Composite Temperature Humidity Test Chamber Per IEC 60068 2 38

Two stresses in one cycle

Test Z/AD folds two environments into a single 24-hour cycle.

Through much of the day it behaves like a damp heat test, swinging between a warm plateau near +65 degrees and a cooler point around +25, all in air held near 93 percent humidity. On certain cycles it adds a cold excursion that takes the specimen down to about -10 degrees. The standard runs ten of these 24-hour cycles, with the sub-zero dip falling on the cycles the method names.

The freeze is the twist a damp heat test leaves out.

During the humid phase, water works into seams, coatings, and any path moisture can find. The cold step then freezes that trapped water. Ice takes up more room than the water it came from, so it pushes outward, opening cracks and lifting coatings, and the next humid phase drives fresh water into the gaps the ice just made.

Two climates in one box

A damp soak and a deep freeze rarely share a chamber. This test folds them into one cycle, and the pairing is the whole idea.

Ice is the wedge

Water alone soaks a part. Frozen, it expands and pries, turning the moisture the warm phase left behind into a mechanical tool.

Why the combination is harsher

Each stress on its own is survivable for most parts, and the cruelty of Test Z/AD is in how the two work together rather than in either alone. The humid phase does the first half of the damage quietly: warm, saturated air drives water into the small flaws of a part, under a seal that breathes, into a hairline crack in a coating, along the gap where a lead enters a body. Then the freeze springs the trap. Water that has crept into those flaws turns to ice, and as it freezes it expands, prying open the very gap it entered and widening the crack a degree at a time. A seal that holds against steady damp heat can be jacked apart by the ice that forms in the moisture it let past; a coating that merely blistered under humidity is lifted clean off when the trapped water freezes beneath it. That coupling is the point of the test. Run the damp and the cold one after the other on the same part, cycle after cycle, and each humid phase pushes a little more water into the openings the last freeze made, so the damage compounds in a way neither stress reaches on its own. The chamber has to deliver both faithfully, the warm saturated soak that wets the part and the deep freeze that turns that water to a wedge, and it has to move between them on a schedule sharp enough that the water is still in the flaws when the cold arrives.

What the chamber has to do

A composite chamber carries the full kit. It heats to +65, humidifies to 93 percent, and refrigerates to -10, all in one working space, and it moves between those states on the schedule the cycle sets. A plain damp heat box cannot reach the freeze, and a plain temperature box cannot hold the humidity, so Test Z/AD needs a machine built to do both at once.

The refrigeration sets the cost.

Adding a cooling system that reaches -10 and recovers quickly turns a humidity chamber into a far more complex and costly machine. The compressor, the evaporator, and their controls join the humidifier and the heaters, and the box has to switch cleanly from making moisture to making cold without the two systems fighting each other.

Humid on the way up, dry at the bottom

Humidity control stops where the water would freeze. Above freezing the chamber holds the saturated condition the damp phase needs. As the temperature falls toward zero, holding a relative humidity loses its meaning, since the moisture begins to frost out of the air, so the cold excursion runs without a humidity target. The standard sets the conditions for each leg, wet and warm at the top, simply cold at the bottom.

The transition is where the damage forms. Water already absorbed during the warm phase is what freezes during the dip, so the chamber has no need to hold humidity at -10 for the test to work. The moisture is already inside the part.

Where the test came from

Test Z/AD has a long pedigree.

It grew out of the moisture-resistance screens that military and aerospace programmes leaned on for decades, where a humid soak paired with a freeze sorted the parts that could take a hard climate from the ones that could not. IEC 60068-2-38 carries that approach into a shared standard, so a composite cycle run in one lab matches one run in another. The method keeps the bite of the older screens while pinning down the numbers a modern qualification needs.

The ten cycles and where the cold falls

The standard frames the test as ten 24-hour cycles, and the cold excursion does not land on every one. Several cycles run as a warm, humid swing alone, building moisture into the part, and the sub-zero dips fall on the cycles the method sets out, so each freeze acts on a specimen that has already taken on water. Spacing the freezes this way lets moisture gather between them, which gives each freeze more water to expand than a freeze on a dry start would find.

