ASTM B762-90(1999)
(Test Method)Standard Test Method of Variables Sampling of Metallic and Inorganic Coatings
Standard Test Method of Variables Sampling of Metallic and Inorganic Coatings
SCOPE
1.1 This method provides sampling plans that are intended for use in the inspection of metallic and inorganic coatings on products for the purpose of deciding whether submitted lots of coated products comply with the specifications applicable to the coating.
1.2 The sampling plans are variables plans. In plans of this type, several articles of product are drawn from a production lot. A characteristic of the coating on the drawn articles is measured. The values obtained are used to estimate the number of articles in the lot that do not conform to a numerical limit, for example a minimum thickness. The number is compared to a maximum allowable.
1.3 Variables plans can only be used when the characteristic of interest is measurable, the test method gives a numerical measure of the characteristic, and the specification places a numerical limit on the measured value. It is also necessary that the variation of the characteristic from article to article in a production lot be normally distributed (see Appendix X2). Each article must be tested in the same way (for example, coating thickness must be measured at the same location, see X2.7) so that the values from article to article are comparable. If one or more of these conditions are not met, a variables plan cannot be used. Instead, an attributes plan must be used. These are given in Method B 602 and Guidelines B 697.
1.4 This standard may involve hazardous materials, operations, and equipment. This standard does not purport to address all of the safety problems associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation:B762–90(Reapproved 1999)
Standard Test Method of
Variables Sampling of Metallic and Inorganic Coatings
This standard is issued under the fixed designation B 762; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope MIL-STD-105 Sampling Procedures and Tables for Inspec-
tion by Attributes
1.1 This test method provides sampling plans that are
MIL-STD-414 Sampling Procedures and Tables for Inspec-
intended for use in the inspection of metallic and inorganic
tion by Variables for Percent Defective
coatings on products for the purpose of deciding whether
2.3 Other:
submitted lots of coated products comply with the specifica-
ANSI/ASQC Z1.9-1979 Sampling Procedures and Tables
tions applicable to the coating.
for Inspection by Variables for Percent Non-
1.2 The sampling plans are variables plans. In plans of this
Conformance
type, several articles of product are drawn from a production
ANSI/ASQC Z1.4-1981 Sampling Procedures and Tables
lot. A characteristic of the coating on the drawn articles is
for Inspection by Attributes
measured.Thevaluesobtainedareusedtoestimatethenumber
of articles in the lot that do not conform to a numerical limit,
3. Terminology Definitions
for example a minimum thickness. The number is compared to
3.1 destructive test—a test that destroys the tested article or
a maximum allowable.
makes it nonconforming to a requirement.
1.3 Variables plans can only be used when the characteristic
3.2 nondestructive test—a test that neither destroys the
of interest is measurable, the test method gives a numerical
tested article nor makes it nonconforming to a requirement.
measure of the characteristic, and the specification places a
3.3 inspection lot—a collection of articles of the same kind
numerical limit on the measured value. It is also necessary that
that is submitted to inspection for acceptance or rejection as a
the variation of the characteristic from article to article in a
group.
production lot be normally distributed (see Appendix X2).
3.4 sample—articles randomly selected from an inspection
Each article must be tested in the same way (for example,
lot whose quality is used to decide whether or not the
coating thickness must be measured at the same location, see
inspection lot is of acceptable quality.
X2.7) so that the values from article to article are comparable.
3.5 standard deviation—a measure of dispersion equal to
If one or more of these conditions are not met, a variables plan
the square root of the mean of the squares of the deviations
cannot be used. Instead, an attributes plan must be used. These
from the arithmetic mean of the distribution (see 9.2.6).
are given in Test Method B 602 and Guide B 697.
