WO2002041094A1

WO2002041094A1 - A humidity control device

Info

Publication number: WO2002041094A1
Application number: PCT/GB2001/005046
Authority: WO
Inventors: Geoffrey Russell-Smith
Original assignee: Russell Smith Geoffrey
Priority date: 2000-11-18
Filing date: 2001-11-16
Publication date: 2002-05-23
Also published as: GB2369070A; AU2002223809A1; GB0028208D0; GB2369070A8

Abstract

A humidity control device (12) for a closed container (10) comprises a hygroscopic desiccant medium (24) contained within a chamber (14). The chamber (14) includes at least two closeable vents (32, 34) controlled in response to ambient temperature. One of the vents (32) is openable to allow the desiccant medium (24) to absorb moisture from air entering the chamber (14) via that vent (32)when ambient temperature is below a first predetermined temperature value. The other of the vents (34) is openable to allow the desiccant medium (24) to expel moisture via that vent (34) when ambient temperature exceeds a second predetermined temperature value.

Description

A HUMIDITY CONTROL DEVICE

The present invention relates to a humidity control device for a closed container.

When goods are transported from one location to another, they are often packed into a packaging container. If the packaging container is sealed, changes in ambient temperature can lead to the creation of condensation within the container from moisture present in the air surrounding the goods. This effect is commonly referred to as "sweating", and can occur in, for example, plastic bags, wooden boxes, metal containers, mobile vehicles, mobile homes and the like.

The creation of condensation within a packaging container is undesirable since it can damage goods packed within the container, particularly those goods which are susceptible to, for example, rust or rot.

Ventilation is often used in an attempt to combat "sweating". Such ventilation may be provided by means of a grille or louver having no moving parts. If moving parts are provided, they are usually only manually adjustable to provide a set airflow rate over a long period of time.

Permanent ventilation does not however eliminate the problems caused by unwanted moisture. This is because in the event that the air surrounding the packaging container has a higher humidity than the air within the container, the humidity within the container will increase in order to reach equilibrium with the environment outside the container. Other methods of controlling the humidity of air within packaging containers include the provision of flap-like mechanisms that respond to ambient temperature and humidity changes.

Adsorptive and absorptive desiccants have been used extensively to control moisture in closed containers. There are, however, two particular disadvantages associated with using desiccants. These disadvantages are:

(i) a desiccant medium can only absorb or adsorb a predetermined amount of moisture such that once the desiccant medium has absorbed or adsorbed this amount, it cannot absorb or adsorb any more moisture; and

(ii) in the event that the temperature within the container increases, adsorptive desiccants will expel moisture contained therein, which in turn increases the humidity within the container.

There are desiccants that only release moisture when the temperature is greater than might naturally occur. Such desiccants are however expensive.

According to a first aspect of the invention there is provided a humidity control device for a closed container comprising a hygroscopic desiccant medium contained within a chamber, the chamber including at least two closeable vents controlled in response to ambient temperature such that one of the vents is openable to allow the desiccant medium to adsorb moisture from air entering the chamber via that vent when ambient temperature is below a first predetermined temperature value and the other of the vents is openable to allow the desiccant medium to expel moisture via that vent when ambient temperature exceeds a second predetermined temperature value. An advantage of the invention is the provision of a device for controlling ^• moisture levels in a closed container, using a desiccant medium, which has a mechanism to prevent the ingress of moisture into the container.

The device permits automatic regeneration of the desiccant without any human intervention when certain atmospheric temperature conditions prevail.

The device is designed to provide a relatively low cost, but permanent, solution to the problem of controlling the creation of condensation within packaging containers, the cost being relatively low in comparison to traditional electrically operated dehumidifϊers.

In embodiments of the invention, the first and second temperature values are equal so that one vent opens as the other vent closes, and vice versa.

The vents are preferably arranged so that neither of the vents is open, or starts to open, unless, or until, the other vent is closed. This prevents air from the exterior of the container communicating directly with the interior of the container.

Studies have shown that "sweating" in closed containers is greatest when the temperature difference between day- and nighttime is high. It is in these situations that the invention will be most effective in absorbing and expelling moisture.

