Note: Descriptions are shown in the official language in which they were submitted.
~E~UC'I`lON O~ ST~NGT~1 LO~S
~UKIL~'G ~'IBLR P~O~SSING
~acKyround of tne ~ ention
-
l. Field of the Inverltion
~ e ir-vention is concer1led Witi1 glass fiber oE a
form referred to dS "silica" based. E~iber of conse~uer1ce
fro1m tne standpoil1t ot ~1lis invenLiorl is ottnerwise irnbued
~i~h certdln cnaracteristics assvciated witt~ optical grade
10 Illaterlal used in con~ unications. Characteristics so
~ lied in~lude a "~risti1le" strength, thdt is a tensile
strenytr1 uyon drawiny, wnich is typically at a level of
41.37 x l0~Pa (60~,000 p~i) or hi~her.
2. De~cription of tne Prior Ar~
'1'he rapid growth of fiber lightwave
com1nunicatior1s from its be~innings a decade ago to a
signiflcant indusLry is well documented. Terrestrial and
underwater syste1ns, some base on 1i1ultimode fibers, some on
single 111ode, carryir1y voice as well as data are in
com111ercial use~ More arnbitious plans are in the formati~e
stages, and it is reliably estifl~ated that optical fioer
will largely replace copper conductor and become the
domir1ar1t factor in communications.
Man~ of ti1e initial problems in fiber
~5 fabrication: coatiny, sneathin~3, spliclr1g, etc., have been
overcome. Fibefs in regular ~ro~uction today are
cnardcterized by low insertion loss, high bandwidtt1, high
stren9t1l, and generally in characteristics that already
result in eco1lomic advantage over traditional rra1lsinission
:~0 Inedia~
As the technology matures attention is being
~irecte~ to secondary problems, some concerned with
economic considerations such as fiber yield and throughput.
~th~rs concerr- mecr1anical ~roperties of particular interest
~5 duriny inst~llatio1l. E~Or example, ~ensile streng~h is a
particularly slgnificar1t parameter during layiny of
2~
submarine cable from shipboard.
It has been recognized for some time that pristine
fiber strength characteristically at a level of 55.16 x
108Pa (800,000 psi) is substantially reduced in practical
cabled fiber. Studies have indicated that some part of
tensile strength loss is associated with high temperature
processing. Even before optical fiber became a practical
reality it had been observed that annealing, expected to
increase strength, in fact resulted in strength loss. See,
for exampie, Proceedin~ _ the Royal Society of London,
Vol. 297, pp. 534-551, 1967. The conclusion reached in
that reference assigned loss of strength to surface
contamination by dust particles.
Subsequent experience continues to show loss in
strength with thermal processing. Studies attributed
strength loss to a variety of causes, e.g., surface damage
from coating and handling, localized devitrification from
surface contamination, as well as mechanical strain. See
17 Electronic Letters, page 23~ (1981) and references cited
therein.
The situation was clarified early in 1981 when it
was shown that tensile strength values below about 27.58 x
108Pa (400,000 psi) were the result of mechanical handling.
Electronic Letters, Vol. 17, pp. 232-233 (1981). This work
attributes major loss to mechanical stripping of the usual
organic coatings normally produced as an in-line step
following fiber drawing. Usual organic coatings must be
removed to permit thermal processing at temperatures at
which such coatings degrade. The problem is complicated by
the fact that as-drawn fiber is not of uniform strength
resulting in the practice of removing weak spots and fusion
splicing to join remaining sections. Fusion splicing is a
form of thermal processing which itself degrades strength.
The need for splicing i5 graphically illustrated by
consideration of distributing of strength values. It is
convenient to refer to pristine fiber strength at, for
example, a nominal value of 55.16 x 108Pa (300,000 psi).
X
V2~
-- 3 --
in fact, this strength is a median strength with
distribution including as much as lO percent of fiber
length at less than 13.79 x 103Pa (200,000 psi) as tested
on lengths greater than 5 ~ilometers. ~imilarly, nominal
splice strength of 27.58 x 108Pa (400,000 psi) resulting
from elimination of poor mechanical handling practice is
also a median value with a distribution of perhaps 1 percent
of all splices at 6.89 x 103Pa (300,000 psi).