1.4 This standard does not purport to address all of the
4. Summary of Test Method
safety concerns, if any, associated with its use. It is the
4.1 The plans in this test method provide the same protec-
responsibility of the user of this standard to establish appro-
tion as the attributes plans in Table 1, Table 2, and Table 3 of
priate safety and health practices and determine the applica-
Test Method B 602 and are interchangeable with them when
bility of regulatory limitations prior to use.
the conditions necessary for variables sampling exist. This
2. Referenced Documents method has no plan comparable to Table 4 of Test Method
B 602, because variables plans are subject to an excessive
2.1 ASTM Standards:
probability of error when the number of nonconforming
B 602 Test Method for Attribute Sampling of Metallic and
2 articles in a lot is expected to be approximately 1% or less as
Inorganic Coatings
it is for the Table 4 plan. Also for this reason, comparable
B 697 Guide for Selection of Sampling Plans for Inspection
2 variables plans are not given for the smallest lot sizes of Table
of Electrodeposited Metallic and Inorganic Coatings
1 and Table 2 of Test Method B 602. The plans of Table 4,
2.2 Military Standards:
Table 1, and Table 2 in Test Method B 602 are described as
This method is under the jurisdiction ofASTM Committee B-8 on Metallic and
Inorganic Coatingsand is the direct responsibility of Subcommittee B08.10on
General Test Methods. AvailablefromStandardizationDocumentsOrderDesk,Bldg.4SectionD,700
Current edition approved Oct. 15, 1990. Published November 1990. Originally Robbins Ave., Philadelphia, PA 19111-5094, Attn: NPODS.
published as B 762 – 86. Last previous edition B 762 – 86. Available from theAmerican National Standards Institute, 11W. 42nd St., 13th
Annual Book of ASTM Standards, Vol 02.05. Floor, New York, NY 10036.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
B762
TABLE 2 Level II—Sampling Plans for Nondestructive Tests,
Level I, Level II, and Level III respectively. For consistency,
A
Standard Deviation Unknown
Table 1 and Table 2 of this method are described as Level II
Inspection 50/50
since they are comparable to Table 1 of Test Method B 602,
nk AQL LQL AOQL
Lot Size Point
and Table 3 and Table 4 are described as Level III.
91 through 16 1.663 1.0 12 4.8 2.4
4.2 The main advantage of a variables sampling plan over
an attributes plan is that fewer articles need to be inspected to
281 through 29 1.649 1.7 10 5.0 2.6
obtain the same protection. For example, a sample of 12 using
501 through 40 1.713 1.7 8.2 4.3 2.2
variables can give the same protection as a sample of 50 using
1 200
attributes.Ontheotherhand,moreexpensivetestmethodsmay
1 201 through 61 1.704 2.1 7.4 4.4 2.5
3 200
be required to yield the measurements required by variables
3 201 through 92 1.778 2.0 5.9 3.8 2.2
sampling.
10 000
4.3 Generally, thickness is the only characteristic of a
10 001 through 137 1.825 2.0 4.9 3.4 2.0
35 000
coating that meets the conditions of a variables plan given in
Over 35 000 223 1.893 1.9 4.0 3.0 1.9
1.3. For that reason, the plans in this method are designed to be
A
The AQL, LQL, 50/50 Point, and AOQL are in percent.
used when the specification for the characteristic in question is
aminimumvalue,whichistheusualcaseforcoatingthickness.
TABLE 3 Level III—Sampling Plans for Nondestructive Tests,
Variables plans can be used when the limit is a maximum and
A
Standard Deviation Known
when there are both a minimum and a maximum. Plans for
Inspection 50/50
these cases are given in the references.
nk AQL LQL AOQL
Lot Size Point
4.4 The sampling plans in Table 1 and Table 2 of this
51 through 6 1.432 1.8 18 7.6 3.8
methodareconsideredtobestandardfornondestructivetesting
and will be used unless the buyer specifies otherwise. Table 5
151 through 10 1.411 2.7 16 7.9 4.1
andTable 6 will be used for destructive testing; these plans use
281 through 14 1.470 2.8 13 7.1 3.5
smaller samples to reduce the cost of inspection with a
resultant reduction of the ability to distinguish between con- 501 through 23 1.492 3.3 11 6.8 3.8
1 200
forming and nonconforming lots.
