Damp Heat Cyclic Test Chamber Conditions Per IEC 60068 2 30 Db

What the Db cycle looks like

Test Db runs on a 24-hour clock. The temperature climbs from a low point near +25 degrees to an upper plateau, holds there in saturated air, then falls back, and the chamber repeats that swing for the number of cycles the spec calls out, often one, two, six, twelve, or twenty-one. Through all of it the humidity stays near 93 percent, and across parts of the cycle the air sits at full saturation.

The shape of the curve carries the test.

During the rise, warm humid air meets a specimen still cool from the low phase, and moisture condenses straight onto its surfaces. That condensation is the active agent. It wets seams, runs into joints, and pools where a steady soak would leave a part merely damp.

The dew does the damage

The cycle is built to make water condense on the part. That film, rather than the warm air alone, is what wets the seams and starts the harm.

Steady would miss it

A part can sit at constant humidity for weeks and stay dry inside. A handful of cycles finds what the steady soak never would.

Two variants, two upper temperatures

The standard offers two variants that differ in the upper plateau. Variant 1 climbs to +55 degrees, the harsher of the two, and suits parts headed for hot, humid service. Variant 2 tops out at +40 degrees for milder conditions. A test plan names the variant alongside the cycle count, and the chamber has to hit both the temperature and the humidity at every step of the swing.

Condensation by design

A cyclic damp heat chamber earns its place by making condensation land on the specimen and nowhere else. If the walls and ceiling run cooler than the specimen, water condenses on them and drips onto the part, which the standard rules out. The chamber heats its walls and ceiling a touch above the air, keeps the specimen the coolest surface through the rise, and lets the moisture gather there on purpose.

Dripping points to a chamber fault rather than a test result.

A drop falling from the ceiling onto a board is contamination rather than the controlled condensation the method wants. Heated walls, careful air circulation, and a ceiling shaped to run any stray water off to the sides keep the test honest.

The breathing action

Cycling does something a steady soak cannot, and it is the quiet reason the Db method finds faults that a constant ninety-three percent never reaches. As the chamber warms, the air inside a part and inside any sealed or vented cavity expands and pushes out; as it cools, that air contracts and draws fresh, humid air back in, so the part breathes with every cycle, inhaling moist air on each fall in temperature and exhaling on each rise. A steady soak lets moisture diffuse slowly through the skin of a part, but this breathing pumps it bodily inward, drawing damp air deep into an enclosure, into the space behind a connector seal, into the cavity under a potted module, far faster and further than diffusion alone would carry it. Each cycle adds another lungful, and the water that rides in on the inhale condenses where it cools and stays, building up inside the part over days in places a steady test would take months to wet, if it ever did. That pumping is why cyclic testing finds failures in seals, vents, and enclosures that pass a constant-humidity soak untouched, since the steady test never moves the air and so never drives the water through the small openings the way real day-night cycles in the field do. The chamber serves that mechanism by ramping the temperature on a controlled curve, neither so fast that it shocks the part nor so slow that the breathing fades, so every cycle draws its full breath.

How the chamber holds the humidity

Holding 93 percent humidity at temperature takes a humidity system built for it. Many cyclic chambers raise humidity with a steam generator or an atomizing head, feeding moisture into the circulating air until the wet-bulb reading matches the target. The control loop trims the moisture against temperature, since the same water content reads as a different relative humidity as the air warms or cools.

Water quality matters.

The humidifier runs on deionized or reverse-osmosis water, since tap water leaves scale on the heater and minerals on the specimen that would skew a corrosion result. A drain carries the condensate away, and the chamber keeps its own water loop clean so the only thing reaching the specimen is pure water.

Reading the part while it is wet

A cyclic damp heat test often measures the specimen during the cycle as well as after. Insulation resistance and leakage current get checked at the end of a cold or a hot phase while the part is still damp, since a fault driven by surface moisture can vanish minutes after the part dries. The standard sets when those measurements happen so a wet-state failure does not slip away before anyone records it.

