How to Choose a Turbidity Meter
ISO 7027 nephelometric, ratio & range
Turbidity (reported in NTU) is a proxy for suspended solids and a direct drinking-water and effluent compliance parameter in most standards. The measurement is deceptively subtle: the choice between ISO 7027 and EPA 180.1 optics, whether you use ratio (dual-angle) measurement to reject color, and how you set the range decide whether your number is defensible or just noisy.
1. What NTU measures
Turbidity is the scattering and absorption of light by suspended particles. It is an optical proxy for suspended solids, not a mass measurement — two waters with the same TSS can read different NTU if particle size or color differs. But because it is fast and standardized, NTU is written directly into permits.
2. ISO 7027 vs EPA 180.1
- ISO 7027. Monochromatic infrared at 860 nm, nephelometric (90° scatter). International standard; the 860 nm wavelength is less affected by sample color.
- EPA 180.1. White (broadband) light around 400 nm. Common in US methods. More color-sensitive.
Use whichever your standard names; ISO 7027 is preferred internationally and for colored waters.
3. Ratio / dual-angle measurement
A ratio turbidimeter compares the 90° scattered light against transmitted (or a second-angle) light. This mathematically rejects color and stray absorbance, so a brown effluent and a clear one of equal particle load read closer to the same NTU. For any colored industrial or final effluent, ratio is worth the small premium.
4. Form factors
- Benchtop. Lab reference, wide range (0–1000+ NTU), best accuracy.
- Portable. Field, IP67, battery, for surveys and outfalls.
- Online. Process probe with self-cleaning for continuous effluent monitoring.
5. Range & calibration
Pick the range to your water: low-range (down to 0.01 NTU) for drinking / ultrapure water, high-range for wastewater. Calibrate with formazin or AMCO EP standards, ideally multi-point across your working range. A single high-point calibration hides low-range error.
6. Matrix pitfalls
Air bubbles are the most common false-high source — de-bubble samples and avoid turbulent draws. Color (rejected by ratio), particles, and cell fouling also matter; keep optics clean.
7. Buying checklist & NTU vs TSS
Choose ISO 7027 (or your named method), ratio if color is present, the right range, and proper standards. Remember: NTU ≠ TSS. NTU is optical; TSS is a gravimetric mass. They correlate per site but the relationship is not universal — if your permit demands TSS, measure it gravimetrically, do not estimate from NTU.
Related buying guides
Frequently asked questions
ISO 7027 vs EPA 180.1?
ISO 7027 uses monochromatic 860 nm infrared nephelometric optics (international, less color-affected); EPA 180.1 uses broadband ~400 nm white light. Both are valid — use the one your standard names, and prefer ISO 7027 for colored waters.
Why ratio turbidity?
A ratio meter compares 90° scattered light to transmitted (or second-angle) light, rejecting sample color. For colored industrial or final effluent this prevents false-high readings from color rather than particles.
NTU or TSS?
NTU is an optical proxy; TSS is a gravimetric mass. They correlate per site but not universally. If your permit requires TSS, measure it by gravimetric method — do not estimate it from NTU.