Dissolved Oxygen (hydro-loop)

Definition

Dissolved Oxygen (DO) is the mass of molecular oxygen dissolved per litre of recirculating nutrient solution, in milligrams per litre. It is sensed directly by optical (luminescent-quench) DO probes on the PhytoSense Mesh, inline in the hydro return and at the reservoir, each temperature- and pressure-compensated. Because oxygen solubility falls sharply as water warms, DO is always read alongside root-zone temperature (VC-MET-109) and reported with its percent-saturation equivalent.

Why it matters

Roots respire, and in water their only oxygen is what is dissolved around them. When DO falls, root respiration and active nutrient uptake stall, root tips die back, and the anaerobic root zone becomes a breeding ground for Pythium — the single most common collapse mode in hydro culture. DO is therefore a leading safety signal for the hydro-loop crops: CHLORA drives aeration and oxygenation against it, and a falling DO with a rising root-zone temperature is treated as an imminent root-rot condition, not a slow drift.

Formula

A directly measured signal. Reported DO is the probe reading after temperature, salinity, and pressure compensation, with saturation reported alongside:

DO = DO_raw · k_T · k_S · k_P

  DO_raw  optical-probe reading (mg·L⁻¹)
  k_T     temperature compensation (T from VC-MET-109)
  k_S     salinity correction (from EC, VC-MET-107)
  k_P     pressure correction (from VC-MET-112)

Saturation reference (reported alongside):
  DO_sat(T) ≈ solubility at T, S, P
  %sat = 100 · DO / DO_sat(T)

Coupling alarm:
  dDO/dt < −0.5 mg·L⁻¹ per 15 min while Trz rising ⇒
  pre-anoxia precursor advisory.

Inputs

Units & Scale

Reported in milligrams per litre to one decimal place, with percent-saturation alongside. DO is intensive and not summed; the operational figure is the return-line probe, with the reservoir probe as a cross-check. Confidence degrades to DEGRADED when return and reservoir DO diverge by > 1.5 mg·L⁻¹ (dead leg or a fouled optical cap) or when the probe's luminescent cap is past its service life and reads sluggishly.

Sampling & Source

• Sampled every 5 minutes from the PhytoSense Mesh optical DO probes inline and at the reservoir.
• Temperature-, salinity-, and pressure-compensated; saturation reported with concentration.
• Stale handling: a probe silent > 15 min is dropped; loss of the return probe fails over to the reservoir probe with a DEGRADED flag.
• Calibration: monthly single-point air-saturation check; optical cap replaced on schedule or when response lags.

Thresholds

Recent Trend

Hydro-loop return-line DO, last 14 sampling roll-ups (mg·L⁻¹):

Interpretation Guidance

Related Metrics

Related SOPs

• SOP Library — root aeration & hydro-loop oxygenation procedures.
• Anoxia & Pythium response on DO CRIT — see Environment SOP set in the SOP Library.
• Aeration/oxygenation interlock & alarm escalation — Monitoring Systems.