How turbidity is measured optically as cloudiness from suspended solids, and what it means for a treatment program.
Turbidity is the cloudiness of water caused by fine particles — silt, clay, corrosion products, precipitated scale, and microbial growth. The particles scatter light, and a turbidimeter measures that scattered light.
- Nephelometric measurement. A light beam passes through the water, and a detector set at 90° to the beam measures the light scattered sideways by particles. More particles, more scattered light.
- Units. The result is reported in nephelometric turbidity units (NTU). Instruments using an infrared (860 nm) source to ISO 7027 report in FNU (formazin nephelometric units); for most field purposes NTU and FNU read similarly, but they are not strictly the same.
- Light source. US EPA Method 180.1 uses a white (tungsten) lamp; ISO 7027 uses an infrared LED. Infrared is less affected by the color of the water. Readings from the two methods can differ on the same sample.
- Ratio designs. Some instruments add detectors at other angles and combine the signals, which reduces the effect of color and lamp ageing and extends the range.
¶ What the reading does and does not represent
- Turbidity is an optical property, not a weight of solids. It correlates with total suspended solids (TSS, mg/L) in a given water, but the relationship changes with particle size, shape, and color. Do not convert NTU to mg/L without a site-specific correlation.
- In a treatment program, a rising turbidity in cooling water can point to dirt load from the air, filter problems, scale precipitating, corrosion products, or biological growth. It is a trend signal: look for changes rather than a single number.
- Air bubbles scatter light just like particles and give falsely high, spiky readings. This is the most common problem with inline turbidity.
- Fouling of the optical windows by biofilm, scale, or oil. It usually makes the reading drift up, though some designs drift down.
- Color in the water absorbs light and makes the reading low — mainly on white-light instruments.
- Condensation on the outside of cold sample cells or windows scatters light.
- Stray light entering the measuring chamber, from sunlight or a clear flow cell.
- Settling. In low or stopped flow, heavy particles settle out of the measuring zone and the reading falls, then spikes when flow restarts and stirs them up.
- Scratched cells or windows scatter light permanently.
- Remove air first. Use a flow cell or bubble trap as the manufacturer supplies, and avoid sampling from the top of a pipe or right after a pump or valve that draws in air.
- Steady, low-velocity flow in the manufacturer’s range: fast enough to keep particles suspended, slow enough to let bubbles rise out.
- Keep light out. Mount the flow cell out of direct sunlight, with its cover fitted.
- Take the sample from the side of the pipe, not the top (air) or the bottom (sediment).
- Short sample lines. Long lines let particles settle or grow deposits, which delays and changes the reading.
¶ Maintenance
- Clean the optics with a soft cloth or swab and mild detergent. Remove scale with a mild acid. Never use abrasive on the windows or sample cells.
- Calibrate with primary or secondary standards. Formazin is the primary standard. Stabilized formazin or polymer-bead standards are more convenient and last longer. Many instruments also use a sealed secondary (check) standard for quick verification between calibrations.
- Zero with particle-free water. Filtered deionized water is the usual low reference.
- Mix standards gently. Invert formazin standards to resuspend them; do not shake, which adds bubbles.
- Check against a lab or bench turbidimeter from time to time, remembering that different methods and light sources can disagree.
- Service life. Light sources age; LED sources commonly last years, tungsten lamps less.
Formazin is made from hydrazine sulphate and is treated as a suspected carcinogen. Handle formazin stock with gloves and eye protection and follow Chemical Handling.
Models documented in this wiki:
- ISO 7027-1, Water quality — Determination of turbidity — Part 1: Quantitative methods
- US EPA Method 180.1, Determination of Turbidity by Nephelometry