U.S. patent application number 09/888535 was filed with the patent office on 2002-03-28 for surface cleanliness measurement procedure.
This patent application is currently assigned to General Electric Company. Invention is credited to Beadie, Douglas Frank, Schroder, Mark Stewart, Woodmansee, Donald Ernest.
Application Number | 20020035869 09/888535 |
Document ID | / |
Family ID | 24686863 |
Filed Date | 2002-03-28 |
United States Patent
Application |
20020035869 |
Kind Code |
A1 |
Schroder, Mark Stewart ; et
al. |
March 28, 2002 |
Surface cleanliness measurement procedure
Abstract
A procedure and tools for quantifying surface cleanliness are
described. Cleanliness of a target surface is quantified by wiping
a prescribed area of the surface with a flexible, bright white
cloth swatch, preferably mounted on a special tool. The cloth picks
up a substantial amount of any particulate surface contamination.
The amount of contamination is determined by measuring the
reflectivity loss of the cloth before and after wiping on the
contaminated system and comparing that loss to a previous
calibration with similar contamination. In the alternative, a
visual comparison of the contaminated cloth to a contamination key
provides an indication of the surface cleanliness.
Inventors: |
Schroder, Mark Stewart;
(Hendersonville, NC) ; Woodmansee, Donald Ernest;
(Simpsonville, SC) ; Beadie, Douglas Frank;
(Greenville, SC) |
Correspondence
Address: |
NIXON & VANDERHYE P.C.
1100 North Glebe Road, 8th Floor
Arlington
VA
22201-4714
US
|
Assignee: |
General Electric Company
|
Family ID: |
24686863 |
Appl. No.: |
09/888535 |
Filed: |
June 26, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09888535 |
Jun 26, 2001 |
|
|
|
09669574 |
Sep 26, 2000 |
|
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Current U.S.
Class: |
73/432.1 ;
15/143.1; 73/864.71 |
Current CPC
Class: |
G01N 1/02 20130101; G01N
2001/028 20130101; B08B 1/00 20130101; G01N 21/94 20130101 |
Class at
Publication: |
73/432.1 ;
73/864.71; 15/143.1 |
International
Class: |
G01P 001/02 |
Goverment Interests
[0001] This invention was made with Government support under
Government contract No. DE-FC21-95-MC31176 awarded by the
Department of Energy. The Government has certain rights in this
invention.
Claims
What is claimed is:
1. A tool for measuring surface cleanliness comprising: a main body
portion; a hand grip portion fixedly disposed with respect to said
main body portion so that motion of said hand grip portion effects
motion of said main body portion; a debris collecting component
detachedly coupled to an operative surface of said main body
portion, whereby said debris collecting component can be disposed
in contact with and moved relative to a target surface, and
thereafter detached from said main body portion for at least one of
storage and evaluation.
2. A tool as in claim 1, wherein said debris collecting component
comprises a swatch of cloth material.
3. A tool as in claim 2, wherein said cloth material is mounted to
a resilient component.
4. A tool as in claim 1, wherein a resilient component is
interposed between said debris collecting component and said main
body portion.
5. A tool as in claim 4, wherein said resilient component comprises
a layer of foam material.
6. A tool as in claim 5, wherein said layer of foam material is
substantially permanently attached to said main body portion.
7. A tool as in claim 4, wherein said debris collecting component
is detachedly secured to said resilient component.
8. A tool as in claim 7, wherein said debris collecting component
is detachedly secured to said resilient component with a reusable
adhesive layer.
9. A tool as in claim 1, wherein an elongated pole component
defines said main body portion and said hand grip portion, said
hand grip portion being defined at a proximal end of said elongated
pole component and said operative surface of said main body portion
being disposed adjacent a distal end of said pole component.
10. A tool as in claim 9, wherein said elongated pole component is
formed from at least one of plastic, metal, and wood.
11. A tool as in claim 1, wherein said main body portion and said
hand grip portion each have a generally circular cross-section.
12. A tool as in claim 11, wherein said hand grip portion has a
diameter less than a diameter of said main body portion.
13. A tool as in claim 11, having a generally uniform diameter
along a length thereof.
14. A tool as in claim 1, wherein said operative surface is
generally spherically convex.
