double type fzero error in vapor pressure calc

%%%Hi, pretty new with MatLab but think I understand what I'm doing, for some reason keep getting an error about 'double' type arguments in the fzero function.

%%%I made this mfile to define function for which I need the root:

function y=deltaphi(Pguess)

Tc=507.6;%K

Pc=30.25;%bar

w=0.301;

T=341.9%K

R=8.3144598*10^(-5);%m3 bar / K mol

f=0.480+1.574*w-0.176*w^2;

ac=0.42748*(R^2)*(Tc^2)/Pc;

a=ac*(1+f*(1-(T/Tc)^(1/2)))^2;

b=0.08664*R*Tc/Pc;

syms V

SRK=Pguess==R*T/(V-b)-a/(V*(V-b));

V=solve(SRK,V,'Real',true)

V=V(V>0)

Vl=double(min(V))

Vv=double(max(V))

Zl=R*T/(Pguess*Vl)

Zv=R*T/(Pguess*Vv)

phi_l=exp(Zl-1-log(Zl)-log(1-b/Vl)-(a/(b*R*T))*log(1+b/Vl))

phi_v=exp(Zl-1-log(Zl)-log(1-b/Vv)-(a/(b*R*T))*log(1+b/Vv))

y=phi_l/phi_v-1

end

%%%%Then I wrote this script file to find a good starting point for the fzero function: R=8.3144598*10^(-5);

T=341.9;

a=3.*10^(-5);

b=1.2088*10^(-4);

syms V

SRK=R*T/(V-b)-a/(V*(V-b));

Vext=solve(diff(SRK,'V')==0,V,'Real',true);

Vext=Vext(Vext>b);

Vmax=max(Vext);

Vmin=min(Vext);

Pmin=max(10*(-7),R*T/(Vmin-b)-a/(Vmin*(Vmin-b)))

Pmax=max(R*T/(Vmax-b)-a/(Vmax*(Vmax-b)))

Pguess=Pmin:0.99*Pmax

Pvap=fzero(@deltaphi,double(Pguess))

%%%Any help would be much appreciated. This is the exact error message

"Error using fzero (line 412) Second argument must be a scalar or vector of length 2.

Error in pvap (line 14) Pvap=fzero(@deltaphi,double([Pmin:0.99*Pmax]))"

Answer 1

[Torsten]

And - is double(Pguess) a scalar or a vector of length 2 ? No, it isn't.

As the second argument in the call to fzero, you have to supply a starting guess or a starting interval for the solution, thus a scalar or a vector of length 2.

Best wishes

Torsten.