Instrumentation Report

8/10/2019 Instrumentation Report

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BENG (HONS) MECHANICAL/ELECTRONICS &

ELECTRICAL ENGINEERING (3+0)

Department of Mechanical Engineering

MEASUREMENT AND INSTRUMENTATION


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Question 1

Voltmeter reading: 70V, Range: 100V

Ammeter reading: 80mA, Range: 150mA

P = VI

=

= 5.6 watts

The magnitude for limiting error,

Therefore, the limiting error for ( )

The magnitude of limiting error,

Therefore, the limiting error for ( )

The limiting error for the power =

Question 2

(a). Rotary variable differential transformer (RVDT).

Definition:

A Rotary Variable Differential Transformer (RVDT) is an electromechanical transducer that

provides a variable alternating current (AC) output voltage that is linearly proportional to the

angular displacement of its input shaft.


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Construction:

Figure 2.1

RVDT is almost identical in the construction with LVDT. The ferrite core in LVDT is replaced

by heart or car diode shaped core as shown in the figure 2.1 above.

Operation:

This core is mounted on the shaft or plunger which is capable of rotating. This rotationalmovement of the shaft creates the imbalance in the external circuit and this imbalance signal is

used to measure the angular displacement of the shaft. RVDT give linear output up to

displacement of +-40 degree. Then output starts becoming nonlinear.

Advantages:

*Relative low cost due to its popularity.

*Solid and robust.

* Capable of working in a wide variety of environments

* No permanent damage to the RVDT if measurements exceed the designed range.


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Disadvantages:

* The core must be in contact (directly or indirectly) with the measured surface which is not

always possible or desirable.

* RVDT provides linear output for about +-40 degree which limits its usability.

(b). Hall Effect transducer.

Definition: A Hall Effect transducer varies its output voltage in response to a magnetic field. Hall

Effect sensors are used for proximity switching, positioning, speed detection, and current sensing

applications.

Construction:

Figure 2.2

Hall Effect Sensors consist basically of a thin piece of rectangular p-type semiconductor

material such as gallium arsenide (GaAs), indium antimonide (InSb) or indium arsenide (InAs)

passing a continuous current through itself.


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Disadvantages:

* May be affected by external interfering magnetic field

* Large temperature drift

* Large offset voltage

(c). Magnetostrictive Transducers.

Definition:

Magnetostrictive transducers are used with sonar equipment to change an alternating current to

sound energy at the same frequency and to form the sound energy into a beam; its operation

depends on the interaction between the magnetization and the deformation of a material having

magnetostrictive properties.

Construction:

Figure 2.4


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Construction (continue)

Based on the Figure 2.4 the main components of the magnetostrictive transducer can be

categorized as follows:

* Waveguide

* Position magnet

* Electronics

* Strain pulse detection system

* Damping module

A magnetostrictive transducer consists of three main components, a ferromagnetic material thathas magnetostrictive properties, a static bias magnetic field needed for transducer operation, and

a coil that applies time varying magnetic fields to the ferromagnetic material in order to produce

guided waves in the material. The coil also plays a role of inductively detecting the magnetic

flux change caused in the material by the incoming guided waves. Ferrous steel is

magnetostrictive.

Operation:

Figure 2.5

The operation flow of a magnetostrictive transducer shown in the flow diagram in Figure 2.5:

a) Signal generator (SigGen): it represents the electronics responsible with the electrical signal

that is generated and transmitted to the electrical coil.


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b) The coil has two tasks:

*To transform the electrical signal and current into a suitable shaped magnetic field (the applied

field H) in transmission mode.

* In acquisition mode, it transforms the acquired variable magnetic flux into alternate voltage.

c) The ferromagnetic strip has two tasks:

*In transmission mode to transform the helical resulting field (H+H0) generated by the coil and

the permanent magnet into torsional vibration through Widemann effect and transmit it to the

pipe wall.

*In acquisition mode, to transform torsional vibrations coming from the pipe wall into variable

magnetic flux through the Mateucci Effect;

d) Digital Acquisition Card (DAC) has the task of acquiring the variable voltage from the coil in

acquisition mode and change it into digital signal and memorize it in the computer memory for

further signal processing.

Advantages:

*Since it is non contact (the position magnet does not touch waveguide) there is no wear and

friction. So there is no limitation on the number of operating cycles and is not affected by

vibrations.

*Linear measurement.

* High Reliability

Disadvantages:

*Dead band on both side of the sensor. (Some manufacturers can reduce the dead bands based on

your requirement but cannot make it to zero).


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Question 2 (continuation)

(d). Orifice plate and Venturi tube as used in fluid flow measurement.

Definition:

* An orifice plate is a device used for measuring flow rate. Either a volumetric or mass flow rate

may be determined, depending on the calculation associated with the orifice plate.

* Venturi tube is a device for measuring fluid flow, consisting of a tube so constricted that the

pressure differential produced by fluid flowing through the constriction gives a measure of the

rate of flow.

