A Spherical Void Electrodynamic Levitator Trap (SVELT) was used to measure the water activity in the supersaturated aqueous solutions of six materials, NaCl, (NH4)2SO4, KH2PO4, NH4H2PO4, KAl(SO4)2 12H2O and glycine. The relationship between chemical potential and concentration was obtained using a fifth order polynomial fit. A comparison of the order of the crystal growth rate obtained from the use of chemical potential difference and concent- ration difference was made. The order of crystal growth rate calculated from concentration difference was found to be close to that obtained from chemical potential difference at low supersaturation, while at higher supersaturation a deviation was noted.