After the development of digital density meters for use in the quality control laboratory in the 1960s, there was an immediate demand for a portable version of these devices for use in the field. Packaging the oscillating U-tube technology into a small, robust model brought a number of challenges, including:
- Making the housing robust enough to withstand outdoor use (knocks, spills) without becoming too heavy for portable use
- Making the whole instrument sealed enough to allow use in hazardous environments and ideally achieve a protection class of IP54
- Ensuring adequate filling outside of a laboratory environment
- Having a display that is readable, even in dark or badly lit areas
- Ensuring the device is light enough for one-handed use and small enough to measure in hard-to-reach places (i.e. from tanks and batteries).
- Using power supplies independent of plugs e.g. battery power
- Having a design that makes the instrument intrinsically safe so it can be used in areas where it is needed
Modern digital hydrometers typically use a W-shaped oscillator made of a borosilicate glass. They have a long filling tube to reach into tanks and containers. Additional features may include:
- Gesture control (a way of operating the instrument via hand movements which makes operation easier in dirty environments or when wearing gloves).
- An interface for reading RFID tags on containers for quick and traceable sample identification.
- The possibility of replacing the cell on-site in case of rupture after dropping it.
- Models which are intrinsically safe and therefore approved for use in hazardous areas (i.e. for measurements on petroleum products, battery acid, and other chemicals).
- Wireless transfer of measured data via Bluetooth®.