Comparative pressure measurement between a capacitive pressure gauge and a Pirani sensor can define the endpoint. Whereas the capacitive pressure gauge functions independently of the gas type and always indicates the true pressure in the drying chamber, the Pirani sensor is calibrated to a specific gas and measures only relative figures according to gas type. Calibrated to N2, the Pirani sensor indicates higher values than the capacitive pressure gauge in response to the presence of water molecules in the chamber. As the concentration of water molecules decreases, the detected value of the Pirani sensor reading progressively becomes closer to that of the capacitive pressure gauge until they become equal at the end point.
Pressure rise test is accomplished by closing the valve between the product and condenser chambers. When the two chambers are isolated, the pressure in the product chamber rises due to the on-going sublimation of the water molecules. The pressure rise curve progressively levels off as the water molecule concentration decreases. The end point is the point at which no further pressure rise is dicscernible.