NTU vs FNU: Turbidity Units Compared (EPA 180.1 vs ISO 7027)

NTU and FNU both describe the same physical effect — light scattered by suspended particles — and both trace back to the same formazin standard. Yet the same water sample can legitimately show different numbers on the two scales, and for compliance reporting, mixing them up is a real problem. This page explains NTU vs FNU without the folklore: where each unit comes from, when the readings diverge, and which one your application should use.

Two standards, two acronyms

The physics is identical: a detector at 90° measures how much light the particles scatter, referenced to formazin polymer suspensions. What differs is the light source written into the standard — a broadband tungsten lamp for EPA 180.1, a monochromatic 860 nm infrared LED for ISO 7027.

Are NTU and FNU interchangeable?

Because both scales are anchored to the same formazin reference, 1 NTU and 1 FNU are often treated as roughly equal — and for clean, colorless water with similar particle sizes, readings usually land close together. But they are different measurement methods, not a unit conversion. The differences grow exactly where it matters:

• Colored water. Dissolved color absorbs light. A broadband tungsten source is affected across its whole spectrum; an 860 nm infrared source is largely outside the visible absorption band, so ISO-type instruments read less biased in colored samples.
• Particle size distribution. Scattering intensity depends on the ratio of particle size to wavelength. A fine clay suspension and a coarse floc can swap rankings between the two methods.
• Very low ranges. In drinking-water-grade measurements (below a few NTU), instrument design dominates and cross-method agreement is at its weakest.

Which one should your sensor use?

Follow your market and your medium, in that order. If your customer base and permits live under EPA rules, specify an EPA 180.1 instrument. If you are in an ISO market — or buying online process sensors — you will mostly encounter ISO 7027 / FNU designs, because the 860 nm infrared source tolerates sample color better and suits continuous industrial duty. This is why most multi-parameter optical probes specify their turbidity channel to ISO 7027 even when the display can show either unit.

Whatever you choose, stay consistent within one data set. A trend built on EPA 180.1 readings cannot be continued with ISO 7027 values without a documented cross-over comparison on your own samples.

A note on turbidity versus TSS

Turbidity is an optical scattering measurement; total suspended solids (TSS) is a mass concentration from filtration. There is no universal conversion — the relationship is site-specific and depends on the particles themselves. If you need mg/L estimates from an optical sensor, build a local correlation with paired grab samples, and re-validate it when the water character changes.

Frequently asked questions

Is 1 NTU equal to 1 FNU?

Numerically they both trace to formazin, so the values are usually close. But they come from different measurement standards with different light sources, and readings can diverge for colored or unusual particle populations. Treat them as related methods, not as the same unit.

Why does my online infrared sensor disagree with the lab’s bench meter?

Most likely a method difference: many bench meters follow EPA 180.1 with a white light source, while online process sensors are commonly built to ISO 7027 with 860 nm infrared. Check the method each instrument declares before comparing numbers.

Can sample color affect turbidity readings?

Yes. Absorption by dissolved color biases white-light (EPA-type) instruments more strongly. The 860 nm infrared source used in ISO 7027 instruments largely avoids the visible absorption band, which is one reason it dominates online monitoring.

How do I convert turbidity to TSS?

There is no universal factor. Develop a correlation on your own water with paired laboratory TSS results, and refresh it whenever the suspended solids character changes.