1. 003
Temperature and Pressure
Term used to measure the degree of hotness or coldness of a thermodynamic substance with
reference to standard value. Thermometer is the instrument used to measure temperature.
Arbitrary scale, t:
Centigrade or Celsius (oC)
On the Centigrade or Celsius scale the boiling point of water at standard atmospheric
pressure is 100oC and the freezing point is 0oC.
Fahrenheit (oF)
On the Fahrenheit scale the boiling point of water at standard atmospheric pressure is
212oF and the freezing point is 32oF.
Conversion:
o
C
5
9
 F  32
o
Absolute Temperature, T:
Kelvin (K) = oC + 273
Rankine (oR) = oR+ 460
ZEROTH LAW OF THERMODYNAMICS
Zeroth Law of thermodynamics states that if two closed system with different temperatures
are brought together in thermal contact with a third system, the heat will flow from the system with a
high temperature to the system with low temperature until the bodies reach thermal equilibrium with
each other.
PRESSURE
The pressure of a fluid is defined as force per unit area, measured in Pascal defined as 1
Newton per square meter.
F N
P  ; or kPa
A m2
Gage Pressure,
(+) Pgage
PRESSURE
Atmospheric
Pressure, Patm
Absolute pressure, Pabs
Vacuum
Pressure, (-) Pvac
Absolute
Pressure, Pabs
Absolute zero pressure
FIGURE 1-5: A graphical representation of pressure
Absolute Pressure
Absolute pressure is measured from the datum of absolute zero pressure or perfect vacuum.
Atmospheric Pressure
Atmospheric pressure is the pressure caused by the weight of the atmosphere.
Pabs  Patm  Pgage
At standard sea-level, the atmospheric pressure is:
1 atm = 101.325 kPa = 14.7 psi = 760 mmHg
Gage Pressure
Gage pressure is a pressure difference between the system and the atmospheric pressure. An
ordinary pressure gage will have a reading of zero this simply means that there is no excess pressure
other than the atmospheric pressure.

2. Vacuum Pressure
Vacuum pressure is a pressure which is significantly lower than atmospheric pressure.
Pabs  Patm  Pvacuum
Pressure and Depth of the Fluid
If an object P is placed in a container, the object experiences a pressure acting normal all over
its surface. This pressure is due to the weight of the fluid above it and increases proportionally as the
object goes deeper inside the container.
P  Pa tm  h
Patm
P  Pa tm  h
h
P  h
P
Where:  = specific weight of the fluid
PROBLEM SET:
1.1 Consider a vertical spring-loaded piston cylinder. The gas inside the cylinder has a pressure of 130
kPa. And the spring exerts a downward force of 200 N on the top of the piston. The diameter of
the piston is 10 cm and the atmospheric pressure is 1 bar. Determine the mass of the piston that
will maintain equilibrium. Neglect the friction effects between the surface of the piston and the
cylinder.
Air
1.2 An open tank is filled with 6.0 meters of water and 3.2 meters of gasoline with specific gravity of
0.90. Determine:
(a) The interface pressure between the water and gasoline
(b) The pressure at the bottom of the tank.
(c) The force exerted by the fluids at the bottom when the diameter of the tank is 1.3 meters.
1.3 A certain boiler has a pressure of 300 psi. If the barometric pressure is 743 mm of Hg, determine
the absolute pressure, in kPa.
1.4 A tank contains water having a height of 12 m and oil (SG=0.88) with 4 m above water surface.
Calculate the pressure of water at the bottom of the tank.
1.5 The pump has a suction pressure of 120 mm Hg vacuum. If the atmospheric pressure is 12 m of
water, determine the absolute pressure, in m of water.
1.6 Determine the equivalent of 35oF temperature change in oC.