+44 203 286 8189 steve.jones@westside-int.com

Rosahl membrane selection

ROSAHL was developed for dehumidifying (air drying) steady-state cabinets and enclosures up to 4m³ or 140 cu ft.  Steady-state means the enclosure should be sealed to avoid the ingress of moisture under normal operation-in the same ways as a domestic refrigerator.  It is understood that the cabinet will be opened from time to time to rearrange items but the expectation is it will be closed most of the time.  For enclosures larger than 4m³ multiple Rosahl membranes are used.

The precise drying time will depend on a number of variables such as starting and target humidity, air temperature, volume, leakage and so on.  The following membrane selection tables have been prepared to give users an indication of drying times.  We recommend you undertake your own tests.  If you are interested in scientific models for estimating drying times use the contact us section to obtain the relevant links.

membrane selection

Economy of selection

For different enclosure sizes, we recommend different models of Rosahl.  If the initial drying time is not critical; the enclosure remains closed for most time, or you have a lower starting humidity, it is often possible to use a smaller membrane.

It is also important to consider the volume of air inside the enclosure rather than the external dimension.  For example, if your enclosure is 50% full by volume of non-porous material, then the space to be dehumidified will be reduce by a similar amount. Finally, try and design your application to ensure you are dehumidifying the smallest volume possible



Rosahl has a half-life of about five years when operated 24/7 at an average operating temperature of 18°C.  This means that a membrane that has operated for five years has 50% of the capacity of a new membrane. It is recommended that a larger membrane is selected for the application to give a longer effective operating life.

The half-life is reduced at average annual operating temperatures above 18°C.  It is extended at lower average annual operating temperatures and when operating for shorter periods of time.  When the membrane is unpowered or in storage, the operating life is not affected.



There are many variables in selecting a suitable dehumidifier, including volume, temperature and humidity variations, target humidity and leakage.  Information is provided for guidance only.  For example, if the contents of the enclosure are porous, drying times may be considerably longer than indicated in the drying curves below.  When it is necessary to cool the enclosure it is important to dehumidify it before the cooling process starts.   See notes on humidity

ROSAHL’s dehumidifying capacity (cabinet internal humidity) is determined from the volume of air in the cabinet and its humidity.  From the table below, select a suitable model for the volume and target humidity. For example, if your cabinet has a volume of 0.5m³ and your target humidity is 50% then select the M-5J1R or MDL-5 membrane.  This table is based on an ambient humidity of 90%, but also gives selection information for ambient not exceeding 70% RH.


Selection of membrane by volume and target humidity

For <90% RH ambient
Target humidity
30% RH
Target humidity
50% RH
Target humidity
75% RH
For <70% RH ambient
Target humidity
25% RH
Target humidity
40% RH
Target humidity
60% RH
Membrane selection
PD4, PD50.25l0.5l1l
PD2, PD3, RD3, RD4 0.5l1 l2 l
M-2J1R60 l 125 l250l
M-3M1R90 l180 l350 l
M-3J1R & MDL-30.125 m³0.250 m³0.5 m³
M-5J1R & MDL-5 0.25 m³0.5 m³1 m³
M-7J1R & MDL-70.5 m³1 m³2 m³
M-10J1R1 m³2 m³4 m³

Drying time curves

The following curves indicate drying performance under defined conditions.  The characteristics of the graphs will vary depending on the material in the container and the degree of air-tightness/leakage.  If the container contains absorbent material, the membrane induces release of moisture from the material, which may slow dehumidification inside the container.  The dehumidifying capacity of the membrane varies according to the absolute water amount inside the container.  Changes in the ambient humidity may also affect the performance, and the curves are provided for guidance only.

Typical drying curve for PD4 & PD5 series


Image of membrane dehumidifier drying curve

Typical drying curve for RD3, RD4 & PD3 series


Typical MDL Series drying curve for a 50 litre container at 30°C 90% RH


Typical M-Series drying curve for a 50 litre container at 30°C 90% RH


Image of Rosahl drying performance for a 50 litre enclosure

Important user notes:

Refer to the user manuals in the documentation section of this site. In addition, please note the following points to help you use Rosahl correctly.  If you are in any doubt about the suitability of Rosahl for your application, please contact us.

– Rosahl is not intended as a domestic product and must be installed by a qualified person

– Rosahl is designed for dehumidifying steady-state (sealed) enclosures or cabinets

– The membranes have a half-life of five years operating 24/7 at an average annual operating temperature of 18°C.  Half-life declines at higher average operating temperatures.  When unpowered, the operating life is not affected by temperature

– You can increase the effective operating life by over-sizing the membrane; using a humidity controller, or operating at lower average temperature

– The device will be effective from -20 to +50C (manufacturer has tested at over 50C for storage to a maximum of 80C).

– When unpowered, the membranes do not provide a barrier to moisture from entering the enclosure.

– The devices require a power supply of 3V DC with constant current-limiting protection. Some fold-back type PSUs are not suitable. All hiccup mode PSUs are unsuitable.

– Power consumption figures are the expected daily average based on our evaluation

– Unless fully vulcanised, silicone rubber sealants should not be used as they adversely affect membrane performance.


Refer to the FAQ section or contact us for specific applications. For further information see Membrane Selection

Go here for typical Rosahl user guide







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