In general, industry has yet to fully appreciate
and to incorporate the most recent findings. As a result,
fiber systems are generally designed for fiber strengths of
the order of 6.89 x 108Pa ~lO0,000 psi), a value well
below nominal pristine strength. Annealing processes which
show promise for substantial strength recovery have gener-
ally not been used commercially, to some extent due to lack
of reproducibility and sometimes to actual strength loss.
Summary of the Invention
According to the invention there is provided
preparation of glass fiber which is permissibly coated
with a water-pervious coating said glass fiber having a
glass surface composed of at least 95 weight percent silica
and having a tensile strength of at least 600 ksi as drawn
in which said preparation includes thermal processing
defined as resulting in an attained elevated temperature at
least for a portion of the said fiber with said elevated
temperature equal to at least lO0 degrees C for a temper-
ature and time equivalent to 600 degrees C for a period of
at least lO seconds in accordance with an Arrhenius
relationship characterized in that a procedure is introduced
to lessen water-derived species on said fiber at least
within the said portion at least during final thermal pro-
cessing for at least that interval of time over which the
said portion is heated under the said temperature-time
conditions except that any portion undergoing viscous flow
during such final thermal processing need be exposed to
,,.v ~
- 3a -
said procedure only during that interval within which the
said temperature-time conditions are met subsequent said
flow said water-derived species being defined as molecular
water or other product present in said ~iber resulting from
exposure to molecular water.
Improved fiber strength results from avoidance of
fiber contact with water or by removal of water-derived
species resulting from exposure. Strength improvement is
the result of reduced thermal degradation, that is,
degradation in strength at a temperature-dependent rate.
Latent damage results even from short term exposure of
coated or uncoated fiber to ordinary ambient air. Since
damage due to exposure proceeds most rapidly at elevated
temperature, it is important that fiber be protected at all
times prior to thermal processing.
The alternative of removin~ water-derived species
is Eor the same reason most desirably practiced prior to
attainment of maximum elevated temperatures during thermal
processing. Since expedient removal is a kinetic process,
it is most effectively carried out at elevated temperatures.
It has been found that glass flow conditions, as
during fusion, heal "thermally damaged" fiber. While it is
expected that commercial practice will entail protection of
fiber continuously from fabrication at least through final
thermal processing, protection may be omitted prior to
attaining flow conditions.
Fiber protection may take the form of water
exclusion, e~g., by use of vacuum, protective inert gas, or
dried ambient, or alternatively, use may take the form of
gaseous chlorine SOC12, HCl, or other medium which
chemically displaces water or water-derived species in the
fiber. Removal of water-derived species depends again on
use of a medium which displaces water-derived species. A
prime example is chlorine. Similar results may be obtained
by relatively long term annealing under dry conditions,
e.g., in vacuum.
The invention is concerned with avoidance or
removal of water-derived species (which constitute latent
strength degradation). It is not directed to recovery of
strength in already degraded fiber. Since degradation is a
temperature dependent phenomenon, desired term of protection
may depend upon conditions of use. Strength requirements
sufficient to permit installation, e.g., of submarine
cable, may result from water exclusion or water-derived
species removal, as described. Long term strength may
require continued protection during use. Assuming proper
practice of the invention during processing, long term
strength is assured, for example, by metal or other
impervious coating or in properly designed filled or
protective cable structures. An alternative coating may
take form of "low melting point" inorganic glass which may
be self-healing at temperatures of interest. Examples
include chalcogenide glasses.
An important aspect of the inventive teaching
involves so-called "pristine" strength values. It has been
commonly assumed that median strengths of 55.16 x 108Pa
(800 ksi) measured on "as drawn" fiber represents the
inherent strength of the fiber. In fact, normal fabrication
includes drawing in air thereby resulting in strength
degradation -- some as measured upon drawing, so~e latent
to result in further degradation at a rate depending
329
upon temperature. Exclusion of water during drawing results
in improved strength. Real fiber strengths in excess of
68.95 x 108Pa (1,000 ksi) have been measured. Other
experiMents serve as the basis for estimations as high as
137.9 x 108Pa (2,000 ksi). Ul~imate fiber strength
re~uires total or near total exclusion of "water-derived
species". Normally such species results from exposure to
water. Other possibilities include -OH introduction from
organic coatings as during crosslinking.
