Dry Heat Test Chamber Configuration Per IEC 60068 2 2

The high side of the ladder

IEC 60068-2-2 is the heat counterpart to the cold test. Where the cold standard walks down a ladder of low temperatures, this one climbs a ladder of high ones, with preferred steps at +30, +40, +55, +70, +85, +100, +125, +155, +175, and +200 degrees, and further rungs reaching higher for parts that face a furnace or an engine bay. A specification names a step, and the chamber holds the specimen there for the soak.

The high end sets the build.

A box that reaches +85 for consumer gear is a different machine from one that holds +200 for an under-hood part. The elements, the insulation, the seals, and the inner finish all change as the target climbs, since materials that shrug off +85 begin to fail at +200, and a box pushed past its rating drifts out of band long before it reaches the number.

Heat is the easy part

Reaching a high temperature is simple. Holding it flat and even across the whole space, hour after hour, is where the work hides.

The guard behind the controller

A stuck controller could cook a load. An independent over-temperature cutoff stands behind it, killing the heat before anything chars.

Dry air, by definition

The word dry carries weight. This test does not control humidity. As air heats, the same water spreads thinner through it, so the relative humidity falls on its own as the temperature climbs, and the chamber neither adds moisture nor pulls any out. That sets a dry heat box apart from a composite temperature-and-humidity chamber, which carries a humidifier, a water supply, and the controls to hold a damp setpoint.

A dry heat chamber drops all of that.

No humidifier, no water tank, no dew-point sensor. The build stays simpler on the moisture side and spends its effort instead on reaching a high temperature evenly and holding it flat for hours.

Forced air and the working space

Heat from an element does not spread evenly on its own. A dry heat chamber circulates the air with a fan so the working space sits within tolerance corner to corner. Air drawn across the heating elements, pushed through the space, and returned for another pass keeps the gradient tight, and the standard sets the gradient and the fluctuation the box has to hold, commonly inside plus or minus 2 kelvin once it settles.

Fan placement and the baffle behind it decide whether the box meets that band. The elements sit out of the working space, usually behind a baffle, so a specimen never meets the raw radiant glow of a hot coil. The moving air carries the heat instead, which keeps a sample from cooking on the side that faces an element while its far side lags behind.

The heat-dissipating catch

The trickiest part of IEC 60068-2-2 is deciding what the air around the specimen is allowed to do, and it turns on whether the part makes heat of its own. For a dead, non-dissipating specimen the rule is simple: the chamber surrounds it with still or barely moving air at the set temperature, lets it soak through until its core reaches that temperature, and the air and the part settle to the same reading. A part that makes its own heat breaks that simplicity. A powered device sitting in still air warms the layer of air clinging to it and ends up hotter than the chamber set point by however much its own dissipation adds, so the temperature it actually reaches depends not only on the chamber but on how fast the air carries that self-made heat away. That makes air velocity a test parameter in its own right, because faster air strips the heat off and brings the part closer to the chamber temperature while still air lets it run hot, and a result quoted without the airflow it was measured in means little. The standard handles this by splitting its methods, holding still air for the dead specimens and a defined moving airflow for the live ones, and by having the lab record the conditions the part itself created rather than only the air it sat in. The chamber serves both by giving steady, even heat and an airflow it can hold to a known speed, so the only heat left unaccounted for in the answer is the heat the part was meant to make.

Gradual or rapid change

IEC 60068-2-2 also splits its methods by how fast the temperature changes.

A gradual method ramps the specimen up slowly, so the part warms through without a thermal shock clouding the result. A rapid method moves it between conditions quickly, to probe the stress of a fast rise. Dissipating or not, gradual or rapid, those choices combine into the lettered methods, Ba, Bb, Bc, and Bd, that a test plan calls out by name so two labs run the same thing.

The soak clock and thermal lag

As with the cold test, the soak counts from the moment the specimen reaches temperature, not from the moment the air hits the setpoint. A thin foil follows the air almost at once. A thick aluminium housing lags by many minutes, so a lab with a heavy part waits for the core to reach the target before it starts the dwell, often two, sixteen, or seventy-two hours long depending on the spec.

Sensor placement settles the argument. A probe on or inside the specimen tells the lab when the part itself has arrived, while the chamber air sensor only reports the air around it.

Mounting and airflow

How a specimen sits in the box shapes the result. A part crammed against a wall or stacked tight blocks the airflow that holds the gradient, so a hot pocket forms and the specimen there runs above the band. The standard expects specimens spaced so air reaches every face, clear of the walls and the return duct, and a lab that overloads the working space hands back the uniformity the chamber was rated for.

What the heat brings out

The point of a dry heat test is the damage heat draws out, and the failures gather around a handful of mechanisms. Plastics soften and sag under their own weight or a light load. Adhesives and potting compounds outgas, leaving deposits and losing their grip. Solder near its limit creeps, and a joint already stressed can let go. Lubricants thin and run, then bake down to varnish. Different materials expand by different amounts, and that mismatch pries at bonds and press fits.

Colours shift, labels curl, and a display darkens.

Outgassing earns its own note.

At high temperature, plastics and coatings release vapours that condense on cooler surfaces, including the chamber walls and the specimen itself. A dry heat box used for outgassing-prone parts gets cleaned between runs, and some carry a small exhaust so those vapours leave the space rather than settling on the next sample.

Heating, insulation, and the inner finish

A dry heat chamber heats with electric resistance elements, sized so the box reaches its top temperature with margin and recovers fast after the door opens. Thick insulation keeps that heat inside and keeps the outer skin safe to touch. The inner liner runs in stainless steel for the high-temperature range, since a painted or plated finish would discolour, flake, or outgas at +200 and contaminate the very test it serves.

