Pressure, Displacement, Flow and Temperature Measurement Flashcards
What are potentiometers
They can produce a linear potential output as a function of the displacement
3 types of potentiometers
- Translational
- Single turn
- Multi turn
How does strain gages work
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
for most metals, poisson’s ratio is
0.3
Gage factor
useful in comparing various strain gage materials
What is the gage factor for most metals
1.6
What is the gage factor for semiconductor materials
Approx. 50-70 times that of metals
What are unbonded gages
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.
Semiconductor strain gages disadvantages
- More temp. sensitive and inherently non-linear than metal strain gages
Semiconductor elements
Can be used as bonded, unbonded or integrated strain gage units which have the advantage of high gage factor
What is piezoelectric effect
When pressure is applied to a piezoelectric along a specified direction, electric charges are developed by the crystal
What are the some commonly used crystals
- Quartz (SiO2)
- Rochelle Salt
- PZT
- PVDF
Working principle of piezoelectric sensors
- 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
Examples of piezoelectric sensors
- A quartz crystal tonometer for measuring pressure from cornea
- Ultrasound transducer used in ultrasound imaging
The total charges q developed is directly proportional to?
force applied
q=kf where k is the piezoelectric constant
Define flow
flow is measured as a rate at which a volume crosses a surface
Working principle of electromagnetic flowmeter
It is based on the principle that when a fluid containing electric charges flows in a magnetic field, an electromotive force is generated
What is the formula for electromagnetic flowmeter
Q=(piUd^2)/4=(piVd)/4B
Working principle of thermocouple
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.
Law of homogeneous circuits
One cannot maintain an electric current in a circuit composed of a single homogeneous metal.
Law of Intermediate metals
the net EMF in a circuit consisting of an interconnection of a number of unlike metals (maintained at the same temperature) is zero
Law of Successive of intermediate temperatures
Draw.
E(T1,T3) = E(T1,T2) + E(T2,T3)
What are thermistors
- 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
Formula for the relationship between the thermistor resistance and absolute temp.
refer to notes
