Nitrate Sensors: Online UV Nitrate Measurement and Selection
Reagent-free, continuous nitrate (NO3-N) monitoring without sample preparation: method, optical path, and model selection.
Three Decisions That Determine Nitrate Sensor Selection
The reagent-free way to continuously monitor nitrate in water and wastewater is UV absorption: nitrate strongly absorbs ultraviolet light, and the absorbed amount gives the concentration. The sensor is an immersion type placed directly into a channel, basin, or by-pass line. Three questions determine the right model:
- In what range will you measure? The optical path length determines the range. A short path suits high-nitrate water such as wastewater, while a long path suits low-nitrate water such as drinking water.
- Nitrate only, or other parameters as well? NICO measures only the nitrate group, NICO plus measures SAC254 and organic equivalents in addition to nitrate, OPUS measures nitrate, nitrite, and organic load via full spectrum, and OPUS aero measures nitrate and nitrite in aeration basins.
- Where and how will it be connected? Immersion or by-pass FlowCell; Ethernet or Modbus RTU; or a TriBox controller when an analog signal is required.
TriOS Optical Sensors for Nitrate Measurement
Values are taken from the TriOS product catalog (2022), the NICO plus brochure (11/2025), and the OPUS aero brochure; see the TriOS brand page and online analyzers for details.
| Model | Measurement method | Nitrate range | Other measured parameters | Optical paths | Response (T100) |
|---|---|---|---|---|---|
| NICO nitrate photometer | Xenon flash lamp, 4 photodiodes + filter; attenuation method | 1 mm: 0.5–60 mg/L NO3-N 10 mm: 0.05–6 mg/L NO3-N | NO3-N, NO3, NOx-N, NOx (calibrated with nitrate standard solution) | 0.3 · 1 · 2 · 5 · 10 · 50 mm | 20 s; measurement interval at least 10 s |
| NICO plus multi-parameter UV photometer | Xenon flash lamp, 4 photodiodes + filter; with turbidity compensation | 0.3 mm: 0–165 · 1 mm: 0–50 · 5 mm: 0–10 · 10 mm: 0–5 · 50 mm: 0–1 mg/L NO3-N (at 10 mm NO3: 0–22 mg/L) | UVT254, SAC254, COD, BOD, TOC, DOC equivalents, turbidity, TSS | 0.3 · 1 · 2 · 5 · 10 · 20 · 50 mm | 40 s; measurement interval at least 20 s |
| OPUS spectral sensor | Xenon flash lamp, 256-channel spectrometer, 200–360 nm | 1 mm: 0–100 mg/L NO3-N 10 mm: 0–10 mg/L NO3-N | NO2-N, COD, BOD, DOC, TOC, TSS equivalents, SAC254 | 0.3 · 1 · 2 · 5 · 10 · 50 mm | 2 min; at least 1 min |
| OPUS aero basin-type spectral sensor | Xenon flash lamp, 256-channel spectrometer, 200–360 nm | 0.3 mm: 2.4–120 mg/L 1 mm: 0.7–36 mg/L 2 mm: 0.35–18 mg/L | Depending on calibration, nitrate only (NO3-N) or nitrate + nitrite (NO2-N: 0.3 mm 4.4–220, 1 mm 1.3–67, 2 mm 0.65–33.5 mg/L) | 0.3 · 1 · 2 mm | 2 min; at least 1 min |
In all four models, the protection class is IP68, power supply is 12–24 VDC (plus/minus 10%), and the calibration/maintenance interval is 24 months; for the specified measurement accuracy, the sample temperature must be +2…+40 °C. Power consumption is at most 7 W for NICO and NICO plus, and at most 8 W for OPUS and OPUS aero.
How Does UV Absorption Nitrate Measurement Work?
Reagent-free and without sample preparation
The sensor is immersed in the water; water passing through the gap (optical path) between the light source and detector absorbs UV light. There are no chemical reagents, sampling lines, or waste, and the measurement is real-time. Consequently, consumable cost and sample preparation error are eliminated.
Photodiode or full spectrum?
NICO and NICO plus measure specific wavelengths with filtered photodiodes; this is simple, economical, and fast. OPUS and OPUS aero scan the 200–360 nm range with a 256-channel spectrometer (0.8 nm/pixel). In the TriOS spectrum, nitrate, nitrite, organic matter, and suspended solids yield distinct absorption curves in the UV region; the full spectrum separates these components from one another.
Optical path determines the range
When the path length increases tenfold, the measuring range narrows tenfold: in NICO, 0.5–60 mg/L on a 1 mm path and 0.05–6 mg/L on a 10 mm path; in OPUS, 0–100 on a 1 mm path and 0–10 mg/L NO3-N on a 10 mm path. Thus, the same sensor body adapts to different water types using different optical path adapters.
NO3-N and NO3 conversion
According to TriOS documentation, 1 mg/L NO3-N corresponds to 4.43 mg/L NO3, and 1 mg/L NO2-N corresponds to 3.28 mg/L NO2. The reporting unit (as nitrogen or as ion) should be selected according to the contract or regulatory limit.
Which Nitrate Sensor for Which Water?
Drinking water, wells, and distribution networks
Low nitrate levels require a long optical path: NICO measures 0.05–6 mg/L NO3-N on a 10 mm path; NICO plus offers ranges of 0–5 on a 10 mm path, 0–2.5 on a 20 mm path, and 0–1 mg/L NO3-N on a 50 mm path. If an organic matter indicator is also desired in the same sensor, NICO plus provides SAC254 (0–60 1/m on a 10 mm path) and UVT254 values as well.