Factors to consider when choosing the or each temperature value at which opening and closing of the vents is effected include: (i) expected temperature levels ; (ii) expected specific humidity levels; and (iii) expected swings in maximum and minimum temperatures.

Selecting the or each temperature value is important because the expelation of moisture may not otherwise take place as required. In certain extreme conditions, particularly very hot and humid climates for example, the desiccant could actually transfer moisture into the container rather than transfer moisture out of the container.

When a hygroscopic desiccant medium is exposed to air, at a constant temperature, it will adsorb or desorb moisture from the air until it attains equilibrium with the air. If the ambient conditions of the air change, then the desiccant medium will seek to move to a new equilibrium. Equilibrium is achieved when the vapour pressure of moisture in the desiccant medium and the surrounding air are equal.

The vapour pressure of moisture present in a desiccant medium relative to the vapour pressure of moisture present in the air determines whether the desiccant medium will adsorb or desorb when it is exposed to the air.

If the vapour pressure of moisture present in the desiccant is greater than the vapour pressure of moisture in the air, the desiccant will expel moisture until the vapour pressure of moisture in the desiccant and in the air is equal, and vice versa.

Relative humidity is inversely proportional to temperature at a specific humidity, as shown in Figure 1. The relative humidity of air, and thus the vapour pressure of moisture in the air, will therefore decrease as temperature increases, and vice versa. This means that if the vapour pressure of moisture in the desiccant medium is less than the vapour pressure of moisture in the air entering the vent that is open when ambient temperature exceeds a predetermined temperature value, the desiccant will adsorb moisture from the air rather than expel it. If the ambient temperature then drops, causing the other vent to open, there is a risk that the vapour pressure of moisture in the desiccant may be greater than the vapour pressure of moisture in the air entering the chamber via that other vent. The desiccant would then expel moisture via that vent rather than adsorbing moisture.

In a particularly preferred embodiment of the invention opening and closing of the vents may be further controlled by the degree of hydration of the desiccant medium.

In such embodiments, one of the vents may be openable to allow the desiccant medium to adsorb moisture from air entering the chamber via that vent when ambient temperature is below a first predetermined temperature value and the degree of hydration is below a first predetermined hydration value. The other of the vents may be openable to allow the desiccant medium to expel moisture via that vent when ambient temperature exceeds a second predetermined temperature value and the degree of hydration exceeds a second predetermined hydration value.

Controlling the opening and closing of the vents in dependence on the degree of hydration of the desiccant, as well as the ambient temperature conditions, assists in ensuring that the desiccant will adsorb or expel moisture, as required, when the respective vent is open. This is because it ensures that the vents only open when the degree of hydration of the desiccant is such that it will adsorb or expel moisture, as required. In such embodiments, the first and second predetermined hydration values may be equal.

The vents may be mechanically linked and may be arranged to move together so that when one vent opens the other vent closes, and vice versa.

This may be achieved by mounting the vents in an offset manner on opposite sides of the chamber.

In some embodiments, operation of the vents may include a fully closed position in which both of the vents are closed before one of the vents opens. Such simultaneous movement permits the desiccant to exchange moisture via only one of the vents at a time.

For the humidity control device to provide an effective performance in terms of the amount of moisture that it can remove from or introduce to the container, the characteristics of the desiccant medium are paramount.

The desiccant medium is preferably chosen to have as high a capacity for moisture as possible. The desiccant device is also preferably quick at responding to changes in humidity.

In a preferred embodiment, the desorption characteristic of the desiccant medium closely follows the adsorption characteristic of the desiccant medium.

This helps to ensure that the performance of the desiccant medium does not deteriorate over time by gradually retaining more and more moisture through repeated use of the device. It also assists in maximising the ability of the desiccant medium to desorb moisture, and therefore regenerate, during favorable ambient temperature and humidity conditions.

The adsorption and desorption characteristics of two different desiccant mediums are shown in Figures 2a and 2b.

It can be seen from Figure 2a that the first desiccant medium adsorbs moisture in a relatively uniform manner. For example, in response to a change in the relative humidity of the surrounding air from 30% to 80% the mass of the first desiccant medium increases by approximately 50% of the initial mass of the desiccant. This increase in mass is created by adsorption of moisture.

