OPEN SOURCE pH METER — v0.1
Metrological-grade pH meter, fully open source. 100% Analog Devices architecture, sub-0.001 pH resolution, buildable for ~350 EUR. No more 3000 EUR black boxes.
Schematic, Gerbers, BOM, firmware and docs published. Fork, modify, fabricate.
Full ADI architecture: ADA4530-1 + AD7177-2 + LTC6655. Resolution < 0.001 pH.
Standard 4-layer PCB, assemblable in lab or via JLCPCB / PCBWay. Minimal BOM.
The chemistry behind hydrogen ion concentration and why measuring it precisely matters.
pH stands for "potential of hydrogen". It is a logarithmic scale that quantifies the acidity or basicity of an aqueous solution. Defined by Søren Sørensen in 1909, it measures the activity of hydrogen ions (H+) in solution.
WHERE [H+] IS THE HYDROGEN ION CONCENTRATION IN MOL/L
Because the scale is logarithmic, each unit represents a 10-fold change in H+ concentration. A solution at pH 4 has 10x more H+ ions than pH 5, and 100x more than pH 6. This enormous dynamic range is why precision instrumentation is critical.
In pure water at 25°C, [H+] = [OH-] = 10⁻⁷ mol/L, giving pH = 7.0 (neutral). Adding an acid increases [H+] (lower pH), while adding a base decreases it (higher pH).
A 0.1 pH error at pH 5 corresponds to a 26% error in H+ concentration. At pH 2, it's a 26% error in a solution that's 100,000x more acidic than water. Precision matters at every point on the scale.
DRAG THE SLIDER OR CLICK A SOLUTION TO SEE THE ELECTRODE RESPONSE
A glass membrane that generates voltage proportional to hydrogen ion activity.
A pH glass electrode is a potentiometric sensor. Unlike resistometric sensors (like load cells), it generates its own voltage — no excitation needed. The voltage depends on the difference in H+ activity between the two sides of a special glass membrane.
The glass membrane (typically lithium-silicate glass, ~0.1 mm thick) has a unique property: when immersed in a solution, a hydrated gel layer forms on the surface. H+ ions from the solution exchange with alkali ions (Li+, Na+) in the gel, creating a charge separation — and therefore a measurable potential.
Inside the electrode, a reference solution of known pH (usually pH 7.0 buffer with KCl) bathes the inner side of the glass. The voltage across the membrane depends on the pH difference between the inner reference and the outer (unknown) solution.
Click the button to immerse the electrode in a pH 4.0 buffer solution and watch the voltage develop across the glass membrane.
Why temperature compensation is essential for accurate pH measurement.
The voltage generated by a pH electrode follows the Nernst equation. The slope of the response (mV per pH unit) depends directly on the absolute temperature. At 25°C, the ideal slope is 59.16 mV/pH. At 0°C it drops to 54.20 mV/pH, and at 80°C it rises to 70.07 mV/pH.
Without temperature compensation, a reading at 50°C would show a systematic error proportional to the temperature deviation. This is not about the solution's pH changing with temperature (it does, but that's a chemical reality), but about the electrode's sensitivity changing.
pHenix solves this with a NTC 30kΩ thermistor integrated into the InLab Expert Pro probe. The MAX32672 MCU reads the temperature via a second ADC channel and applies the Nernst correction in firmware, ensuring accurate readings from 0°C to 80°C.
R = GAS CONSTANT | T = TEMPERATURE (K) | F = FARADAY CONSTANT
The slope changes with temperature: 0.0% deviation from the ideal 25°C value. This is why pHenix includes a NTC 30kΩ temperature sensor for automatic compensation.
From glass electrode to digital value — every component chosen for a reason.
The pH electrode has an output impedance of 100-1000 MΩ. Drawing even picoamps of current would shift the voltage. The ADA4530-1 has an input bias of < 20 fA — 5000x less than a typical op-amp.
With a 59 mV/pH slope and sub-0.001 pH target, we need to resolve ~59 µV. A 32-bit sigma-delta ADC with >24 noise-free bits gives us 0.15 µV resolution — 400x better than required.
The ADC converts voltage relative to its reference. A noisy reference = noisy measurement. The LTC6655 has 0.25 ppm noise and < 2 ppm/°C drift — the ruler must be more precise than what it measures.
ARM Cortex-M4F with FPU for Nernst equation and temperature compensation calculations. SPI at 50 MHz for ADC communication. UART output for datalogging. 1 MB flash for calibration data.
Ultra-low noise (0.8 µVRMS) linear regulator for the analog signal chain. Power supply noise directly corrupts the measurement — this LDO is quieter than the signal we're measuring.
Separate supply for the MCU and digital logic. Isolating analog and digital power domains prevents switching noise from the processor from contaminating the measurement path.
The #1 design challenge in pH instrumentation: measuring voltage without drawing current.
A pH electrode is not like a battery — it has an internal resistance of 100 to 1000 MΩ. If the measurement circuit draws even 1 nA (one billionth of an amp) through a 500 MΩ electrode, it creates a voltage drop of:
= 8.5 pH UNITS OF ERROR!
That's why the ADA4530-1 is critical. With an input bias current of 20 fA (0.00002 nA), the error drops to:
= 0.0002 pH — NEGLIGIBLE
But achieving fA-level performance isn't just about the chip — the PCB itself leaks current across its surface (humidity, flux residue, contamination). This is where the guard ring technique becomes essential.
The ADA4530-1 has an integrated guard ring. By driving the guard at the same potential as the signal, there is zero voltage difference across the PCB surface — so no leakage current can flow. Input bias stays at 20 fA.
The wort pH during mashing must be between 5.2 and 5.6 for optimal enzymatic conversion. A 0.1 pH deviation alters the beer flavor profile. pHenix enables real-time monitoring with datalogging.
Litopenaeus vannamei shrimp require pH between 7.5 and 8.5. Below 7.0, mortality rate increases dramatically. UART output for automated 24/7 monitoring.
Experiments in phosphate buffer (PBS) requiring stable pH at 7.40 +/- 0.02. Auto temperature compensation and sub-mV resolution eliminate systematic uncertainties.
In NFT (Nutrient Film Technique) systems, pH must stay between 5.5 and 6.5 for N, P, K, Fe absorption. pHenix coupled with a dosing pump (GPIO MAX32672) automates correction.
Creams and injectable formulations must respect precise pH (5.5 skin, 7.4 IV). ISO 10523 norm: uncertainty < 0.05 pH. The ADA4530-1 + AD7177-2 chain exceeds this requirement.
The EU Water Framework Directive (WFD) mandates pH monitoring of water bodies. pHenix on battery with datalogging enables autonomous field campaigns. CSV export for regulatory reports.
Ag/AgCl, BNC, XEROLYT, NTC 30kΩ
ALL INCLUSIVE, PROBE INCLUDED
VS COMMERCIAL INSTRUMENTS
2,000 — 5,000 EUR
METTLER TOLEDO, METROHM, HANNA