Pressure, Displacement, Flow and Temperature Measurement

Question 1

What are potentiometers

Answer 1

They can produce a linear potential output as a function of the displacement

Question 2

3 types of potentiometers

Answer 2

• Translational
• Single turn
• Multi turn

Question 3

How does strain gages work

Answer 3

When a fine wire is stained within its elastic limit, its resistance changes due to the changes in diameter, length and resistivity. The resulting resistance change -> measure extremely small displacements

Question 4

for most metals, poisson’s ratio is

Answer 4

0.3

Question 5

Gage factor

Answer 5

useful in comparing various strain gage materials

Question 6

What is the gage factor for most metals

Answer 6

1.6

Question 7

What is the gage factor for semiconductor materials

Answer 7

Approx. 50-70 times that of metals

Question 8

What are unbonded gages

Answer 8

wire grid, metal foil, or film cemented on a base. When the strain gauge is connected to the Wheatstone bridge circuit, at initial preload, the strain and resistance of the four arms are approximately equal. The resultant output voltage of the bridge is equal to zero. Application of pressure causes a small displacement causes a decrease in resistance of the strain gauge. This results in an unbalance in the output voltage of the Wheatstone bridge which is proportional to the small displacement hence the pressure.

Question 9

Semiconductor strain gages disadvantages

Answer 9

• More temp. sensitive and inherently non-linear than metal strain gages

Question 10

Semiconductor elements

Answer 10

Can be used as bonded, unbonded or integrated strain gage units which have the advantage of high gage factor

Question 11

What is piezoelectric effect

Answer 11

When pressure is applied to a piezoelectric along a specified direction, electric charges are developed by the crystal

Question 12

What are the some commonly used crystals

Answer 12

• Quartz (SiO2)
• Rochelle Salt
• PZT
• PVDF

Question 13

Working principle of piezoelectric sensors

Answer 13

• When pressure is applied to a piezoelectric sensor, electric charges are developed by the crystal
• Piezoelectric sensor measures changes in pressure
• Piezoelectric sensor converts pressure to an electrical charge

Question 14

Examples of piezoelectric sensors

Answer 14

• A quartz crystal tonometer for measuring pressure from cornea
• Ultrasound transducer used in ultrasound imaging

Question 15

The total charges q developed is directly proportional to?

Answer 15

force applied

q=kf where k is the piezoelectric constant

Question 16

Define flow

Answer 16

flow is measured as a rate at which a volume crosses a surface

Question 17

Working principle of electromagnetic flowmeter

Answer 17

It is based on the principle that when a fluid containing electric charges flows in a magnetic field, an electromotive force is generated

Question 18

What is the formula for electromagnetic flowmeter

Answer 18

Q=(piUd^2)/4=(piVd)/4B

Question 19

Working principle of thermocouple

Answer 19

When the junction of 2 dissimilar metals are kept at different temp., the generated voltage is a function of the temperature difference between the junctions. For accurate measurement, the reference junction should be kept constant.

Question 20

Law of homogeneous circuits

Answer 20

One cannot maintain an electric current in a circuit composed of a single homogeneous metal.

Question 21

Law of Intermediate metals

Answer 21

the net EMF in a circuit consisting of an interconnection of a number of unlike metals (maintained at the same temperature) is zero

Question 22

Law of Successive of intermediate temperatures

Answer 22

Draw.

E(T1,T3) = E(T1,T2) + E(T2,T3)

Question 23

What are thermistors

Answer 23

• Made of ceramic materials that are thermal resistors with a high negative temperature coefficient (increase in T -> decrease in R)
• Small in size, sensitive to temperature change, and operates at very low power

Question 24

Formula for the relationship between the thermistor resistance and absolute temp.

Answer 24

refer to notes