Wastewater treatment plant: aeration basin
OPUS aero is designed for continuous monitoring of nitrate and nitrite inside the basin; on a 0.3 mm path, nitrate is measured at 2.4–120 mg/L. If only nitrate is needed, NICO (0.5–60 mg/L on a 1 mm path), described by TriOS as a low-cost nitrate photometer, may be sufficient.
Treatment plant effluent and discharge point
When organic load (COD, BOD, TOC equivalents) and suspended solids must be monitored alongside nitrate, OPUS provides these parameters in a single sensor using the full spectrum.
Surface water and environmental monitoring
Rivers, lakes, and monitoring stations are among the applications listed in the TriOS catalog for NICO, NICO plus, and OPUS. When multiple sensors are installed on a station panel, the TriBox3 (4 sensors) or TriBox mini (2 sensors) controller collects and logs the data.
Mounting, Cleaning, and Communication
Mounting
- Pressure rating: 30 bar with SubConn connector, 3 bar with fixed cable, 1 bar inside FlowCell (2–4 L/min).
- Sample flow velocity 0.1–10 m/s; sample and ambient temperature +2…+40 °C.
- Body in stainless steel (1.4571 / 1.4404); a titanium (3.7035) option is also available for NICO, NICO plus, and OPUS.
- Dimensions: approximately 470 mm × 48 mm (on 10 mm path); stainless steel body approximately 3 kg, titanium approximately 2 kg.
Cleaning and maintenance
- Optical windows are nano-coated, delaying fouling.
- AirShot2: cleans in 10 seconds with a compressed air blast, triggered as frequently as every 5 minutes, operates with TriBox3 (at most 7 bar).
- For sensors with an optical path of 10 mm and below, FlowCell FC 48/10 USC: ultrasonic cleaning, 2–4 L/min, at most 1 bar.
- Typical supervision requirement at most 0.5 hours per month; calibration/maintenance interval 24 months.
Communication
Thanks to the TriOS G2 interface, configuration is performed via a standard web browser without additional software; the instruments include a data logger (approximately 2 GB in NICO, NICO plus, OPUS, and OPUS aero). Digital interfaces are Ethernet and Modbus RTU.
Controller and analog signal
TriBox3: 4 sensor inputs (M12), 6 x 4-20 mA outputs, 1 SPDT relay, 7-inch touchscreen, Modbus TCP, and WiFi. TriBox mini: 2 sensor inputs, 2 x 4-20 mA outputs, 1 relay, 3.5-inch screen.
Related pages: Online analyzers · TriOS brand · Spectral water quality sensors
Frequently Asked Questions About Nitrate Sensors
UV nitrate measurement, optical path, unit conversion, maintenance, and connectivity.
How does an online nitrate sensor work, and does it require reagents?
No. The nitrate molecule strongly absorbs ultraviolet light; the sensor passes UV light through the water and calculates the nitrate concentration from the absorbed amount. There is no sampling, reagent, or waste. For this purpose, TriOS NICO uses a xenon flash lamp and a 4-photodiode + filter detector, while OPUS and OPUS aero scan the 200–360 nm range with a 256-channel spectrometer.
What is the difference between NO3-N and NO3?
NO3-N expresses nitrate as nitrogen, whereas NO3 expresses it as the nitrate ion. According to TriOS documentation, 1 mg/L NO3-N corresponds to 4.43 mg/L NO3. Pay attention to which unit the sensor operates in and which unit your regulation or process limit is written in; NICO and NICO plus can output both units.
How is the optical path length selected?
The optical path is the distance the light travels through the water. A short path is for high-concentration samples (wastewater), and a long path is for low-concentration samples (drinking water). In NICO, a 1 mm path measures 0.5–60 mg/L NO3-N and a 10 mm path measures 0.05–6 mg/L NO3-N; when the path is ten times longer, the range becomes ten times narrower. Choose the path where your expected value falls near the middle of the measuring range.
Is nitrate measurement accurate in turbid water or water with a high organic load?
These sensors are designed to account for optical interference caused by turbidity and organic matter: NICO performs turbidity compensation, while OPUS and OPUS aero compensate for turbidity using the full spectrum. For NICO, accuracy is specified as plus/minus (5% + 0.1 mg/L NO3-N) on a 10 mm path and plus/minus (5% + 1 mg/L NO3-N) on a 1 mm path.
Can I measure nitrate and nitrite with the same sensor?
Yes, depending on the sensor family. OPUS measures nitrate (NO3-N) and nitrite (NO2-N) separately; OPUS aero measures either nitrate alone or nitrate plus nitrite depending on calibration and is designed for wastewater aeration basins. NICO is suited for the nitrate group (NO3-N, NO3, NOx-N, NOx), while NICO plus covers SAC254 and organic equivalents in addition to nitrate.
How much maintenance does the sensor require?
Across all four models, TriOS specifies a typical supervision requirement of at most 0.5 hours per month and a calibration/maintenance interval of 24 months. Optical windows are nano-coated; the TriOS AirShot2 compressed air system can trigger a 10-second cleaning blast as frequently as every 5 minutes. For optical paths of 10 mm and below, the FlowCell FC 48/10 USC with ultrasonic cleaning can be used.
How does the sensor connect to a PLC or SCADA?
NICO and NICO plus operate via Ethernet (TCP/IP) and RS-485 Modbus RTU, while OPUS and OPUS aero operate via Ethernet and RS-232 or RS-485 Modbus RTU; configuration is performed from a standard web browser. If a 4-20 mA analog signal is required, the TriBox3 controller offers 6 x 4-20 mA outputs, 1 SPDT relay, and 4 sensor inputs; the TriBox mini is a 2-channel unit.
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