Capstone Project

ENGR 696 & 697 Senior Project I & II

San Francisco State University

Brewing beer at home has become very popular in the past few years. As craft brewing at home has gained popularity, it has created a demand for newer technology. There are two main stages in making beer; the brewing stage and the fermentation stage. Most, if not all, of the technology has been created to help the homebrewer with the first stage of brewing.

For example, homebrewers want to create the best possible brew to share with others and are turning to online forums asking for newer technologies and methods to exacting their brew to perfection. Many homebrewers have specifically asked for a digital hydrometer. Which the responses have been that they do not know of any in existence. This project aims to address these types of needs with user-friendly tools that aid in the learning process of fermenting.

The digital hydrometer will allow the homebrewer to view real-time specific gravity information while the liquid is fermenting. This will allow the brewer to identify if the yeast is properly attenuating at the rate specified. The brewer will then be able to identify when the primary fermentation has been completed and will then transfer the beer to the secondary vessel for proper conditioning. This is done so the beer will have the best flavor possible. The digital hydrometer will also allow the homebrewer with the ability to calculate their projected ABV percentage.

Currently there are not any digital hydrometers designed specifically for homebrewer use available for purchase. Homebrewers now use the analog hydrometer, a bulb placed in a sample of the liquid with a scale for the brewer to identify where the liquid level is. This has been widely used but is lacking because the brewer may be uncertain of the accuracy of reading it and will not be able to use this data for an automated fermentation vessel.

Available for purchase are professional digital hydrometers that are used in scientific laboratories. These are very accurate but very costly, upwards of $2,000. The average homebrewer will not be willing to pay this amount, especially since they are not designed specifically for the homebrewer.

To answer the homebrewer’s need, a couple of digital hydrometers are currently being developed. The specifics of the sensor devices are not disclosed by the startups. Two of these were posted on Kickstarter, and each needed a pledge of $25,000 and $30,000 to get up and running. Both of these startups have the homebrewer specifically in mind.

The scope of our project is beyond just the digital hydrometer. We have successfully designed a digital hydrometer with further integration in mind. Soon we will integrate it with a temperature-controlled system that will account for varying temperatures and provide accurate, specific gravity information.

Brewers are more concerned with specific gravity than the density of a liquid because specific gravity is a dimensionless quantity. Specific gravity is the ratio of the density of a given liquid to the density of water, making it a universal measurement across systems of units. This simplifies calculations as the percentage of alcohol in a given brew can be calculated easily with predetermined formulas with minimal conversion.

Our original goal for this project was to design and produce a self-contained and self-regulating dual-stage home fermentation unit for the home beer brewing community; to be able to take as much user error out of the beer brewing equation as possible. However, as the project progressed and time constraints grew tighter, we realized that an entirely self-contained system was beyond the scope of our abilities (e.g., learning TIG welding for stainless steel).

We decided to focus the project on a specific part of our original self-contained design: update the home brewer’s old-fashioned analog hydrometer with current technology to make a digital hydrometer for use in the home brewing of beer. Updating the hydrometer will take error out of the beer brewing process by making checking the brew’s specific gravity a clean and hands-off process, reducing the possibility of fouling a home brew's time and effort.

We decided to utilize a top-down, double float type system operating under Archimedes' Principle to make the hydrometer. Archimedes' Principle states that "the weight of an object in liquid is the weight of the liquid displaced." Having calibrated the system once for water, knowing the displacement of the measurement buoy relative to the reference, the mass of the measuring buoy, and the measurements of the buoy, the specific gravity of the liquid it is in can be extrapolated from these parameters.

A top-down system means that the system sits at the top of the liquid and measures down from a reference. The double float refers to a stationary, reference float that sits at the top of the fluid and utilizes a measurement float that moves relative to the reference buoy with the change in the specific gravity of the liquid.

An LVDT is utilized to measure the measurement buoy's displacement relative to the reference. An LVDT is a linear measurement device that utilizes a moving metal core to measure displacement. The LVDT produces an output voltage approximately linearly proportional to the position of the metal core within the device. This voltage can be interpreted to measure displacement, and as stated before, with the known mass and dimensions of the measuring buoy, extrapolate the specific gravity of the liquid.

One important parameter of the LVDT is the minimal friction of the measuring unit. The core that moves inside the LVDT is not designed to interface with anything and relies on electromagnetic principles for measurement. This means that there is minimal friction in the measurement unit itself and therefore introduces minimal error in measuring overall.