What damp heat cycling brings out

The damage from cyclic damp heat is the slow kind. Moisture drives corrosion on bare metal and under coatings. An electric field across a wet, contaminated surface grows dendrites, fine metal whiskers that bridge conductors and short them. Insulation resistance falls as water tracks across a board. Adhesives and laminates swell and delaminate as water works into them. Organic materials grow mould given enough warmth and time.

The cyclic part sharpens all of it. Each condensation event refreshes the water film, each breathing stroke carries more moisture inward, and the repeated wetting and partial drying concentrates contaminants where they do the greatest harm.

Cycle count and severity

Severity rides on the number of cycles. A short plan of one or two cycles screens for gross faults. A long plan of six, twelve, or twenty-one cycles probes the slow corrosion and migration that need repeated wetting to show. The spec names the count, and a chamber set up for cyclic work runs unattended through all of them, holding each swing to the same shape so cycle twenty looks like cycle one.

Where it differs from steady-state damp heat

A steady-state damp heat test holds one high temperature and one high humidity without moving, and it answers a different question, how a part copes with constant moisture. The cyclic test adds the temperature swing and the condensation and breathing that ride along with it, which makes it the harder test for anything sealed or coated. A plan that has to find moisture ingress reaches for the cyclic method, while one checking steady absorption stays with the constant one.

Chamber upkeep for cyclic work

A cyclic chamber lives a wet life, and the upkeep follows.

The humidifier scales and needs cleaning. The wet-bulb wick, on chambers that use one, has to stay clean and fed or the humidity reading drifts. Seals see constant moisture and get checked for the leaks that would let the chamber lose its grip on humidity. A chamber kept up holds 93 percent through a long plan, while one left to scale and drift wanders off the band partway through and spoils the run.

The ramp that makes the dew

The rate of the temperature rise is the part of Test Db that has to be right. Climb too slowly and the specimen warms in step with the air, never sitting cool enough for water to condense on it, so the test loses its bite. Climb at the rate the standard sets and the specimen lags behind the warming air, its surface stays below the dew point of the saturated room, and a film of water forms across it.

The chamber earns its result in those first hours of each cycle.

Mass decides how much dew forms.

A heavy part holds the cold of the low phase longer and gathers more condensation than a light one, so two specimens in the same run can wet differently. A lab notes the part mass and arranges the load so the airflow reaches each piece, since a part shadowed behind another warms unevenly and skips the dew the test leans on.

Air movement and uniformity

A cyclic damp heat chamber circulates its air gently. The flow has to spread the heat and the moisture evenly so every corner of the working space reads the same temperature and humidity, and it has to stay slow enough that it does not dry the condensation off a specimen before the method means it to. A blast of air would strip the film the rise just laid down.

Uniformity is harder in a wet box than a dry one. Humidity varies more across a space than temperature does, and a cool spot near a wall can sit at a different relative humidity from the centre. The fan layout, the baffles, and the wall heating work together to hold the whole working space inside the band the standard allows, commonly a few percent around 93.

Measuring humidity at temperature

Reading humidity to a few percent at a warm, saturated condition is its own challenge. Many chambers use the wet-and-dry-bulb method, a pair of sensors where one stays wet from a wick, and the difference between the two gives the relative humidity. Others use a capacitive sensor that reads humidity directly. Each drifts in its own way, and a saturated, swinging environment is the hardest place to keep either one honest.

The wick is the weak point on a psychrometric chamber.

If the wick dries out, fouls, or runs on hard water, the wet-bulb reading climbs and the chamber decides the air is drier than it is, so it over-humidifies. A lab on a wet-bulb chamber keeps the wick clean, fed with the same pure water as the humidifier, and trimmed to length, since the humidity the specimen sees rests on that small piece of cloth.

A box built to stay wet

The chamber itself has to survive the conditions it creates. Saturated air at +55 degrees corrodes ordinary steel, so the interior runs in stainless throughout, with welds and fasteners chosen to match. The floor slopes to a drain so condensate leaves the box rather than pooling, and the door seal holds a tight line against the warm, moist pressure inside.

