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Adsorption is not Absorption

The “d” in Adsorption is not a typo mistake to be replaced with a “b” to spell Absorption. These two terms describe two different scientific concepts. Adsorption is when water is held by a physical bond on the surface of a material, but Absorption is when water is integrated with the material.

Adsorption “d” is used for desiccants like dry clay, silica gel or molecular sieves. They have many small pores that give these materials very large surface areas that can bind water.

Absorption “b” is used for Chemisorbents that form chemical bond with water, like Calcium oxide (CaO), also known as quicklime or burnt lime.


There are many types of desiccants. Liquids like acetone and alcohols are dehydrating agents that can pull water vapor from the air. Solid salts like NaCl can also pull water vapor from air. Desiccants like these are chemically reactive and therefore unsuitable for many applications, but are very suitable for other applications.

We offer natural Activated alumina, Clay, Molecular Sieve and Silica Gel, that can be in direct contact with goods and not be harmful. We also offer Calcium Chloride with Starch, that bind the salt when it’s wet and prevent it from leaking. Calcium Chloride is a very cost efficient desiccant that’s suitable for container transports, but it’s chemically active and it expands in size, so it’s unsuitable for many applications. Clay is the most cost efficient alternative (for sensitive applications), when large amounts are needed during normal conditions. Silica Gel is the preferred alternative when smaller amounts are needed during normal conditions. Molecular Sieves is the preferred alternative when extra dry conditions must be met and when when conditions are cold or hot. Activated alumina is a durable alternative when the desiccant shall be regenerated many times.

  • Activated alumina: A porous desiccant with similar properties as Silica Gel when drying. The advantage with Activated alumina is the high crush resistance, and its physical and chemical stability. It’s therefore useful in applications where the desiccant will be regenerated many times.   
  • Calcium Chloride: A salt with formula CaCl2 that suitable as a desiccant from 0 °C (or freezing point) to +80 °C. It can absorb many times it’s own weight in moisture, and will then form a liquid brine. This brine is chamically active and it’s therefore important to store it in a leak proof bag/container, and/or bind it with Starch to a gel.
  • Clay: A natural, safe all-rounder, suitable for standard applications in all areas of industry. See link
  • Silica gel: Commonly found in small packaging used in industry, pharmaceutics, diagnostics and the food sector due to its high adsorption capacity. In the United States its often packed with food and pharmaceuticals, as Silica Gel has been approved for direct contact by the FDA (U.S. Food & Drug Administration). See link
  • Blue/Orange Silica gel: Silica gel with a color indicator to show if it’s dry or wet. Cobalt (II) chloride is deep blue when dry and pink when wet. Methyl violet can be formulated to change from orange to green, or orange to colorless. See Link
  • Molecular sieve: Suitable for pharmaceutics and diagnostics, as well as special technical applications Molecular Sieves can be produced with different pore size, from 3 Ångström (Angstrom) and larger. Common pore sizes are: 3Å, 4Å, 5Å, 10Å / 13X and they are often written 3A, 4A, 5A, 10A / 13X. Silica Gel and Clay have many different pore sizes and will therefore adsorb most chemicals, but Molecular Sieves with small pores can only adsorb small molecules like water and will not adsorb large molecules. When packed together with Silica Gel, the Molecular Sieves can remove moisture from the Silica Gel. See link

Here below are graphs comparing Silica Gel, Clay and Molecular Sieve under different conditions.

NOTE: Constant absolute humidity 10 g/m3 from +20°C to +300°C in graph above

Desiccant Polymers

Desiccants can also be combined with polymers, so they can be moulded into shape. The adsorption speed of the desiccant is slowed down by the polymer and this is a desired characteristic for some applications, as the desiccant will not be fully saturated in short time when a container is open for inspection. The speed of adsorption is a function of the thickness and desiccants deep inside a moulded piece will therefore never adsorb any water. This limits the useful thickness for moulding. Note also that the polymer has high viscosity when moulding, and this sets a limit on its ability to fill narrow moulds. 

