- Summary
The OIWM measures the level of hydrocarbons in water systems using a photoacoustic technique. The technique measures the acoustic response of hydrocarbons to a pulsed laser source input, the level of acoustic response being proportional to concentration of oil in water. The monitor will measure the oil content in the range 200-2000ppm. In the existing prototype unit the electronics are packaged in a rack unit and the instrument head in a flanged mounting. The head contains pressure and temperature sensors to provide on-line data which facilitates ppm algorithms. In the subsea unit the electronics will be packaged with the sensor head in a single pressure vessel.

 - The PA Process
A laser fires pulses of light through a sapphire window into the fluid. The optical energy is absorbed causing sudden local heating. The subsequent thermal expansion of molecular bonds in the analyte generate a pressure wave which can be detected with a sensitive acoustic transducer. A photoacoustic transducer (PZT) in the head detects the resulting acoustic response which is termed the PA response. In order to confirm that the energy delivered in the laser pulse is consistent, a beam splitter is used to reflect a small proportion of the light and a photodiode converts this to an electronic pulse, which is termed the energy detection. In real time, the PA and energy detection pulses are indistinguishable from the noise, however after averaging a series of repetitions (around 1500) a recognisable AC pulse is measured.