capacitive sensor principle
Defining Terms
Admittance (Y): The reciprocal of impedance.
Immittance: A response function for which one variable is a voltage and the other a current. Immittance is a general term for both impedance and admittance, used where the distinction is irrelevant.
Impedance (Z): The ratio of the phasor equivalent of a steady-state sine-wave voltage to the phasor equivalent of a steady-state sine-wave current. The real part is the resistance, the imaginary part is the reactance.
Now, let's give an example on Aluminum Oxide Sensors. The sensors are formed when a layer of porous, moisture sensitized aluminum oxide is deposited on a conductive substrate and the oxide is coated with gold. The technology has been refined to take advantage of semiconductor manufacturing techniques and higher purity substrates (such as ceramic) to produce sensors with consistent responses to moisture changes in the oxide.
A typical aluminum oxide sensor is in essence a capacitor. The conductive base and the gold layer become the capacitor’s electrodes, forming what is essentially an aluminum oxide capacitor. Water vapor penetrates the gold layer and is absorbed by the porous oxide layer. The number of water molecules absorbed determines the electrical impedance of the capacitor. For example, aluminum oxide hygrometers respond to the vapor pressure of water over a very wide range of vapor pressures. The strong affinity of water for this oxide, combined with the large dielectric constant of water, makes this device highly selective toward water. It does not respond to most other common gases or to numerous organic gases and liquids.
In situations in which aluminum can be chemically attacked, silicon sensors with a tantalum metal base could be used as an alternative. Such sensors are the most inert, although their sensitivity is somewhat less. The oxide layer is generally described in the form of a mass of tubular pores running up from the metal base to the exposed surface. Change in the size of these tubules with time is presumed to be the cause of the slow shifts in calibration often experienced with these sensors. Water is absorbed in these tubules in amounts directly related to the moisture content of the gas in contact with it. The amount of water is sensed electrically by measuring the change in capacitance and/or admittance produced by this water. Because of the radius of the pores in the aluminum oxide, the sensor is virtually specific for water molecules.
So, if you measure capacitance with the sensor electronics .. you have a capacitive probe, if you measure admittance (the reciprocal of impedance) you have an admittance probe, if you measure both, you have a hybrid ..
Hope it helps ..