'Open the Visual Basic Editor from Excel
'Create a module if one is not already opened,
'Copy this file to the VBA module 

'AirPTWCvEng(Pressure (psia), Temperature (deg F), HUmidity Ratio) returns specific heat at constant volume (Btu/lbwet/deg F)

'AirPTWCvSIF(Pressure (MPa), Temperature (deg K), HUmidity Ratio) returns specific heat at constant volume (KJ/kgwet/deg K)

Declare Function MyAirPTWV Lib "TAIR32.DLL" Alias "AirPTWV" (ByVal P As Double, ByVal T As Double, ByVal W As Double, ByVal U As Long) As Double
Declare Function MyAirTVWP Lib "TAIR32.DLL" Alias "AirTVWP" (ByVal T As Double, ByVal V As Double, ByVal W As Double, ByVal U As Long) As Double
Declare Function MyAirPTWC Lib "TAIR32.DLL" Alias "AirPTWC" (ByVal P As Double, ByVal T As Double, ByVal W As Double, ByVal U As Long) As Double

Public Function AirPTWCvSIF(ByVal P As Double, ByVal T As Double, ByVal W As Double) As Double
    Dim V As Double
    Dim V1 As Double
    Dim P1 As Double
    Dim T1 As Double
    Dim dVdTp As Double
    Dim dPdTv As Double
    Dim Cp As Double
    
    
    V = MyAirPTWV(P, T, W, 19)
    T1 = T + 1
    V1 = MyAirPTWV(P, T1, W, 19)
    dVdTp = (V1 - V) / (T1 - T)
    P1 = MyAirTVWP(T1, V, W, 19)
    dPdTv = (P1 - P) / (T1 - T)
    Cp = MyAirPTWC(P, T, W, 19)
    AirPTWCvSIF = Cp - T * dVdTp * dPdTv * 1000
    
    
End Function

Public Function AirPTWCvEng(ByVal P As Double, ByVal T As Double, ByVal W As Double) As Double
    Dim V As Double
    Dim V1 As Double
    Dim P1 As Double
    Dim T1 As Double
    Dim dVdTp As Double
    Dim dPdTv As Double
    Dim Cp As Double
    Dim US As Integer
        
    US = 16
    
    V = MyAirPTWV(P, T, W, US)
    T1 = T + 1.8
    V1 = MyAirPTWV(P, T1, W, US)
    dVdTp = (V1 - V) / (T1 - T)
    P1 = MyAirTVWP(T1, V, W, US)
    dPdTv = (P1 - P) / (T1 - T)
    Cp = MyAirPTWC(P, T, W, US)
    AirPTWCvEng = Cp - (T + 459.67) * dVdTp * dPdTv * 144 / 778.169262
    
    
End Function