Oil-in-Water Measurement Sensors: Oil and PAH Monitoring by UV Fluorescence
Reagent-free, continuous oil-in-water measurement: principle, model selection, turbidity compensation and installation.
Three Questions When Selecting an Oil-in-Water Sensor
Continuous monitoring of oil in water is based on UV fluorescence: aromatic compounds in oil are excited by UV light, and their emitted fluorescence is measured. No reagents or sample preparation are needed, and response time is within seconds. Model selection depends on these three questions:
- How much oil do you expect? Leak detection in drinking water and cooling water requires ppb-level PAH sensitivity (for example, 0–50 ppb), whereas refinery and bilge water require ppm-level oil ranges (for example, 0–150 ppm or 0–1000 ppm).
- How turbid and colored is the water? The Pyxis HM-500 compensates internally for turbidity and color; the TriOS enviroFlu and microFlu V2 HC are used in clean to moderately fouled water, with an additional sensor when turbidity compensation is needed.
- Where will it be installed? TriOS for manhole, open-channel or inline immersion; Pyxis HM-500 for pipeline mounting via an inline tee; enviroFlu for type-approved marine scrubber washwater monitoring.
Oil-in-Water and PAH Sensors
Values are taken from the TriOS product catalog (2022), enviroFlu operating manual (04/2025) and Pyxis HM-500 datasheet. "Typical" oil values are the typical ranges stated by the manufacturer. The TriOS enviroFlu is also offered in a BT version for BTX (benzene, toluene, xylene) measurement: 0–1000 and 0–10000 µg/L.
| Model | Measuring range | Detection limit / accuracy | Compensation | Output | Installation |
|---|---|---|---|---|---|
| TriOS enviroFlu-HC 500 | PAH: 0–50 and 0–500 ppb Oil (typical): 0–1.5 and 0–15 ppm | Detection limit PAH 0.3 µg/L, oil 9 µg/L (oil-dependent); accuracy plus/minus (5% + detection limit) | No built-in turbidity compensation (possible with TTurb via TriBox3); cross-sensitivity: turbidity, dissolved organic matter | RS-232 (TriOS), 4-20 mA, 0-5 V | Immersion; 311 mm × 68 mm; IP68 |
| TriOS enviroFlu-HC 5000 | PAH: 0–500 and 0–5000 ppb Oil (typical): 0–15 and 0–150 ppm | Detection limit PAH 0.5 µg/L, oil 15 µg/L (oil-dependent); accuracy plus/minus (5% + detection limit) | No built-in turbidity compensation | RS-232 (TriOS), 4-20 mA, 0-5 V; MB version: RS-485 Modbus RTU | Immersion; 311 mm × 68 mm; IP68 |
| TriOS microFlu V2 HC compact | PAH: 0–5000 ppb Oil (typical): 0–150 ppm | Detection limit PAH 5 ppb, oil 0.15 ppm (typical); accuracy 10% of full scale | No built-in turbidity compensation | RS-485 Modbus RTU; 4-20 mA (optional 0-5 V, 0-10 V) | Immersion or inline; approximately 162 mm × 48 mm; IP68 |
| Pyxis HM-500 / HM-520 / HM-510 | 10 ppm / 100 ppm / 1000 ppm (referenced to Marine-Offshore Oil) | Detection limit 0.1 / 0.5 / 0.5 ppm; repeatability 0.1 ppm or at most 3% | Automatic color and turbidity compensation: up to 150 NTU and 10 ppm iron | 4-20 mA and RS-485 Modbus RTU | Inline tee (¾ inch FNPT); CPVC 7 bar, 304 stainless steel (SS) 21 bar; IP67 |
Oil-in-Water Measurement by UV Fluorescence
How does it work?
