Abstrict A desiccant package is provided that includes a housing having
at least one opening. A desiccant material is located in the housing.
A sealing element, which is secured over the opening, includes a
water-permeable membrane. The sealing element reduces the rate at
which moisture enters the desiccant container and prevents fine
particles from escaping from the package.
Claims What is claimed is:
1. A desiccant package comprising:
a housing having at least one opening;
a desiccant material located in said housing; and
a sealing element secured over said at least one opening, said
sealing element having a particular thickness and including a water-permeable
membrane such that said particular thickness gives rise to a desired
permeation rate for water vapor, said sealing element having a first
and second wire mesh elements, wherein said membrane being located
between said first and second wire mesh elements, at least one O-ring
for sealing said sealing element in said opening, a retaining ring
located on an outer surface of said sealing element for securing
said sealing element in said opening.
2. The desiccant of claim 1 wherein said membrane is polyethylene
terephthalate.
3. The desiccant of claim 2 wherein said membrane has a thickness
of between 3 and 6 microns.
4. The desiccant of claim 1 wherein said housing has a cylindrical
shape.
5. The desiccant of claim 4 wherein said at least one opening is
located on one end of said cylinder.
6. The desiccant of claim 4 wherein said housing has first and
second openings on opposing ends of said cylinder and further comprising
a second sealing element secured over said second opening.
7. A desiccant package, comprising:
a housing having at least one opening;
a desiccant material located in said housing; and
means for allowing moisture to be conducted through said opening
at a prescribed rate while preventing particles larger than about
0.5 microns from being conducted through said opening, said conduction
means including a first and second wire mesh elements wherein said
membrane is disposed therebetween, an O-ring for sealing said sealing
element in said opening, and a retaining ring located on an outer
surface of said sealing element for securing said sealing element
in said opening.
8. The desiccant of claim 7 wherein said conduction means includes
a polyethylene terephthalate membrane.
9. The desiccant of claim 8 wherein said membrane has a thickness
of between 3 and 6 microns.
10. The desiccant of claim 7 wherein said housing has a cylindrical
shape.
11. The desiccant of claim 10 wherein said at least one opening
is located on one end of said cylinder.
12. The desiccant of claim 10 wherein said housing has first and
second openings on opposing ends of said cylinder and further comprising
a second sealing element secured over said second opening.
13. The desiccant of claim 8 wherein said membrane has a selected
thickness that gives rise to a desired permeation rate for water
vapor.
Description FIELD OF THE INVENTION
The invention relates generally to desiccant packages, and more
particularly to a desiccant package that may be used in a high reliability
environment for extended periods of time such as an optical fiber
splice box used in undersea transmission systems.
BACKGROUND OF THE INVENTION
Desiccants are commonly employed to maintain the moisture content
of a closed system at a low level. Desiccant materials such as silica
gel and molecular sieve, for example, have a large surface area
for adsorbing moisture. However, conventional desiccant packagings
may not be suitable for high reliability applications that require
a clean environment. For example, one problem with conventional
desiccant materials is that they can release fine particles, which
can contaminate the clean environment.
Another problem arises when the closed system is to be operational
for an extended period of time (e.g., one or more decades) without
being available for maintenance. In such situations the capacity
of the desiccant should not be exhausted during the operational
life of the system since there is no opportunity to replace the
desiccant material. The rate at which the desiccant capacity is
exhausted is of particular concern when the closed system must undergo
an assembly process that can last for hours or even days. While
the system is exposed to the larger environment during assembly,
and hence more moisture will be able to reach the desiccant in a
shorter period of time, prematurely exhausting the desiccant capacity
even before the system is closed. Common desiccant materials absorb
water rapidly in an open environment, approximately 50% of their
capacity in one or two hours.
