Instrumentation Assignment: Sensors, Signal Conditioning, Control Systems

INSTRUMENTATION ASSIGNMENT
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Question 1
Accuracy refers to the closeness with which a
measured value approaches the true value of the measured
quantity.
Sensitivity is the ability of an instrument to respond to small
changes in the quantity that it measures.
An error is a difference between the measured value and the
true value of a quantity being measured.
Linearity is an expression of the extent to which the actual
measured curve of a sensor departs from the ideal curve.
Span refers to the arithmetic difference between the highest
and lowest values of the input being sensed.
Hysteresis is the measure of the ability of a sensor to follow
the changes of the input parameter regardless of the direction
of change.
Question 2a
Pressure sensor
A pressure transducer is a device that is used to
convert measured pressure into an electrical or mechanical
output. An example is a piston type pressure sensor whose
construction is depicted in figure 1 below.
Figure 1: Piston pressure transducer
In its construction, the sensor has a piston, spring,
and pressure input. When pressure is exerted as shown, the
piston moves as to compress the spring. As a result, the piston
displacement will be directly proportional to the applied
pressure. Then, outside of the spring piston arrangement is a
meter that measures the corresponding amount of pressure.
Notably, the device is made to withstand shock, vibrations and
impulsive pressure changes making it an exceptional sensor in
hydraulic applications. For instance, measuring pressure drops
across separators, strainers, and filters.
Temperature
A temperature sensor is used for measuring and
monitoring hotness and coldness of a particular locality or spot
in industries. An example is a bimetallic thermometer that
follows two principles of operation. First, a metal tends to
contract or expand when exposed to temperature changes.
Then, the dimensional change observed depends on metal
types since each type possesses a different temperature
coefficient.
The bimetallic strip has two dissimilar metals that are
riveted or welded together to form a spiral-shaped or a twisted
helix as shown in figure 2.
Figure 2: A bimetallic strip adapted from [6]
Temperature change makes the free end to expand or
contract linearly. For this reason, the deflection experienced at
the end can be read by fixing a pointer. It is common to get
different shapes of these devices like cantilever type, spiral or
flat type. When working in an oven, the top blue metal tends
to expand faster than the red one but contract slower when
used in a refrigerator. The strip will then bend upwards
completing an electrical circuit to initiate current flow. In fact,
one can control the temperature by adjusting the gap between

the wire and the strip. The working bimetallic thermometer is
shown in figure 3 below.
Figure 3: Working bimetallic thermometer [6].
Flow rate
Liquid flow rate can be measured by an ultrasonic
flowmeter. It is evident that fluid velocity and amplitude are
affected by pressure waves. As such, electronic flowmeters
use specialized techniques to measure these wave changes
especially those with frequencies larger than 20kHz. Figure 4
below shows the implementation of the sensor. In the device,
piezo-crystals P1 and P2 are mounted conveniently so that
they work as a transmitter-receiver pair. Then, the ultrasonic
signals are transmitted through the piezo-crystals and the
liquid. Switch “sw” is used to supply P1 and P2 from the
oscillator thereby connecting the detector to the piezo-crystals.
Also, the detector uses phase shift to measure the upstream
and downstream transit time. If the ultrasonic waves travel at a
velocity C while the liquid flows at velocity v for a separation
distance of b between P1 and P2, the phase-shifts are
Pd 1= wb
C+ v and Pd 2= wb
C−v
The difference between the two phase shifts represents the
flow rate of the fluid.
Figure 4: Ultrasonic Flowmeter [6]
Fluid Level
A differential pressure (DP) level sensor measures
the fluid level by computing the difference between bottom
tank pressure and head pressure as shown in figure 5 below.
The sensor is used in industries to measure liquid levels when
desired.
Figure 5: Level sensor
Question 2b
Table 1: Piston Type sensor specifications
Parameter Specification
Accuracy ±2.5 % of F.S. (Ascending )
Range 0 to 0.5 Kg/cm2 up to 10 Kg/cm2
Static Pressure 200 / 400 Kg/cm2
Dial Size 63 / 100 & 150mm
Connection ¼” BSP / NPT (F)
Table 2: Bimetallic thermometer specifications
Parameter Specification
Accuracy ±1% Full-Scale Accuracy
Range 50° to 1,000°F
Stem Lengths 24"
Dial Size 3" and 5"