It can also be seen from Figure 2a that the desorption characteristic of the first desiccant medium closely follows the adsorption characteristic of the medium, thereby forming a relatively narrow hysterisis loop.

This shows that the first desiccant medium expels moisture at almost the same rate as it adsorbs moisture when the relative humidity of the surrounding air changes. This means that the first desiccant material expels almost the same amount of moisture, when the relative humidity of the surrounding air decreases from x to y, as it adsorbs over the corresponding upward change from y to x, regardless of the actual values of x and y.

This in turn means that the first desiccant medium is less likely to accumulate a residual amount of moisture as the relative humidity of the air to which it is exposed switches between first and second values.

It can be seen from Figure 2b that the second desiccant medium does not adsorb moisture in a uniform manner. For example, in response to a change in the relative humidity of the surrounding air from 30% to 80% the mass of the second desiccant medium increases by approximately 18% of the initial mass of the desiccant. This increase in mass is much less than the increase in mass of the first desiccant medium over the same change in relative humidity. The second desiccant medium does not adsorb moisture at a quick rate until the relative humidity of the surrounding air exceeds 80%.

It can also be seen from Figure 2b that the desorption characteristic of the second desiccant medium does not follow the adsorption characteristic of the medium, thereby forming a relatively wide hysterisis loop.

This shows that at high relative humidities the second desiccant medium expels moisture at a much slower rate than it adsorbs when the relative humidity of the surrounding air changes. This means that the second desiccant material does not expel the same amount of moisture, when the relative humidity of the surrounding air decreases from a to b, as it adsorbs over the corresponding upward change from b to a, when the values of a and b are high.

This means that the second desiccant medium is likely to accumulate a residual amount of moisture as the relative humidity of the air to which it is exposed switches between the first and second values, if those values are high.

A large difference between the first and second values would be required to achieve efficient regeneration of the second desiccant material.

However, as referred to above, Figure 2b shows that unless the relative humidities are very high the second desiccant medium is very poor at adsorbing moisture. It would be undesirable therefore to use the second desiccant medium in a humidity control device because it is poor at adsorbing moisture at average values of relative humidity. It is also poor at expelling moisture, and therefore regenerating, at average values of relative humidity.

As an indication of a good desiccant medium, the maximum difference between the change in mass of the desiccant medium when it is adsorbing moisture compared to when it is desorbing moisture, at any value of relative humidity, is 12% or less of the initial mass of the desiccant.

Given that the adsorbing and desorbing process is largely dependent on temperature, in an embodiment of the invention the thermal expansion and contraction of materials may be used to facilitate opening and closing of the vents.

Other advantageous features of the invention are set out in dependent Claims 6-16.

According to a second aspect of the invention there is provided a humidity control device for a closed container comprising a hygroscopic desiccant medium operably linked to a closeable vent, the vent being controlled in response to ambient temperature and the degree of hydration of the desiccant medium such that the vent is closed when the ambient temperature is below a first predetermined temperature value and the degree of hydration is below a first predetermined hydration value, and the vent is open when the ambient temperature exceeds a second predetermined temperature value and the degree of hydration exceeds a second hydration value. A preferred embodiment of the invention will now be described, by way of a non-limiting example, with reference to the accompanying figures in which:

Figure 1 is a graph showing the relationship between the relative humidity of air and ambient temperature;

Figures 2a and 2b are graphs showing the adsorption and desorption characteristics of two different desiccant materials;

Figure 3 is a schematic cross-sectional view of a closed container incorporating a humidity control device according to an embodiment of the invention;

Figure 4 is a perspective view of the closed container of Figure 1; and

Figures 5-7 are schematic cross-sectional views of the humidity control device during use.

A closed container 10 incorporating a humidity control device 12 according to an embodiment of the invention is shown in Figures 3 and 4.

The humidity control device 12 includes a chamber 14 formed from a solid material, such as mild steel. The chamber 14 is sealingly fixed within an aperture 16 defined in a sidewall 18 of the otherwise closed container 10 by means of a seal 20. The seal 20 is held under compression by fixing screws 22 (Figure 4) that secure the chamber 14 to the sidewall 18 of the container Referring now to Figure 5, a hygroscopic desiccant medium 24 is located within the chamber 14. The desiccant medium 24 is suspended in a desiccant holder 25, from the roof 26 of the chamber 14, by means of a spring 28. In other embodiments another resilient member may be used in place of the spring 28.

