function [outstruct] = WTP_vT(v,T)
% -----------------------------------------------------------------
% function [outstruct] = WTP_PT(v,T)
%
% written by Francois Brissette in April 2005, 
% based on original work by Philippe Daigle, May 2002
%
% calculates basic thermodynamic properties for water, 
% follows IAPWS-97 formulation, Region 3 and 5 have NOT
% been implemented. This has been programmed for use by
% engineering students and regions 1, 2 and 4 cover the
% useful range.  See www.iapws.org for the paper describing
% the formulation in details
%
% you are free to use, modify and distribute the functions as long
% as authorship is properly acknowledged
%
% INPUTS:
% v specific volume in m3/kg
% T in C
%
% OUPUT:
% outstruct - a structure of strings that contains the following:
% pressure in kPa, temperature in C, specific volume in m3/kg,
% internal energy in kJ/kg, enthalpy in kJ/kg, entropy in kJ/kg/K
% as well as two messages giving info such as the state or error message
% in the case of incorrect data entry
%
% NB: This function does not use the reverse equations of the formulation
% but iterated using the direct equations for P and T.  Results will
% be identical up to the tolerance defined in the function.  Default is
% 10^-7
%


Pg=0; Tg=0; vg=0; ug=0; sg=0; hg=0;
Pf=0; Tf=0; vf=0; uf=0; sf=0; hf=0;
info_msg='';
info_msg_2='';

CK=273.15;
T=T+CK;    % C to K

% first step - establish the thermodynamic state by supposing first that
% consitions are saturated
%

P=Thermo_State(T);

if T <= 623.15
    [Pg,Tg,vg,ug,sg,hg] = property_PT(P,T,2); % Region 4 for steam
    [Pf,Tf,vf,uf,sf,hf] = property_PT(P,T,1); % Region 1 for supercooled liquid
    if v < vg & v > vf
        state=3;    % saturated
    end
    
    if v < vf
        [Pf,Tf,v_test,uf,sf,hf] = property_PT(100,T,1); 
        state=1;   % supercooled liquid
    end
    
    if v > vg
        state=2;   % superheated vapour
    end
    
     [Pf,Tf,v_test2,uf,sf,hf] = property_PT(0.000001,T,2); 
    if v > v_test2
        state=5;   % v will result in P too close to zero
    end
end

if T > 623.15  % either it's superheated vapour, or it is out of range
    n1=348.05185628969;
    n2=-1.1671859879975; 
    n3=.0010192970039326;
    Ptest=n1+n2*T+n3*T^2;   % calculate P in boundary between region 2 and 3
    if Ptest>100
        Ptest=100;
    end
    [Pg,Tg,vg,ug,sg,hg] = property_PT(Ptest,T,2); % Region 2
    
    if v > vg
        state=2;   % superheated vapour
    end
    
    if v < vg 
        state=4;    % region 3 or P > 100 000
    end
    
end


if T < 273.15001 | v<0  % basic quality control
    state=5;
end

    

if state==1 % T < Ts, Liquide comprim