Brief Description of the Drawinq
Figures 1 and 2, on coordinates of probability of
failure (upon tensile testing) and strength in units of
thousands of pounds per square inch (ksi) [1 ksi =
~,894,757Pa] r depict data for fiber: as drawn (FI~. 1,
curve 1; FIG. 2, curve 10), as conventionally spliced with
mechanical stripping (FIG. l, curve 2), as thermally
processed without the application of the inventive teaching
~FIG. 1, curve 3), as spliced with chemical stripping (FIG.
2, curve 11), and finally, fiber as treated to remove
water~derived species during splicing (FIG. 2, curve 12).
Detailed Description
1. GLOSSARY - While all terms used in the
description of the invention are known, it is useful to
assign specific, sometimes more quantitative meanings.
1. ~ Silica Fiber - This term which
encompasses fiber, desirably processed, in accordance with
the invention, is characterized by a surface under or
before any protective coating which contains at least 95
percent by weight silica, whether physically admixed or
chemically combined. For these purposes, "surface"
connotes a thickness corresponding with total surface
roughness in turn definng the maximum depth of contact by
ambient gas. Typical thickness is a micrometer or less.
Usual communications-grade optical fiber structures include
core regions which may contain as much as ~0 weight percent
or more of germania or other dopant material with silica,
and clad regions of silica sometimes containing small
-- 6
amounts of dopant such as fluorine or boron oxide.
While communications-grade fiber certainly con-
stitutes the aLea of greater significance from the inventive
standpoint, the invention is successfully practiced on fiber
designed for other purposes, for example on reinforcing
fiber to be included in plastic composites.
2. High Stren~th Fiber - Fiber of a pristine
median strength of at least 41.37 x 103Pa (600 ksi).
Contemplated fiber has been drawn and permissibly coated.
Coating as contemplated is generally by organic polymeric
material. Permissible coatings may be cured as by
irradiation cross-linking. Some types of commercial optical
fiber are coated with two or more layers having differing
elastic moduli.
Strengths reported in this description may be
obtained from failure on direct ~dynamic) tensile testing
or on proof testing which involves winding from one drum to
another under prescribed tension. Appropriate procedures
are described in Optical Fiber Telecommunications, Academic
Press, Inc., 1979, edited by A.G. Chynoweth and S.E. Miller,
Chapter 12.
In general, fiber of lesser pristine strength is
not beneficially treated by the invention since failure is
dominated by other mechanisms.
3. Thermal Processing - Processing in which the
fiber, or at least some surface region of the fiber,
attains a temperature of at least 600 C for a period of at
least 10 seconds or equivalent. The temperature-time
relationship if Arrhenius 1 in K so that an
temperature
equivalent time for a temperature of 700 C is 50 milli-
seconds. In fact, real splicing generally requires tem-
peratures approaching 2000 C at times of 10 seconds.
Experimentally, it has been determined that these conditions
reult in maximum damage. (Further increase in time does
not result in greater damage).
4. Fusion Splicing - Procedure by which fiber
___
ends are joined by contacting at sufficiently high
X
~L~9~2~
temperature to result in flow and finally by joinder upon
cooling. Sufficient flow occurs within a period no greater
than a few minutes. For pure silica, splicing is accom-
plished at a tempera~ure of 1800 C or higher. In general,
high silica fiber as contemplated attains a temperature of
at least 1800 C, at least at the surface during splicing.
Fusion splicing may be accomplished by a variety of tech-
niques, for example, by torch, arc or laser heating.
5. Fiber - Elongated body of a major cross-
sectional dimension -- usually diameter of up to 300
micrometers. Usual communications fiber has a thickness of
from 100 to 2no ~m. Strength fiber may be somewhat thinner,
for example, of a diameter of 50 micrometers or less. Fi~er
may be manufactured by a number of methods usually involving
drawing from a preform body which in turn may be produced
by deposition within a tube, e.g., by CVD or MCVD, or by
deposition of hydrous particles ("soot"), by deposition on
the outside or end of a rod. An alternative involves
continuous formation from a melt as by double crucible.
Strength member fiber is sometimes produced by drawing from
a single crucible.
6. "Water-Derived Species" - Chemically detectable
compositional change observable at a glass fiber surface by
infrared spectroscopy due to exposure to water vapor.
Detected species include molecular water, isolated Si-OH,
hydrogen bonded Si-OH. Description is here in terms of
"adsorption" by silica, the main constituent of the glass
fiber at the surface. Inclusion by ingredients other than
silica within the specified 5 weight percent limit may
result in other species.
