When you’re managing harmful algae blooms (HABs), understanding what your sensors are measuring is essential. That’s why AquaRealTime has developed a short video to help water quality professionals interpret relative fluorescence units (RFUs), chlorophyll-a, and phycocyanin readings and then convert these measurements into actionable water quality insights.
Watch the full video with AquaRealTime CEO Christopher Lee to learn how to turn raw sensor signals into reliable, lab-grade insights for better water management.​​
What Are RFUs?
Relative fluorescence units (RFUs) are the raw signal output your sensors record. Chlorophyll-a and phycocyanin sensors use the principle of fluorescence. They shine light into the water at a specific wavelength. Algae molecules respond by emitting light at another wavelength.
For example, when a sensor sends a blue light into the water, chlorophyll molecules emit a red light back. The more red light detected, the higher the chlorophyll concentration — and therefore, the higher the algae density in the water.
However, each manufacturer’s sensors have different optics and electronics, meaning RFU readings are not standardized across brands. RFUs provide a reliable relative measure of algae trends but not a universal value for concentration.
From RFUs to Micrograms Per Liter
Micrograms per liter (µg/L) — also expressed as parts per billion (ppb) — represents the concentration of chlorophyll or phycocyanin in the water. It’s the absolute unit that scientists and regulators use for comparison, and a fundamental reference point for data-driven algae management.
But converting RFUs to µg/L isn’t simple. Factors include:
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Algae species and cell wall thickness
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Growth phase and light conditions​
These elements all affect fluorescence behavior. That’s why AquaRealTime’s platform provides two options for estimating µg/L values within your dashboard.
Two Ways to Estimate Concentration
Generic conversion: AquaRealTime’s default conversion uses a rule-of-thumb factor derived from controlled experiments with multiple strains of cyanobacteria and algae. This gives a reasonable estimate of µg/L values and allows users to quickly toggle between RFU and concentration views.
Field calibration tool: For site-specific precision, users can input lab-verified values from field samples into the dashboard. This one-point calibration creates a new, corrected time series aligned to your sample date and time without needing to retrieve or recalibrate the unit.
You can perform as many field calibrations as needed throughout the season, preserving each dataset for long-term comparison.
Setting Smart Alert Thresholds
Threshold selection depends on your application. While chlorophyll-a reflects all algae, phycocyanin targets cyanobacteria — the blue-green algae that can produce toxins or taste and odor compounds. Although not federally regulated, several risk-based frameworks provide guidance:
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Recreational waterways: The EPA considers <10 µg/L chlorophyll-a low risk, 10–50 µg/L moderate risk, and >50 µg/L high risk for recreational use.
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Drinking water utilities: The State of Kansas uses 10 µg/L chlorophyll-a as a benchmark for elevated taste and odor risks*. Staying below this level typically results in smoother plant operations.​​
*This is not their recommendation, just something they found that helps their process.
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With AquaRealTime’s dashboard, you can set custom alert thresholds and receive automatic email alerts when values exceed those or other limits.
The Chlorophyll-to-Phycocyanin Ratio
AquaRealTime also displays the chlorophyll-a to phycocyanin ratio, an advanced diagnostic indicator of community balance.
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A ratio near 10 suggests healthy conditions dominated by green algae or diatoms.
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A ratio near 1 indicates cyanobacteria dominance and a higher risk of toxins or taste and odor events.
Monitoring this ratio over time helps managers detect early shifts in ecosystem dynamics and verify whether treatment interventions are steering conditions toward improvement.
Bringing Science to the Surface
Real-time monitoring takes the guesswork out of algae management. By combining automated data capture, intuitive visualization, and tools like field calibration and threshold alerts, AquaRealTime delivers continuous water quality insights that turn scientific precision into practical decision-making.
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Learn how AquaRealTime can help you establish calibrated, real-time monitoring across your reservoirs, lakes, or ponds. Schedule a call today!