15. A tool as in claim 1, wherein said operative surface is a
generally continuously curved, non-planar surface.
16. A method of measuring surface cleanliness comprising: providing
a sampling device; wiping a prescribed area of a target surface
with said sampling device; measuring a reflectivity of a surface of
said sampling device to determine a cleanliness of the target
surface.
17. A method as in claim 16, wherein said step of wiping is
repeated at least once before said measuring step.
18. A method as in claim 16, further comprising providing a mask
component having an opening defined therein to define said
prescribed area of said target surface for being wiped with said
sampling device and wherein said step of wiping comprises wiping an
area of the target surface delineated by said opening in said mask
component.
19. A method as in claim 16, wherein said sampling device comprises
a smear component secured to a tool.
20. A method as in claim 19, further comprising detachably securing
said smear component to said tool.
21. A method of measuring surface cleanliness comprising: providing
a sampling device; wiping a prescribed area of a target surface
with said sampling device; comparing a debris loaded surface of
said sampling device to a particulate contamination key to
determine a cleanliness of the target surface.
22. A method as in claim 21, wherein said step of wiping is
repeated at least once before said comparing step.
23. A method as in claim 21, further comprising providing a mask
component having an opening defined therein to define said
prescribed area of said target surface for being wiped with said
sampling device and wherein said step of wiping comprises wiping an
area of the target surface delineated by said opening in said mask
component.
24. A method as in claim 21, wherein said sampling device comprises
a smear component secured to a tool.
25. A method as in claim 24, further comprising detachably securing
said smear component to said tool.
Description
BACKGROUND OF THE INVENTION
[0002] The invention relates to a procedure and tools for
conducting a robust, quantitative measurement of surface
cleanliness for field installation, factory fabrication, and
assembly of mechanical systems such as power plant equipment. More
specifically, the invention provides a quantification of the
qualitative white glove test often used by cleanliness
inspectors.
[0003] Surface particulate contamination is a well-known source of
mechanical system failures. Particulate contamination can cause
abrasion at the interface between moving parts, contamination of
fluids flowing through the system and erosion of structures in high
velocity fluid flow path(s) and/or create deposits that either
reduce desired flows or insulate against desired heat transfer.
While the provision of filters and the like can control the flow of
particulate contaminates into a system during operation, in some
systems the presence of particulate contaminates on parts during
assembly can substantially contribute to particulate accumulation
in the field and the resultant risk of poor performance and/or
reduced component operating life. Moreover, filters are always
specified with effectiveness, which means that a small percentage
of undesirable particles will always pass through.
[0004] A variety of approaches to determine surface cleanliness are
known and used for various components and surfaces. However, none
of those approaches deal with directly measuring the presence and
amount of particulate on the surface of a large component during
factory assembly. Instead, these techniques focus either on
measurement of particulate concentration in fluids used to wash
parts or on determining the presence of organic films. The fluid
concentrations are usually determined using light attenuation or
refraction, often using laser beams. The organic films are
determined by creating a water film on the surface and determining
the formation of droplets or breakup of fluid films into rivulets,
or the organic film thickness is determined by refraction of light
shined at the film. The fluid based approaches cannot be adapted
for practical, economic use on a mechanical assembly floor and the
second mentioned technique is not relevant since the particles, not
an organic film, is at issue.
[0005] In another known approach, surface replicas are used. With
such replicas, the surface to be sampled is covered with either an
adhesive tape or is covered with a curable material, which
subsequently replicates the surface topography while also capturing
loosely held surface particulate contamination. The surface is then
scanned manually or with sophisticated optical recognition software
to count numbers of particles and sizes. This approach is indeed
used scientifically but it cannot give instantaneous results on the
factory floor and in most cases would be considered prohibitively
expensive. Moreover, the sample area is of necessity, exactly equal
to the size of the removed sample. No amplification can be obtained
this way. In other words, it is not possible to sample a larger
area than that of the sampling device itself.
BRIEF SUMMARY OF THE INVENTION
[0006] The invention provides a special procedure using a set of
specialized equipment to quantify the particulate contamination of
a component surface. In a preferred embodiment of the invention,
the contamination can be amplified to provide for a more accurate
reading, by sampling a larger component area then the area of the
sampling device itself. With the sampling devices of the invention,
a measurement is provided that has visual meaning to the operator
making it and provides a convenient archival form of the
contamination sample for later process audit, if necessary or
desirable.
