CO₂ injected through an inline diffuser and held against a morning degassed baseline, with hardware safety limits.
Published
Purpose
To document how CO₂ is controlled on a tank kept at or near zero carbonate hardness, where the usual pH/KH reference table does not apply.
Background
The common way to infer dissolved CO₂ is a pH/KH table. On active aquasoil run without a buffer, carbonate hardness sits at or near zero, which makes that table unusable, because a tiny KH error produces an enormous CO₂ error. What remains repeatable is the change in pH between degassed water and water under injection. This project therefore uses pH drop as an operational control signal rather than an absolute dissolved CO₂ measurement, and publishes no estimated ppm figure.
Methodology
01Capture the degassed pH each morning, before injection starts, as that day's baseline.
02Hold a target drop below that baseline through the photoperiod rather than aiming at an absolute pH.
03Drive the solenoid through a virtual output so the relay is not switched repeatedly.
04Keep the hard limits on the controller itself: a pH floor, a permitted time window, and a leak sensor.
05Log pH continuously so the day's curve can be reviewed rather than sampled.
Observations
·Holding the drop rather than an absolute pH has been stable across water changes that shift the baseline.
·Keeping the safety limits on the controller rather than in software means the tank survives a restart or a broken script.
Scope
·Why the pH drop is used as a control signal rather than a pH/KH table at near-zero KH
·How the degassed baseline is captured each morning
·Which limits live on the controller rather than in software