Damp Heat Steady State Test Chamber Parameters Per IEC 60068 2 78 Cab

A constant condition, held

Test Cab keeps temperature and humidity fixed for the length of the test. Where a cyclic method swings the conditions and a composite one adds a freeze, Cab does neither. It picks a point, a warm temperature paired with a high relative humidity, and holds it flat from the first hour to the last. The damage comes from constancy, the slow, steady arrival of moisture into the part.

Steadiness is the method.

A part under Cab meets no shock and no change. It simply sits in warm, damp air long enough for water to diffuse into materials, settle on surfaces, and reach the places a brief exposure would never touch.

Steady is the point

No swing, no freeze. The chamber picks a warm, damp condition and holds it flat from the first hour to the last.

The two are linked

Relative humidity rides on temperature, so a flat damp condition means holding both at once rather than one and then the other.

The parameters that define a run

Cab is named by its parameters, and three numbers set a run: the temperature, the relative humidity, and the duration. The standard offers preferred temperatures, often 30 or 40 degrees, paired with a high humidity, commonly 93 percent, and a duration chosen from a ladder that runs from hours to many days. A test plan names the three, and the chamber has to deliver each one and hold it.

The pairing is deliberate. A given temperature and humidity together fix the amount of water in the air and the dew point, so naming both pins the exact damp condition the part faces rather than only how warm or how wet it is on its own.

Choosing the severity

The severity comes from the world the product will live in, not from a wish to make the test hard. Cab offers a menu of temperature and humidity pairs and a menu of durations, and choosing among them is a matter of matching the run to the climate the part will actually meet over its life. A part bound for an air-conditioned office asks for a mild pairing held for days; a part bound for a tropical roof, a ship hold, or an outdoor cabinet asks for the warm, near-saturated settings held for weeks. The duration matters as much as the level, because steady damp heat works slowly, drawing moisture into a package and along an interface over time, so a soak too short ends before the water has reached the places that fail while a soak of the right length lets the moisture arrive and do its work. The standard lays out preferred values rather than leaving the choice open, both so results from different labs can be compared and so a designer reaching for a number is nudged toward one that means something. Picking the severity well is the quiet skill of the whole test: set it too gently and a real weakness slips through, set it too harshly and good parts are condemned for a climate they will never see, and only a pairing drawn from the product true service life gives a verdict worth trusting.

Tolerance, the band around the setpoint

Holding a number means holding it within a band. The standard sets a tolerance on the temperature, often a couple of degrees, and on the humidity, a few percent, and the chamber has to keep the working space inside both for the whole run. A drift outside the band changes the stress the part feels, so a long Cab test rests on a chamber that holds its setpoint flat for days without wandering.

Temperature and humidity are linked, which makes the band harder to hold. Relative humidity depends on temperature, so a small temperature drift moves the humidity even while the water content stays the same. A chamber that lets its temperature sag overnight will see its humidity climb, and the part slides off the condition the plan named.

Uniformity across the working space

The band has to hold across the whole working space, everywhere a specimen sits rather than at one sensor alone. The standard ties its tolerance to the working space, the central volume where parts go, and pulls the measuring points clear of the walls, the door, and the air inlets. A chamber that reads 40 and 93 at its control sensor can still run warm or dry in a far corner, so a careful test maps the space and places the parts where the condition holds.

Loading the chamber for a steady soak

How the parts go in shapes whether they all meet the same condition.

The standard expects the specimens spaced so the damp air reaches every face, clear of the walls and the return duct, and a tray crammed full blocks the gentle airflow that holds the humidity even. A part shadowed behind another sits in stiller, drier air and ages less than its neighbours, so a careful load follows the pattern the chamber allows and leaves room for the air to move.

Fluctuation over time

Steadiness over hours is its own parameter. Fluctuation is how much the reading wobbles at a point once the box has settled, and a chamber that cycles its heater or its humidifier in coarse steps lets the condition ripple even while its average sits on target. A long Cab run wants a tight, smooth hold, so the control trims its heat and moisture in fine steps and the part meets a flat line rather than a gentle wave.

Why condensation stays away

Steady-state damp heat wants the moisture as vapour on and around the part, with no liquid water condensing on it. If a surface drops below the dew point, water beads on it and changes the test, bridging terminals and pooling where vapour alone would never gather. The chamber holds its walls and the load above the dew point, warms the space evenly, and keeps the air drifting gently, so the part stays damp without ever turning wet.

Water and the long hold

A Cab run can last days or weeks, and the humidity has to hold the whole time. The chamber raises humidity with steam or an atomising head and feeds it from deionised or reverse-osmosis water, since minerals in tap water would scale the system and settle on the parts. Over a long run the box draws a steady flow of that pure water and drains the condensate, holding the humidity flat without anyone opening the door.

The humid box and its build

A steady-state damp heat chamber is built around the long, wet hold. Its interior runs in stainless so the constant damp does not corrode it, its walls carry gentle heat to keep condensation off the load, and its humidity system is sized to top up moisture steadily for days rather than to chase a swinging setpoint. The fan moves the air softly so the working space stays uniform without drying the part, and the whole box is sealed and drained to live a damp life run after run.