Anything that rusts inside the box contaminates the test. A spot of corrosion on a shelf or a fastener sheds particles and ions into the water film on the specimen, which can start a corrosion site the part would never have grown on its own. A chamber kept clean and rust-free keeps that contamination out of the result.

The two ways down

How the cycle comes back down matters as much as how it climbs. One route lets the temperature fall while the air stays saturated, holding the wetting through the descent so the part spends the cool hours dripping rather than drying. Another adds a drier, cooler rest at the bottom before the next climb, giving the surface a chance to shed some water before the dew forms again. The two read very differently to the specimen: the saturated descent keeps corrosion and migration working around the clock, while the drier rest lets the part recover a little and tests instead the repeated wetting and drying that loosens coatings and works moisture into seams. The route a plan picks shapes how long the specimen stays wet and how much it dries between cycles, and a single degree or a few minutes off the profile changes the dose. The chamber has to follow the chosen shape exactly, cycle after cycle across the whole plan, because a run that drifts halfway through is no longer the test the report claims it is.

Powered during the soak

Some damp heat cyclic plans run the specimen with power applied, so the test catches faults that only show under a working voltage. A small bias across a wet, contaminated gap drives the metal migration that grows dendrites far faster than moisture acting on its own, and a few tenths of a volt can be the difference between a board that survives and one that grows a short across a gap by the final cycle. A powered part also warms itself, and that self-heat shifts where condensation lands, sometimes sparing the hot component and soaking the cool connector beside it instead. A chamber set up for powered work carries sealed feed-throughs for the cables, a way to hold the wiring clear of the condensate that runs down the inside walls, and often a means to switch or watch the load from outside so a technician never has to open a wet box. The point is to let the power reach the part without handing the moisture a path to ground, because a leak through the test rig tells a lab nothing about the specimen and ruins the run.

Recovery and the final look

After the last cycle the specimen comes back to standard room conditions under a controlled recovery, and the lab makes its final measurements and inspection there. Some checks happen wet, at the end of a cycle, and some happen dry, after recovery, and the standard sets which is which so a result reads the same from one lab to the next. A part gets opened and examined for corrosion, for water that breached a seal, and for the swelling or delamination that moisture leaves behind.

Where Db parts end up

A product earns a cyclic damp heat plan when its life involves warm, wet air and the daily swing that brings dew. Equipment for the tropics, gear stored in unconditioned warehouses, outdoor enclosures that heat in the sun and cool at night, and marine fittings all face the breathing and condensation the test recreates. The cycle count on the plan reflects how long and how punishing that service is.

The pumping that finds the leak

Cycling does something a steady soak cannot, and it comes down to the air inside a sealed part. As the chamber heats, the gas trapped in an enclosure expands and pushes out through seams and vents; as it cools, the enclosure draws air back in, and that returning air is wet. Over many cycles the part breathes, pumping moist air into spaces a constant humidity would never reach, so water ends up behind seals, inside connectors, and under conformal coating where it can corrode and bridge. That breathing is why a cyclic plan finds the moisture-ingress faults a steady test walks straight past.

The rising edge of each cycle adds the second mechanism.

During the climb, warm humid air meets a specimen still cool from the low phase, and moisture condenses straight onto its surfaces, a film that runs into joints and pools where a steady soak would leave the part merely damp. Each cycle refreshes that film and the breathing carries more of it inward, so the wetting and the pumping feed each other across the run. A chamber built for the cyclic test holds its walls above the dew point so the water lands on the part rather than dripping from the ceiling, and follows the ramp the standard sets so the condensation forms on schedule, cycle after cycle.

Pulling it together

Cyclic damp heat is a weather machine more than an oven. IEC 60068-2-30 sets a daily swing of temperature inside saturated air so condensation lands on the specimen, breathing pulls moisture into sealed spaces, and corrosion, migration, and falling insulation resistance get their chance to show. A chamber built for Test Db heats its walls to keep the water on the part, holds 93 percent on clean water through every cycle, and lets a lab read the specimen while it is still wet. Set up that way, a run of cycles tells a maker whether a product will live through a monsoon season, a tropical warehouse, or a year of dew on cold mornings.