ph-tester-dat
the output should go to ADC-dat
apps
display-dat and mcu-dat == stm8-dat
the control board
the sense board
What kind of op-amp IS required?
You need an electrometer-grade or FET-input op-amp.
Suitable op-amps for pH measurement
| Op-amp | Input bias current |
|---|---|
| LMP7721-dat | 3 fA |
| LMC6001-dat | 25 fA |
| TL072-dat / TL082-dat | ~65 pA (borderline) |
| OPA129-dat | 75 fA |
| AD8605-dat | 1 pA |
Rule of thumb:
Input bias current should be < 1/1000 of electrode current
Typical pH front-end architecture
pH electrode
│
│ (very high impedance)
▼
[ FET-input buffer ]
│
├─ Gain & offset
│
├─ Temperature compensation
│
▼
ADC / MCU
- First stage must be unity-gain buffer
- Guard rings + clean PCB required
- Shielded cable mandatory
Ibias
- Ibias ≤ 1 pA → good pH front end
- Ibias 1–10 pA → acceptable with care
- Ibias 10–50 pA → hobby / lab demo only
- Ibias >50 pA → no
probes
| Feature / Observation | White / Clear Probe | Grey / Duller Probe |
|---|---|---|
| Typical role | pH sensing electrode | Reference electrode |
| Sensitive to pH | Yes | No |
| Voltage behavior | Changes ~59 mV per pH (25 °C) | Nearly constant |
| Tip appearance | Clear to milky white glass | Grey / matte ceramic or dull glass |
| Internal element | Ag/AgCl + fixed pH solution | Ag/AgCl + KCl electrolyte |
| Purpose | Converts H⁺ activity to voltage | Provides stable reference voltage |
| Reaction to buffer change | Voltage shifts with pH | Minimal change |
| Use with ADC | Must be buffered (ultra-high Z) | Must be buffered (ultra-high Z) |
| What happens if swapped | pH reading flips sign | pH reading collapses or drifts |
| Can be used alone | No | No |
| Common mistake | Connected directly to ADC | Used as sensing electrode |
Electrode Function
Sensing electrode Detects H⁺ ion concentration (pH) in soil water or moisture. Usually made from special glass or ISFET.
Reference electrode Provides a stable, fixed voltage to compare against. Usually Ag/AgCl or a solid-state equivalent embedded in the probe.
ISFET = 离子敏感场效应晶体管
Simplified probes
1) Basic principle
pH measurement depends on the activity of H+ ions, which produces a potential difference. Traditional pH probes use:
- A glass membrane: selective to H+ ions
- A reference electrode: provides a stable reference potential
If you instead use metal rods or metal-coated probes, the measured potential difference will depend on the metals and the solution composition. In this case you are not measuring true pH but rather a relative ORP (oxidation-reduction potential) or a trend in acidity/alkalinity.
Limitations:
- Very low accuracy (±1–2 pH or worse)
- Can only indicate "acidic vs alkaline" trend, not precise pH
- Electrodes oxidize or become contaminated easily; short lifetime
2) Example metal combinations
| Anode (positive) | Cathode (negative) | Notes |
|---|---|---|
| Platinum (Pt) | Ag / AgCl coating | Closest to lab ORP electrodes; corrosion resistant |
| Copper (Cu) | Zinc (Zn) | Shows electrochemical reactions; trends observable |
| Nickel (Ni) | Copper (Cu) | Inexpensive but short-lived |
Key points:
- Use one electrode as a reference (Ag/AgCl preferred for stability)
- Use one electrode as the sensing probe (inserted into soil or water)
- Adding a small amount of saline improves conductivity
Color-based identification
| Electrode color | Common materials | Notes |
|---|---|---|
| White / silver | Silver (Ag), silver-plated copper, platinum (Pt), nickel (Ni) | Bright, corrosion-resistant; often used for reference or coated sensing surfaces |
| Gray / dark | Lead (Pb), titanium (Ti), stainless steel, zinc (Zn), galvanized metal | Darkens with oxidation; hard; often used for sensing probes or protective housings |
Parallel two-probe operation (side-by-side probes)
Typical setup:
- One white probe (likely Ag/AgCl or silver-plated)
- One gray probe (stainless steel, copper, or platinum)
- Insert both into soil or into the sample liquid
How it works:
- Soil/water contains moisture and dissolved ions (H+, OH-, Na+, Cl-, etc.).
- A small electrochemical potential forms between the two metal surfaces, like a tiny battery.
- The potential difference depends on metal types, conductivity, and acidity/alkalinity.
- Measure the voltage between the two probes with a high-impedance ADC or electrometer. The voltage trend roughly reflects soil acidity/alkalinity.
Important: this is not a true pH measurement because the system lacks a glass membrane or ISFET that selectively responds to H+ activity.
Characteristics of side-by-side placement
| Feature | Description |
|---|---|
| Potential difference | Local half-cell potentials form between each metal and the sample, creating a measurable voltage difference |
| Soil moisture sensitivity | High moisture → lower resistance → more stable signal. Low moisture → high drift and unstable readings |
| Probe spacing | Too close → electrochemical interference. Too far → weaker signal. Optimize spacing for your setup |
| Durability | Metals oxidize, especially the non-coated (darker) probe, reducing lifetime |