Example 1: Tropack Packmittel GmbH – TROPApuck 2 g

Example 2: Tropack Packmittel GmbH – Active-Film M-0026


Humidity can be measured as gram of water vapor per cubic meter of air. When hot air cools down, then it will reach a temperature when water droplets starts to form. This temperature is the Dew point. The relative humidity is 100% RH when dew starts to form.

The highest natural Dew point was measured 2003 in Dharan, Saudi Arabia, it was +35°C and the air was then saturated with 40 grams of water vapor per cubic meter. The highest Dew point measured in USA was +30°C (30 gram/m3), but this happens rarely. An upper limit for most installations is +25°C (23 gram/m3) and on average one can use 15 gram/m3 when making calculations.

Note that condensation can start on cold surfaces in a building when the relative humidity is only 60-70% RH. This can cause mold, corrosion and other humidity related problems. Relative humidity that’s too low may cause wood to shrink, paint to crack and static electricity to form. Too dry air may also cause human health problems and an indoor humidity of 25-60% RH is therefore recommended for human comfort.

Outdoor summer weather 
40% RH perfect summer day
50% RH comfortable
65% RH uncomfortable

Humidity in package – 4 sources
1) Water vapor in materials inside package
2) Water vapor on the walls of the package
3) Water vapor in air inside package
4) The permeation of water vapor into the package

Damages caused by humidity 
1) Pharmaceutical formulas and reagents for diagnostics often degrade faster in humid conditions, there are however exception like the gel coating on capsules that can also degrade when conditions are too dry, causing the gel coating to become cracked and milky in appearance.
2) Moisture can promote growth of mold, mildew and fungus.
3) Polymers often swell in humid conditions and the presence of water can weaken the bonds between polymeric chains. Polyester has the same tensile strength when wet, but Nylon, Rayon and many other polymers become weaker.
4) Moisture can penetrate into the crystal structure of some materials and form hydrates. Some materials are also water soluble and too high humidity will cause irreversible damage.
5) Electronic component that are not dry enough, will be permanently damaged when they are sent through a solder oven. The moisture inside will become steam and the component will explode/crack. This is called the Popcorn effect. Sometimes the damage is obvious and easy to repaid, but sometimes its just a small crack and the component will work when the electronic circuit is tested, but will fail later when installed. The cost of replacing the component is relative small if its detected at the factory, but the cost to replace the component when its already installed in site is much higher and its also very hard to find the problem it the component is causing an intermittent fault. 
6) Moisture can cause electronics to short circuit, when the electric current is conducted by water rather than the intended circuit. This can cause a dramatic failure, but could also be very hard to detect if there’s only small current taking an alternative path.

Humidity Indicator Cards

The most common humidity indicator cards use cobalt (II) chloride and change color from blue (less than indicated RH level) to pink (greater than indicated RH level). These are available in many versions and are easy to read. They can be used for many years, but will degrade over time if exposed in direct sunlight (UV-light) and they will also degrade if exposed to liquid water or condensation, as cobalt (II) chloride is a salt that will wash away with water.

The humidity indicators are inexpensive, so you cannot compare their accuracy with that of expensive electronic instruments. The tolerance is +/– 5% relative humidity at 20°C (+/– 2°C) and deviations are approximately 2.5% relative humidity per 5°C above and below 20°C. The cobalt (II) chloride color changes later in temperatures above 20°C, and earlier in temperatures below 20°C.

Although the European Community (EC) has issued a directive that classifies items containing cobalt (II) chloride as toxic, it has not banned these indicator cards. There is however a desire to replace these with Cobalt free humidity indicator cards. One example is based on copper (II) chloride and its Brown when dry and Azure when wet.

