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Modern water faucets can be considered functions of these two inputs, which get mapped to the two lower-level controls: the apertures of the hot and cold pipes. Old-fashioned faucets gave users direct control of these two apertures, and in doing so had simpler mechanical implementations but were functionally inferior interfaces; for example, it would be difficult for a user to keep temperature constant while varying flow rate.
However, good interface design does not stop there. Many modern faucets fail to address the following.
•It should be easy to control mixture and aperture independently. Users may want to be able to leave temperature fixed and simply turn the faucet on and off; likewise, users may want to adjust temperature while the faucet is kept fully on. However, I have seen faucets that make it difficult or impossible to adjust one factor without adjusting the other. The faucet below forces you to center the handle (discarding your desired mixture setting) when turning the faucet off.
•Mixture (hot/cold) generally requires a knob or lever having a very wide angle of motion, whereas aperture (on/off) generally needs not have very wide motion. Temperature in most places is slow to respond to input, whereas flow responds immediately. If you’ve taken any classes on control systems you learn that a feedback controller (in this case, the human) will have poor control of a system when its output signal is delayed. Designs like the one below are effective because they offer a wide range of motion to maximize control of temperature.
•Aperture is the primary control; temperature is secondary. Not adhering to this, your design will be unintuitive. We all subconsiously understand that in order to move water where we want it, we must exert effort upward against the force of gravity. Since the beginning of recorded history of mechanisms that move water around, vertical motion has always been the driver. Secondary to this is choosing our temperature; we may carry a pail of hot water in our left hand and a pail of cold water in our right hand. Now, by resisting nature, the design below suffers from an unintuitive interface in spite of the degree of control it may offer in the end.
sink faucet, bath faucet, bidet faucet, basin mixer, angle valve, stop valve