The spring 28 allows the desiccant holder 25 to move within the chamber 14 in response to changes in the mass of the desiccant medium 24 caused by changes in the degree of hydration of the desiccant medium 24.

The desiccant holder 25 is formed with a spigot 27 extending downwardly from a bottom surface 29 thereof.

The chamber 14 also includes first and second vents 32,34 formed in opposite sidewalls 36,38 thereof. Each of the vents 32,34 is openable and closeable by means of a vent member 40,42 slidingly mounted on the inner surface of each of the walls 36,38. Each of the vent members 40,42 includes an aperture therein which, when aligned with the corresponding vent 32,34 formed in the adjacent sidewall 36,38, effectively opens the vent and permits air to flow into and out of the chamber 14.

When the first vent 32 is open it allows the ingress of air from the interior of the container 10. When the second vent 34 is open it allows the ingress of air from the exterior of the container 10.

The vent members 40,42 are interconnected by means of a rigid bush member 44. This means that sliding movement of one of the vent members 40 along its adjacent sidewall 36 causes sliding movement of the other vent member 42, in the same direction, along its adjacent sidewall 38. The vents 32,34 formed in the chamber walls 36,38 are aligned with each other, whereas the openings formed in the vent members 40,42 are misaligned. This means that when one of the vents 32,34 is open, the other of the vents 32,34 remains closed. The alignment is also such that neither of the vents 32,34 starts to open until the other of the vents 32,34 has closed.

The vent members 40,42 are mounted on inner surfaces of the sidewalls 36,38 of the chamber 14 to sit flush against those surfaces. They are coated with a material having a low coefficient of friction to reduce friction between the vent members 40,42 and the inner surfaces of the sidewalls 36,38 of the chamber 14 such that the vent members can slide over the sidewalls 36,38 easily.

The rigid bush 44 interconnecting the vent members 40,42 includes an aperture 46 formed centrally therethrough. The aperture 46 is aligned with the spigot 27 provided on the desiccant holder, such that the spigot 27 protrudes into the aperture 46.

The aperture 46 formed in the rigid bush 44 is provided with a wall 47 that protrudes below the lower surface 49 of the bush 44 in order to form a channel 51.

Two pawls 48,50 are pivotally mounted in apertures formed in opposite sides of the channel wall 47. The pawls 48,50 are spring-loaded to pivot towards the centre of the channel 51.

The chamber 14 further includes a thermally expandable cylinder 52 secured to the base 54 of the chamber 14. The thermally expandable cylinder 52 expands and contracts in response to temperature changes, thereby adjusting the position of a plunger member 56 of the cylinder 52.

A mechanical link member 58 is mounted on an end of the plunger member 56 that is remote from the thermally expandable cylinder 52. The mechanical link member 58 defines a cup corresponding in size to the channel 51 protruding below the lower surface 49 of the bush 44 such that the cup is slidingly engageable over the channel wall 47.

Operation of the humidity control device will now be described with reference to Figures 5 to 7.

Figure 5 shows the humidity control device in an initial condition when the ambient temperature is below a predetermined value.

The desiccant medium 24 is dry and is therefore located towards the top of the chamber 14.

The vent members 40,42 are positioned so that the first vent 32 is open into the interior of the container 10, and the second vent 34 is closed.

The relative humidity of air entering the chamber 14 from the interior of the container 10 is relatively high as a result of the low ambient temperature.

The desiccant medium 24 is contained within the desiccant holder 25 within, for example, a bag (not shown). Such a bag may include a plurality of holes therethrough that are smaller than the smallest size of desiccant crystals. The desiccant holder 25 may be formed from a wire mesh. The desiccant is therefore exposed to the air entering the chamber 14 from the container 10. To enable the vapour pressure of moisture contained in the desiccant medium to reach equilibrium with the vapour pressure of moisture contained in the air entering the chamber 14 from the interior of the container 10, the desiccant medium 24 adsorbs moisture from the air, as shown by arrow A in Figure 5. This increases the degree of hydration of the desiccant medium 24 and reduces the relative humidity of the air. It also increases the mass of the desiccant medium causing the desiccant holder 25 to move downwards within the chamber 14.