More information about Humidity Indicator Cards. See link

Immersion Proof Breathers

Immersion Proof Breathers are also called protective vents or “free breathers”. They are based on a membrane that keep liquid water and water droplets out, but will let water vapor and air through for pressure equalization. The hydrophobic properties of the membrane prevent the breakthrough of liquid water, but allow permeation of water vapor. Other gases with a similar molecular size (~3.1Å or less) to water vapor also permeate through the membrane. Some membranes work equally good in both directions, but some membranes have one side that is designed to be out and one side to be in. We can help you determine what type to use and help you calculate the right dimension.

Mini Breather

The Immersion Breather above can be submerged 1 meter under water for 2 hours without leakage of liquid water. But air and water vapor can pass the membrane and the flow rate depends on the pressure differential. Nominal Flow Rate is 30cc/min @ 1″ Wg and 85 cc/min @ 6″ Wg, where Wg is Inch Water Gauge, a Non-SI unit for pressure and cc/min is cubic centimeters per minute of air and water vapor at standard pressure.

Example: Imagine that we have a water tight box and it can handle a maximum pressure of 6″ Wg. Imagine that the temperature increase in this box at a rate of +1°C per minute. Imagine that this box is vented by one Immersion Breather with flow rate 85 cc/min @ 6″ Wg.

Question: How large internal air volume can this box have, if the maximum expansion of air shall be less than 85 cc/min @ +25°C?

Calculation: At normal pressures and temperatures the gas will have a linear expansion when we compare to the absolute temperature in Kelvin (K), where temperature Kelvin is the temperature in Celsius + 273.

Temperature +25°C = (25+273) K = 298 K

Volume (V) * 1(°C/min)/298°K < 85 cc/min

Volume (V) < 298 * 85 cc

Volume (V) < 25330 cc

Result: The box must be less than 25 liter (25330 cc) for one Immersion Breather

Moisture Vapor Transmission Rate (MVTR)

(Water Vapor Transmission Rate (WVTR) or Water vapor permeability)

Molecules of water vapor are very small and can therefore permeate through materials. The type of material and the thickness will affect how fast. Materials like metal and glass are hard for water vapor to permeate, but materials like fabrics and plastics are easy to permeate. Adding a layer of aluminum to a plastic bag will therefore drastically improve the ability to prevent water vapor permeating. The most common international unit for the MVTR is g/m²/day. Rates in aluminium foil laminates can be as low as 0.001 g/m²/day, but the rate in fabrics can be several thousand g/m²/day. The MVTR value will depend on Relative Humidity and Temperature. High humidity and temperature will give higher MVRT value. For storage in a normal warehouse it can be of interest to know the MVTR value at +25°C and 40% RH, but for storage in tropical conditions it can be of interest to know the MVTR value at +35°C and 85% RH.

Example: Imagine that we have a Moisture Barrier Bag (MBB) made out of laminated aluminum foil. MVTR for this bag is 0.01 g/m²/day at +25°C and 40% RH. The inner area of the front and back of the bag is 2 x 100 x 150 mm = 30000 mm2 = 0.03 m2. Imagine that we like to store an item in a normal warehouse for 5 years and we like to have less than 10% RH inside the bag all these years.

Question: How much desiccant do we need in the bag?

Calculation: The amount of water vapor permeating the bag can be calculated as:

MVRT value * the area * number of days

0.01 * 0.03 * (5×365) = 0.5475 gram

Molecular Sieves is a better alternative than Clay or Silica Gel if we like to keep 10% RH in the bag. The capacity of Molecular Sieve is 15 % adsorption at +25°C and 10% RH (as shown in previous graph). We also need to subtract 2% adsorption, as desiccants can have this when new from factory. The amount of Molecular Sieve we need can therefore be calculated as: 

Amount > 0.5475/((15-2)%)

Amount > 0.5475/0.13

Amount > 4.2 gram

Pressure Valves

Pressure Valves, also known as Pressure Relief Valves, Pressure Release Valves or Breather Valves. These are spring loaded valves that protect sealed containers from excessive pressure or vacuum. The cost, weight and size of these containers can therefore be reduced. These spring loaded valves offer higher flow rates than Immersion Proof Breathers. They are also 100% sealed when the pressure difference is low, as compared to Immersion Proof Breathers.