[0007] According to the well-known white glove test, an inspector
wears a bright white glove and wipes a glove finger or hand across
the target surface for some distance and then observes the glove
where it touched the surface. The inspector then makes an arbitrary
decision about whether the removed contamination is sufficiently
low as to not require a recleaning of the surface. The invention
provides equipment and procedures to quantify this conventional
white glove test. In an embodiment of the invention, a bright white
cloth swatch is rubbed on a known clean surface similar in surface
roughness to the surface that is to be sampled. The surface wipe is
preferably accomplished by securing the swatch to a tool adapted to
be grasped by the operation and swiped on a target surface area
with a generally repeatable amount of pressure. Most preferable the
target area is defined, and limited by a mask. The effectively
still clean swatch or smear is then measured for its reflectivity
(brightness) using a reflectivity instrument. The smear is then
rubbed on the surface to be tested to collect particulate
contamination and the reflectivity is again measured. The loss of
reflectivity is then related, for example, through imperical
calibration, to the amount of contamination that was transferred to
the smear in the second rubbing.
[0008] In another embodiment, a reflectivity instrument is
calibrated based on the reflectivity of the clean surface rubbed
swatch so that only a single rub of each swatch thereafter, on the
part to be tested, is required.
[0009] In yet a further embodiment of the invention, a
contamination key is provided illustrating the appearance of the
cloth wipe or swatch exhibiting different amounts of particulate
contamination so that following a test swipe of the component, the
swatch can be compared to the key to visually approximate the level
of contamination.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a perspective view showing a cleanliness
measurement procedure embodying the invention;
[0011] FIG. 2 is a schematic cross sectional view of an exemplary
cleanliness measurement tool embodying the invention;
[0012] FIG. 3 is an elevational view of another cleanliness
measurement tool embodying the invention;
[0013] FIG. 4 is a delta reflectance calibration curve; and
[0014] FIG. 5 is a particulate contamination key embodying the
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0015] The surface cleanliness measurement technique embodying the
invention is a quantitative adaptation of the "white glove"
technique, which has been used throughout history to spot check
surface cleanliness. The invention provides a robust, quantitative
measurement of surface cleanliness for field installation, factory
fabrication and assembly of mechanical equipment. The technique is
applicable to a variety of manufacturing procedures such as
checking for cleanliness on close mating components to ensure
debris does not interfere with the fit-up. It can also be used to
monitor cleanliness of manufactured components and to monitor
cleanliness compliance in clean rooms.
[0016] The invention proposes to measure surface cleanliness by
sampling a prescribed area of a surface of the component and
determining the amount of particulate contamination transferred to
the sampling device. According to a preferred embodiment of the
invention, the sampling protocol specifies certain features of the
swiping process important to accurately quantify particle
contamination, including sampling device area in contact with the
target surface, the area sampled by the sampling device, the force
of the wipe, the uniformity of pressure distribution, and the
amount of wiping done to collect the sample. The amount of
contamination is then quantified by a calibration technique that
relates the reduction in light reflectance of the cloth sample and
the mass of the dirt type that was collected on the cloth
sample.
[0017] A key to effective sampling is to properly mount the
debris-collecting device on a tool in such a way that the
contamination is transferred to the debris-collecting device as
much as possible. Two basic tools are provided in an exemplary
embodiment of the invention to perform the contamination
measurement. The first tool is a hand tool, which carries the
debris collecting material or smear while the target area is wiped
for contamination. The hand tool is preferably equipped with a
spring that is compressed during wiping to apply a known force
during the wipe of about 170 g. The second tool is a pole tool for
sampling, e.g., the inside of a pipe. The area sampled inside the
pipe is determined by wiping around the complete inner
circumference over a known axial distance. Providing indicia along
at least a portion of the length of the pole allows the axial
distance of the swipe to be monitored. Like the hand tool, the pole
tool is equipped with a spring to apply a uniform force during the
wipe. In a presently preferred exemplary embodiment, the spring
component for both the hand tool and the pole tool is a layer of
foam interposed between the body of the tool and the smear to more
uniformly distribute the wiping pressure, as described in greater
detail below.
