While most automotive sensors are submersible, zirconia-based sensors require a very small supply of reference air from the atmosphere.

Called charged coupled devices, or C.C.D.'s, these sensors require a set of several chips and components.

These sensors require a sufficiently small air gap and need to be particularly reliable.

Because the sensor does not require physical contact it is particularly useful for applications where access presents challenges or where dirt is prevalent.

The sensors require recalibration or replacement every six months.

Individual sensors may require a trusted platform or hierarchical trust structures.

Most sensors currently used require heating to at least 400 degrees Fahrenheit.

The electrochemical sensor is sensitive to alcohol vapors, and requires recalibration over time.

Today's state-of-the-art sensors are very sensitive, but require an expert to operate and are bulky and expensive.

The sensor requires cleaning if debris accumulates on the mirror.