Water Vapor Saturation Data (Tsat, Pg) and the Psychrometric Chart

The following Water Vapor Saturation Data is required in order to plot the Saturation Curve (Relative Humidity Φ=100%) of the Psychrometric Chart.

 

Temp

Pressure
 

Temp

Pressure

Tsat (°C)

Pg (kPa)
 

Tsat (°C)

Pg (kPa)

0.01

0.6117

 

25

3.1699

1

0.6571

 

26

3.3639

2

0.7060

 

27

3.5681

3

0.7581

 

28

3.7831

4

0.8136

 

29

4.0092

5

0.8726

 

30

4.2470

6

0.9354

 

31

4.4969

7

1.0021

 

32

4.7596

8

1.0730

 

33

5.0354

9

1.1483

 

34

5.3251

10

1.2282

 

35

5.6290

11

1.3130

 

36

5.9479

12

1.4028

 

37

6.2823

13

1.4981

 

38

6.6328

14

1.5990

 

39

7.0002

15

1.7058

 

40

7.3849

16

1.8188

 

41

7.7878

17

1.9384

 

42

8.2096

18

2.0647

 

43

8.6508

19

2.1983

 

44

9.1124

20

2.3393

 

45

9.5950

21

2.4882

 

46

10.099

22

2.6453

 

47

10.627

23

2.8111

 

48

11.177

24

2.9858

 

49

11.752

25

3.1699

 

50

12.352

 

The basic information used to construct a psychrometric chart is the water vapor saturation data (Tsat, Pg) which is obtained from steam tables over the range from Tsat = 0.01°C through 50°C. The specific humidity ω is then evaluated using the relative humidity Φ as a parameter to produce the various relative humidity curves (blue lines) as follows:


where P is the standard atmospheric pressure 101.325 [kPa].

The saturation curve (100% relative humidity) is also known as the dew point curve is drawn as a red line. Notice that on the saturation curve the wet and dry bulb temperatures have the same values.

For more information refer to Chapter 10b on the Psychrometric Chart

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Source of the (Tsat, Pg) data: NIST Chemistry WebBook - accessed Feb 2008

The major simplifying assumption in the construction of the chart is that the enthalpy of the mixture is assumed to be constant throughout the adiabatic saturation process (described in Chapter 10a). This implies that the evaporating liquid added does not significantly affect the enthalpy of the air-vapor mixture, leading to the constant slope wet bulb temperature / enthalpy lines defined by:


Note that this simplyfied linear equation results in an error of less than 1% over the temperature range 0.01°C to 50°C.

Finally the specific volume curves which complete the chart are given by:

Refer to the complete MATLAB program for Plotting the Psychrometric Chart using these equations.

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Engineering Thermodynamics by Israel Urieli is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License