The pattern is part of the severity.

A plan that froze a dry part on cycle one and never let it soak would miss the point. The method interleaves the wetting and the freezing so the two feed each other across the full ten days.

Pulling a wet box down to the freeze

The hardest moment for the refrigeration is the drop from a warm, saturated box to -10. The air is full of water, and that water carries latent heat the cooling system has to remove before the temperature will fall, so the early part of the descent runs slowly while the moisture condenses and then freezes out. A composite chamber sizes its refrigeration for that wet pull-down rather than for a dry one, since a system chosen for dry cooling would stall partway down.

Why the freeze stops at minus ten

The cold leg goes to about -10 degrees, deep enough to freeze the water a part has absorbed without turning the test into something else. Push the freeze far colder and the run starts to behave like a thermal cycling test, where the stress comes from the temperature swing itself rather than from ice. The composite method keeps the freeze shallow on purpose, so the damage stays tied to the moisture the humid phase put there.

The transitions and their timing

How fast the box moves between states shapes the test. The standard sets the ramp from warm to cold and back, so the specimen freezes through and thaws on a known schedule rather than at the chamber's whim. A box that crawls between states robs the cycle of the sharp freeze that does the work, while one that slams between them risks a thermal shock the method never asked for.

Recovery between cycles matters too. The part has to return to the humid starting condition cleanly so the next cycle begins where the last one did, and the chamber holds each of the ten cycles to the same shape so cycle ten stresses the part as hard as cycle one.

Condensation, frost, and the wet box

Like any humid chamber, a composite box has to keep its own condensation off the specimen during the warm phase and manage the frost that forms during the cold one. Heated walls keep water from dripping onto the part while it sits warm and wet. On the cold leg, frost gathers on the coldest surfaces, the cooling coil among them, so the box runs a defrost between excursions to keep the refrigeration breathing.

Water quality carries over from damp heat practice. The humidifier runs on deionized or reverse-osmosis water so no minerals reach the specimen, and a drain clears the condensate and the melt that follows each freeze.

Holding humidity through the warm swing

Through the warm part of the cycle the chamber works like a damp heat box, and the same demands apply. It has to hold near 93 percent at +65 while the air circulates gently enough that it does not dry the film off the specimen, and it has to keep its walls a touch warmer than the part so condensation gathers on the specimen rather than dripping from the ceiling. The humidity has to be there, and steady, for the part to drink the water the freeze will later turn against it. A chamber that wavers on humidity in the warm phase sends a drier part into the freeze, and the test softens without anyone intending it.

Surface frost or buried ice

Two kinds of ice form, and they do different work. Frost on the outside of a part is largely harmless, a coat that melts away on the next warm phase and leaves nothing behind but a little surface water. The ice that matters is the water that soaked into a seam, a crack, or the space under a coating before the freeze caught it. Liquid water swells by close to a tenth of its volume as it turns to ice, and when that happens inside a cavity the part cannot give, the wall of the cavity does, prying the opening a fraction wider. That buried ice works cycle after cycle, and the gap it leaves draws in more moisture on the next warm, wet phase, which freezes and widens it again. The damage is cumulative and quiet, hidden from the outside until a seal finally lets go or a track corrodes through, and it is the reason the test runs ten cycles rather than proving a point in one.

What Test Z/AD brings out

The failures gather where moisture and ice meet. Coatings delaminate as frozen water lifts them from below. Seals split as ice widens the gap they were meant to close. Cracks that a humid soak only wetted grow as the ice inside them expands. Corrosion still runs, as it does in any damp heat test, and the freeze adds a mechanical prying on top of the chemical attack.

The result reads as a tougher screen than steady or cyclic damp heat. A part that passes a constant humidity soak, or even a freeze-free cyclic one, can still fall to Test Z/AD, because the composite cycle adds a mechanism the gentler tests leave out.