Calibration before a long run

A test that runs for weeks rests on sensors that read true the whole time.

Before a long Cab soak the lab calibrates the temperature and humidity sensors against a traceable reference, since a humidity probe that reads two percent high quietly shifts the condition the parts feel for the entire run. The chamber gets a probe it can trust and a hold it has proven, because there is no way to recover a soak that drifted unseen for a fortnight.

The test that runs for weeks

A Cab soak at the long end of the ladder ties up a chamber for a fortnight or more, so the run has to survive a building's daily rhythm without a stumble. A power blip, a water-supply hiccup, or a door opened by mistake can break the steady condition and put the whole soak in question, so the chamber runs on a reliable supply, logs its condition throughout, and alarms on a drift before it spoils the result. The log that travels with the parts proves the condition held for every one of those hours.

Conditioning and recovery

A Cab test has a beginning and an end the standard frames as carefully as the soak between. Before the run, the parts are conditioned at standard room conditions so every specimen starts from the same known state. After the soak, a controlled recovery brings them back, and the standard names whether the measurements happen at the end of the damp period, while the part is still warm and wet, or after recovery once it has returned to room conditions. That timing decides whether a moisture-driven fault is caught or missed.

What the soak brings out

Constant damp heat works slowly and broadly. Moisture diffuses into plastics and swells them. Insulation resistance falls as water tracks across surfaces and into materials. Thin films of water let corrosion start on bare metal. Adhesives soften, labels lift, and absorbed water shifts the electrical behaviour of components. None of it is sudden, which is the point, the test asks how a part holds up under steady moisture rather than how it survives a shock.

Reading insulation resistance

For many parts the headline reading is insulation resistance, and it falls as moisture tracks across and into a material. The standard sets when it is measured, warm at the end of the soak or after recovery, since a value taken while the part is still damp can sit far below one taken once it has dried. A drop that recovers as the part dries points to a surface effect, while one that stays points to moisture that reached deeper and did lasting harm.

Sealed and unsealed parts

How a part is built changes how the soak reaches it. An open board takes moisture across its whole surface at once, wetting through in hours and reaching equilibrium with the chamber early in the run. A sealed enclosure resists, holding the damp out until water finds a path through a seam or diffuses slowly through a gasket, and then traps it inside long after the chamber is switched off. The standard's steady condition suits both, giving the open part a uniform exposure and the sealed one the patient time its slow ingress demands. A run long enough to drive moisture into a well-sealed box can be many times what an open board needs to show the same effect, and that gap is the reason the duration ladder stretches out to weeks rather than stopping at a few convenient days. The build sets the clock as much as the climate does.

Why steady beats a quick check

A short exposure tells little about steady moisture.

Water moves into a material slowly, so a part pulled after an hour looks fine while the same part after ten days has taken on enough moisture to fail. The steady soak gives the water the time it needs to reach equilibrium with the part, and Cab durations stretch to days and weeks to allow it. The result reflects where the part settles in damp air rather than where it sits on the way there.

Where Cab sits among the methods

Test Cab is the general, steady member of the damp heat family. The cyclic method swings temperature to drive condensation and breathing. The composite method adds a freeze. The component-focused steady test pushes the temperature and humidity higher to age small parts fast. Cab holds the broad middle, a constant warm and humid soak at a chosen severity, used across equipment and components alike when the question is how a product copes with steady moisture over time.

Holding a flat condition for weeks

The hard part of a steady damp soak is endurance rather than extremity. A run can hold one warm temperature and one high humidity for hundreds or thousands of hours, and any drift over that span shifts the stress the parts feel, so the chamber has to keep the same number flat while a building's day and night swing around it. The humidity and the temperature are bound together, since relative humidity depends on temperature, and a box that lets its heat sag overnight watches its humidity climb off target without a drop of extra moisture entering. Meeting the standard means holding both loops steady at once, week after week, on a supply of pure water that never runs dry.

Then the band has to hold across the whole working space rather than at the control sensor alone. The standard ties its tolerance to that space, pulling the measuring points clear of the walls, the door, and the air inlets, and a chamber that reads its target at the sensor can still run a few percent off in a far corner. A careful run maps the loaded space, places the parts where the condition holds, and trims its humidifier in fine steps so the reading sits flat rather than rippling. The whole value of a long soak rests on that quiet steadiness, since a condition that wandered partway through ages the parts to a number no one can trust.

Pulling it together

Steady-state damp heat is a test of patience, and its result lives in the parameters.

IEC 60068-2-78 holds a part at a chosen temperature and humidity for a chosen time, and the value of the run depends on how exactly the chamber keeps those numbers, inside the band, uniform across the working space, and steady hour to hour, with no condensation and pure water behind the humidity. Set and held that way, a Cab soak tells a maker how a product will fare after a long spell in warm, damp air, the climate of a tropical store, a humid basement, or a season of wet heat.