Water activity refers to the amount of water in a system that is available for microbial growth. Commercial water activity meters are precision instruments with the ability to determine water activity to within 0.001 units and carry prices of $10,000 or more. The purpose of the study was to build a robust water activity meter from commercially available components and measure the water activity of liquids commonly used in compounded formulations. SHT-85 sensors were connected to an Adafruit Feather HUZZAH microcontroller. Standard salt slurries and common oral liquid vehicles were monitored in airtight containers until equilibrium was reached. Standard curves were used to convert sensor outputs to water activity values. The standard curves were linear with R2 >0.99. Oral liquid vehicles showed water activity values between 0.62 and 0.99. Samples equilibrated within 9.5 hours in 16-ounce jars or 2.5 hours in 20-mL vials. Stirring the sample during measurement reduced equilibration time in 16-ounce jars, but not in 20-mL vials. The inexpensive meter was accurate and precise in measuring the water activity of standards and selected oral vehicles. An accurate and precise water activity meter was constructed at a cost of approximately $150. Common oral formulation components have a water activity of >0.6, the United States Pharmacopeial threshold for requiring a preservative. Pharmacists should use caution when diluting preserved vehicles.
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