1 201 through 30 1.551 3.2 9.4 6.0 3.5
4.5 Additional variables plans are given in Appendix X3.
3 200
Alsofoundthereareinstructionsforthecalculationofplansfor 3 201 through 44 1.618 3.1 7.7 5.3 3.2
16 000
needs that are not covered.
16 001 through 66 1.680 3.0 6.4 4.6 3.0
35 000
5. Significance and Use Over 35 000 103 1.719 3.0 5.6 4.4 2.9
A
The AQL, LQL, 50/50 Point, and AOQL are in percent.
5.1 Sampling inspection permits the estimation of the over-
all quality of a group of product articles through the inspection
TABLE 4 Level III—Sampling Plans for Nondestructive Tests,
of a relatively small number of product articles drawn from the
A
Standard Deviation Unknown
group.
Inspection 50/50
5.2 The specification of a sampling plan provides purchas-
nk AQL LQL AOQL
Lot Size Point
ers and sellers a means of identifying the minimum quality
51 through 12 1.433 1.7 19 7.6 3.8
level that is considered to be satisfactory.
5.3 Because sampling plans yield estimates of the quality of
151 through 19 1.410 2.6 16 7.9 3.7
a product, the results of the inspection are subject to error. 280
281 through 29 1.470 2.8 13 7.1 3.8
Through the selection of a sampling plan, the potential error is
known and controlled.
501 through 48 1.494 3.3 11 6.7 3.8
1 200
1 201 through 66 1.551 3.2 9.4 6.0 3.5
3 200
TABLE 1 Level II—Sampling Plans for Nondestructive Tests,
3 201 through 102 1.618 3.1 7.7 5.3 3.2
A
Standard Deviation Known
16 000
16 001 through 159 1.680 3.0 6.4 4.6 3.0
Inspection 50/50
nk AQL LQL AOQL
35 000
Lot Size Point
Over 35 000 248 1.717 3.0 5.6 4.3 2.9
91 through 7 1.664 1.1 12 4.8 2.4
A
The AQL, LQL, 50/50 Point, and AOQL are in percent.
281 through 12 1.649 1.7 10 5.0 2.6
501 through 16 1.712 1.7 8.2 4.4 2.3
5.4 Sampling inspection is used when a decision must be
1 200
madeaboutwhattodowithaquantityofarticles.Thisquantity
1 201 through 25 1.704 2.1 7.4 4.4 2.5
may be a shipment from a supplier, articles that are ready for
3 200
3 201 through 36 1.778 2.0 5.9 3.8 2.2
a subsequent manufacturing operation, or articles ready for
10 000
shipment to a customer.
10 001 through 52 1.829 2.0 4.9 3.4 2.1
5.5 In sampling inspection, a relatively small number of
35 000
Over 35 000 82 1.893 1.9 4.0 2.9 1.9
articles(thesample)isselectedrandomlyfromalargernumber
A
The AQL, LQL, 50/50 Point, and AOQL are in percent. of articles (the inspection lot); the sample is inspected for
B762
TABLE 5 Sampling Plans for Destructive Tests, Standard
butthereisasmallprobabilitythatthesamplewillindicatethat
A
Deviation Known
the lot is unacceptable. The larger the proportion of noncon-
50/50
forming articles in an inspection lot, the more likely it will be
Inspection Lot Size nk AQL LQL
Point
that the sample will indicate that the lot is unacceptable. If
26 through 1 200 5 1.262 2.3 25 10
every article in an inspection lot is nonconforming, a sample
1 201 through 35 000 10 1.411 2.7 16 7.9
will always indicate that the lot is unacceptable.
Over 35 000 14 1.519 2.5 12 6.5
A
The AQL, LQL, and 50/50 Point are in percent.
NOTE 1—Throughout this method, it is assumed that no mistakes are
made in sampling, measurement, and calculation.
