An investment casting shell substrate sensor method and apparatus comprising a hollow probe 10 having an aperture 30, the aperture 30 covered by a water proof breathable membrane (WPBM) 20 that is constructed and arranged to repel liquid from entering the aperture 30, while remaining permeable so as to allow vapor and air to pass through the aperture 30, thereby creating a liquid free internal void 50 within the hollow probe 10 that receives a sensor 40 for measuring the conditions (i.e. temperature and humidity) within the internal void 50, which are accurately indicative of the conditions at the shell substrate at the aperture 30 location. The disclosed sensor assembly provides target specific information at selected substrates within the shell real time, during the shell building process.