Construction:

Orifice plate:-

Figure 2.6

An orifice plate is a restriction with an opening smaller than the pipe diameter which is

inserted in the pipe; the typical orifice plate has a concentric, sharp edged opening, as

shown in Figure 2.6. Because of the smaller area the fluid velocity increases, causing a

corresponding decrease in pressure.


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Construction (continue)

Venturi Tube:-

Figure 2.7

* The entry of the venture is cylindrical in shape to match the size of the pipe through which

fluid flows. This enables the venture to be fitted to the pipe.

*After the entry, there is a converging conical section with an included angle of 19 to 23.

*Following the converging section, there is a cylindrical section with minimum area called as the

throat.

*After the throat, there is a diverging conical section with an included angle of 5 to 15.

*Openings are provided at the entry and throat (at sections 1 and 2 in the diagram) of the venturemeter for attaching a differential pressure sensor (u-tube manometer, differential pressure gauge,

etc) as shown in Figure 2.7.


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Operation:

Orifice Plate:-

Figure 2.8

As shown in the Figure above, when the liquid reaches the orifice, it is forced through a narrow

hole in the center of the plate. By reducing the flow path like this, we can see changes in the

speed and pressure of the liquid - on one side of the plate the pressure is high, while on the other

side the pressure is comparatively low. The faster the liquid moves, the greater the difference in

pressure. This differential pressure is measured via impulse lines by a differential pressure

transmitter which converts it into an analogue or digital signal which can be processed to provide

a display of the instantaneous rate of flow.


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Operation (continue):

Venturi Tube:-

*The fluid whose flow rate is to be measured enters the entry section of the venturi meter with a

pressure P1.

*As the fluid from the entry section of venturi meter flows into the converging section, its

pressure keeps on reducing and attains a minimum value P2 when it enters the throat. That is, in

the throat, the fluid pressure P2 will be minimum.

*The differential pressure sensor attached between the entry and throat section of the venturi

meter records the pressure difference(P1-P2) which becomes an indication of the flow rate of the

fluid through the pipe when calibrated.

*The diverging section has been provided to enable the fluid to regain its pressure and hence its

kinetic energy. Lesser the angle of the diverging section, greater is the recovery.

Advantages:

Orifice Plate:-

*Suitable for both liquids and gasses

*Extremely Accurate

*No Moving Parts

* Low cost

Venturi Tube:-

*Less changes of getting clogged with sediments

*Coefficient of discharge is high.

*Its behaviour can be predicted perfectly.

*Can be installed vertically, horizontally or inclined.


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Disadvantages:

Orifice Plate:-

* Not suitable for liquids with suspended solids or gasses.

* Requires a constant pressure and temperature in gas applications in order to be accurate- unless

you use a multi- variable transmitter.

Venturi Tube:-

*They are large in size and hence where space is limited, they cannot be used.

*Expensive initial cost, installation and maintenance.

*Cannot be used in pipes below 7.5cm diameter.

Question 3

Resistance k, between k and k

Tolerance kk 0.06, k k

Tolerance k

Percentage of tolerance = ( k)

Coefficient of temperature on resistance =

= (()

At


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Question 4

The circuit in FIGURE Q4 is a differential amplifier driven by a bridge. Find Vo.

Figure Q4

From the figure,

k (k k)

k k

So the current that flows in this part of the circuit is,

I =

= 6.452 A

The voltage flowing across the k and k is,

V= I =

With this we can calculate the voltage flow across the k resistor,

V80k= 0.8 m

This voltage is also same as the voltage that goes into the input of the op amp. Let V1 be the

voltage to the left of the k resistor on the upper part of the circuit shown in Figure Q

and we can write a node equation at that node.

[(V1 ) ()+ [V1 ] +[(V1)/ ] = 0

3V1 +V1+1.5V1


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Q4(Continuation)

V1= 3.255mV

So the current that goes through the k resistor is ,

I20k= (3.255m-1.9352)/ 20k = 6.599 A

So the output voltage Vo is,

Vo = 1.9352m(

Vo= 1.9252m

Question

Resolution of the conversion =

=

= 3.91mV

LSB:

Vin(min)=

()

3.91mV

MSB:

Vin(max) =()

= 496.1mV

V(fs) = 1111111 = Vmax S= 500 = 496.1mV


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Question 6

Figure Q6(a) Ammeter directly in series with the load

(a). RA , I A,

R

R

Figure Q6(b) Voltmeter connected across the load

(b) Er = IR

500V = I ()


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Q6 (Continuation)

I = 0.505A

In 1000V range voltmeter has 10k/V sensitivity, so

In range voltmeter has k sensitivity

Rv= sensitivity

Rv M

E= Iv v

Iv=

Iv= 0.2mA

Ammeter would indicate I+ Iv= 0.505A + 0.2mA= 0.5052A

Voltmeter would indicate Ep= 0.505A 499.95V

(c) Referring to Figure Q6(a) and Q6(b), the circuit in Figure Q6(a)is the best

circuit when measuring high resistance value. The circuit in Figure Q6(b) is

the best when measuring low resistance values.


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