In this work, the Photopyroelectric (PPE) technique using a Thermal Wave Resonator Cavity (TWRC) is used to measure the thermal diffusivity of agar. We, determine the liquid to gel phase transition temperature as a function of agar concentration, detecting a shift in that temperature. As agar concentration decreases, the phase transition temperatures get lower. The thermal diffusivity of agar as a function of temperature was measure by means of a cavity length variation due to thermal expansion of the TWRC. It was compared against the typical mechanic length variation
with a micrometer.