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

Rosahl frequently asked questions (FAQs)


Rosahl FAQ categories are listed below.  Click the category to jump to the section required:


– Application

– Availability

– Electrical Installation

– Operation

– European Directives and CE Marking

Application FAQs


What is the smallest dehumidifier available?


The world’s smallest electrical dehumidifier is the PD3 from Rosahl.  It is about 25%  the size of a regular AA dry cell; runs from a 3V dc source and is used in in steady state containers up to 2 litres (0.5 US Gal). The RD3 and RD4 have the same capacity but the RD3 is installed on the outside of the container whereas the RD4 is installed on the inside.

What do you mean steady state applications?

ROSAHL is designed for air drying in steady state cabinets up to 8m³ or 280 cu ft in size.  This means the cabinets should be sealed to avoid the ingress of moisture under normal operation-in the same ways as a refrigerator.  Of course the cabinet will be opened to move things in and out, but the expectation is that it will be closed most of the time.  As an indicator, drying curves are given in the selection section.

Metric volumes-SI version

The volume information for ROSAHL is based on cubic metres (m³).

So one cubic meter (1m³) is equal to any and all of the following:

1,000 l or litres

100,000 cl or centilitres

1,000,000 ml or millilitres

1,000,000 cm³ or cubic centimetres

For other conversions this website seems to convert almost anything to anything.
Volume Conversion

Can I use silicone as a sealant?

No.  During the curing process silicone rubber gives off gasses which can adhere to the surface of the membrane and reduce the efficiency of the electrolysis process.  Silicone based sealants can however be used in the enclosure provided the membrane is attached after the silicone has fully cured.

An example of silicone free sealants include MS Polymer from Foerch, Latex Caulk (painter’s caulk), Betafill from Dow, or similar products.

Can the Rosahl be mounted on the outside of a cabinet or enclosure?

Rosahl membranes can be mounted on the either the inside or outside as appropriate.  Please take adequate precautions to protect the membranes from mechanical damage or wetting.

What are the disadvanages of Rosahl?

When selected correctly there are no disadvantages in using Rosahl

What is the operating temperature range for Rosahl?
The operating temperature range for Rosahl is -10C  to +50C.

The surface temperature of the Rosahl membrane depends on the usage and environment.  Under typical operating conditions the membrane’s surface temperature can increase by up to 5° C  over ambient in cases of high humidity.

Rosahl has been tested at -20° C and operates with slightly reduced performance.

Where can I get a better understanding of relative humidity

If you’re looking for a basic introduction to relative humidity, try Wikipedia

Specifically in relation to using Rosahl dehumidifiers you might also try our guide called Understanding Humidity

Is there an oxygen build-up inside the enclosure?

Based on application information from the manufacturer, oxygen levels inside the container can rise by about 1% over background levels: for example from 20% to 21%.

It is not simple to predict due to number of variables and depends on factors such as ambient and target humidity levels, and leakage area of the container.  Understandably, as the internal humidity falls, less oxygen is produced until the process stops.

Is there an MDL mounting kit available for Rosahl?

A mounting retainer and packing gaskets are available individually.  The retaining plate is a cut and pre-drilled stainless steel.  See catalogue page 7 or go here for more details.

The M-Series membranes are mounted on a pre-drilled stainless steel plate and supplied with a packer/gasket.  No additional mounting kit is required.

When might Rosahl not be suitable?

When might Rosahl not be suitable?

Rosahl is suitable for use in a wide range of applications, and by taking suitable precautions, its use will be extended. In the following situations, please contact us with details of your application and we will offer further guidance. For example:

  • Applications with a high average ambient temperature
  • Where you are unable to seal the enclosure to prevent moist air from entering
  • Where you require a short drying cycle, particularly when using a large enclosure
  • In applications where the membrane is likely to become wet, or in aggressive environments such as with VOCs present
  • Applications below -20°C
  • Applications with high pressure differentials
  • Military applications

Availability FAQs

Where can I buy ROSAHL micro dehumidifiers?

ROSAHL is available only from Westside International, a UK based limited company.  We ship products to worldwide destinations.  Use the contact us page or contact us at:-

Westside International Ltd
Farbeld House
Station Road
Oxfordshire OX12 7PE

Tel: +44 203 286 8189
E-mail: information@westside-int.com

How much does Rosahl cost?

Rosahl prices are based on quantity per order.  The smallest membranes start from GBP 17.20 (UK Pounds) and there are some great quantity discounts available.  See price list or use the contact us page for more information

Can I buy a power supply from you?

Using a power supply with the correct overload protection is important for the reliable operation of Rosahl flat panel membrane dehumidifiers. The manufacturer recommends a constant current limiting overload characteristic.  Power supplies using a hiccup mode are NOT suitable.  Power supplies with fold-back overload may be used provided the output voltage under fault conditions does not fall below 2V DC.