An example of a closed system of the previously mentioned type
is a splice box, which is employed in undersea optical communication
systems. One type of splice box serves as a housing for securing
cable connectors such as cable-to-cable connectors and cable-to-repeater
connectors. Since undersea cables require electrical continuity
across their spans and thus across connectors, the splice box must
be appropriately insulated. For this reason the splice box undergoes
an overmolding process to encase it in polyethylene during its final
assembly. The splice box is designed to function undersea for twenty-five
or more years without requiring service.
Some splice boxes house additional components such as a gain equalization
filter (GEF), for example. This filter is used to adjust the gain
among the various optical channels after they undergo amplification
in the repeater. To avoid corrosion of the filter, the moisture
content of the splice box needs to be maintained at or below a relative
humidity of 50%. Since the splice box does not provide a hermetic
seal that prevents moisture from entering, the GEF splice box requires
a desiccant.
Accordingly, there is need for a desiccant that does not release
fine particles or dust and which has a substantially reduced moisture
adsorption rate.
SUMMARY OF THE INVENTION
In accordance with the present invention, a desiccant package is
provided that includes a housing having at least one opening. A
desiccant material is located in the housing. A sealing element,
which is secured over the opening, includes a water-permeable membrane.
The sealing element advantageously reduces the rate at which moisture
enters the desiccant container and prevents fine particles from
escaping from the package. By reducing the rate at which moisture
enters the container, the amount of moisture adsorbed during assembly
of the splice box is reduced, thus preserving the desiccant's capacity
for undersea use.
In one particular embodiment of the invention, the water-permeable
membrane is fabricated from polyethylene terephthalate and has a
thickness, for example, of between 3 and 6 microns. The housing
may advantageously have a cylindrical shape and the opening is located
on one end of the cylinder.
The sealing element may include first and second wire mesh elements,
with the membrane being located therebetween. At least one O-ring
is employed for sealing the sealing element is the opening. A retaining
ring is located on an outer surface of the sealing element for securing
the sealing element in the opening.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an embodiment of a desiccant package constructed in
accordance with the principles of the present invention.
FIG. 2 shows the permeability of various polymer films to water
vapor at 25 degrees Celsius.
FIG. 3 shows the rate of permeation of water vapor through two
Mylar films 3 microns and 6 microns thick.
FIG. 4 shows the permeation of water vapor through polycarbonate
membranes having pores sizes of 0.08 and 0.22 microns.
FIG. 5 shows the permeation rate of water vapor for Saran film
and for Mylar 6 microns thick.
DETAILED DESCRIPTION
FIG. 1 shows an embodiment of a desiccant package constructed in
accordance with the principles of the present invention. The package
includes a cylindrical shaped housing 10 fabricated from a nonporous
material such as aluminum, stainless steel or plastic. The particular
shape and materials of the housing are application specific and
the configuration shown in FIG. 1 is not to be construed as a limitation
on the invention. The housing contains the desiccant material for
adsorbing moisture. Any desiccant material may be used, including
but not limited to, silica gel and molecular sieve.
Moisture enters the housing through the two opposing planar end
surfaces 2 and 4 of the housing 10. The end surfaces 2 and 4 are
each covered by a sealing element 6.
In accordance with the present invention, the sealing element 6
includes a water-permeable membrane 8 that reduces the rate at which
moisture enters the desiccant container and prevents fine particles
from escaping from the container. Since the diffusion rate of water
through the membrane depends on the membrane material and thickness,
the choice of material dictates the rate at which reaches the desiccant
material. The diffusion rate of materials for a particular gas or
vapor is measured in terms of its permeability.
FIG. 2 shows the permeability of various polymer films to water
vapor at 25 degrees Celsius. The water-permeable membrane 8 may
selected from among these materials. As shown , the permeability
constant of these materials extends over three orders of magnitude,
thus offering a wide range of choices that can be determined based
on the application.
Among the materials listed in FIG. 2 a particularly suitable material
from which the membrane 8 may be fabricated is polyethylene terephthalate,
also known as MYLAR. MYLAR is an attractive choice for use in a
desiccant employed in a splice box because the splice box is raised
to a temperature of 140.degree. C. during the overmolding process
and MYLAR can withstand such a high temperature. Moreover, MYLAR
is resistant to tearing, thus enhancing its reliability.