The voltage produced by the LVDT is sent through signal conditioning that isolates the LVDT from the circuit through a voltage buffer and then conditions the signal to the input range of an analog-to-digital converter (ADC). The buffer was found to be necessary as the ADC seemed to want to draw more power from the LVDT than could be supplied. The conditioning removes the slight error the buffer introduces to the signal and amplifies the analog voltage so that the ADC can better utilize its 5-volt range. After this conversion, the data is sent to a microprocessor.

The microprocessor takes the ADC data and runs the number through an equation that was derived related to the analog voltage output of the LVDT to the specific gravity of the fluid. Then this number (the specific gravity) is output to an array of 7-segment LED displays, where the user can easily read the specific gravity of the fluid.

Before we began designing our digital hydrometer, we asked ourselves “What requirements does the home brewer workaround?”. We wanted our hydrometer to integrate into and be compatible with a home brewer’s current brew setup and equipment. This meant that our hydrometer needed to be able to:

• It must fit into a minimum of a 5-gallon fermentation tank; brews are usually made in batches of 5 gallons and a plastic 5-gallon fermentation tank (5-gallon plastic bucket) is commonly used by most brewers.

• The output voltage of the sensor (the LVDT) must be in DC voltage, the use of DC voltage was decided for practical reasons; using a sensor that preconditions its output to DC voltage makes designing a circuit and handling the data output easier because we don’t need signal conditioners to translate the output data.

• The sensor should be designed to have a “floating” reference when beginning the fermenting process. This means there should be a one-time calibration for each new fermenting process the sensor oversees because the volume of water changes as the brew ferments. The floating reference should also not impose friction on the buoy’s travel within the core of the LVDT.

• The buoy should be sized to have less than 2 inches of displacement to allow for the full range of specific gravity we want to measure.

What Worked

The overall design provides accurate results, but the system is bulky, and it would be nice to have a smaller system to work around. The smaller system we tried to create did not provide enough buoyant force on the reference tube, producing inaccuracies in the measuring device.

For this to be a real marketable product for consumers (the home brewing community) to purchase we would need to remake all the plastic components to be made out of food-grade plastics so that we can be sure poisonous chemicals are leaching into the food product.

Design Tradeoffs 

A major trade-off we made for the sake of time was making the prototype out of non-food-safe plastics and non-food-safe adhesives, such as electrical tape. This trade-off was a practical necessity because we believe having proof of concept is more important than material construction. Now that we know our design not only works but works accurately we can further refine the design and the construction of our prototype or construct better and more advanced prototypes.

Another trade-off was to use the LVDT to simplify our proof of concept. The LVDT is expensive and significantly added to the cost of this project. However, using this approach allowed the project to move forward quickly and as smoothly as possible. Now that the proof of concept has been established, our team is looking into sensor alternatives that will reduce the overall cost of each module.

Comparison of Results

The digital hydrometer created is nothing more than an electronic version of the analog hydrometer. The convenience brought by the digital hydrometer is that it can sit in the fermentation tank and samples are taken without having to remove and replace some of the brew. As such, it needs to be as accurate as the analog hydrometer, which the demonstration video shows it to be.

A comparison of the two shows that our system can sit in a sealed tank, negating the user from opening the sealed container to take samples. This is an advantage as every time the unit is opened, the brew is potentially exposed to bacteria or other contaminants which can render the entire batch unusable. Our product is superior in this regard, and it provides a convenience factor as the brewer can simply look down at the top of the tank to determine the current specific gravity instead of going through an involved process.

As mentioned before, this is a product the market is currently lacking. We intend to bring a product to market that is convenient, safe to use, and affordable to the home brewer.

Legal, Economic, Ethical or Societal Issues

After successfully testing our prototype we discussed what would need to be changed in the final design to safely and ethically market this device to the home brewing community. All the plastics used would need to be exchanged for plastics deemed food safe by the FDA, these plastics will not leach out harmful chemicals like BPA, benzene, or other harmful hydrocarbons into the fermenting beer. These hydrocarbons may destroy the taste of the fermenting beer but also poison anyone consuming the beer.

Further product and patent research is needed to ensure that the design of this unit doesn't conflict with the patents on file today. Specifically, our concern is with lab-grade hydrometers as the digital hydrometers marketed toward brewers are still in development.