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Physical Construction

The physical construction of the digital hydrometer must meet the following criteria:

  • The electronic components must fit within the housing
  • There is a water-tight housing around the electrical components
  • It is able to withstand the wet, acidic, and alcoholic environment of home brewing
  • It must be food safe and not release contaminants
  • The battery and other electronic components must be accessible for recharging/servicing
  • The the device should fit within the opening of different fermentation vessels
  • The construction shall not interfere with wireless signals required for operation
  • It should be able to be sanitized with home brewing sanitation chemicals
  • It should continue to operate after reasonable drops, falls, and bumps

Preform PET Bottle (“Petling”)

For all of these criteria, a readily available solution has been found and tested by the iSpindle community: a “petling” or preform PET bottle. A preform is an intermediate product that is subsequently blown into a polyethylene terephthalate (PET) container. Once heated in a mold the containers are blown into a variety of markets to create a variety of container shapes, sizes, thicknesses, colors, etc. They’re commonly used in the geocaching community as durable containers to hide small caches in, and therefore are available in geocaching marketplaces relatively cheaply. Common ones in geocache shops, (Geocache One for example) are around 13cm in length and have an outer diameter of 2.5cm and cost ~$2.00 USD. Most of them weight between 10g and 50g at their heaviest/largest.

Polyethylene terephthalate is the most commonly recycled plastic. It can be approved as safe for contact with foods and beverages by the FDA, Health Canada, EFSA & other health agencies, has an excellent resistance to alcohols and diluted acids, and is known for its good gas (oxygen, carbon dioxide) and moisture barrier properties. Given that PET fermentation vessels are often used in home brewing, it is safe to say that petlings can be sanitized and will not release contaminants.

The caps of petlings are usually high density polyethylene (HDPE), another plastic that can be approved as food safe. Although it has properties that should ensure its resistance to breaking down in a brewing environment, if ballasted correctly, the cap of the petling should have limited contact with the must.

PET Centrifuge Tube

Similar to the petling, a plastic centrifuge tubes (often used for medical lab testing) are also readily available and are inexpensive, water-tight, come in a variety of sizes.

Glass Test Tubes and Vials

An alternative to PET preforms could be glass vials and test tubes that include a high density polypropylene cap with a foam or rubber gasket. These have all of the advantages of the PET bottles, with added scratch resistance and avoidance of plastic pollution. Disadvantages include being more fragile, more expensive, and heavier.

Without testing, it’s difficult to know for sure if a glass alternative to petlings would be suitable, although I have a general aversion to plastics.