Sizu Riatto
Flush Hour · Chapter 7

Air You Can Count

See why the tub fogs, how little water the air holds, and how much fresh air the mushrooms want. Temperature and humidity in; dew point, VPD and liters a minute out.

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Opens on the book's tub. Change any number to your own and press Calculate.

The air in the tub

Fresh air

Carbon dioxide

Enter the temperature and the humidity, then press Calculate.

The formula

  • Saturation vapor pressure es = 0.6108 × e^(17.27 T ÷ (T + 237.3)) kPa (FAO-56, T in °C): it climbs steeply as the air warms
  • Actual vapor pressure e = es × RH ÷ 100; vapor pressure deficit (VPD, the air's drying power) = es − e
  • Dew point: g = ln(RH ÷ 100) + 17.27 T ÷ (T + 237.3), then Td = 237.3 g ÷ (17.27 − g). Any surface colder than Td fogs
  • Water in the air (g a m³) = e (Pa) ÷ (461.5 × (T + 273.15)) × 1,000
  • Fresh air: flow (L an hour) = air changes an hour × tub volume; air changes = flow × 60 ÷ volume
The book's worked example, Chapter 7. At 18 °C (64.4 °F) and 90% the air could hold es = 2.064 kPa; it holds 1.858. The dew point is 16.3 °C: any wall about 1.7 °C cooler than the air fogs. The VPD is 0.206 kPa. The tub's 55.44 L of air hold only about 0.77 g of water, against 4.2 kg in the straw. Six air changes an hour (all the air every 10 minutes) is 332.64 L an hour, 5.544 L a minute, through 10 holes of 49.09 cm². Keep CO₂ under 800 ppm, 1.88 × the 425 ppm outdoors.
Check it. Call the humidity inside your closed tub after an hour, then read a hygrometer: cheap ones read a few percent off, so check yours in a sealed bag with damp salt (it should read near 75%).

Honest limits. The formula is the FAO-56 form; other published forms agree within about 0.1 °C here. A tub is not a sealed lab: lids, holes and opening it move every number. Above about 1,200 ppm of CO₂ oysters grow long stems and small caps. The book's tub, its straw and its harvests were worked out on paper from published figures, not grown by us: weigh your own. Prices are illustrative, as seen in November 2026 (US mid prices, with UK beside where they differ), shown without a currency sign; put in your own. No yield or profit is promised. Everything stays in your browser: no sign-up, nothing stored.