Sonde vs CTD, what's the difference?

CTD stands for Conductivity, Temperature, Depth, the three measurements at the core of classical oceanographic water-column profiling. A CTD is built around those three sensors, often extended with add-on channels for dissolved oxygen, pH, turbidity, chlorophyll or current.

Sonde is the broader industry term for a submersible multiparameter probe. In practice, most commercial sondes (YSI EXO, In-Situ Aqua TROLL, and similar) are CTD-based instruments with optional add-on sensors, so "sonde" and "CTD-based multiparameter instrument" are often used interchangeably in marketing copy. But the terms are not strictly identical: a sonde is any integrated multiparameter probe, and not every sonde-class instrument includes a conductivity channel.

Why this distinction matters when choosing an instrument

If a project needs salinity or precise water-chemistry profiling (conductivity, dissolved oxygen, pH), a CTD-based sonde is the right tool, that is specifically what those instruments are built to measure.

If a project's real questions are about the physical environment, how much light reaches a crop, how warm the water is, how much sediment is suspended, how the current moves, a full CTD-based sonde brings capability (and cost) that goes unused.

Where GrowProbe fits

PEBL's GrowProbe is a sonde-class logger in the general sense (an integrated multiparameter probe), but it is not a CTD: it has no conductivity/salinity channel. It measures turbidity, temperature, light, current speed and current direction, the parameters most seaweed, shellfish and restoration monitoring projects track day to day, at a fraction of the cost of a configured CTD-based sonde. See the specification and the pricing comparison for how it stacks up against full sondes from YSI, In-Situ and similar makers.

Need help?

Get in touch at hello@pebl-cic.co.uk.