The TriOS enviroFlu excites the water using a xenon flash lamp and a 254 nm optical filter; a photodiode and 360 nm filter measure the fluorescence emission. The microFlu V2 HC applies the same principle using a 255 nm LED. Aromatic compounds such as naphthalene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene and pyrene are selectively detected. Because the method is far more sensitive than infrared techniques, even trace PAH levels are detected.
ppb PAH vs ppm oil
The measured quantity is PAH fluorescence (ppb). Among hydrocarbons, only aromatic compounds fluoresce; because the ppm oil value is derived from this as a typical factor, the composition of the oil being monitored affects the result. The TriOS enviroFlu is calibrated against a phenanthrene-based standard. On the Pyxis HM-500 series, the output is in ppm referenced to Marine-Offshore Oil, and calibration for specific oils can be written to the sensor via the uPyxis app.
Color and turbidity
The main sources of error in oil measurement are turbidity and color. The Pyxis HM-500 uses additional photoelectric components to monitor water color and turbidity simultaneously and correct for them automatically (up to 150 NTU and 10 ppm iron). The TriOS enviroFlu and microFlu V2 HC do not include internal compensation; in lines that frequently become turbid, this difference determines the selection.
Short flow channel, easy cleaning
TriOS uses a nano-coating on its optical windows to reduce oil and biofilm adhesion. The Pyxis HM-500 is easy to clean thanks to its short flow channel; the uPyxis app diagnoses sensor fouling in approximately 5 seconds, after which a 2-point calibration is performed directly on the sensor.
Which Sensor for Which Line?
Heat exchanger and cooling water leak detection
For early detection of oil ingress into condensate and cooling water, the low-range enviroFlu-HC 500 (PAH 0–50 ppb) is suitable. The Pyxis HM-500 series is also listed for heat exchanger and oil tank leak detection.
Drinking water, UF/RO feed and desalination
To catch hydrocarbon contamination at raw water intakes and upstream of membranes, the enviroFlu (drinking water, desalination) or Pyxis HM-500 (UF and RO feedwater, reinjection water) is used.
Refinery, airport, road runoff and industrial wastewater
The TriOS enviroFlu and microFlu V2 HC are listed for refinery and pipeline monitoring, and the microFlu V2 HC for airport, fuel station and road surface drainage trench applications. The HM-500 is suitable for industrial wastewater and stormwater; for high oil loads, the HM-510 (0–1000 ppm) is selected.
Bilge water and marine scrubber washwater
The TriOS enviroFlu is listed for bilge water and for washwater monitoring on exhaust gas cleaning systems type-approved under resolutions MEPC.259(68) and MEPC.340(77). The EGC Water Analyzer combines the enviroFlu PAH sensor with pH and turbidity sensors and the TriBox3 in a single system.
Mounting, Power Supply and Maintenance Interval
TriOS enviroFlu
- Power supply 12–24 VDC (plus/minus 10%), at most 3.5 W; T100 response time at most 10 s, measurement interval at least 5 s.
- Sample +2…+40 °C, ambient −5…+55 °C; flow velocity 0.1–10 m/s; IP68 (NEMA 6P).
- Pressure: 30 bar with SubConn, 3 bar with fixed cable, 1 bar (2–4 L/min) inside FlowCell.
- Stainless steel (1.4571 / 1.4404) approximately 2.7 kg or titanium (3.7035) approximately 1.9 kg. A stainless steel body is not suitable for continuous use in seawater; select titanium for seawater.
- The standard measuring head (POM, synthetic quartz glass) is not suitable for water below pH 4; an acid-resistant PPS head version is available.
- Portable operation is possible with an optional handheld meter.
TriOS microFlu V2 HC and Pyxis HM-500
- microFlu V2 HC: 12–24 VDC, typical at most 0.6 W; T90 response time 6 s, minimum measurement interval 3 s; stainless steel approximately 650 g.
- Pyxis HM-500 series: 24 VDC (plus/minus 2), 65 mA; operating temperature 4–40 °C; IP67; CPVC models 0.37 lb (approximately 170 g), SS models 2.5 lb (approximately 1.1 kg).
- For Pyxis, the OIW-500 synthetic oil-in-water secondary calibration standard (500 ppm) can be used; dilute with deionized water for lower values.
