Autumn brings the first run of cold, still nights, and with them the first mornings when a shed floor is wet under a roof that did not leak, or a tractor is beaded with water on a day that looks dry. Condensation sits behind a surprising share of rural maintenance problems: rust on stored machinery, mould in sheds, corroded terminals in switchboards and pump controllers, and electronics that fail inside sealed boxes that never saw a drop of rain. The physics is simple, and it comes down to a number most sites do not log: dew point.
Relative humidity is the wrong number to watch
Relative humidity describes how close the air is to saturation at its current temperature. Warm air can hold more water vapour than cool air, so relative humidity changes whenever the temperature does, even when the amount of water in the air has not changed at all.
On a typical still night the moisture content of the air barely moves, but the air cools, so relative humidity climbs from a moderate figure in the afternoon to the high nineties by dawn, then falls again as the sun comes up. A chart of relative humidity looks alarming every night and tells you very little.
Dew point is the temperature to which air would have to cool, with its moisture content unchanged, to become saturated. It tracks the actual amount of water in the air. It changes when moist air arrives or drier air replaces it, with rain, a change in wind direction, a front or irrigation, and not simply because night has fallen.
Most weather stations and loggers calculate dew point for you. If yours does not, a useful approximation when relative humidity is above about 50 per cent is to subtract one fifth of the gap between relative humidity and 100 from the air temperature. Air at 20°C and 70 per cent relative humidity has a dew point of roughly 14°C.
Condensation happens on surfaces, not in the air
Water condenses on any surface that is colder than the dew point of the air touching it. So the useful question is never “is it humid?” but “is anything colder than the dew point?”
Three things commonly make a surface colder than the air around it:
- Radiative cooling. On clear, still nights, anything exposed to the sky loses heat by radiation and can fall several degrees below the air temperature. Uninsulated metal roof sheeting is the classic case: its underside drops below dew point, water forms, and it drips as though it had rained inside.
- Thermal mass. A tractor, a steel frame, a concrete slab or a full tank lags behind the air. After a cold night, warmer and moister morning air meets metal that is still cold, and the metal sweats. That is why equipment can be wet mid-morning on a day that looks dry.
- Ground contact. Slabs and anything stored directly on them stay cold for longer than the air above.
What to measure
A useful condensation setup has three parts.
- Air temperature and humidity, in a shaded, ventilated radiation shield away from walls and roofing, with dew point calculated from them. Ideally one set outside and one inside the building you care about.
- Surface temperature of the thing at risk: the underside of a roof sheet, a machinery panel, the lid of an enclosure. A contact probe bonded to the surface works, provided it is shaded, because a probe in direct sun measures the sun.
- The margin, which is surface temperature minus dew point. This is the number to chart and alert on. A comfortable positive margin means dry. A margin closing towards zero overnight means condensation is coming. A negative margin means it is happening now.
Allow a buffer of a couple of degrees before you call it safe. Consumer humidity sensors are typically good to a few per cent, they become less reliable close to saturation, and some read high for a while after they have been wetted. Treat readings near 100 per cent with caution and cross-check against a second instrument occasionally.
Sheds and stored equipment
Once you are watching dew point, the fixes become easier to choose.
- Ventilate only with drier air. Opening a shed to warm, moist morning air while its contents are still cold makes the problem worse. Bring outside air in when its dew point is lower than the dew point inside and lower than the coldest surface you are protecting.
- Insulate the roof. An anti-condensation blanket or sarking under metal roofing keeps the inner surface closer to air temperature, and is usually the most effective fix for a dripping shed.
- Get things off the slab. Store equipment and product on pallets or racks, and cover machinery with breathable covers rather than plastic sheeting, which can trap moisture against cold steel.
- Dehumidify enclosed spaces where power is available and the contents justify it.
- Mind stored product. Fertiliser, seed and anything else that takes up moisture belongs off the floor and away from the part of the shed that drips.
Electronics enclosures
A sealed enclosure looks safe, but an IP rating describes resistance to dust and water under test conditions, not years of daily temperature cycling. As the box warms in the day and cools at night, the air inside expands and contracts. Seals and cable glands are rarely perfect, so the box breathes, and each breath on a cool evening draws in moist air. The moisture has nowhere to go. It condenses on the coldest internal surface, often the lid, and drips onto whatever is below.
Practical measures, roughly in order of value:
- Measure inside the box. A small temperature and humidity sensor inside the enclosure, logged alongside the outside readings, shows moisture accumulating over weeks. A steadily rising internal dew point finds a slow leak before the leak finds the electronics.
- Control the breathing. A pressure-equalising membrane vent lets the box breathe through a filter that passes air but not liquid water, instead of drawing air in through the glands.
- Use desiccant carefully. It works in an enclosure that is genuinely sealed, but it saturates. Add a humidity indicator card and a replacement schedule. In a box that breathes freely, desiccant is exhausted quickly.
- Keep the electronics warm. A board slightly warmer than the enclosure wall stays dry. A device’s own heat often does this; a small thermostatically controlled heater is the industrial answer, at an energy cost that matters on solar.
- Plan the drainage. Cable entries at the bottom, drip loops on every cable, and a mounting orientation that sends any water away from the boards. Conformal coating is a sensible backstop.
Reading the data over a season
The real value comes from logging for long enough to see the pattern: which nights bring the margin close to zero, what weather sets them up, and whether a change you made actually helped. Clear, still nights after rain are the usual suspects. After a season you have a condensation calendar for the site, and you can act ahead of the risk rather than after the rust.
Where Bizix Agritech fits
Our field nodes log air temperature, humidity and dew point, and the same edge compute can track the condensation margin against a surface or in-enclosure sensor and raise an alert before the drip starts. The weatherproof enclosures we deploy are specified with the same problem in mind. Designed and supported in Australia.