Rosahl | frequently asked questions (FAQ)


Search the FAQ for the top questions about Rosahl ionic membranes or leave a new question in the comment area of the reply section below and we will respond within a couple of days.

Rosahl FAQ categories are: 

– Application

– Availability

– Electrical Installation

– Operation.


Application (13)

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). Larger sizes are available for up to 8 m³ (280 cu ft)

Rosahl is a solid state device that requires no maintenance, has no moving parts and is very economical over its long useful life.

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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.

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Where can I get a ROSAHL range overview?

ROSAHL is available in various sizes and and installation options including membrane only, membrane with integrated power supply, and membrane with integrated power supply and humidity control.  For an overview go to:-


Selecting the correct membrane

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Why isn’t Rosahl suitable for domestic applications

In fact ROSAHL can be used in most environments including homes however its purpose is air drying enclosed spaces such as cabinets or gun safes. It is sold as a device and needs to be installed with a suitable power supply and control circuit to ensure a long service life.  Contact us if you have specific applications questions.

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Metric volumes-SI version

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

So one cubic meter (1m³) is equal to any 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



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Metric volumes-confused version

The volume information for ROSAHL is based on cubic metres also spelt (or spelled)  meters and shortened correctly to m³ as opposed to cu. m.  To confuse matters further it is sometimes also known as the stère (not widely used any more according to Wikipedia) or the kilolitre/kiloliter.  Clear so far?

So 1m³ equals all of the following:

1,000 l or litres/liters)

100,000 cl or centilitres/centiliters

1,000,000 ml or millilitres/milliliters

1,000,000 cm³ or cubic centimetres/centimeters or sometimes also called cc

…and if you don’t believe me go here as it seems to convert almost anything to anything.
Volume Conversion



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Can I use silicone as a sealant?


During the curing process silicone rubber gives off siloxane gas, which can adhere to the surface of the membrane and reduce the efficiency of the electrolysis process.  However it can be used provided the membrane is attached after the silicone has fully cured.

Ask your local supplier for a silicone free sealer like MS Polymer from Foerch, Latex Caulk (painter’s caulk), Betafill from Dow, or similar,

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

The Rosahl membrane can be mounted on the either the inside or outside as appropriate.  Specify at time of ordering

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What are the disadvanages of Rosahl?

When selected correctly there are no disadvantages in using Rosahl

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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.

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Where can I get a better understanding of relative humidity

If you’re looking for a basic introduction to relative humidity, try the HyperPhysics at http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/relhum.html

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


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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 fall, less oxygen is until the process stops.

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Is there an MDL mounting kit available for Rosahl?

A mounting retainer and packing gasket are available to order.  The retaining plate is a cut and pre-drilled plate made from plated mild steel.  See catalogue page 7 or go here for more details

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Availability (4)

Where can I buy ROSAHL micro dehumidifiers?

ROSAHL is only available from Westside Internationa based in the UK from where it is shipped worldwide.  Use the contact us page or contact them at:-

Westside International Ltd
Farbeld House
Station Road
Oxfordshire OX12 7PE

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


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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

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Can I buy a power supply from you?

As electrical regulations vary so much around the world, we normally supply only the membrane.  The equipment maker needs to mount the membrane, supply an appropriate humidity controller and suitable power supply.  Because Rosahl it is a passive 3V dc component it is not required to be CE marked.  It does conform to the European RoHS directive.


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What is the cost price of a Rosahl ionic membrane dehumidifier?

The cost price of the Rosahl ionic membrane dehumidifier is based on the unit and quantity ordered.  Prices start from 34 Euros and more information can be found on the basic Rosahl price list .  Units are generally stocked in our UK facility  and shipped worldwide.  Please contact us for a quotation if required

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Electrical installation (7)

Rosahl inrush currents

The inrush current taken by Rosahl on power up is related to the size of membrane; 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 Documents 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
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What is the Rosahl membrane voltage tolerance?

The MDL 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.

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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.

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Are ROSAHL membranes RoHS compliant?

The Rosahl membranes are RoHS compliant.

RoHS stands for the Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment. The European RoHS Regulations came into force on 1 July 2006.

The RoHS Directive bans the placing on the EU market of new electrical and electronic equipment containing more than agreed levels of lead, cadmium, mercury, hexavalent chromium, polybrominated biphenyl (PBB) and polybrominated diphenyl ether (PBDE) flame retardants.

Also see Are Rosahl membranes CE marked

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Are Rosahl membranes CE marked?

The three European Directives for electrical and electronic equipment that have implications for Rosahl membranes are the: Low Voltage Directive; EMC Directive, and RoHS 2 Directive.  The Rosahl membranes are not required to be CE marked as explained below:

Low Voltage Directive 2006/95/EC

Answer: Rosahl membranes are not required to meet the European Low Voltage as they are out of scope according to Article 1

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 2004/108/EC

Answer: Rosahl membranes are not required to meet the European EMC Directive as they are out of scope according to Article 1 Section 3.

Article 1 Section 3 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 Directive 2011/65/EU

Answer: Rosahl membranes meet the requirements of European RoHS  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.”

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

Rosahl membranes are out of scope of the European LV Directive 2006/95/EC. It is also considered out of scope according to Article 3.1 of the EMC directive 2004/108/EC.  Rosahl membranes are compliant with RoHs Directive  but as components do not need to be CE marked


See Are Rosahl membranes CE marked

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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

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Operation (9)

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.

For more go to http://www.rosahl.co.uk/rosahl/how-does-rosahl-work/

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Is the ROSAHL membrane dangerous?


No, the 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 download section

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Can this be used for home environment to reduce humidity?

Thanks for your question, Unfortunately ROSAHL is not suitable for the application you have in mind as its moisture extraction rate is too low.
It is designed to protect equipment in closed cabinets.

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Will ROSAHL operate at -20°C?

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

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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.  On continuous use at 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 may reduce.  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.

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How low can Rosahl reduce the humidity?

Depending on the size of the enclosure, the sealing against moisture ingress the size of membrane installed, Rosahl can reduce humidity to below 5%.  However, this may be too low for some substances and a humidistat should be used.

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How does a Rosahl solid state dehumidifier work?

It works by electrolysis, by 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

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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.

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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.

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