[0018] An example of a hand tool that may be used for swiping a
target surface is a wooden drawer pull having approximately 2 inch
diameter major surface to which a layer of foam is applied. This
tool provides a simplified approach with which a firm but not
excessive force can be used to obtain the smear sample. In FIG. 1,
a hand tool 10 such as an adapted drawer pull is shown during a
contamination collection procedure. As illustrated, a mask 12 is
preferably provided, as described in greater detail below, to limit
the area that can be swiped with the hand tool. A schematic cross
sectional view of this exemplary hand tool is shown in FIG. 2. As
illustrated, the hand tool includes a hand grip portion 14 which in
the case of a drawer pull is composed of the reduced diameter neck
16 of the drawer pull and the base 18, and an operative portion or
main body 20 including an operative surface 22 to which the smear
24 is mounted.
[0019] As mentioned above, in the presently preferred embodiment, a
spring, such as a layer of foam 26 is interposed between the main
body 20 of the tool 10 and the smear 24. In the presently preferred
embodiment, the layer of foam has a thickness of about 1/8 to 1/4
inch. The foam is preferably fixedly secured to the hand tool main
body for example with an adhesive suitable for bonding foam to the
material, e.g. wood, plastic, or metal, of the hand tool main body
20. The smear 24 is preferably detachably coupled to the distal
surface of the foam. In an exemplary embodiment, an adhesive layer
28 is provided on the smear that is releasable and reapplyable so
that the smear can be releasably attached to a supply sheet,
removed from the supply sheet and adhered to the foam of the tool
and then removed from the tool and applied to an archival sheet.
Suitable adhesives that will releasably attach the smear to the
foam so that the smear remains in place during the swiping
operation but which is detachable from the foam and maintains
sufficient adhesion to be applied to an archival sheet are known
and suitable such adhesives can be readily identified by those
skilled in the art.
[0020] As an alternative to providing a resilient material on the
hand tool, the smear can be supplied with a resilient backing
adhered thereto so that the swipe and resilient backing are
together applied and removed from the hand tool.
[0021] As schematically illustrated in FIG. 3, the pole tool is
preferably in the form of an elongated pole component or dowel 30
or, e.g., wood, plastic or metal material, having a diameter of at
least about 1/2 inch and a length on the order of from about 8
inches to about 4 feet long. A smear 32 is preferably adhered to a
resilient material (not shown in FIG. 3) such as a foam layer (not
shown) provided on an operative surface 34 of a main body 35 of the
tool, at the distal end 36 thereof. The proximal end of the pole
tube is used as a hand grip portion 38. As noted above, the pole
tool may have indicia 40 along its length to indicate the depth of
insertion into a pipe or tube and so that the operator can swipe
the pipe interior through a prescribed axial length.
[0022] Accordingly, to conduct a contamination sampling, a tool is
selected for receiving the smear and swiping a target region of the
component surface. In one exemplary embodiment, typified by the
hand tool 10 illustrated in FIG. 2, the sampling tool has an
operative face 22 generally corresponding to but slightly greater
in diameter than the smear 24 to ensure substantially full surface
contact of the smear with the surface to be sampled. As also
explained above, the tool operative surface is also preferably
resilient to control and increase the contact area between the
smear and the target surface. In a preferred embodiment, then, the
smear is mounted on a small, 2 inch outer diameter tool face which
is generally continuously curved and preferably approximately
spherically convex. As noted above, to control and increase the
contact area between the smear and the target surface, the tool has
a thin layer of foam or other resilient material fixedly secured on
the tool face, under the smear.
[0023] In a preferred embodiment, the smear is a bright white
circular cloth disk 24,32 having a diameter of about 1 to 21/2
inches, more preferably about 11/2 to 2 inches and most preferably
about 13/4 inches in diameter. To securely but removably secure the
cloth to the tool 10,30, the backside of the cloth swatch
(hereafter referred to as a smear) has a multiple-use contact
adhesive 28 applied thereto that is used to selectively adhere the
smear to a supply sheet, to the sampling tool, and then to a
convenient archival form for later process or audit, should that be
necessary. Suitable smears of 13/4 inch diameter having a
multiple-use contact adhesive on one side can be obtained from DA
Services, Inc. Defense Apparel, 247 Addison Road, Windsor, Conn.