Westside International Ltd offers a suitable range of open board type, and enclosed power supplies.  Go here for more information, or contact manufacturer ETA

Electrical installation FAQs

Rosahl inrush currents

The inrush current taken by Rosahl on power-up is related to the size of membrane, the RH of the enclosed space, and water vapour in the membrane.  It can draw up to 12A in extreme cases

We recommend the use of current limiting power supplies for a number of reasons: it avoids nuisance tripping;  it is less stressful for the membrane, and a smaller power supply is cheaper.

The recommended power supply requirements are given below, and in addition to limiting the available current it is important to maintain the voltage to – 0.1 V and +0.3 V.  Power supply requirement are in the user datasheets and are found in the Documentation section of this website.  They are summarised below.

Recommended power supply rating       
  MDL-3 MDL-5 MDL-7
Rated output voltage (all models) 3V dc
Rated output current 2A 3A 5A

Hiccup power supplies are not suitable

What is the Rosahl membrane voltage tolerance?

The membranes are designed to operate at 3 V d.c.  The supply should not fall below 2.9 V and not exceed 3.3 V d.c.

What happens if I reverse the membrane polarity?

It is important to avoid this.  When the polarity is reversed the dehumidification process will also be reversed.  More importantly the porous electrode will be depleted by the chemical reaction leading to irreparable failure of the membrane in a relatively short time.

Can I use a hiccup power supply?

The membrane user guides give the rating and characteristics of the power supplies. For the MDL types a constant current limiting type is recommended.  Hiccup PSUs are not suitable.  Fold-back may be used provided the voltage to the device does not fall below 2V DC

Operation FAQs

How does Rosahl dehumidify a cabinet?

By mounting a Rosahl membrane on the inside wall of a cabinet and providing vent holes it uses electrolysis  to remove moisture and expels it to the outside.

When 3V dc is applied to the Rosahl membrane the moisture on the inside of the cabinet breaks down into hydrogen ions (H+) and oxygen: the hydrogen ions migrate through the membrane to be discharged on the outside where they react with oxygen in the air, resulting in water vapour being discharged.

How does Rosahl work?

Is the ROSAHL membrane dangerous?

No.  ROSAHL membrane is not dangerous.

There are warnings about careful handling in the installation instructions to avoid contact with the operating faces of the membrane but this is to avoid contamination and ensure a long working life.

For more information see the data-sheets in the documentation section

Can this be used for home environment to reduce humidity?

ROSAHL is not generally suitable for domestic applications as the moisture extraction rate is relatively low.
It is designed to protect equipment in sealed cabinets cabinets, and requires installation by a qualified electrical professional.

Will ROSAHL operate at -20°C?

ROSAHL will work normally at temperatures of -20°C. Contact us if you require performance data.

What is the life of a Rosahl dehumidifier membrane?

Dehumidification performance of the devices will gradually decrease with the use. The life expectancy is based on accelerated tests conducted by the maker.  Working 24/7 at an average operating temperature of 18 ℃, the performance half-life is about 6 years.  This is extended if the device is not used continuously for example when a humidistat is installed.  At higher annual average operating temperatures the lifecycle is reduced.  Contact us for specific requests.

We recommend you oversize the membrane to allow for a decline in performance over time. The membrane is unlikely to suffer catastrophic failure but will degrade over time.  A typical effective operating life of about eight years can be expected when suitable precautions are taken.

How low can Rosahl reduce the humidity?

Depending on the size of the enclosure, the size of membrane installed, and the sealing against moisture ingress , Rosahl can reduce humidity to below 5%.  This may however be too low for some natural substances such as wood, and a humidistat should be used for control.

How does a Rosahl solid state dehumidifier work?

Rosahl works by electrolysis, removing moisture at a molecular level.  To do this Rosahl uses a solid polymer membrane with a porous dehumidifying side (anode).  When a voltage (3V dc) is applied the moisture dissociates into hydrogen atoms (H+) and oxygen.  The hydrogen migrates through the membrane and is discharged on the cathode side where is reacts with oxygen to form water vapour.

For more go here

What is a Rosahl solid state dehumidifier?

A Rosahl  solid state dehumidifier is a solid polymer membrane (SPE) that is used to electrolytically decompose and remove moisture in the air using a low voltage (3V dc) source.  No liquid water is involved in the process

What is the operating temperature of Rosahl?
The operating temperature range for Rosahl is -10C  to +50C

The surface temperature of the Rosahl membrane depends on the usage and environment.  Under typical operating conditions the membrane’s surface temperature can increase to ambient +20° C  in cases of high humidity.