FIG. 3 shows the rate of permeation of water vapor through two
MYLAR films 3 microns and 6 microns thick. The data was obtained
by the conventional weighted-cell method at a temperature of 30.degree.
C. and 75% relative humidity. A dried desiccant package containing
silica gel was placed inside a glass bottle which was capped tightly
with the polymeric film, which had an exposed area of about 3.8
cm.sup.2. The bottle was hung above distilled water containing excess
sodium chloride inside a canning jar. The sealed jar was placed
inside a temperature controlled oven maintained at 30.degree. C.
By monitoring the weight change of the bottle containing the desiccant,
the rate of water adsorption was measured in a straightforward manner.
Although the weighted-cell method for experimentally determining
water vapor permeability is simple, it is subject to errors, including
the possibility of an additional diffusion barrier arising from
stagnant air layers inside the cell. It is important to demonstrate
that no significant barrier was present that could distort the data.
As seen in FIG. 3 the initial permeation rates (at <50% adsorption
capacity) of the package without film, with the 3 micron film, and
with the 6 micron film are, respectively, 5.3 1.2 and 0.7 mg/hr.
The permeation rate without any MYLAR film is at least four times
greater than those with the films. No significant diffusion barrier
error was evident. The permeation rate of the 3 micron film was
close to double the rate for the 6 micron film, indicating that
the primary rate-limiting step was the permeation through the MYLAR
film.
Returning to FIG. 1 the sealing element 6 includes the membrane
8 and a series of other elements that support the membrane over
the end surfaces 2 and provide a seal to prevent moisture and other
particles from entering or exiting the housing 10 without passing
through the membrane 8. More specifically, the membrane is sandwiched
between two wire mesh elements 12. An O-ring (not shown) is located
about the inner surface of end surfaces 2 and 4. The seal is established
when the mesh 12 is pressed against the O-ring. A retaining ring
14 located on the side of the mesh 12 remote from the O-ring secures
the sealing element 6 in the end surfaces 2 and 4 of the housing
10.
Based on the data shown in FIG. 3 the membrane advantageously
may be a MYLAR film. The thickness of the film will depend on the
rate of permeation that is desired for a given application, but
from on the above analysis a thickness of 3-6 microns generally
should be sufficient. The initial permeation rates for a desiccant
container sealed with a MYLAR sheet three microns and six microns
thick are approximately 0.4% and 0.24% of the adsorption capacity/hour,
respectively. For the desiccant container without the MYLAR the
initial permeation rate is about 1.9%. Accordingly, the MYLAR film
creates a diffusion barrier that reduces the water adsorption rate
by about four to eight times. The MYLAR also effectively prevents
fine particles exceeding about 0.5 microns from escaping from the
desiccant housing, thus avoiding contamination to the closed system
(e.g., a GEF splice box) in which the desiccant is employed.
For certain applications it may be desirable to increase the water
adsorption rate over that obtained with a MYLAR film. For example,
polycarbonate films having small pores can provide an increased
water permeation rate and yet limit the release of particles. FIG.
4 shows the permeation of water vapor through polycarbonate membranes
having pores sizes of 0.08 and 0.22 microns. FIG. 4 also shows the
permeation of water vapor through the desiccant container without
any membrane. As shown, the permeation rates are all similar to
one another and have a value of about 5.3 mg/hour when the adsorption
capacity is below 50%.
In applications that require a permeation rate lower than that
provided by MYLAR film, a polymer may be selected from FIG. 2 that
has a permeation rate below that of MYLAR. For example, polyvinylidene
chloride, known by the tradename SARAN Wrap, has a permeability
of about 0.3-1.0. FIG. 5 shows the permeation rate of water vapor
for SARAN film and for MYLAR 6 microns thick. The permeation rate
for the SARAN film is 0.036 mg/hr, or about twenty times less than
the MYLAR film. |