Chapter 13 431 homogeneous system. This evaluation method can also be used for heterogeneous systems to determine such interactions as the effect of shaking or the agglomeration of suspended particles. Additional methods employing these clear package types include freeze/thaw stability and cloud point analysis. The freeze/ thaw evaluation determines the compatibility of a system after exposure to low temperatures (≤ -10°C) followed by thawing to room temperature over three cycles. A cloud point analysis is performed to determine the temperature at which a clear homogenous system (such as a hair spray) begins to cloud over prior to precipitate formation. An important parameter to consider in the preparation of these test units is to provide for the head space that will be represented in the final system. The liquefied gases exist in equilibrium within the liquid and gas phases therefore, if there is excess headspace, the equilibrium will shift which may result in incompatibility. In order to calculate the proper fill ratios to provide for a desired head space, the density of the concentrate and propellant as well as the volume of the test pack are required. (See Figure 5.) Figure 4. The use of clear plastic coated Wheaton bottles to determine aerosol system compatibility