The seals earn attention too. A door gasket has to stay flexible and tight at the top temperature, run after run, and a gasket chosen for a +85 box hardens and leaks in a +200 one. A viewport, where the box has one, uses high-temperature glass set in a frame that takes the expansion without cracking.

Over-temperature protection

A box that holds +200 carries a risk a cold box does not. A stuck controller, a welded heater contactor, or a sensor that fails reading low can drive the heat past the set point until something inside chars, melts, or ignites, and a chamber full of powered electronics gives a fire plenty to feed on. A dry heat chamber answers with an independent over-temperature limiter: a second sensor and a hard cutoff, wired apart from the main loop, that kills the heaters the moment the air climbs past a safe ceiling, regardless of what the primary controller believes is happening. The two channels share no common point of failure, so the fault that blinds one cannot also disable the other. That limiter belongs in the configuration from the first drawing, set a sensible margin above the highest test point and no higher, and a lab checks its trip point as carefully as it checks the calibration, because it is the one feature standing between a runaway and a burned-out chamber with the night shift gone home.

Heat-up rate and recovery

A specification often names how fast the box has to climb as well as where it lands. A gentle ramp suits a gradual method and a delicate part. A steep ramp tests a part that meets heat suddenly in service. The heating elements get sized for the steeper of the two, since a box that ramps fast can always slow down, while a box built only to hold cannot suddenly find the power to climb.

Recovery matters as much as the ramp.

Every time the door opens, a slug of room air drops the working space, and the elements have to bring it back without a long sag and without overshooting on the way up. A chamber tuned for a tight band recovers in minutes and settles flat, while a weak one hunts above and below the target for a quarter of an hour before it steadies, eating into the soak the standard counts.

Sensors and calibration at the hot end

Heat changes what a sensor can be trusted to read. A platinum resistance probe stays accurate and stable across the dry heat range and suits the working-space measurements. A thermocouple, cheaper and tougher, reaches the highest steps where a resistance probe would struggle, at the cost of a little accuracy. The choice follows the temperature and the precision the test calls for.

Calibration carries weight here. A half-degree sensor error at +200 swallows a quarter of a 2-kelvin band, so a lab calibrates the chamber probes against a traceable reference before a qualification run and maps the working space with a grid of sensors, often nine for a small box and more for a large one. The map shows where the hot and cool corners sit, and the lab sets the specimen in the zone that holds the band rather than trusting the whole volume blindly.

Reaching the top of the ladder

The preferred steps climb well past +200 for parts that face genuine furnace heat.

Past a point, forced-air resistance heating gives way to radiant panels or a muffle design, the insulation grows thick enough to swallow much of the cabinet, and the inner liner moves to alloys that hold their shape in the glow. Those builds blur the line between a test chamber and an industrial oven, and the standard still asks for the same discipline: a known temperature, a held band, and a soak counted from the part.

The heat the box gives off

A chamber that holds a high temperature for days pours heat into the room around it. The cabinet skin stays cool to the touch through insulation, and the warmth still leaks, so a row of dry heat boxes can lift a lab's ambient enough to upset other equipment. A well-set lab gives these chambers room to breathe, vents the heat they shed, and watches that a hot summer day does not push a box to the edge of its ability to cool back down between runs.

Recovery and reading the result

After the soak the specimen comes back toward room conditions under a controlled recovery, and the standard sets how the lab inspects and measures the part once it has cooled. Pulling a hot part straight into a cool room can crack it through thermal shock or warp a housing as it cools unevenly, so a measured recovery keeps that from inventing a failure the heat alone never caused.

Reading a heat failure

Some heat damage shows at once, and some hides until the part cools. A housing that sagged at temperature may set into a new, wrong shape on the way down and look merely odd rather than failed. A seal that crept open reseats as it cools and hides the leak it sprang in the heat. A solder joint that went soft and shifted can freeze back into a connection that tests fine on the bench yet sits one thermal cycle from breaking. The standard asks the lab to inspect and measure during the soak where practical, and again after recovery, and to note which faults showed hot and which stayed once cool, because the two tell different stories about the part. A pass that rests only on a cooled-down look can wave through a unit that failed at temperature and quietly recovered its shape, and that is the failure a fielded product will repeat the first hot afternoon it meets. The reading taken at heat, awkward as it is, is often the one that counts.

Why the air has to move

Heat from an element does not spread evenly on its own, so a dry heat chamber lives or dies on how it moves its air. A fan draws the air across the heating elements, pushes it through the working space, and returns it for another pass, and the placement of that fan and the baffle in front of the elements decide whether the box holds its band corner to corner. The elements sit out of the working space, behind a baffle, so a specimen never meets the raw radiant glow of a hot coil; the moving air carries the heat instead, which keeps a sample from baking on the side that faces an element while its far side lags behind.

The heat-dissipating specimen turns that airflow into a parameter of its own.

A powered board or a small motor adds warmth to the air around it, and a fast airstream strips that warmth away, so the part reads cooler than it would in service. For those parts the standard limits the air speed near the specimen, and the chamber runs its fan slower or routes the air around the part rather than straight across it, so the forced circulation does not carry off the self-heating the test is meant to capture. The same box that blasts air for an even soak on a dead sample has to ease off for a live one, and a chamber built for dry heat has to do both.

Pulling it together

A dry heat chamber looks like a plain oven from across the room, and the configuration is where the standard makes its demands. IEC 60068-2-2 turns dry heat into a set of choices a box has to get right: a temperature off the high ladder, a forced-air design that holds the band, an air speed tuned to whether the specimen heats itself, a soak counted from the part, and an over-temperature guard standing behind it all. A chamber built and set up to those choices gives a heat result a lab can defend, run after run, on a part headed for a hot roof, a sunlit dashboard, or the inside of a machine that never gets a chance to cool down.