

Analogue Digital

Displays

Pointer- scale indicator

LED CRT LCD EL

Recorder/ Printer

Chart

recorder

Paperless

recorder

Laser

printer

Data Presentation Elements

Displays

Recorder/ Printer

Measuring Device: Analog Multi-meter

Permanent magnet moving coilgalvanometer

Permanent magnet

Lower control spring

Upper control spring

Permanent magnet

Pointer

Scale

Voltage, Vor

Current, I

PMMC galvanometer

Input variable

Transducer element Signal conditioning element

Data presentationelement

Current carrying

coil

Control spring

Pointer and scaleT q0

DC source: Voltage or

Current

Transduced Torque

Output:angle

Scale deflection

Measurement Process flow:1. The transducer element converts the

current I in amperes (A) into a torque T(N.m) with a transfer function gain KT(N.m/A)

2. The signal conditioning elementconverts the torque T (N.m) intoangular displacement ) with transferfunction gain Ks /(N.m).

3. The data-presentation elementconverts the angular displacement

4. into scale deflection q0 with transferfunction gain KD (/mm)

Block diagramPMMC galvanometer

Input variable

Transducer element Signal conditioning element

Data presentationelement

Current carrying

coil

Control spring

Pointer and scaleT q0

DC source: Voltage or

Current

Transduced Torque

Output:angle

Scale deflection

Block diagramPMMC galvanometer

The transfer function gain can be express as:

Note: The measurement is assumed operate for a steady-state system. Transfer function gain also known as sensitivity or amplification of respective functional element.

0 =

Overall sensitivity function

= OR0/ =

Measurement element calculation:

Exercise 1

An elastic type of pressure-measuring instrument is ofdiaphragm type. The central deflection of the diaphragmwas found to be 0.25 mm of an applied pressure of 106

Pa. The output displacement of diaphragm has been fedto an LVDT (linear variable transducer) with a built-inamplifier having a sensitivity of 40 V/mm. Finally, theoutput is displayed on an analog voltmeter which has aradius of scale line as 60 mm and has a voltage rangefrom zero to 10 volts in an arc of 150o.

Determine the sensitivity of the given diaphragm gaugein terms of mm/bar.

Given: 1 bar = 105 Pa

[10 Marks]

Exercise 1 - Solution

P

Transducer element Signal conditioning element

Data presentationelement

Diaphragm type of pressure

transducer

LVDT with built-in

amplifier

Analogvoltmeterdx V q0

Pressure, Pa Displacementmm

Voltage, V Pointer & scale

Determine the sensitivity of the given diaphragm gauge interms of mm/bar.

The central deflection of the diaphragm was found to be0.25 mm of an applied pressure of 106 Pa.

The output displacement of diaphragm has been fed to anLVDT (linear variable transducer) with a built-in amplifierhaving a sensitivity of 40 V/mm.

Finally, the output is displayed on an analog voltmeterwhich has a radius of scale line as 60 mm and has a voltagerange from zero to 10 volts in an arc of 150o.

Exercise 1 - Solution

P = 106 Pa

Transducer element Signal conditioning element

Data presentationelement

Diaphragm type of pressure

transducer

LVDT with built-in

amplifier

Analogvoltmeterdx = 0.25 mm V = ? volt

q0 = ?scale line as 60 mm

Pressure, Pa Displacementmm

Voltage, V Pointer & scale

KT = ? mm/Pa Ks = 40 Volt/mm KD = ? mm/Volt

Since the deflection of diaphragm gauge is 0.25 mm for an applied pressure of 106 Pa, thus the gain, KT is

=

=0.25

106= 2.5 107 /

The output for signal conditioning element can be calculated as:

= = 40 0.25 = 10

Therefore, Gain KD for data presentation element is computed by:

=0=60 150

2 360

10= 15.7 /

Finally, the sensitivity (Overall gain) Koverall of the measurement element is determined as:

= = 15.7 /

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: 2.5 105 /2

To be continuedClassification of instruments