7. Water-Derived Stren~_ - Tensile strength
resulting from thermal treatment as defined for fiber
containing water-derived species without application of
the inventive procedure. the same strength may result
after longer times at lower temperature. Such strength
to be meaningful is measured on fiber which has not been
subjected to mechanical damage, for example, damaged by
mechanical stripping. Water-derived strength is typically
at a median value of about 27.58 x 103Pa (400 ksi).
Strengths of a median value of less than about 20.68 x
108Pa (300 ksi) indicate substantial mechanical damage
and do not benefit substantially from applicati~n of the
invention.
In a preferred embodiment the fiber is protected
from water at least through final thermal processing so that
the water-derived strength is not a measurable value.
8. Improved Stren~h Level - Medium tensile
strength representing improvement over "water-derived
strength" due to practice of the invention. Use of an
inventive procedure on exposed fibers following thermal
processing results in return to a 'Irecovery strength".
Recovery strength is at least 20 percent higher than water-
derived strength. Typical values of improved strength are
found to be at or above 34.47 x 108Pa ~500 ksi) and
preferred species of the invention are so defined.
9 Threshold Temperature - That temperature
.
attained during thermal processing which results in measur-
able loss of strength in the absence of the invention. For
these purposes time is assumed to be the shortest practical
interval for such processing. This time may vary somewhat
depending on the type of thermal processing; as an example,
arc splicing perhaps one of the rapid processes contemplated
requires three to four seconds to complete a splice as
presently practiced. Strength loss, as well as improvement
realized by the practice of the invention are measurable
for thermal processing in which the fiber is maintained at
600 C for three to four seconds, although maximum strength
loss at this temperature is bound to occur only after
perhaps four minutes.
10. ~ Temperature - Temperature above which
some part of strength loss exemplified by water-derived
strength is decreased resulting in recovery of some
X
- 9
part of the pristine strength. Again identifying a tempera-
ture level implies a corresponding time. For temperatures
effective in healing silica rich compositions (temperatures
of 1500 C or higher) times as short as one second or less
suffice. "Strength loss" is a fiction for aspects of the
invention in which treatment avoids introduction of water-
derived species.
3. The Drawing
FIG. 1 on Coordinates of Probability of Failure
(~) on the ordinate and fracture stress in ksi (1 ksi =
6894.757 x 103Pa) on the abscissa is a plot of data
reflecting the relationship of these two parameters for
three coated fibers which are identical in composition and
cross-section, but have undergone different processing
histories. Curve 1 contains data for fiber following usual
drawing in air; that is for a fiber of "pristine" strength.
Curve 2 corresponds with a fiber which has been damaged by
mechanical stripping of organic coating preparatory to
fusion splicing. Curve 3 corresponds with a fiber which
has been thermally processed (without application of the
inventive teaching).
Median strengths for the three fibers are: 55.16
x 108, 4.83 x 108 and 27.58 x 108Pa (>800 ksi, 70
ksi, and 400 ksi), respectively. While distribution is
characteristically different for the fibers as evidenced by
the different slopes of the three curves, it is clear that
meaningful strength loss results from mechanical handling
(Curve 2). Loss for fiber which is thermally processed
while avoiding mechanical damage also shows a statistical
strength loss which is quite significant. Distribution is
tightest for pristine fiber (Curve 1). From Curve 2, it is
seen that loss due to thermal processing of mechanically
damaged fiber is largely the result of mechanical damage
itself. The median strength of approximately 4.83 x 108Pa
(70 ksi) is not significantly reduced upon thermal
processing following mechanical damaging.
-- 10 --
FIG. 2 again contains three Curves; Curve 10 is
identical to Curve 1 of FIG. 1. Again median s~rength is
55.16 x 108Pa (800 ksi). Measurements were again made on
fiber which had been coated with an organic composition by
an in-line process before reeling. The fiber of Curve 11
has a median strength similar to that of Curve 3 of FIG 1.
