| Deciding factor | 90-degree scatter | Transmission |
|---|---|---|
| Typical range | 0.01 – 400 NTU | 1 – 4000 NTU |
| Resolution at the clean end | down to 0.01 NTU | poor below ~1 NTU |
| Linearity at the turbid end | rolls off above ~400 NTU | usable up to ~4000 NTU |
| Coloured sample behaviour | partial absorbance cross-talk | absorbed light reads as turbidity |
| Optical geometry | receiver at 90° to emitter | emitter and receiver in line |
| Dependence on path length | independent of cell depth | Beer-Lambert, depth dependent |
Can one turbidity instrument cover both principles?
Some transmitters house a scatter receiver and an in-line detector together and cross over between the two channels as concentration rises, trading one optical weakness for the other instead of asking the user to know the sample beforehand.
Does sample colour affect both optical methods?
Pigment that absorbs light removes energy from any path, so colour biases the transmission channel directly and also reaches the scatter receiver as reduced side signal; the two channels need a sample-specific calibration when colour is expected.
Decision rule
Nephelometric scatter decides while the sample sits at the low end, because a receiver set at 90 degrees counts particles against a dark field and keeps sensitivity where a clear beam barely dims; switch to transmission once the concentration is high enough that scatter multiplies and folds back on itself, because attenuation across a fixed path still yields a usable signal exactly where the scatter channel saturates.
All figures on this page are illustrative sample values, not measured specifications.