TABLE 6 Sampling Plans for Destructive Tests, Standard
A
5.11 The probability of accepting an inspection lot that
Deviation Unknown
contains nonconforming items is often described in terms of
50/50
Inspection Lot Size nk AQL LQL
Point the Acceptable Quality Level (AQL) and the Limiting Quality
Level (LQL).TheAQLis the quality level that is considered to
26 through 1 200 9 1.181 2.8 27 12
1 201 through 35 000 19 1.412 2.5 16 7.9
be acceptable. The LQL is a quality level that is considered to
Over 35 000 34 1.497 2.8 12 6.7
be barely tolerable.Asampling plan is selected that has a high
A
The AQL, LQL, and 50/50 Point are in percent.
probability of accepting lots of AQL quality and of rejecting
lots of LQL quality. In this method, the AQL given for a
sampling plan is the quality level of lots (expressed as the
conformance to the requirements placed on the articles. Based
percentage of nonconforming articles) that have a 95 % prob-
on the results, a decision is made whether or not the lot
ability of being accepted. The LQL is the quality level of lots
conforms to the requirements.
that have a 10 % probability of being accepted or, in other
5.6 Since only a portion of a production lot is inspected, the
words, a 90 % probability of being rejected. The tables in this
qualityoftheuninspectedarticlesisnotknown.Thepossibility
methodgivetheAQLandLQLofeachplan.Theyalsogivethe
exists that some of the uninspected articles are nonconforming.
50/50 point, the quality level of a lot that is just as likely to be
Therefore, basic to any sampling inspection plan is the will-
accepted as rejected.
ingness of the buyer to accept lots that contain some noncon-
5.12 The disposition of nonconforming inspection lots is
forming articles. The number of nonconforming articles in
beyond the scope of this method because, depending on the
accepted lots is controlled by the size of the sample and the
circumstances, lots may be returned to the supplier, kept and
criteria of acceptance that are placed on the sample.
used, put to a different use, scrapped, reworked, or dealt with
5.7 Acceptance sampling plans are used for the following
in some other way. An alternative is rectifying inspection in
reasons:
which rejected lots are screened and used.
5.7.1 When the cost of inspection is high and the conse-
5.13 In rectifying inspection, when an inspection lot is
quences of accepting a nonconforming article are not serious.
rejected, all of the articles in the lot are inspected and
5.7.2 When 100 % inspection is fatiguing and boring and,
nonconforming ones are removed. They may be replaced with
therefore, likely to result in errors.
conforming articles. The now 100 % conforming lot is ac-
5.7.3 When inspection requires a destructive test, sampling
cepted. With this practice, the average quality level for a series
inspection must be used.
of lots taken as a whole will be better because of the addition
5.8 In acceptance sampling by variables, the coating char-
of the 100 % conforming lots. When the incoming lots are of a
acteristic of each article in the sample is measured. Using the
good quality level, the average quality level of a series of lots
arithmetic mean of these values, the standard deviation of the
will be even better when the rejected lots are screened and
process, and the factor k that is found in the Tables, a number
resubmitted. When incoming lots are of a poor quality level,
is calculated (see 9.3). If this number equals or exceeds the
the average quality of a series of accepted lots will again be
specified minimum, the inspection lot conforms to the require-
good because many of the incoming lots will be rejected and
ments. If it is less, the lot does not conform. If the standard
upgraded. At intermediate quality levels of incoming lots, the
deviationoftheprocessisnotknown,thestandarddeviationof
average quality level of a series of accepted lots will again be
the sample is calculated and used.
improved, but it will not be improved as much as in either of
5.9 Theuseofasamplingplaninvolvesthebalancingofthe
the above cases; and there will be an intermediate quality level
costs of inspection against the consequences of accepting an
where the degree of improvement is the least. This improved
undesirablenumberofnonconformingarticles.Thereisalways
quality level is called the Average Outgoing Quality Limit
a risk that
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