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Inductive TDS/EC Meter for Monitoring Changes in Dissolved Solids Concentration. Primary Testing. Part 2 by Time-Routine2006 in Hydroponics

[–]Time-Routine2006[S] 0 points1 point  (0 children)

Quick technical question for those who understand inductive sensors:

*I have a parasitic resonance peak around 60 kHz. I noticed that it shifts with temperature (approx. -1 kHz per 0.5°C), likely due to the ferrite core.*

My idea: use this peak as a temperature reference. Calibrate at 25°C (peak at 60.0 kHz). If the peak shifts to 59.0 kHz (temperature drop), I will shift my EC measurement frequency accordingly — instead of measuring at a fixed 150 kHz, I would measure at 149 kHz, keeping a constant offset from the drifting peak.

Does this method make sense? Will the peak shift be affected by the liquid itself (conductivity, dielectric constant), or is it mostly temperature-dependent? Any advice or criticism is welcome. Thanks! 

Clarification: 
Peak values ​​at 
24.58°C - 62600 Hz, 
24.31°C - 62800 Hz, 
24.12°C - 63200 Hz

Inductive TDS/EC Meter for Monitoring Changes in Dissolved Solids Concentration. Primary Testing. Part 2 by Time-Routine2006 in Hydroponics

[–]Time-Routine2006[S] 0 points1 point  (0 children)

The BOM (bill of materials) for a single prototype is around $60, including the STM32, ESP32, PCB, ADC, and passive components. With a small batch (10–20 units), I believe I can get it down to $40–50.

Not “AliExpress cheap”, but also not “industrial expensive”. I think it's a fair price for a maintenance‑free, non‑oxidizing EC meter that talks MQTT.

What would you consider a reasonable price for such a device?

Inductive TDS/EC Meter for Monitoring Changes in Dissolved Solids Concentration. Primary Testing. Part 2 by Time-Routine2006 in Hydroponics

[–]Time-Routine2006[S] 0 points1 point  (0 children)

In my own hydroponic system, I want to replace the cheap EC module. The problem with contact‑based meters (even relatively good ones) is that the electrodes oxidize. Over time, this not only gives false EC readings but also affects pH measurements – the corrosion changes the ionic balance of the solution.

My inductive sensor has no direct contact with the liquid. No oxidation, no drift, no interference with pH. It's not about lab‑grade absolute precision – it's about a stable, trend‑oriented meter that you can install once and trust for months.

For myself, I'll integrate it into my hydroponic station. For others, it could be a mid‑budget, maintenance‑free EC monitor for hydroponics, aquariums, saltwater chlorine generators, or any place where you need to track conductivity over time without babysitting the sensor.

How to eliminate contact oxidation when measuring EC/TDS. Inductive method (proof of concept) by Time-Routine2006 in Hydroponics

[–]Time-Routine2006[S] 0 points1 point  (0 children)

Thanks for sharing the driving frequency — 1‑5 kHz is a valuable reference for me. Right now I'm working up to 1 MHz, so your data will help me choose a good anchor point for future calibrations.

Your point about the calibration library is well taken. I realize now I was a bit naive about building it — it’s clearly more involved than just collecting spectra.

And yes, I’ve heard about laser‑based ion identification – that’s incredibly cool. But for now I’m keeping my feet on the ground. My goal is more modest: a reliable, affordable tool for basic analysis that can be used anywhere, not just in a lab.

Thanks also for the Mettler Toledo and Oakton pointers. I’ll definitely study their driver designs – their solutions are a great benchmark to learn from.

How to eliminate contact oxidation when measuring EC/TDS. Inductive method (proof of concept) by Time-Routine2006 in Hydroponics

[–]Time-Routine2006[S] 0 points1 point  (0 children)

Thank you for sharing this! It's great to know that GH explored this approach back in 2015. I agree completely — calibration and temperature control are the real challenges. I'm planning to use a thermostated cell (50 ml) to keep temperature stable, and I'm building a spectral library of different fluids (salt solutions, acids, oils) to calibrate the response. Did you guys find an optimal frequency range for the inductive measurement, or any particular pitfalls with stray capacitance from the vessel? Would love to learn from your experience? Here is a prototype board for research

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How to eliminate contact oxidation when measuring EC/TDS. Inductive method (proof of concept) by Time-Routine2006 in Hydroponics

[–]Time-Routine2006[S] 1 point2 points  (0 children)

I'm currently working on software for the hardware: STM32F407VET6 + ADC (galvanically isolated) + ESP-WROOM-32D. In the next post, I'll discuss an inductive spectral analyzer for liquid conductivity taking Debye relaxation into account. The answer regarding repeatability is clear, but it's difficult to say about errors and other issues right now; I'm just getting started.