Faucet Sensing Technologies:Capacitance sensing

Faucets with capacitive sensing technology have an omni-directional detection zone surrounding the entire spout, rather than the small sensor window commonly used with AIR faucets. It is important to note that capacitance sensing is not the same as “thermal” sensing, as temperature has nothing to do with its operation.

Capacitance sensing, which detects the presence or absence of a conductive object, works by measuring changes in an electrical property called capacitance. Capacitance describes how two conductive objects with a space between them respond to a voltage difference applied to them. In typical capacitance sensing applications, the probe or sensor is one of the conductive objects; the target object – in this case the user’s hands – is the other.

A faucet employing capacitance sensing acts as the probe that creates a low power electrical field round the faucet – the spout becomes an antenna. The target object is a human body. This works because every person typically has 22-picofarads of capacitance, which is derived from the body’s water composition. This ultra-low electrical current that we all have running through our bodies completes the circuit when we get close enough. In essence, water starts to flow when a hand approaches the spout from any direction and stays on as long as the hands are within range.

As with any antenna, this system needs to be tuned and cannot be readily adapted to just any spout. While it eliminates the need to locate an infrared sensor “somewhere,” it does not readily adapt to different geometries as small changes in spout design can have a rather dramatic effect on the sensing area.

Capacitance is proportional to the area of overlap and inversely proportional to the separation between conducting objects. The closer the objects are to each other, the greater the capacitance. A capacitance sensing faucet picks up a palm easier than a finger. In other words, sensing distance will be longer when sensing a palm.

The sink material is also a factor. The faucet using capacitance sensing must be isolated from its surroundings, particularly if installed on a metal sink (such as stainless steel or cast iron). To isolate the faucet, an isolation layer needs to be employed that is greater than 0.5 inches when installed on a metal sink or deck. If not properly isolated, the entire sink unit will become the antenna, leading to unpredictable operation. Depending on the faucet geometry, the faucet may not work at all on stainless steel. Additionally, metal mounting hardware and the capacitance wire must not be within 0.5 inches of the metal sink either.

Also note that the flow rate of the faucet, water quality and water supply connections can slightly influence the operation of the faucet using capacitance sensing. Remember, water is electrically conductive, and depending on the local water quality (mineral content, dissolved solids), it can become a very effective conductor. This could alter the performance of the sensing zone and thus the proper activation of the faucet.

This does not mean that capacitance sensing does not have application advantages. These issues can be addressed through improved sensing algorithms. Because there is no sensor window as usually found on AIR-sensing faucets, a prime vandalism point is removed. Additionally, all of the mechanical and electrical components are concealed below the deck; the spouts are extremely vandal resistant.

Capacitance sensing faucets do not fit every application. They are a good fit, however, for any transient, high-traffic restroom.

False triggers are common for any of these technologies. However, the system will usually prevent water from running indefinitely, even if a false trigger happens. Possible sources of false triggers in AIR faucets are objects are left in sink, infrared light coming from other devices in the room or a high infrared noise from CFL (compact fluorescent lighting). Capacitance sensing faucets’ false triggers can be due to a change in the environment, such as a wireless signal of the right frequency or if conductive objects are placed around unit.