Example Data Table
Illustrative calibration using a linear fit: C = 2.10·T + 0.15. Values are examples only.
| Turbidity (NTU) | Blank (NTU) | Dilution | Corrected T (NTU) | Concentration (mg/L) |
|---|---|---|---|---|
| 5.0 | 0.2 | 1 | 4.8 | 10.23 |
| 25.0 | 0.2 | 1 | 24.8 | 52.23 |
| 80.0 | 0.5 | 5 | 397.5 | 834.90 |
Formula Used
Turbidity is an optical scattering signal and concentration depends on particle size and composition. This calculator converts NTU to concentration using a user-supplied calibration curve.
T = (T_raw − T_blank) · Dfactor = 1 + k · (T_sample − T_ref)C_ref = C_model(T) / factor
- Linear:
C = a·T + b - Quadratic:
C = a·T² + b·T + c - Power:
C = a·T^b - Exponential:
C = a·e^(b·T) + c - Logarithmic:
C = a·ln(T) + b
Units: NTU is unitless by definition; concentration output is labeled mg/L by convention. Use coefficients fitted to your own standards to keep units consistent.
How to Use
- Measure sample turbidity and, if available, a blank turbidity.
- If you diluted the sample, enter the dilution factor.
- Select the calibration model that matches your fit.
- Enter coefficients from your standard-curve regression output.
- Optional: add a temperature coefficient to normalize readings.
- Optional: enter uncertainties to estimate concentration uncertainty.
- Press Calculate to show results and export options.
This article explains how turbidity-to-concentration conversion is built, validated, and reported in practical laboratory and field workflows. It also highlights common pitfalls, recommended checks, and documentation that make exported results audit-ready.
1) What turbidity represents
Turbidity (NTU) is a nephelometric measure of light scattered by suspended particles. It responds to particle size, refractive index, color, and detector geometry, so identical mass concentrations can produce different NTU values. That is why a local calibration is essential for defensible concentration estimates.
2) Why concentration needs calibration
If you prepare standards at known concentrations (for example 0, 10, 25, 50, 100 mg/L) and record their NTU, you obtain a mapping between optics and mass. The best-fit curve becomes your conversion model. Without standards, NTU can still rank samples, but not quantify mass reliably.
3) Choosing a model shape
Many systems are linear at low turbidity, then bend as multiple scattering increases. A quadratic fit can capture curvature, while power laws often describe broad dynamic ranges. Exponential fits may match sensor electronics rather than physics. Select the simplest model that keeps residuals small across your operating range.
4) Blank and dilution handling
Blank subtraction removes baseline scatter from clean water, reagents, cuvettes, or biofilm. Dilution restores the original sample scale: a 10× dilution multiplies corrected NTU by 10 before conversion. In high-turbidity samples, dilution is often the cleanest way to stay within an instrument’s linear response window.
5) Temperature and optical sensitivity
Temperature can shift readings through viscosity changes, bubble formation, and small optical drift. If your calibration was built near a reference temperature, a small fractional coefficient k (per °C) can normalize results. Keep k conservative and laboratory-validated; otherwise, temperature correction can add more error than it removes.
6) Uncertainty and reporting
This calculator uses first-order uncertainty propagation to estimate concentration uncertainty from turbidity repeatability and coefficient fit errors. Report results as C ± σ, and keep the model type and coefficients with the data. When uncertainty is large or models are highly nonlinear, Monte Carlo sampling is a stronger validation approach.
7) Quality checks that improve trust
Run a mid-range check standard each day and watch for drift. Keep replicate readings (three is common) and reject obvious bubble spikes. Track the coefficient set version, instrument serial, and cuvette type. If you change particle chemistry or size distribution, rebuild the calibration because NTU-to-mass mapping will shift.
8) Practical ranges and interpretation
For many natural waters, turbidity below about 5 NTU indicates low suspended solids, 5–50 NTU suggests moderate loading, and values above 50 NTU often reflect storm runoff or disturbed sediments. Convert to concentration for mass-based comparisons, but always interpret results alongside particle characteristics and site conditions.
1) Can one NTU-to-mg/L conversion work for every water source?
No. Particle size, color, and composition change scattering efficiency. Always calibrate with standards prepared from the same material class as your samples to avoid large bias.
2) What model should I choose for my calibration curve?
Start with linear for low-range work. If residuals grow at higher turbidity, try quadratic or power. Choose the simplest model that keeps residuals small across your full measurement range.
3) Why does the calculator ask for a blank turbidity?
A blank removes baseline scatter from clean water, cuvettes, reagents, and instrument drift. Subtracting it improves accuracy, especially for low-turbidity samples near the detection limit.
4) When should I use dilution factor?
Use dilution when the raw sample exceeds the instrument’s reliable range or shows nonlinearity. Enter the total dilution ratio so the corrected turbidity reflects the undiluted sample value.
5) What does the temperature coefficient k do?
k applies a small fractional correction per degree to normalize readings to a reference temperature. Use k only if you validated temperature sensitivity for your instrument and sample type.
6) Why is uncertainty sometimes not computed?
If required inputs are missing, or the model is invalid for the corrected turbidity (for example log or power with T ≤ 0), the uncertainty calculation is not meaningful and is skipped.
7) Are the output units always mg/L?
The label is mg/L by convention, but units depend on how you built the calibration. If your standards were in g/L or ppm, your coefficients will produce those units instead.