These valves come in different sizes for different air flows, and also with different opening pressure. For Two-Way Pressure Relief Valves the opening pressure can be different for over pressure, as compared to vacuum. Some containers can withstand high overpressure, but are very sensitive to vacuum. It’s therefore important that calculations are made to determine what pressure valves to use. We can help you with these calculations or you can follow the calculation guide in this brochure from AGM Container Controls – Breather Valves

Example: The AGM valve TA238-30-30-R has for over pressure a cracking pressure range of 3.00-4.00 PSID and a reseal pressure after crack of minimum 3.00 PSID. Minimum flow rate is 2 SCFM at 4.5 PSID. For under pressure (vacuum) the same valve has a cracking pressure range of 3.00 – 4.00 PSID and a reseal pressure after crack of minimum 3.00 PSID. Minimum flow rate is 2 SCFM at 4.5 PSID. Note that the flow rate is not fixed for all pressures, but follows a curve and here below are the Flow Rate v.s. Pressure curves for AGM valve TA238-30-30-R.

AGM manufactures a variety of Breather Valves, which include valves that keep dust, water, and blowing sand from entering containers. Some are also protected against Radio Frequency Interference (RFI) to keep radio signals inside or outside the container. Some high flow versions are magnetic valves, and they open faster to full air flow. These Breather Valves meet SAE AS27166 (replaces cancelled MIL-V-27166) and MIL-DTL-27166 which is the re-instated version of MIL-V-27166.

Demonstration – Train Tanker Implode from Vacuum 


Transportation and storage has always been a problem. For most of history humans have needed to store food for at least a year, to make sure they would not starve before the next harvest. There’s also been a need to prevent metals from corrosion and fabrics from mould. This has been extra important for the military, as they need to store supplies for a very long time and then also transport and store these in climate zones that offer many different problems for storage. The US Department of Defence (DoD) was therefore early in setting a MIL-standard. Other standards that are similar in many ways have followed. In France they have the AFNOR-standard and in Germany a similar DIN-standard and the BWB TL-standard. For electronic manufacturing there’s also the Standard Organisation JEDEC and they publish industry specific standards for storage and handling.

AFNOR NF H 00321
1 Unité of the French AFNOR standard can bind 100 grams of moisture at 23°C and 40% RH. This is equivalent to 16 Units US MIL-D 3464 E or 16 Einheiten according German DIN55473.

BWB TL 6850-0008 
The German military standard BWB (Bundesamt F. Wehrtechnik und Beschaffung)

DIN 55473
16 Einheiten according German DIN55473 is equivalent to 16 Units US MIL-D 3464 E, or equivalent to 1 Unité of the French AFNOR standard that can bind 100 grams of moisture at 23°C and 40% RH.


MIL-standards used by the US military can often be found by QuickSearch

The US Department of Defence (DoD) released the MIL-D-3464C standard in 1963, covering the use of desiccants in bags for packaging and static dehumidification.

In 1966 the standard was updated to MIL-D-3464D to also include adsorption capacity, the size of desiccant particles, rate of adsorption and how dust tight, strong and corrosive the desiccant bags were.

In 1987 the standard was updated to MIL-D-3464E download

From 1988 replaced with MIL-STD-2073-1C

From 1988 replaced with MIL-STD-2073-1C

From 1991 replaced with MIL-STD-2073-1C

From 1991 replaced with MIL-STD-2073-1C


Bags with German DIN 55473 standard, TL 6850-0008, French AFNOR NFH 00321 and US MIL-D 3464 E. They conform to EU laws and some are approved by FDA, the US Food and Drug Administration, for use with pharmaceuticals and food.