The greater the amount of moisture that is adsorbed by the desiccant medium 24, the greater the increase in mass of the desiccant medium 24 and the further downward the desiccant holder 25 moves.

As the desiccant holder 25 moves downward, the spigot 27 protrudes further into the aperture 46 in the bush 44, and into the channel 51 defined by the channel wall 47.

When the degree of hydration of the desiccant medium reaches a predetermined hydration value, the spigot 27 engages the pawls 48,50 protruding into the channel 51. On engagement, the pawls 48,50 are forced to pivot so as to protrude outwardly from the channel 51, as shown in Figure 6.

The desiccant medium 24 will continue to adsorb moisture from air entering the chamber 14 from the interior of the container 10 until the vapour pressure of moisture in the desiccant medium 24 and the air is equal. If the ambient temperature then rises, the thermally expandable cylinder 52 expands and causes the plunger 56 to move upwards in the chamber 14. This in turn moves the mechanical link 58 upwards.

If the ambient temperature exceeds a predetermined temperature value, expansion of the cylinder 52 causes the mechanical link 58 to slidingly engage over the channel wall 47 and contact the pawls 48,50 protruding therefrom. This engagement prevents further upward movement of the mechanical link 58 relative to the channel wall 47 such that further upward movement of the mechanical link 58 pushes the bush 44 upwards in the chamber 14.

Upward movement of the bush 44 causes sliding movement of the vent members 40,42 upwards, closing the first vent 32 and opening the second vent 34, as shown in Figure 7.

The stroke length of the plunger 56 of the thermally expandable cylinder 52 is chosen so that it provides the required amount of movement to move the vent members 40,42 so that the first vent 32 is closed and the second vent 34 is opened.

If the desiccant material 24 does not adsorb sufficient moisture via the first vent 32 to cause the spigot 27 to engage the pawls 48,50, expansion of the thermally expandable cylinder 52 has no effect on the position of the vent members 40,42. The first vent 32 therefore remains open even though the ambient temperature has increased. The mechanism therefore eliminates the risk of the desiccant medium adsorbing moisture via the second vent in the event that the vapour pressure of moisture in the air to the exterior of the container 10 is greater than the vapour pressure of moisture in the desiccant medium 24. On opening of the second vent 34, air from the exterior of the container 10 enters the chamber 14.

The relative humidity of air entering the chamber 14 from the exterior of the container 10 is relatively low as a result of the higher ambient temperature. This, in combination with the fact that the degree of hydration of the desiccant material 24 is high, means that the desiccant material 24 will expel moisture into the air in order to reach equilibrium with the air. Moisture from the desiccant material 24 will therefore be expelled via the second vent 34, as shown by arrow B in Figure 7.

As the desiccant material 24 expels moisture, the degree of hydration of the desiccant medium 24 decreases, reducing the mass of the desiccant material 24 and causing the desiccant holder 25 to move upward within the chamber 14.

Such regeneration of the desiccant material 24 continues until the vapour pressure of moisture in the desiccant and the air entering the chamber 14 from the exterior of the container 10 is equal, or until the desiccant material 24 is dry.

If a sufficient amount of moisture is expelled from the desiccant material 24, the resulting upward movement of the desiccant holder 25 disengages the spigot 27 from the pawls 48,50. As a result the spring bias applied to the pawls 48,50 causes the pawls 48,50 to pivot back to a position where they protrude into the channel 51.

Movement of the pawls 48,50 back to a position where they protrude into the channel 51 disengages the pawls 48,50 from the mechanical link 58. Springs 60,62 are secured between the vent members 40,42 and the base 54 of the chamber 14 such that on disengagement of the pawls 48,50 from the mechanical link 58, the springs 60,62 pull the vent members 40,42 downwards.

This downward movement of the vent members 40,42 closes the second vent 34 and opens the first vent 32.

The default position of the vent members 40,42 is therefore such that in the event that the degree of hydration of the desiccant medium 24 is below a predetermined value, the first vent 32 is open. The second vent 34 only opens when both the degree of hydration of the desiccant medium 24 exceeds the predetermined value and the ambient temperature exceeds a predetermined value. The second vent 34 therefore only opens when it is certain that the desiccant medium 24 will expel moisture and thereby regenerate.

