Surface Acoustic Wave (SAW)

Surface Acoustic Wave (SAW)

The Fuel Sniffer employs a Surface Acoustic Wave (SAW) Vapor Microsensor to measure the concentration of fuel in used lubricating oil samples by sampling the “headspace” in the sample bottle. The instrument assumes (based on Henry’s Law) that the fuel vapor concentration is directly related to the fuel present in the oil sample. A pump inside the instrument draws headspace vapors across the SAW sensor which detects absorbed hydrocarbons by a change in frequency of a surface acoustic wave.  

A SAW sensor consists of a piezoelectric substrate that has an interdigital electrode lithographically patterned on its surface. The surface of the SAW sensor has a polymer coating that offers solubility to fuel vapors. The mechanism of detection is a reversible absorption of the fuel component into the polymer. When this device is excited by external RF (Radio Frequency) voltage, a Rayleigh wave (waves perpendicular to the surface) generates on the surface of the device. When fuel contamination comes in contact with the SAW sensor surface it will absorb into the polymer coating.

This absorption into the polymer causes a mass change, producing a corresponding change in the amplitude and velocity of the surface wave. When used in a self resonant oscillator circuit, the change in Rayleigh wave velocity resulting from vapor absorption into the polymer coating causes a corresponding change in oscillator frequency. This change in frequency is the basis of the Fuel Sniffer’s detection. The absorption is semi-selective based on the properties of the polymer coating and the partition coefficient (solubility of the chemical and polymer) of the chemical of interest. 

Prior to the Fuel Sniffer, customers relied on GC-MS and Flash Point technologies to detect fuel dilution.  GC-MS is a method that combines the features of gas-liquid chromatography and mass spectrometry to identify different substances within a test sample. It is known as the “gold standard” for forensic substance identification because it is used to perform a specific test. A specific test positively identifies the actual presence of a particular substance in a given sample.  Yes it is powerful, but it is also expensive. The typical cost of a GC-MS is almost 10 times the cost of the Fuel Sniffer to provide good “witness” results on a portable, fast and reliable platform. Also, a highly skilled chemist is required to run the GC-MS instrument to obtain good results.

Flash Point tests can be relatively fast and accurate if an automated flash point tester is used, but it is not as fast or as convenient as the Fuel Sniffer. Flash Point systems do not present percent fuel dilution calculations except when using correlation tables generated by running many samples containing a known percentage of fuel.

In comparison, the Fuel Sniffer offers an improved safety factor over Flash Point methods because it  reduces exposure to fire and inhalation hazards from heating fuel samples. In addition, generating correlation tables to obtain the known percentage of fuel is not required with the Fuel Sniffer.

FTIR testing of fuel relies heavily on individual fuel calibrations and careful baselining of the blank oil matrix. The absorbance peak in the spectra regions used to detect fuel is not very sensitive and careful calibration needs to be done by highly trained professionals. Also, different fuels refined from around the world will exhibit different absorbance bands that can sometimes be missed by the calibration peak. The Fuel Sniffer does not suffer from these calibration and matrix problems.

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