Different batches of dry clay will adsorb different amount of moisture. The amount of clay is therefore changed for each batch. “Weight app.” in the tables here below is therefore not exact, but an “app.” approximate weight.

1 Unité of the French AFNOR standard can bind 100 grams of moisture at 23°C and 40% RH. This is equivalent to 16 Units US MIL-D 3464 E or 16 Einheiten according German DIN55473.

DIN OR MILWeight app.@ 20% RH
1/6 UNIT6 gram+0.7 gram
1/3 UNIT12 gram+1.5 gram
1/2 UNIT18 gram+2.2 gram
1 UNIT35 gram+4.5 gram
2 UNITS71 gram+9 gram
4 UNITS142 gram+18 gram
8 UNITS285 gram+36 gram
16 UNITS570 gram+72 gram
32 UNITS1140 gram+144 gram
DIN OR MILWeight app.@ 40% RH
1/6 UNIT6 gram+1 gram
1/3 UNIT12 gram+2 gram
1/2 UNIT18 gram+3.1 gram
1 UNIT35 gram+6.2 gram
2 UNITS71 gram+12.5 gram
4 UNITS142 gram+25 gram
8 UNITS285 gram+50 gram
16 UNITS570 gram+100 gram
32 UNITS1140 gram+200 gram
DIN OR MILWeight app.@ 80% RH
1/6 UNIT6 gram+1.6 gram
1/3 UNIT12 gram+3.2 gram
1/2 UNIT18 gram+5 gram
1 UNIT35 gram+10 gram
2 UNITS71 gram+20 gram
4 UNITS142 gram+40 gram
8 UNITS285 gram+80 gram
16 UNITS570 gram+160 gram
32 UNITS1140 gram+320 gram

Unit conversion

Here below are some common units listed and compared. There are also other units and most of these can be found at


1 in = 2.54 cm = 0.0254 m
1 ft = 30.48 cm = 0.3048 m
1 yard = 91.44 cm = 0.9144 m
1 mile = 1.6093 Km = 1609.3 m


pascal [Pa]
1 kilopascal [kPa] = 1000 pascal [Pa]
1 bar = 100000 pascal [Pa]
1 psi [psi] = 6894.7572931783 pascal [Pa]
1 Standard atmosphere [atm] = 101325 pascal [Pa]
1 megapascal [MPa] = 1000000 pascal [Pa]
1 newton/square meter = 1 pascal [Pa]
1 millibar [mbar] = 100 pascal [Pa]
1 dyne/square centimeter = 0.1 pascal [Pa]
1 kilogram-force/square meter = 9.80665 pascal [Pa]
1 pound-force/square foot = 47.8802589804 pascal [Pa]
1 pound-force/square inch = 6894.7572931783 pascal [Pa]
1 torr [Torr] = 133.3223684211 pascal [Pa]
1 millimeter mercury (0°C) = 133.322 pascal [Pa]
1 inch mercury (32°F) [inHg] = 3386.38 pascal [Pa]
1 inch mercury (60°F) [inHg] = 3376.85 pascal [Pa]
1 millimeter water (4°C) = 9.80638 pascal [Pa]
1 inch water (4°C) [inAq] = 249.082 pascal [Pa]
1 foot water (4°C) [ftAq] = 2988.98 pascal [Pa]
1 inch water (60°F) [inAq] = 248.843 pascal [Pa]
1 foot water (60°F) [ftAq] = 2986.116 pascal [Pa]
1 atmosphere technical [at] = 98066.500000003 pascal [Pa]


°F = °C*1.8 + 32
°C = (°F – 32) / 1.8
°R = °F + 459.67
°K = °C + 273.15


1 in3 = 16.387 cm3
1 ft3 = 0.0283 m3
1 us f.oz = 29.574 ml
1 imp f.oz = 28.41 ml
1 us gal = 3.7854 l
1 imp gal = 4.546 l


1 OZ = 28.35 g
1 lb = 0.4536 kg = 453.6 g
1 imp ton = 1016 kg

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