When the ambient temperature decreases, the thermally expandable cylinder 52 contracts such that the mechanical link 58 moves downwards, back towards the base 54 of the chamber 14.

Calibration of the thermally expandable cylinder 52 is possible by adjusting a threaded knob 64 on the cylinder to adjust the volume in which the material contained within the cylinder 52 can expand or contract. This effectively adjusts the temperature at which movement of the mechanical link is sufficient to effect opening and closing of the respective vents 32,34. This temperature may be set to 30°C, for example. The use of a thermally expandable cylinder to effect movement of the vent members 40,42 means that no other power sources such as batteries, kinetic devices or another electrical supply is necessary. There is a risk when using batteries, for example, that the batteries could fail. Batteries also have to be replaced periodically, increasing the running costs of a device employing batteries.

Also, devices which use a fixed electrical power source such as a mains supply or a power supply derived from a ship or motor vehicle engine, are not practical. This is because the humidity control device is intended for use in packaging containers used to ship goods from one place to another. It is therefore important to ensure portability, reduce the amount of human input to operate the device and limit the requirement for making electrical connections during shipping, storage and transit.

The degree of hydration of the desiccant medium 24 that is required to operate the pawls 48,50 may also be calibrated through selection of the spring 28.

The ability to set the conditions required to operate the pawls 48,50 and the temperature required to open and close the vents 32,34 is advantageous. Hygroscopic desiccant materials normally take hours to adsorb significant amounts of moisture and days to show any physical changes. Any mechanical devices linked directly to changes in humidity therefore normally have a slow incremental response. This is not satisfactory when it is desirable to open and close a vent rapidly in response to changes in atmospheric conditions.

This disadvantage is overcome in the embodiment of the device described herein through the action of the pawl arrangement. As the spring 28 can be chosen such that the pawls 48,50 engage the mechanical link 58 at a set degree of hydration of the desiccant, a clear and controllable signal that the desiccant medium 24 requires regeneration is provided. The time taken for the thermally expandable cylinder 52 to expand fully is less than 10 minutes. Thus, if it is desirable for the desiccant to regenerate, indicated by the protruding pawls 48,50, the time taken to open and close the vents 32,34 is less than 10 minutes. In terms of using non-electrical responses to humidity and temperature conditions, this is a digital-type response.

In embodiments where it is desirable to have high moisture levels inside the container 10, the chamber 14 can be positioned so that moisture is adsorbed from the exterior of the container 10 and expelled to the interior of the container 10.

The device would function in the same manner except that the second vent 34, which opens to the exterior of the container 10, would be open to the exterior of the container 10 when the ambient temperature is below a predetermined temperature value.

In this condition, the thermally expandable cylinder 52 would be in a retracted position and the first vent, which opens to the interior of the container 10, would be closed.

In this condition, the desiccant medium 24 would adsorb moisture from the air to the exterior of the container 10.

On an increase in ambient temperature, so that the ambient temperature exceeds a predetermined temperature value, and the degree of hydration is high enough to facilitate operation of the vent members 40,42, second vent 34 would close and the first vent 32 would open, allowing the desiccant medium 24 to expel moisture into the interior of the container 10.

This may be useful when a product requiring hot and humid conditions, such as, for example, tobacco, is packed in the container 10.

Given also that the humidity control device 12 may be used in all weather conditions, materials are selected to withstand extreme temperature and humidity conditions.

For a mild steel construction, a coating of a dry film lubricant such as MOS2, would be advantageous for use on the vent members 40,42 to reduce the frictional forces as the vent members 40,42 slide to open and close the vents 32,34.

A flexible, breathable and waterproof PTFE membrane may be provided to cover the exterior of the second vent 34 to stop the ingress of water into the chamber from the exterior of the container 10, e.g. when it rains. The permeability of the material is preferably greatest when temperatures are high so that moisture will move from the chamber 14 to the exterior of the container 10.

