Measuring Changes in the Specific Gravity of a Fluid

The hobby of home brewing has become very popular in the United States in recent years, leading to the emergence of a number of commercial products catering to home brewers. One of the most important devices is the hydrometer, which measures specific gravity. Specific gravity changes throughout the brewing process until fermentation is finished. Home brewers must sample the specific gravity of their batches to verify when it stops changing, thus telling them when it is finished fermenting. Most traditional methods of measuring specific gravity require manual sampling of the liquid, a process that is inefficient and wasteful.

To eliminate the need for manual sampling, the Department of Mechanical and Aerospace Engineering at Utah State University set out to design a device that could continuously monitor the changing of specific gravity. The concept that was chosen to accomplish the design objective was to apply a strain gage to a cantilever beam. The beam was attached to a float, whose buoyant force would change depending upon the specific gravity of the fluid. This change in buoyant force would result in a different strain in the beam, which was measured by the gage. Measured strain was calibrated to a known specific gravity value.

There were a number of factors to consider when selecting a strain gage for the project, the first of which was the harsh brewing environment. The device would be submerged in liquid for a four- to six-week period. The measurement also needed to be independent of temperature. Finally, the strain gage needed to be both precise and accurate to make the device reliable. To meet these requirements, the Department chose a Micro-Measurements’ Transducer-Class® strain gage. The device was coated in M Coat W1 wax and food-grade silicone in order to make it waterproof. A full-bridge strain gage was used to negate the effects of temperature change. The resulting device has proven to be a reliable and accurate tool in the hands of home brewers.

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