Foundations — the learning path

Summary

Everything else in this wiki assumes you can read a spec sheet and know what “1 ppm of H₂” physically means, what makes a metal-oxide sensor different from an electrochemical cell, and why a number from a soil probe and a number from a water well don’t compare directly. This section builds that footing from nothing. It is written to be read in order, with each page assuming the one before it, and by the end you should be able to pick up an unfamiliar H₂ instrument, read its datasheet, know which physical trick is inside it, convert its units to anything you need, and judge honestly whether it can do the job you have in mind. That is the jump from “smart non-specialist” to “beginner hardware/geochemist,” and these four pages plus two on the Start-here shelf are the full sequence.

Read these in order

1. Why hydrogen is hard to sense — the idea the field is organized around: H₂’s molecular symmetry leaves a sensor almost nothing to grab, so every technology is a workaround. Start here even before the units page; it tells you why the rest exists.

2. Concentration units: ppmv, ppmm, and everything in between — what “1 ppm of H₂” actually means, and how to convert between every unit an instrument might report: ppmv vs ppm by mass (H₂ reads ~14× smaller by mass, a common source of error), mg/m³, %vol vs %LEL, partial pressure (why the same ppm reads differently downhole), and dissolved units. The conversion cheat-sheet you will come back to constantly.

3. How the sensor families actually differ — what physically changes inside each family when H₂ arrives, and how to stop conflating them: metal-oxide (chemiresistor) vs work-function devices (MIS/MIM/Schottky, including reverse-biased palladium) vs Pd-resistive vs catalytic vs genuinely electrochemical vs thermal vs optical. Only one family is properly called “electrochemical,” and this page explains why.

4. Gas-phase vs aqueous-phase sensing — free soil/mud gas vs H₂ dissolved in groundwater: the Henry’s-law bridge that converts between them, and the geochemical reason the two numbers still tell different stories (gas = physics and migration; dissolved = microbial redox state). When to measure which, and how to act on each.

5. Figures of merit — the standardized axes for comparing any two sensors honestly: sensitivity, limit of detection (and its three incompatible definitions), selectivity, response time, drift, and the modality-specific ones. Every technology page’s Figures of merit section points back here.

6. Reading a sensor datasheet — putting it all together: a line-by-line walk through real electrochemical and metal-oxide H₂ datasheets, translating each spec into what it actually measured and what it means for a field program. This is the last page in the sequence.

Where to go next

Once these are under your belt, the sensor-selection guide and the comparison charts will read like plain English, and any individual technology page — from Raman to electrochemical cells — will slot into the map these foundations give you.