What is the storage temperature of Rosahl membranes
The temperatue range for storage is -40 to +80C
What are the recommended humidities for production and storage environments?
To avoid deterioration and damage of products or to achieve proper process conditions, environment and the indoor climate must often be within certain temperature and humidity limits. Low relative humidity may dry up products and high relative humidity may increase the water activity and growing mould and bacteria in the production or process lines.

  • For specialist information consult the manufacturer or a materials specialis
Will installing a recirculating fan improve dehumidifying performance?

Yes. The dehumidifying performance of the membrane improves when a fan is installed inside the enclosure.


The improved air circulation allows more water vapour to be carried to the element surface than in static enclosures and accelerates the drying process.


 There is no other way to improve the dehumidifying performance other than the above.

European Directives and CE Marking FAQs

           >>Go here for the RoHS Declaration of Conformity
           >>Go here for the REACH/SVHC Declaration of Conformity
Are ROSAHL membranes RoHS compliant?



Yes, all Rosahl membranes are RoHS compliant. The manufacturer has completed a  Declaration of Conformity.


Additional information


RoHS stands for Restriction of the use of Certain Hazardous Substances in Electrical and Electronic Equipment. The original RoHS directive came into force in July 2006 and banned  the placing on the EU market of new electrical and electronic equipment containing more than agreed levels of six regulated substances: lead, cadmium, mercury, hexavalent chromium, polybrominated biphenyl (PBB), polybrominated diphenyl ether (PBDE). It was replaced by Directive 2011/65/EU and referred to as RoHS 2.

The RoHS 3 Directive (EU) 2015/863 came into force from July 2019 and added four additional substances DEHP, DBP, BBP, DIBP to the original six.

Directive (EU) 2017/2102 was adopted from June 2019 solely to address issues addressing practical implications affecting secondary market operations for EEE, involving repair, replacement of spare parts, refurbishment and reuse, and retrofitting, should be facilitated to promote a circular economy in the Union. Certain niche groups were also excluded from the scope (e.g lead organ pipes)


Are Rosahl membranes CE marked?


 The Rosahl membranes are not required to be CE marked as explained below

The three European Directives for electrical and electronic equipment that have implications for Rosahl membranes are the: Low Voltage Directive; EMC Directive, and RoHS 3 Directive.

Low Voltage Directive 2014/35/EU


Rosahl membranes are not required to meet the European Low Voltage as they are out of scope according to Article 1 because they operate on 3V d.c


Article 1 states:
For the purposes of this Directive, ‘electrical equipment’ means any equipment designed for use with a voltage rating of between 50 and 1 000 V for alternating current and between 75 and 1 500 V for direct current, other than the equipment and phenomena listed in Annex II.


EMC Directive 2014/30/EU

Rosahl membranes are not required to meet the European EMC Directive as they are out of scope according to Article 1 Section 2(d).

Article 1 Section 2(d) states:

This Directive shall not apply to:

equipment the inherent nature of the physical characteristics of which is such that:

(a) it is incapable of generating or contributing to electromagnetic emissions which exceed a level allowing radio and telecommunication equipment and other equipment to operate as intended; and

(b) it will operate without unacceptable degradation in the presence of the electromagnetic disturbance normally consequent upon its intended use.


RoHS 3 Directive 2015/863/EU

Answer: Rosahl membranes meet the requirements of European RoHS 3 Directive, but are not required to be CE marked.

According to the guidance notes issued by the EU Directorate-General for the Environment, components are sold as a stand-alone components must meet the substance restrictions according to Art. 4 but do not need CE marking.  In response to Q7.3 ” Do components have to comply with RoHS?” the guidance notes state: “RoHS  provides that the electrical and electronic equipment (EEE) has to meet the requirements of the Directive. Since equipment consists of different components, the EEE itself can only meet the substance requirements if all its components and parts meet the substance restriction requirements of RoHS, including non-electronic or non-electric components like fasteners or the plastic case of a desktop computer. Therefore components being used in finished EEE or for repair or upgrade of used EEE, which is in the scope of RoHS must meet the substance restrictions according to Art. 4 but do not need CE marking.  Components sold as a stand-alone components or if produced to be used in a product benefiting from an exclusion do not have to be CE marked and do not have to comply with the substance requirements.”


Does Rosahl meet European Directives for LVD, EMC and RoHS ?



Rosahl membranes are out of scope of the European LV Directive 2014/35/EU. It is also considered out of scope according to Article 3.1 of the EMC directive 2014/30/EU.

Rosahl membranes are compliant with RoHs Directive  but as components do not need to be CE marked

See FAQ ‘Are Rosahl membranes CE marked?’


Do membranes comply with the European REACH Directive?

Yes. The manufacturer has completed a Declaration of Conformity.

Rosahl membranes conform to the European REACH Regulation 1907/2006/EC set up by the European Chemicals Agency (ECHA) wherein the substances listed do not exceed 0.1% wt % in Substance of Very High Concern (SVHC)







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