function u=xiBO(RHOS,rhoc,Tb,Pib,mu0)

R=8.314;

rhob=mu0*1e3*DIPPR105(RHOS,Tb);

dlnrhosdTb=dlnDIPPR105(RHOS,Tb);
nuSb=(Pib/Tb)/(-R*Tb*rhob*dlnrhosdTb/mu0);
rhobr=rhob/rhoc;
u=1+(log(nuSb+1)-(-0.42704*(rhobr-1)+2.089*(rhobr-1).^2-...
    0.42367*(rhobr-1).^3)).*(-5.87605+6.72002*rhobr...
    -1.27101*rhobr^2).^(-1)./rhobr;
