Notes lectures

1. IIT (BHU), Varanasi
Thermodynamics
(DT-125 and FT-123)
Date: 13th July 2022
Pressure and Temperature
Dr. Akhilendra Pratap Singh
Department of Mechanical Engineering
IIT (BHU), Varanasi, India
Email: akhilendra.mec@itbhu.ac.in

2. Thermodynamics
Recap: Introduction
State, Process, Path, and Cycle
Equilibrium and Stability [C]
Isothermal, Isobaric, and Isochoric Process
Steady and Unsteady State
Some more types of systems [C]

3. Thermodynamics
Pressure
Pressure: Normal component of force per unit area.
/
lim normal
A A
F
p
A

 
  
 
2
3 2
5 2
6 2
1 1 /
1 10 /
1 10 /
1 10 /
1 100 0.1
Pa N m
kPa N m
bar N m
MPa N m
bar kPa MPa




 
1 kgf/cm2 =?? bar = 0.9807bar
1 psi= lbf/in2
1 bar= 14.5 psi

4. Thermodynamics
Pressure: Pascal's Law
The pressure P at a point in a fluid under equilibrium is the same in all directions.
(Pascal's Law)

5. Thermodynamics
Absolute, Gauge (gage), and Vacuum Pressures
Absolute pressure: The actual pressure at a given position. It is measured relative to absolute
vacuum (i.e., absolute zero pressure).
Gauge pressure: The difference between the absolute pressure and the local atmospheric
pressure (Pressures above atmospheric pressure). Most pressure-measuring devices are
calibrated to read zero in the atmosphere, and so they indicate gauge pressure as +ve reading.
Vacuum pressure: Pressures below atmospheric pressure.
Throughout this
text, the pressure P
will denote
absolute
pressure unless
specified otherwise.

6. Thermodynamics
Pressure Measurement: Manometer
Patm
Patm + Pgauge
Patm + Pvac
P P
 Pguage will be positive.
 Pvac will be negative.

7. Thermodynamics
Pressure Measurement: Examples

8. Thermodynamics
Pressure Measurement: Bourdon Gage
(French engineer and inventor Eugene Bourdon (1808–1884))
When fluid under pressure fills the tube, the elliptical section tends to become circular, and the
tube straightens.
This motion is transmitted by the mechanism to the pointer.
By calibrating the deflection of the pointer for known pressures, a graduated scale can be
determined from which any applied pressure can be read in suitable units.
Bourdon tube measures the pressure relative to the pressure of the surrounding existing at the
instrument.
Accordingly, the dial reads zero when the pressure inside and outside of the tube are same
(mostly Patm).

9. Thermodynamics
Pressure Measurement: Bourdon Gage
(French engineer and inventor Eugene Bourdon (1808–1884))

10. Thermodynamics
Pressure Measurement: Pressure Transducers
Piezoelectric Pressure Transducers
(piezoelectric effect): A charge is generated within
certain solid materials (crystalline substance),
when they are deformed.
Strain-gage pressure transducers: Another
important type of sensor employs a diaphragm
that deflects when a force is applied, altering an
inductance, resistance, or capacitance. [C]

11. Thermodynamics
Calibration of Pressure Transducers

12. Thermodynamics