Reading and measuring the part

As with other damp heat methods, a composite test often measures the specimen while it sits in a telling state. Insulation resistance and leakage current get checked at points the standard names, sometimes wet, sometimes cold, since a fault driven by surface moisture or by an ice-opened crack can hide once the part settles back to room conditions. The lab inspects for corrosion, for water that breached a seal, and for the lifting and cracking the freeze leaves behind.

Mould, corrosion, and the slow failures

The dramatic cracks are only half the story. Alongside the split seals and lifted coatings, the warm, wet phases grow the same slow problems any damp heat test does. Corrosion creeps on bare metal and under coatings, feeding on the thin film of moisture the humidity leaves behind on every surface. An electric field across a wet, contaminated surface grows the fine metal dendrites that bridge conductors, and a board that measured clean on day one can carry a leakage path by the final cycles. Insulation resistance falls as water tracks across the laminate and into the glass weave, and once a route forms it tends to widen rather than heal. The freeze does not wipe any of this away. It rides on top of it, driving water deeper into the openings the corrosion has started, so a single specimen can finish a run carrying a cracked joint from the ice, a corroded pad from the damp, and a dendrite across a gap, each logged as a separate finding even though they share one cause.

A box that takes both worlds

The chamber has to survive the same pairing it inflicts. The wet phase corrodes, so the interior runs in stainless with matched welds and fasteners. The cold phase chills the humidity controls and the door seal, so the sensors and the wet-bulb wick, where one is used, sit where they will not freeze and foul, and the seal stays flexible at the bottom of the swing. A box that rusts, or whose seal stiffens in the cold, drifts off its conditions and spoils the run it was built to hold.

Neighbours on the shelf

Test Z/AD sits between two simpler methods.

The cyclic damp heat test swings temperature and humidity without ever crossing freezing, and it answers how a part copes with warm, wet weather alone. The combined cold, dry heat, and low-pressure method stacks a different trio of stresses for altitude work. The composite temperature humidity test holds its own ground as the one that pairs saturated heat with a freeze, and a test plan reaches for it when moisture and ice are the pair a product will face.

Where the composite test fits

A product earns Test Z/AD when its life pairs humidity with cold, or when a maker wants a hard, fast screen for moisture ingress. Gear that ships through cold cargo holds after a humid factory, equipment that swings between a warm wet day and a freezing night, and parts that have to shrug off both extremes get this cycle. It compresses a punishing stretch of weather into ten days of chamber time.

Why the freeze does the work

The composite cycle earns its severity from the order of its stresses.

Through the warm, humid phase a product drinks water into seams, cracks, and the space under a coating, the same slow ingress any damp heat test drives. Then the cold step takes the specimen below freezing, and the water it absorbed turns to ice. Ice takes up more room than the water it came from, so it pushes outward where the part cannot give, opening cracks, lifting coatings, and tearing seals wider, and the next humid phase drives fresh water into the gaps the ice just made. Each stress on its own a part might survive; linked, the moisture from one becomes the wedge in the next.

That coupling is why the chamber has to do two hard jobs at once. It carries the humidity system and heaters of a damp heat box and the refrigeration of a cold one, and it has to swing between a saturated warm plateau and a sub-zero dip on the schedule the standard sets. Humidity control stops where the water would freeze, so the cold leg runs dry while the moisture already inside the part does the damage; the heated walls keep condensation off the load during the warm phase, and a defrost clears the coil between freezes. A box that pairs saturated heat with a freeze exposes the failure that surfaces only when moisture and cold hit one after another.

Pulling it together

The composite temperature humidity test is two climates working together.

IEC 60068-2-38 soaks a product in warm, saturated air, then freezes the water it absorbed, and repeats that pairing until any weak seal, coating, or crack gives way. A chamber for Test Z/AD heats, humidifies, and refrigerates in one space, holds its walls warm to control the wet-phase condensation, defrosts its coil between freezes, and moves between states on the schedule the cycle sets. Built and run that way, ten cycles tell a maker whether a product can survive a world that swings between soaking warmth and a freeze.