06095.
[0024] To limit the area to be sampled to a prescribed area, the
area to be sampled is preferably masked with a clean and stiff but
flexible sheet of material 12 such as Teflon into which a sampling
area 42 has been cut. The sheet stock from which the mask is formed
preferably has a thickness on the order of about {fraction (1/16)}
of an inch with a known open area therein used as a guide for
sampling. In an exemplary embodiment, a circular hole of
approximately 6 inches diameter (15.24 cm) and having a chamfered
edge 44 is formed in the sheet stock thereby defining the sampling
hole to limit the sampling to a controlled region of known area.
Either the hand tool 10 or the pole tool 30 can sample the area
defined by the mask 12.
[0025] To avoid removal of particulate contamination from the
smear, according to a preferred wiping procedure, the forward edge
of the smear tool is lifted as it is moved in ever increasing
circles from the center outward. Moreover, advantageously, as the
smear tool is moved it is so rotated into the direction of motion
as to provide a fresh surface of the smear to pick up
contamination. As the wiping trajectory approaches the inner,
chamfered edge of the mask, the tool is so tilted that the outside
edge rides up slightly over the lip of the mask while the wiping
motion is tangential to the inner edge. This overlap enables the
capture of any material inadvertently rubbed out onto the mask.
Meanwhile, the tangential motion minimizes the tendency to rub
contamination from the sampled area out under the mask.
[0026] Two wipes of the surface are generally sufficient to capture
substantially all the surface contamination on the smear. A second
wipe will not only collect residue from the first wiping but will
also smear the contamination around the cloth to make a more
uniform darkening of the smear. According to the preferred
embodiment of the invention, a sampling area is not rubbed more
than 3 times because of the effects of pushing the contamination
from the surface to the interior of the cloth structure. Also, the
additional rubbing may degrade the cloth sampling surface by
abrasion. These adverse effects on the ultimate reflectivity
measurements can be discounted, however, by consistently following
a selected procedure exactly each time. It is clear that the
accuracy, repeatability and reproducibility of the cleanliness
measurement will largely depend upon the sampling being carried out
using a prescribed procedure, that is a prescribed number of wipes
and prescribed wipe pattern on a prescribed surface area, each time
the measurement has carried out.
[0027] The swiping procedure, the smear and the target surface may
be pretreated to facilitate the removal of contamination to
evaluate the same. For example, it may be beneficial to precoat the
smear with a clear and somewhat tacky coating to enable it to hold
larger amounts of loose particulate matter. That coating may be
applied either initially or after the first wipe procedure or in
advance of each wipe procedure. It may also be of benefit in select
testing procedures to spray the target surface with a clear liquid
cleaner to help release contamination from the target surface and
transfer it to the smear. It is also to be appreciated that the
presence of any remnant oil film from the target surface may
require a re-calibration with that same level of oil film since the
oil, even without particulate will be likely to reduce the smear
brightness itself. As an alternative, the smear can be soaked to
vaporize any organic material leaving only the particulate residue
to reduce reflectivity. These potential enhancements to the
contaminant removal will ultimately depend, at least in part, upon
the purpose for the surface contamination evaluation. When the
testing procedure is provided for the purpose of evaluating the
amount of debris that may, in use, be released from the surface and
collected at deposition points within the apparatus in the field,
then it is desirable for the swipe test to be indicative of the
particulate material that may be removed from the surface and
redeposited during normal use of the component.
[0028] The amount of contamination picked up by the smear is
measured by comparing the reduction in smear reflectivity to a
known amount of contamination in the area being sampled. The
reflectivity meter and sensor are a commercial system, set up to
operate with a blue filter where the specimen (smear) is pressed
against the face of the sensor head. The sensor head has a 3/4 inch
hole in its distal end and contains both the light source and the
photomultiplier, which illuminates and measures reflectance from
objects placed against the 3/4 inch hole. One example of the device
described is a Photovolt Model 577 and can be purchased from UMN
Electronics--Photovolt Division, 6911 Hillsdale Court,
Indianapolis, Tenn. 46250.
