Chlorophyll Sensors: Chlorophyll-a and Algae Monitoring
Reagent-free fluorescence measurement: selecting the right chlorophyll-a sensor for reservoirs, lakes, drinking water intake and cooling water.
Three Questions When Selecting a Chlorophyll-a Sensor
Chlorophyll-a sensors excite water with an LED and measure the fluorescence emitted by algal pigments; there are no reagents, grab sampling steps or laboratory wait times. Selection depends on three questions:
- Which water application? Submersible TriOS microFlu V2 for reservoir, lake, river and drinking water raw intake monitoring; inline Pyxis EM-500 for cooling towers and process lines.
- Which measuring range? TriOS microFlu V2: 0–200 µg/L; Pyxis EM-500: 0.1–50 ppb; EM-510: 0–500 ppb.
- How turbid and colored is the water? The Pyxis EM-500 series compensates up to 150 NTU and 10 ppm iron; the TriOS microFlu V2 does not include internal turbidity compensation.
Chlorophyll-a Sensors
Values are taken from the TriOS microFlu V2 manual (12/2024), the Pyxis EM-500 datasheet and the EM-400 manual. Note that ppb and µg/L are the same unit of concentration.
| Model | Installation | Measuring range | Excitation / emission | Compensation | Output and power | Typical application |
|---|---|---|---|---|---|---|
| TriOS microFlu V2 (chl) | Immersion or flow cell; IP68; approximately 162 mm × 48 mm | 0–200 µg/L; detection limit 0.05 µg/L | 470 nm / 685 nm | No turbidity compensation; electronic daylight compensation; internal LED reference measurement | RS-485 Modbus RTU; 4-20 mA; 12–24 VDC, typical max 0.6 W | Surface water, reservoir, bathing lake, drinking water raw intake, environmental monitoring |
| TriOS microFlu V2 (blue) blue-green algae | Same housing | 0–200 µg/L; detection limit 0.5 µg/L | 620 nm / 655 nm | No turbidity compensation | Same | Cyanobacteria (phycocyanin) monitoring |
| Pyxis EM-500 | Inline tee (¾ inch NPT); UPVC; 172 mm × 36.6 mm; 170 g | 0.1–50 ppb; plus/minus 0.2 ppb | 470 nm / 675 nm | Automatic: 10 ppm iron (color) and 150 NTU turbidity | Isolated 4-20 mA and RS-485 Modbus; 24 VDC, approximately 1 W | Cooling towers and open-loop process water |
| Pyxis EM-510 | Same platform | 0–500 ppb; plus/minus 1 ppb | 470 nm / 675 nm | Same | Same | Waters with higher algal density |
| Pyxis EM-400 handheld | Battery-powered handheld meter; IP67; cuvette-free | 0–50 ppb; resolution 0.1 ppb; accuracy plus/minus 1% or plus/minus 0.1 ppb | 470 nm / 650 nm | Automatic color and turbidity compensation | Bluetooth uPyxis; 4 AA alkaline batteries | Spot checks, guiding algaecide dosing |
Chlorophyll-a Fluorescence Measurement
In-vivo measurement
The sensor measures chlorophyll-a directly inside living algal cells; there is no need to disrupt cells or extract pigments with solvents. Pyxis notes that the reading is used to evaluate living algal biomass in bulk water. With continuous measurement, algal blooms are detected within hours.
Blue-green algae, CDOM, tryptophan
Depending on the factory-installed optical filter combination in the same housing, the microFlu V2 measures different parameters: chlorophyll-a (470/685 nm), cyanobacteria (620/655 nm), CDOM (375/460 nm, 0–500 µg/L), tryptophan (275/360 nm, 0–500 µg/L), rhodamine and fluorescein (470/590 nm, 0–200 µg/L). Each sensor is configured for a single parameter.
Handling color and turbidity
The Pyxis EM-500 series uses additional optical channels to simultaneously measure and compensate for color and turbidity. The TriOS microFlu V2 does not include internal turbidity compensation, but features electronic daylight rejection and an internal reference measurement that corrects for LED aging and temperature effects.
Cleaning and calibration
TriOS uses a nano-coating on its optical windows; the calibration/maintenance interval is 24 months. The short flow channel of the Pyxis EM-500 is easy to clean; fouling status is viewed via the uPyxis app and the sensor is calibrated with CHLORO-10, CHLORO-20 and CHLORO-50 standards. The manufacturer recommends monthly cleaning and calibration.
Which Chlorophyll Sensor for Which Water?