The fiber of Curve 11 had been thermally processed following
stripping in hot concentrated sulfuric acid during which
mechanical stripping was avoided. Curve 12~considered
exemplary of processes in accordance with the invention is
plotted from values of recovery strength. For one set of
experiments such recovery strength represents a real
improvement from the median strength of the water-derived
strength valvue of FIG. 11 [median strength about 27.58 x
10 Pa (~00 ksi)] to a median value of about 41.37 x
108Pa (600 ksi). In this set of experiments the
difference between the values represented by Curves 12 and
11 resulted from inclusion of chlorine during oxyhydrogen
torch fusion splicing.
4. Inventive Procedure
(a) General - Extensive study has characterized
the relative parameters of and resulting strength loss due
to thermal processing. Discussion in these terms is useful
for purposes of description but is not limiting. For
example, as discussed, an important aspect of the invention
involves substantial avoidance of water and therefore of
such loss so that fiber being processed has never been
reduced to its water-derived strength (strength of exposed
fiber resulting from thermal processing.
Strength loss to which the invention is directed
occurs at a temperature of at least threshold value under
conditions which are noted. Since some substantial part of
the damage may be healed by attainment of higher tempera-
tures, damage may be greatest only over fiber regions
maintained with certain temperature minima and maxima
(corresponding with threshold value and flow). In an
exemplary process, e.g. fusion splicing, maximum
X
2~
-- 11 --
temperature is, by definitiont sufficient to bring about
flow. Damage prior to fusion is retained only for cooler
fiber portions (although exposure to air during cooling
introduces damage -- usually maximum damage -- at the
position of the splice).
For certain types of thermal processing, parti-
cularly where heating of substantial fiber length is
essentially uniform, points of greatest stength loss are
randomly located while for other types of thermal processing
maximum strength loss occurs for fiber positions corres-
ponding with temperatures between noted minima and maxima.
This latter category is described in terms of "edge" effect.
This most significant strength loss -- usual point of
failure -- occurs at some location remote from the position
lS of highest temperature attainment during thermal processing.
Regardless of the form of thermal processing damage
to which the invention is directed occurs at or above some
threshold value as defined in the glossary. For usual
purposes this threshold value is 600 C. All processes in
accordance with the invention require treatment of all
fiber regions which attain threshold temperature during
thermal processing. Continued strength require continued
protection, as well.
Treatment in accordance with the inven~ion, e.g.,
use of chlorine during fusion splicing is generally
continued through the entirety of the thermal processing in
which the concerned regions of the fiber are at or above
threshold although inventive treatment may permissibly be
discontinued for regions of the fiber during attainment of
temperatures sufficient to result in healing and if healing
is adequate, for the same regions prior to attainment.
The inventive procedure invariable takes a form
resulting in lessening of introduced water-derived species.
From strength measurements it has been found that exposure
of fiber at any normally encountered ambient conditions,
i.e., to air at room temperature leads to deterioration
g
- 12 -
upon thermal processing. Relative humidity values as low
as 10 percent or lower results in measurable strength loss
upon thermal processing. Procedures in accordance with the
invention contemplate avoidance of or lessening of
introduced water-derived species and are best implemented
by continuous protecting of fiber from drawing at least
through attainment of threshold temperature durinq final
thermal processing (and desirably beyond~. In usual
processing, treatment by an inventive process is initiated
before or on attainment of threshold and is continued
through cooling to threshold.
Exame~
Examples set forth below are chosen to be of
comparable structure processiny history. Fibers are of
the general structure and composition of commercial
interest, iOe.l are high strength, high silica of outer
diameter from 50 to 300 ~m. Some fibers were
communications-grade structures with high index core
regions. Others were uniform compositions test structures.
Strength improvement realized in accordance with the
invention has not been found dependent on such inner
structure provided, of course, that the fiber is of a
pristine median strength of 41.37 x 10 Pa (600 ksi)
thigh strength fiber as defined).
Examples 1 and 2 were conducted on communications-
grade mul~imode graded index fiber of an outer diameter
125 um with an outer surface fo better than 99 percent by
weight silica and with a core region of approximately 55 ~m
diameter, graded with germania, to a maximum value of about
15 weight percent and having a "pristine strength", defined
as the tensile strength realized upon drawing in air, o~
approximate~y 55.16 x 108Pa (800 ksi). In both examples,
splices were produced by fusion splicing using an oxyhydro-
gen torch with maximum attained temperature about 2000 C
as indicated by an optical radiation pyrometer. In both
instances, organic coating produced by an in line coating
procedure was chemically removed without abrading for a
- 13 -
distance of approximately 2 cm. Splicing was accomplished
in the usual manner by butting the unheated fiber ends and
heating with splicing occurring in a period of between 5
and 30 seconds.