[0029] An exemplary Smear Sampling Technique includes the following
steps:
[0030] 1. Measure unused smear reflectivity once or twice to verify
that the smear is new. Record the actual reflectivity value.
[0031] 2. Mount smear on a tool by using tweezers to transfer the
smear from its paper folder to the wiping tool.
[0032] 3. Conduct a pre-wipe to "condition" the smear for its
initial loss of reflectivity. The conduct of a pre-wipe and of a
sample wipe is identical. The smear is wiped completely over the
target surface twice, controlling the applied force where it is
measurable (hand tool). For the hand tool, it is recommended that
the wipe start at the center of the masked area and gradually
spiral outward to slightly overlap the mask. Each pass should take
about 5 seconds, so a complete wipe will take a bit over 10
seconds.
[0033] 4. Using only tweezers, transfer the smear to its paper
folder.
[0034] 5. Make a 5-point measurement of reflectivity, being sure to
move the measurement area 1/4 inch off the smear center in each
direction. Record the measurements.
[0035] 6. Store the smear for later examinations and
comparisons.
[0036] 7. An unused smear should be saved (about once per batch of
surface measurements) to serve as controls for later
remeasurements.
[0037] An example of a calibration procedure procedure is as
follows: a sample of the particulate material to be collected is
placed on a weigh paper in a laboratory balance to obtain its gross
weight. The sample is then distributed within the area of the mask
placed on the calibration target surface. The empty weigh paper is
then reweighed to get the tare weight. Then, a sample wiping
procedure is performed, as described above, and a reflectivity
measurement is performed, as described above. Results are graphed
as the reduction in (delta) reflectivity (value of the smear after
the pre-wipe to after the sample) as a function of the mass loading
(gm/m.sup.2) of contaminant placed inside the masked area. An
example of a calibration curve is provided as FIG. 4.
[0038] As an alternative to measuring the reflectivity of each
smear following the surface wipe, a particulate contamination key,
as illustrated by way of example in FIG. 5, can be prepared to show
the visual appearance of smears that have been used to wipe
surfaces having various known mass loading of contamination. Then,
following a wiping procedure, the smear can be visually compared to
the key to determine the approximate surface contamination of the
component being tested.
[0039] As noted above, the cloth smears that are preferably used in
the surface contamination evaluation process of the invention are
available commercially, but they have been used heretofore to
manually wipe surfaces suspected of having radioactive material on
them. After wiping, the smears have conventionally been placed
under radiation detectors to measure the radioactivity. The smear
cloths are conventionally supplied adhered to wax paper folders to
enabling labeling, sample protection and archiving as is the case
with the preferred embodiment of the invention. As noted, however,
the smear cloths have been manually held to swipe testing surfaces
and no provision has been made to improve collection efficiency by
using a special rubbing tool as disclosed hereinabove.
[0040] It is to be appreciated that the invention has several
benefits for use on the factory floor. The stain or contamination
of the smear is very visual to the operator and subsequent
measurement is quite understandable without technical training.
Also, the reflectivity measurement can be made on the factory floor
within minutes of the sample smear being wiped on the target
surface. This is advantageous in effective implementation of the
technique under the time pressures of a major manufacturing and
assembling operation. It also minimizes the risk of smear
contamination or accumulated particle loss during transport to a
remote detection site. Because the operator can both sample and
measure himself, there is an increased likelihood that the process
will be regularly used and will consequently reinforce the
operator's awareness of cleanliness requirements. As noted above,
the masked area is for example about 6 inches which is
substantially larger than the diameter of the swatch which is
preferably about 1.75 inches. This means that the sampled area is
({fraction (6/1.75)}).sup.2 or almost 12 times larger than the area
of the swatch. This provides a sample concentration that is an
order of magnitude improvement over using replicas, even if the
smear collection was only 90% effective. This increases the
likelihood that even low levels of particulate contamination can be
accurately detected and measured.
[0041] While the invention has been described in connection with
what is presently considered to be the most practical and preferred
embodiment, it is to be understood that the invention is not to be
limited to the disclosed embodiment, but on the contrary, is
intended to cover various modifications and equivalent arrangements
included within the spirit and scope of the appended claims.
* * * * *