In other embodiments, operation of the vent members 40,42 may be facilitated by changes in ambient temperature only. In such embodiments, the mechanical link 58 may be permanently secured to the bush 44. This means that expansion and contraction of the cylinder 52 effects movement of the vent members 40,42, regardless of the degree of hydration of the desiccant medium 24.

Claims

1. A humidity control device for a closed container comprising a hygroscopic desiccant medium contained within a chamber, the chamber including at least two closeable vents controlled in response to ambient temperature such that one of the vents is openable to allow the desiccant medium to adsorb moisture from air entering the chamber via that vent when ambient temperature is below a first predetermined temperature value and the other of the vents is openable to allow the desiccant medium to expel moisture via that vent when ambient temperature exceeds a second predetermined temperature value.

2. A humidity control device as claimed in Claim 1 wherein the first and second predetermined temperature values are equal.

3. A humidity control device as claimed in any preceding claim wherein opening and closing of the vents is further controlled by the degree of hydration of the desiccant medium.

4. A humidity control device as claimed in Claim 3 wherein one of the vents is openable to allow the desiccant medium to adsorb moisture from air entering the chamber via that vent when ambient temperature is below a first predetermined temperature value and the degree of hydration is below a first predetermined hydration value, and the other of the vents is openable to allow the desiccant medium to expel moisture via that vent when ambient temperature exceeds a second predetermined temperature value and the degree of hydration exceeds a second predetermined hydration value.

5. A humidity control device as claimed in Claim 4 wherein the first and second predetermined hydration values are equal.

6. A humidity control device as claimed in any preceding claim wherein opening and closing of the vents is controlled such that one of the vents is open when the other of the vents is closed.

7. A humidity control device as claimed in any preceding claim wherein opening and closing of the vents is effected by an actuating body that expands and contracts in response to changes in ambient temperature.

8. A humidity control device as claimed in Claim 7 wherein the actuating body is calibrated according to ambient temperature in order to synchronise opening and closing of the vents.

9. A humidity control device as claimed in Claim 7 or Claim 8 wherein the actuating body is a thermally expandable cylinder.

10. A humidity control device as claimed in Claim 4 and any claim dependent therefrom wherein a resilient member supports the desiccant medium such that the degree of hydration of the desiccant medium determines the relative position of the desiccant medium within the chamber.

11. A humidity control device as claimed in Claim 10 wherein the resilient member is a spring.

12. A humidity control device as claimed in Claims 9 and 11 wherein the desiccant medium is contained in a desiccant holder that includes a spigot member that engages and actuates spring-biased pawls when the degree of hydration of the desiccant medium exceeds the second predetermined hydration value such that an actuating portion of the thermally expandable cylinder engages the actuated pawls when the thermally expandable cylinder expands in response to an increase in the ambient temperature above the second predetermined temperature value in order to open and close the respective vents.

13. A humidity control device as claimed in any preceding claim wherein the vent that is openable in order to expel moisture from the desiccant medium includes a flexible membrane that prevents ingress of water into the chamber while allowing moisture to leave the chamber.

14. A humidity control device as claimed in Claim 13 wherein the flexible member is a breathable PTFE membrane.

15. A humidity control device as claimed in any preceding claim wherein a hysterisis curve for adsorption and desorption of the desiccant medium is such that the difference between adsorption and desorption curves is not greater than 12% of the possible mass increase at any value of relative humidity.

16. A humidity control device as claimed in any preceding claim wherein operation of the device is controlled by ambient temperature and humidity changes alone.

17. A humidity control device for a closed container comprising a hygroscopic desiccant medium operably linked to a closeable vent, the vent being controlled in response to ambient temperature and the degree of hydration of the desiccant medium such that the vent is closed when the ambient temperature is below a first predetermined temperature value and the degree of hydration is below a first predetermined hydration value, and the vent is open when the ambient temperature exceeds a second predetermined temperature value and the degree of hydration exceeds a second hydration value.

18. A closeable container including a humidity control device as claimed in any preceding claim.

19. A closeable container as claimed in Claim 18 and any of Claims 1 to 16 wherein one of the vents is openable to the interior of the container and the other of the vents is openable to the exterior of the container.

20. A closeable container as claimed in Claim 18 or Claim 19 wherein the container includes a plurality of interlinked walls and the humidity control device is located within one of the walls.