Example 1
Fiber, as described above, spliced in a conven-
tional oxyhydrogen torch operated in air, was found-to have
a median strength of approximately 27.58 x 108Pa (400 ksi),
with a distribution of the form shown in FIG. 1, Curve 3.
~,~
The procedure of Example 1 was repeated, however,
with an outer mantle of chlorine affixed during splicing.
The heat zone in Example 2 (as in Example 1) as defined by
visible radiation extended for a distance of approximately
2 mm in each direction from center (defining an overall
heat zone of approximately 4 mm). This zone also defined
the fiber region at or above threshold temperature (600 C).
The chlorine mantle contacted the fiber over a region at
least equal to the heat zone. Tensile strength, as
measured dynamically, was at a median level of 41.37 x
10 Pa (600 ksi) with a distribution as shown in FIG. 2,
Curve 12.
Example 3, 4
Fiber as in Examples 1 and 2, was spliced by means
of a commercial 3 watt laser (CO2). Continuous heating
times were of the order of 5 seconds duration. Splicing
was attained with a single duration. Resulting median
strengths were 27.58 x 108Pa (400 ksi) and 41.37 x 108Pa
(600 ksi) without and with a heated chlorine mantle,
respectively. The chlorine was heated to a temperature of
about 600 C to simulate conditions inherent to oxyhydrogen
splicing. Unheated chlorine was essentially ineffectual
for this very brief splice time for the specimen fiber
which had been exposed during drawing.
Example 5
Fiber was drawn from a 2 mm diameter silica
rod by use of a laser resulting in a temperature of
X
approximately 2000 C in a vacuum chamber (100 ~m) (so that
air was excluded during the entire temperature tranverse
from maximum to room temperature). Fiber diameter was from
60 to 120 ~m. The fiber was then reheated without having
previously been exposed to air to approximately 1700 C,
again with a vacuum applied over the entire temperature
range during cooling to approximately room temperature~
Tensile strength measurement in air resulted in retention
or pristine strength.
Example 6
The procedure of Example 5 was repeated, however,
with reheating in air resulting in a tensile strength of
approximately 24~13 x 108Pa (350 ksi).
Example 7
Example 6 was repeated, however, with fiber drawn
in air resulting in a substantially unchanged pristine
strength as drawn [approximately 55.16 x 108Pa (800 ksi)]
but in reduced strength upon testing following reheating
[(approximately 24.13 x 108Pa (350 ksi) ] .
Exam~le 8
Example 7 was repeated, however, with reheating
conducted in vacuum. Final strength realized after
reheating was approximately 2~.13 x 108Pa (350 ksi).
Example 9 --
Fiber of a diameter from 150-250 ~m was drawn
from 2mm rod of fused silica of purity of at least 99
weight percent by use of an oxyhydrogen torch resulting
in a maximum attained temperature of at least 2000 C.
Fiber was tensile strength tested as drawn to result in a
measured value of at least 55.16 x 10 Pa (800 ksi).
Example 10
Fiber as produced in Example 9 was reheated to a
temperature of approximately 650 C in air for a period of
about 30 minutes resulting in a water-derived strength of
approximately 32.41 x 108Pa (470 ksi).
29
- 15 -
Example 11
Fiber as drawn in Example 10 was subsequently
heated in a vacuum of approximately 100 ~m to a temperature
of from 750-850 C for a period of about 30 minutes
resulting in a minimum recovery strength of approximately
46.54 x 108Pa (675 ksi).
Example 12
Example 11 was repeated except that final heating
was conducted in a chamber which had first been evacuated
and then backfilled with chlorine. Temperature and time
were as set forth in Example 11. Measured recovery
strength was approximately 46.88 x 108Pa (680 ksi).
Example 12
Example 2 was repeated substituting an HCl gas
mantle for the chlorine mantle. Strength realized was
agian approximately 41.37 x 103Pa (600 ksi).
Example 13
Example 1 was repeated, however, substituting a
chlorohydrogen torch with a result that tensile strength
realized upon splicing was approximately 41.37 x 10 Pa
(600 ksi).
Conversion Fact _
1 psi = 6894.757 Pa
