Every textile test method sends you to a standard atmosphere — for most laboratories, ISO 139's 20 ± 2 °C and 65 ± 4 % RH. Holding that atmosphere is not difficult in principle. Holding it where the specimens are, all day, with the door opening and a full load inside, is what separates a useful cabinet from an expensive humidifier.
This guide is about choosing the equipment. For the conditioning procedure itself — equilibrium by mass, approach from the dry side — see our earlier article on ISO 139.
First decision: cabinet, room, or both
A conditioning cabinet holds specimens at the standard atmosphere until they reach equilibrium. A conditioned room does that and also lets people work inside it.
The difference matters because several methods require the measurement or test, not only the conditioning, to take place in the standard atmosphere. ISO 3759, for example, asks for dimensional change specimens to be measured in the standard atmosphere. A tensile test, a thickness measurement or a shrinkage reading done on an open bench outside the cabinet starts the moment the specimen leaves equilibrium.
| Cabinet | Room | |
|---|---|---|
| Best for | Specimens tested quickly after removal; small and medium volumes | Labs testing many specimens, large items, or methods that require testing in the atmosphere |
| Footprint | Fits in an existing lab | Needs an insulated, sealed space and a room-type unit |
| Weak point | Specimens drift once outside | Doors, people and equipment heat load |
Many laboratories end up with both: a room for the core physical tests and a cabinet for overflow, special atmospheres or preconditioning work.
Second decision: which atmospheres?
Write down every atmosphere your customers' methods call for:
- ISO 139 temperate: 20 °C, 65 % RH. ISO 139 also defines a tropical alternative at a higher temperature.
- ASTM D1776 for textiles in American methods — historically at 21 °C rather than 20 °C, with its own tolerances that have changed between editions.
- Other set points for nonwovens, plastics or packaging that may share the equipment.
Note that tolerances have changed over time. Older documents based on earlier editions of ISO 139 quote ± 2 % RH; the current ISO 139 tolerance is ± 4 % RH. If a customer specification still quotes the old figure, clarify which applies before you choose — and before an auditor asks.
Questions to ask the supplier
Uniformity, not just stability. Stability is how steady the controller reading is over time. Uniformity is how close the conditions are at different points in the working volume — top shelf, bottom shelf, near the door, at the back. Specimens are at those points, not at the sensor. Ask how uniformity is verified and request the data.
Recovery after door opening. A cabinet opened twenty times a day spends part of the day recovering. Ask how long it takes to return within tolerance after a typical opening, with a typical load.
Capacity with air flow. ISO 139 asks for air to have free access to the specimens; AATCC methods ask for specimens laid separately on screens or perforated shelves. The usable capacity is the number of specimens you can lay out singly with air around them — usually far fewer than the nominal volume suggests.
Humidification and dehumidification. Both are needed: in a dry winter the cabinet must add moisture, in a humid summer it must remove it, and at 20 °C it must also cool. Ask what water quality the humidifier needs. Hard water leaves mineral dust on specimens and sensors.
Sensors and calibration. Humidity sensors drift. Ask how the sensor is calibrated, how often, and whether it can be checked in place against a reference instrument.
Independent records. The controller display is not evidence. An independent temperature and humidity data logger, placed among the specimens and read out regularly, gives you a record that stands up when a result is challenged. A ventilated or sling psychrometer is a simple, robust way to spot-check humidity readings.
Mapping before you trust it
When the equipment arrives, map it: place loggers at several positions in the working volume, load it as you would in practice, and record over at least a full working day including door openings. Mark any position that falls outside tolerance and do not use it for specimens. Repeat the mapping after any relocation, repair or major change in use.
A short specification for your purchase request
- Set points and tolerances required (list each standard and edition).
- Uniformity across the working volume, with verification data.
- Recovery time after door opening at typical load.
- Number of specimens that can be laid singly with free air access.
- Humidifier water requirement, cooling and dehumidification capacity.
- Calibration method for sensors, and provision for an independent logger.
Our climatic conditioning chamber is designed around the ISO 139 standard atmosphere, and for laboratories that need to work inside the atmosphere we supply room-type conditioning units in different capacities, together with data loggers and sling psychrometers. The equipment is supplied from our workshop in Istanbul with a one-year warranty.