- Typical inspection: at most 0.5 hours per month for TriOS; calibration/maintenance interval 24 months.
Brand and guide pages: TriOS enviroFlu · TriOS microFlu V2 · Pyxis HM-500 series · TriBox controllers · spectral and optical water quality sensors
Frequently Asked Questions About Oil-in-Water Sensors
UV fluorescence, PAH and oil units, turbidity compensation, installation and maintenance.
Which method is used for oil-in-water measurement?
One of the most sensitive methods in online measurement is UV fluorescence: aromatic hydrocarbons (PAH) are excited by UV light around 254–255 nm and emit fluorescence around 360 nm. TriOS notes that this method is much more sensitive than commonly used infrared scattering or absorption methods, making it possible to measure very low PAH traces even in drinking water and cooling water condensate.
What is the difference between ppb PAH and ppm oil values?
The sensor measures PAH fluorescence and reports it in ppb. Because PAHs differ greatly in solubility and fluorescence yield, TriOS notes that no single universal calibration exists; the enviroFlu is factory-calibrated with the phenanthrene-based TriOS fluorescence calibration standard (TFCS), and the digital reading is divided by 6.2 to obtain the phenanthrene equivalent (PAH-p). Oil concentration is given in ppm as a typical value in the catalog: 0–1.5 and 0–15 ppm for enviroFlu-HC 500, and 0–15 and 0–150 ppm for enviroFlu-HC 5000. Because only aromatic components fluoresce, the ppm value depends on the composition of the oil being monitored; on the Pyxis HM-500 series, calibration for different oils can be written to the sensor via the uPyxis app.
Does an oil sensor measure accurately in turbid or colored water?
It depends on the model. The Pyxis HM-500 measures water color and turbidity with additional photoelectric components and compensates automatically; the manufacturer states that this holds up to 150 NTU turbidity and 10 ppm iron. The TriOS enviroFlu and microFlu V2 HC do not have built-in turbidity compensation; for the enviroFlu, compensation is possible via a separate turbidity sensor connected through a TriBox3 controller.
Is the sensor installed in-line in a pipe or submerged?
Both are possible. The TriOS enviroFlu and microFlu V2 HC are submersible sensors that operate in manholes, shafts, open channels, pipelines or bypass flow cells. The Pyxis HM-500 series installs in-line on the pipe via a ¾-inch FNPT tee; CPVC models can also be used in submersible applications with the MA-102S adapter kit.
With which outputs does the sensor connect to automation?
The enviroFlu-HC provides digital RS-232 (TriOS protocol) along with 4-20 mA and 0-5 V analog outputs; the enviroFlu HC MB version features an RS-485 Modbus RTU interface without an analog output. The microFlu V2 HC offers RS-485 Modbus RTU and 4-20 mA (with optional 0-5 V or 0-10 V). The Pyxis HM-500 series includes both 4-20 mA and RS-485 Modbus RTU together.
How is oil monitored in marine exhaust gas scrubber washwater?
The operating manual for the TriOS enviroFlu HC states that it holds type approval under IMO resolutions MEPC.340(77) and MEPC.259(68) and is listed for marine exhaust gas cleaning (scrubber) washwater monitoring. In this application, turbidity correction is mandatory: the enviroFlu measures simultaneously alongside a turbidity sensor, and correction is performed in the controller software. The TriOS EGC Water Analyzer system combines the enviroFlu PAH sensor, pH and turbidity sensors, and the TriBox3 controller.
What is the maintenance and calibration interval of the sensor?
For the TriOS enviroFlu and microFlu V2 HC, typical inspection effort is at most 0.5 hours per month and the calibration/maintenance interval is 24 months; on the enviroFlu, manufacturer calibration can be extended to 4–5 years using the DryCAL kit. Optical windows are nano-coated. On the Pyxis HM-500 series, sensor fouling is diagnosed in approximately 5 seconds via the uPyxis app, and 2-point calibration is performed directly on the sensor.
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