Reservoirs, lakes, bathing lakes and rivers
The TriOS microFlu V2 is listed for these applications: surface waters, bathing lakes, raw water treatment, drinking water treatment and environmental monitoring. Its low power consumption (typical max 0.6 W; max 70 mW in sleep mode) suits autonomous field stations. A TriBox controller is used in multi-sensor stations.
Drinking water treatment plant intake
The TriOS microFlu V2 is specified for drinking water and raw water treatment intakes. To continuously track algal load in raw water, the chlorophyll and blue-green algae (cyanobacteria) versions can be deployed together; the CDOM version can also be selected for dissolved organic matter.
Cooling towers and process water
The Pyxis EM-500 is designed for continuous monitoring of living algal biomass in cooling towers and open-loop process water systems. Mounted inline via a tee, it feeds data to the controller over 4-20 mA and Modbus so algaecide dosing can be controlled accordingly. See our cooling tower automation guide for details.
Field checks and verification
The handheld EM-400 is used for spot algae checks around the plant and for verifying inline sensors; the sample is poured directly into the measurement cell, delivering a stable reading in 5 seconds, and can be field-calibrated with 0, 10, 20 and 50 ppb standards.
Related pages: TriOS microFlu V2 and TriBox controllers · Pyxis EM-500 / EM-510 · spectral water quality sensors · oil-in-water measurement
Frequently Asked Questions on Chlorophyll-a Sensors
Fluorescence method, blue-green algae, turbidity compensation, measuring ranges and maintenance.
How does a chlorophyll-a sensor work?
Chlorophyll-a is the primary photosynthetic pigment of algae and emits fluorescence when excited by light at a specific wavelength. The sensor excites the water with an LED and measures the emitted light: the TriOS microFlu V2 uses 470 nm excitation and 685 nm emission for chlorophyll-a, while the Pyxis EM-500 uses 470 nm excitation and 675 nm emission. No reagents or sample preparation are required. The reading is evaluated as an indicator of living algal biomass in the water.
What is the difference between chlorophyll-a and blue-green algae (phycocyanin)?
Chlorophyll-a is a pigment present in all algae, whereas phycocyanin is specific to blue-green algae (cyanobacteria). The chlorophyll version of microFlu V2 operates at 470/685 nm and the cyanobacteria version operates at 620/655 nm (excitation/emission). In reservoir and lake applications where cyanobacteria risk is monitored, both are measured with separate sensors.
Is chlorophyll measurement accurate in turbid water?
It depends on the model. The Pyxis EM-500 series automatically compensates for color (up to 10 ppm iron) and turbidity (up to 150 NTU). The TriOS microFlu V2 does not include internal turbidity compensation; instead, it features electronic daylight compensation and an internal reference measurement that corrects for LED aging. In highly turbid surface waters, pairing it with a turbidity sensor is recommended.
Which measuring range is required?
In natural waters (reservoirs, lakes, rivers), the TriOS microFlu V2 chlorophyll version measures over a 0–200 µg/L range with a 0.05 µg/L detection limit. In process and cooling waters, the Pyxis EM-500 provides 0.1–50 ppb, and for denser algae the EM-510 provides 0–500 ppb. The handheld Pyxis EM-400 measures 0–50 ppb. Note that ppb and µg/L represent the same concentration unit.
Can a chlorophyll sensor be used for algae monitoring in cooling towers?
Yes. The Pyxis EM-500 is specified for continuous monitoring of living algal biomass in cooling towers and open-loop process waters; it installs into a pipeline via the ST-001 inline tee and outputs 4-20 mA and RS-485 Modbus. The handheld EM-400 is designed for guiding algaecide dosing and performing spot checks around the plant.
How often should the sensor be cleaned and calibrated?
Pyxis recommends cleaning and calibrating the EM-500 series at least once a month; the interval can be extended in clean water and shortened in fouled water. Calibration is performed using CHLORO-10, CHLORO-20 and CHLORO-50 standards; fouling diagnostics and calibration can be run from the uPyxis app. For the TriOS microFlu V2, typical inspection effort is at most 0.5 hours per month and the calibration/maintenance interval is 24 months.
What is the output signal and how does it connect to a PLC?
The microFlu V2 provides RS-485 Modbus RTU and 4-20 mA (optional 0-5 V or 0-10 V); it is powered by 12–24 VDC and typically consumes at most 0.6 W. The Pyxis EM-500 provides isolated 4-20 mA and isolated RS-485 Modbus; it is powered by 24 VDC and consumes approximately 1 W.
Send the make and model of your current instrument to [email protected] — our engineers will propose the equivalent with the same connection and measuring range.
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