The table to the left compares the two systems. All the
units in thermodynamics can be derived from these base units. Details
of the thermodynamic units will be introduced in the following sections.
Definition of Pressure
The absolute pressure (P) is the force
acting on unit area.
In the SI system, the unit for pressure is Pa, Pascal.
In the English system, it is psi.
Pa = N/m
psi = lbf/in2
Since Pa is a small unit in the SI system, other units are
also used in thermodynamics, such as:
1 bar = 105
1 kPa = 103 Pa
1 MPa = 106 Pa
1 atm = 101325 Pa
The air surrounding the earth can be treated
as a homogeneous gas, called atmosphere. Atmospheric pressure (Pa)
is the pressure due to the force by the atmosphere mass. Standard atmospheric
pressure is 101325 Pa.
Barometer is a device used to measure
the atmospheric pressure.
Pa = ρ
= The density of the working liquid, kg/m3
g = The acceleration of gravity,
h = The height of the working liquid
in the tube, m
Gage Pressure and Vacuum Pressure
Gage pressure (Pg) is the
difference between the absolute pressure and the atmospheric pressure
if the difference is positive. If the difference is negative, it is
vacuum pressure (Pv).
Pg = P -
(P > Pa)
Pv = Pa- P (P < Pa)
Absolute pressure is used in thermodynamic relations
U-Tube Manometer is used to measure pressure
difference. One end of it is open to the atmosphere and the other end
is connected to the equipment whose pressure is needed to be measured.
At the right side,
P1 = Pgas + ρgas g h1
Usually, the second term on the right hand of the previous equation is negligible since the density of the work fluid is much larger than the density of the gas.
P1 = Pgas
At the left side,
P1 = Pa + ρworking fluid g h1
Combine the two equations above, the pressure in the gas
tank can be determined as
Pa + ρworking fluid g h1
Temperature and the Zeroth Law
The Zeroth Law
The measurement of the degree
of hotness or coolness is temperature.
If two bodies at different temperatures are brought together,
the hot body will warm up the cold one. At the same time, the cold body
will cool down the hot one. This process will end when the two bodies
have the same temperatures. At that point, the two bodies are said to
reached thermal equilibrium.
The Zeroth Law of thermodynamics states:
Two bodies each in thermal equilibrium with
a third body will be in thermal equilibrium with each other.
The Zeroth Law of thermodynamics is a basis for the validity
of temperature measurement.
To establish a temperature scale, two
fixed, easy duplicated points are used. The intermediate points are obtained
by dividing the distance between into equal subdivisions of the scale
Fahrenheit Scale (oF)
Freezing Point of Water = 32.0
Boiling Point of Water = 212.0
Celsius Scale (oC)
Freezing Point of Water = 0.0
Boiling Point of Water = 100.0
Thermodynamic Temperature Scale
The pressure of an ideal gas is
zero = 0.0
The Triple Point of Water = 273.16
Relations between Different Temperature Scales
The relations between the above temperature
T (K) = T(oC) + 273.15
T (oF) = 1.8T(oC)
T (oF) = 1.8 (T(K)-273.15) + 32.0
The thermodynamic temperature scale in the English system is the Rankine
scale. The temperature unit on this scale is the rankine, which is
designated by R. The thermodynamic temperature scale in S.I. system
(K) and English system (R) are related by
Thermometers measure temperature,
by using materials that change in some way when they are heated or cooled.
In a mercury or alcohol thermometer the liquid expands as it is heated
and contracts when it is cooled, so the length of the liquid column
longer or shorter depending on the temperature. Modern thermometers are
calibrated in standard temperature units such as Fahrenheit or Celsius.
Three practical points for using thermometer are:
The thermometer should be isolated to everything except the body
which temperature is measured. The general method is to immerse the
thermometer in a hole in a solid body, or directly in a fluid body.
When thermal equilibrium is reached, the thermometer can indicate
its own temperature as well as the body measured. The thermometer
should be small relative to the body so that it only has a small
effect upon the body.
The thermometer must not be subject to effects such as pressure
changes, which might change the volume independently of temperature.
Digital thermometers almost replace the mercury ones
in nowadays because they are more accuracy and more easy to use.