A sustained rise in the breath hydrogen concentration >5 ppm was considered to represent the arrival of the meal at the caecum.
Moreover, the highest hydrogen concentration measured was 0.18%; significantly lower than commercially viable hydrogen storage needs to be.
The sensor detects hydrogen concentrations down to 10 parts per million at room temperature, whereas there is no response to oxygen.
Given a fixed volume, gas has considerably lower hydrogen concentration.
An increase in hydrogen concentration in clay minerals has a similar effect on the count rate.
The minimum hydrogen concentration detectable was 3 ppm, which gave a recorder deflection of 2 mm.
Only the intimate association with the hydrogen-consuming Archaea keeps the hydrogen concentration low enough to allow the bacteria to grow.
Thus they would avoid interstellar debris: suns, dust clouds, hydrogen concentrations... It didn't particularly matter.
Argon-hydrogen gas compositions with hydrogen concentration below the low flammable limit can be used, eliminating the safety issues.
These ignitors burn-off the hydrogen concentration outside the orbiter, near the Shuttle main engines.