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NowYouSea
Field validation - Lake Quily - June 2025

Our first tests in real conditions.

First deployment of a NowYouSea CTD in freshwater: sound speed profile measurement over 46 meters depth, with cross-validation against a professional probe.

Key figures

What the field confirmed.

< 0.35 %
Speed error
vs theoretical reference
63x
Higher resolution
vs standard CTD probe
~ 200 €
Component cost
vs 5,000+ EUR commercially
0.02 m vs 0.5 m
Vertical resolution
25x finer than commercial probe
Our approach

Measure precisely, cost little.

The speed of sound in water depends on temperature, salinity and pressure. Commercial probes take only a few measurements per descent. The NowYouSea CTD is a demonstrator: it proves that a low-cost measurement can approach the performance of very expensive professional instruments.

Performance vs commercial
~ 200 €
NowYouSea CTD
vs
5 000 €+
commercial probe
Sound speed error
< 0.35 %
measured field Quily 2025
Methodology

How we conducted the test.

1
Site: Lake Quily
June 24, 2025, freshwater, max depth 46 m. Site chosen for its accessibility and the stability of its water column.
2
Instrumented descent
Synchronized T/P sensors, slow and controlled descent. Continuous high-frequency sampling throughout the profile.
3
Sound speed calculation
Application of the Chen-Millero equation adapted for freshwater. Point-by-point calculation along the profile.
4
Cross-validation
Comparison with theoretical reference value and Valeport SVP professional probe at common depths.
5
Saltwater extension
The system was also tested at sea, at 100 m depth off Six-Fours (Mediterranean), confirming its mechanical integrity in saltwater.
Profil de célérité du son (0-46 m)
0 5 10 15 20 25 30 35 40 46 Depth (m) 1430 1440 1450 1460 1470 1480 1490 1500 Sound speed (m/s) thermocline +0.35 % +0.16 % -0.10 % NowYouSea CTD Reference probe Sound speed profile - Lake Quily, June 24, 2025
Thermocline: zone of rapid temperature transition between warm surface waters and cold deep waters. It significantly alters acoustic propagation.
Case study - Lake Quily - June 24, 2025
Site
Location
Lake Quily, Brittany
Date
June 24, 2025
Water type
Freshwater
Max depth
46 m
Measurements
Data points
~2 900
Resolution
63 pts/m avg.
Speed error
< 0.35 %
Surface T
~19 °C
Validation
Theoretical ref.
Chen-Millero
Pro probe
Valeport SVP
Result
Validated
Valeport cross-validation in progress. Raw data available on request.
Comparison table

NowYouSea vs alternatives.

Criterion Commercial CTD probe Scientific AUV NowYouSea CTD
Equipment cost 5 000 - 15 000 € 50 000 - 500 000 € ~ 200 €
Profile resolution ~1 pt/m 5-20 pts/m 63+ pts/m (validé)
Deployment Requires boat + trained operator Team + supervision One diver or a line
Open-source Non Non Yes - data and code
Freshwater Some models Depends on config Validated (Quily 2025)
Speed error < 0.5 % (declared) < 0.3 % (high-end) < 0.35 % (measured)
IoT integration No / CSV export Proprietary Natif MQTT / API
Valeport note: cross-validation with additional professional SVP probe. Data consolidation in progress.
Economic model

Accessible at scale.

NowYouSea CTD
~ 200 €
Sound speed error
< 0.35 % measured
Commercial probe
5 000 €+
Sound speed error
< 0.5 % declared

The cost breakthrough we bring is not accidental, it is structural. It unlocks use cases that were previously impossible to budget for.

A network of 10 NowYouSea stations costs less than a single high-end commercial CTD probe. This is not the same market: it is a new market.

Comparison
5 000 €+ commercial probe
~ 200 € NowYouSea CTD
That is 25x cheaper at equivalent performance on this test.
Price excludes labor and infrastructure. Target: native integration into the NowYouSea connected rock for zero additional sensor cost.
Join the project

Interested in this data?

Whether you are a researcher, diver, consulting firm or simply curious: let's talk. Raw Quily data is available on request.

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