CO₂ ppm for grow tents
and hydroponic rooms

Grow tents and small sealed hydroponic rooms have the most extreme CO₂ dynamics of any growing environment, because the ratio of plant biomass to air volume is very high and the enclosure is very tight. Things happen fast, in both directions.

Lights on: the room runs out

Put a dense canopy under strong light in a sealed tent and watch the CO₂. Starting at ambient — 415 to 450 ppm — it falls immediately, because the plants are fixing carbon faster than the enclosure can leak replacement in. Within an hour a well-sealed tent can be at 250 to 300 ppm.

At that point the grow is CO₂-limited. The lights are running at full power, the nutrient solution is perfect, the temperature is correct — and none of it can be converted, because the carbon input has run out. Every additional watt of light is wasted from that moment until fresh air arrives.

This is the single most common unrecognised bottleneck in small sealed grows, and it is invisible: the plants look fine, they are simply growing more slowly than they could.

The band

The important caveat: these bands assume light is not limiting. Under a modest LED with a canopy that is already light-saturated at ambient CO₂, enrichment will do very little. This is Liebig's law again — the scarcest resource sets the rate. Money spent on CO₂ in an under-lit tent is money that should have gone on light.

Lights off: it reverses

When the lights go out, photosynthesis stops but respiration does not. The plants now release CO₂ rather than consuming it, and in a sealed tent it accumulates — sometimes to several thousand ppm by the end of a long dark period, particularly with a heavy canopy in a small volume.

There is no benefit to that. The plants are not using it, and the room is now a space that is unsafe to walk into without ventilating. So the requirement inverts: during dark hours the room needs venting, not enriching.

This is the reason a good grow-room controller needs two thresholds and two outputs:

• Below the lower threshold → switch the CO₂ source on (regulator solenoid, generator).
• Above the upper threshold → switch the exhaust fan on.

With both in place the room self-manages across the full 24 hours without a schedule, which matters because photoperiods change, canopies grow, and a schedule set in week two is wrong by week six.

Why holding the band matters more than the average

This is where cheap enrichment setups fail. A tank on a timer, or a generator on a fixed cycle, produces a sawtooth: the level shoots up, then decays as the plants consume it and the tent leaks, then shoots up again. The average might be 1,200 ppm. The plants experience 2,200 ppm for twenty minutes and 500 ppm for the rest of the hour.

Photosynthetic rate responds to the concentration present at the time. Time spent below ambient is time the crop is limited, and it is not compensated for by the peaks — the response curve flattens at the top, so the excess above 1,500 ppm buys you almost nothing while the trough below 400 ppm costs you real growth.

A threshold controller produces a much flatter profile, because it adds gas in short bursts triggered by the actual level rather than by a clock. Same gas consumption, considerably more time inside the band.

Safety, properly

This section is not boilerplate. Enriched sealed rooms are one of the few horticultural settings with a genuine CO₂ hazard.

• 5,000 ppm is the 8-hour occupational exposure limit; 30,000 ppm is the 15-minute short-term limit. A normal enrichment band is well below both — a stuck solenoid is not.
• CO₂ is odourless and colourless. There is no warning sensation until levels are already very high. You cannot smell your way to safety.
• It is heavier than air and pools at low level in a still room. A small sealed room with a leaking source can develop a dangerous layer near the floor.
• Combustion generators consume oxygen and produce carbon monoxide if they burn incompletely, in addition to CO₂ and a great deal of water vapour. A CO₂ monitor does not detect carbon monoxide.

Practical rules: ventilate before entering an enriched room, mount a monitor that displays the live level where you can see it before you go in, and set the upper threshold so the exhaust fan runs automatically if the level ever overshoots. That last one is the real protection — it means a source failure vents itself instead of filling the room.

Setting it up

1. Fix light first. If your canopy is not light-saturated, spend the money there. CO₂ enrichment amplifies a good grow; it does not rescue a dim one.
2. Seal the room properly. Enriching a leaky tent means paying for gas that leaves. Sealing also makes the control stable, because the level responds to the source rather than to the draught.
3. Put the sensor in the canopy, at plant height, away from the CO₂ outlet and away from the exhaust. A sensor next to the diffuser reads the gas, not the room.
4. Set the band. Source ON below 1,000–1,050 ppm, fan ON above 1,400–1,500 ppm is a reasonable starting point under strong light.
5. Watch the first week's log. If the source runs almost continuously, the room is leaking or the source is undersized. If the fan runs constantly at night, the dark-period accumulation is larger than you assumed.
6. Re-check as the canopy grows. Demand rises through the cycle. A controller handles this automatically; a timer does not.

The summary

A sealed grow room has two opposite CO₂ problems twelve hours apart. Lights-on it depletes and needs adding to; lights-off it accumulates and needs venting. Both are handled by one measurement and two thresholds — and neither is handled well by a timer, because the loads change with the canopy, the photoperiod and the season.

And before any of it: make sure light is not your limiting factor. Enrichment is a multiplier on a grow that is already working.