The present invention relates to coatings for transducers in a sensor that contact a fluid, which is defined herein as a liquid, a gas or combinations thereof, to apply signals and/or measure responses when measuring a fluid property, wherein the fluid property can be a physical or chemical quality or condition.  The invention finds application on transducer surfaces, which can be metal, semi-conductors, polymers, crystalline or amorphous materials or the like, that have, or can be functionalized to have, hydroxyl sites.  The invention can be used with transducers for off-line, on-line or in-line monitoring and/or analysis of fluids such as, for example, lubricants, natural and/or synthetic motor oils, standard additives and/or adjuncts, combustion engine fuels, other hydrocarbon-based fluids used in transportation and industrial applications, and the like.  More specifically, the present invention relates to antifouling coatings that do not substantially interfere with the transducer's ability to apply signals to and/or measure responses from a fluid, while reducing the need for transducer maintenance, such as cleaning. 
Fluids are a critical component for the proper operation of many types of devices and/or processes.  For example lubricants are needed for an internal combustion engine to efficiently provide power over a long service life, metal working fluid is needed in machining equipment for rapid metal removal and maximum tool life, and vapors above liquid can be used to identify the liquid.  Optimum performance is achieved when the fluid in question is of a proper quantity and quality for the application.  For a particular application, a fluid preferably includes an appropriate base fluid and desired performance additives, e.g. corrosion inhibitors, friction modifiers, dispersants, surfactants, detergents, markers and the like.  During use or consumption, a fluid's condition should remain within specified limits, that is, chemical and/or other fluid changes should be within proper performance specifications. 
A fluid's quantity, initial quality and/or continuing condition are often determined using sensors that test particular fluid properties.  Such sensors, whether used off-line, on-line, or in-line, typically have one or more transducers contacting the fluid to transmit a signal, receive a response, or both transmit a signal and receive a response.  A limitation for sensors is that the transducer can be fouled by the fluid, or components of the fluid including contaminants, being tested.  To alleviate a fouling problem, sensors may require regular cleaning or replacement of the transducers, often after each fluid test, in order to provide sufficiently accurate results.  Fouling problems can be particularly acute with sensors that make surface dependent measurements in fluids that are “surface active”.  For example, an attenuated total reflectance (ATR) infrared sensor, which uses an internal-reflectance-element (IRE) in contact with a fluid to determine fluid absorption at one or more frequencies, can show substantial signal drift in a lubricant formulated to provide surface corrosion and wear protection. 
While coatings, such as Teflon®, are known to be used in some applications to protect transducers from fouling, an issue with present coatings is that they can substantially interfere with a signal being sent and/or received by the transducer, thereby limiting the performance of a sensor.  Typical coatings have their own IR attenuation which limits their use in IR sensor applications, and many current coatings have electrical impedance or porosity issues which limit their use in sensors that transmit or receive electrical signals.  Another issue with current coatings on sensor transducers is that the coating thickness is not easily controlled.  Many coatings, with desirable properties, cannot be controllably applied in a sufficiently thin layer; for example, on an ATR infrared sensor's IRE transducer the coating must be on the order of nanometers to allow the optical signal to penetrate through the coating and into the fluid.  Another problem with current sensor coatings is that the coatings do not provide adequate fouling protection for the transducer.  That is, the coating does not substantially decrease transducer maintenance.  Therefore, there is need for an improved transducer coating that does not substantially interfere with the operation of a sensor and that provides an increased transducer maintenance-free operating period. 
The present invention overcomes limitations of previous coatings for transducers used to monitor/analyze fluids.  The invention is a simple, cost-effective coating that has minimal effect on signals being sent and/or received by transducers in contact with a fluid, that can be applied in a controlled thickness, and that substantially decreases or eliminates transducer fouling thereby increasing the transducer's maintenance-free operating period.