This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles
for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D91 − 24
Standard Test Method for
Precipitation Number of Lubricating Oils1,2
This standard is issued under the fixed designation D91; 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 (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope* 3.1.1 For definitions of terms used in this test method, refer
1.1 This test method covers the determination of the pre- to Terminology D4175.
cipitation number of steam cylinder stocks and black oils, and 3.2 Definitions of Terms Specific to This Standard:
can be used for other lubricating oils. 3.2.1 ASTM precipitation number, of lubricating oils, n—the
1.2 The values stated in SI units are to be regarded as number of millilitres of precipitate formed when 10 mL of
standard. No other units of measurement are included in this lubricating oil are mixed with 90 mL of ASTM precipitation
standard. naphtha, and centrifuged under the conditions of the test.
1.3 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the 4. Significance and Use
responsibility of the user of this standard to establish appro- 4.1 Fully refined petroleum oils normally contain no
priate safety, health, and environmental practices and deter- naphtha-insoluble material. Semirefined or black oils fre-
mine the applicability of regulatory limitations prior to use. quently contain some naphtha-insoluble material (sometimes
1.4 This international standard was developed in accor- referred to as asphaltenes). This test measures the amount of
dance with internationally recognized principles on standard- naphtha-insoluble material in the oil. This quantity is reported
ization established in the Decision on Principles for the as the precipitation number.
Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical
5. Apparatus
Barriers to Trade (TBT) Committee.
5.1 Centrifuge Tube, cone-shaped, conforming to the di-
2. Referenced Documents mensions given in Fig. 1, and made of thoroughly annealed
2.1 ASTM Standards:3 glass. The graduations, numbered as shown in Fig. 1, shall be
D4057 Practice for Manual Sampling of Petroleum and clear and distinct, and the mouth shall be constructed in a shape
Petroleum Products suitable for closure with a cork. Scale-error tolerances and
D4175 Terminology Relating to Petroleum Products, Liquid smallest graduations between various calibration marks are
Fuels, and Lubricants given in Table 1 and apply to calibrations made with air-free
D4177 Practice for Automatic Sampling of Petroleum and water at 20 °C.
Petroleum Products
5.2 Centrifuge, meeting all the safety requirements for
3. Terminology normal use and capable of whirling two or more filled
centrifuge tubes at a speed which can be controlled to give a
3.1 Definitions:
relative centrifugal force (rcf) between 600 and 700 at the tips
of the tubes. The revolving head, trunnion rings, and trunnion
1
This test method is under the jurisdiction of ASTM Committee D02 on
cups, including the rubber cushion, shall be soundly con-
Petroleum Products, Liquid Fuels, and Lubricants and is the direct responsibility of structed to withstand the maximum centrifugal force capable of
Subcommittee D02.06 on Analysis of Liquid Fuels and Lubricants. being delivered by the power source. The trunnion cups and
Current edition approved Nov. 1, 2024. Published November 2024. Originally
approved in 1921. Last previous edition approved in 2019 as D91 – 02 (2019). DOI:
cushions shall firmly support the tubes when the centrifuge is
10.1520/D0091-24. in motion. The centrifuge shall be enclosed by a metal shield or
2
This test method has been adopted for use by government agencies to replace case strong enough to eliminate danger if any breakage occurs.
Method 3101 of Federal Test Method Standard No. 791b.
3
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Calculate the speed of the rotating head by means of the
contact ASTM Customer Service at www.astm.org/contact. For Annual Book of following equation:
ASTM Standards volume information, refer to the standard’s Document Summary
page on the ASTM website. speed, r/min 5 1337 =rcf/d (1)
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
1
D91 − 24
8. Procedure
8.1 Add 10 mL 6 1 mL of the oil to be tested in each of two
clean, dry centrifuge tubes at room temperature. Fill each tube
to the 100 mL mark with hexanes and close tightly with a
softened cork (not a rubber stopper). Then invert each tube at
least 20 times, allowing the liquid to drain thoroughly from the
tapered tip of the tube each time. Place the tubes in a water bath
at 32 °C to 35 °C for 5 min 6 1 min. Momentarily remove the
corks to relieve any pressure, and invert each tube again at least
20 times exactly as before. The success of this method depends
to a large degree upon having a thoroughly homogeneous
mixture which will drain quickly and completely from the
tapered tip when the tube is inverted.
8.2 Balance the two centrifuge tubes or pairs of tubes with
their respective trunnion cups and place them on opposite sides
of the centrifuge head. Then whirl them for 10 min at a rate
sufficient to produce a relative centrifugal force (rcf) between
600 and 700 at the tips of the whirling tubes (see 5.2). Repeat
this operation until the volume of sediment in each tube
NOTE 1—For volumetric tolerances see Table 1. remains constant for three consecutive readings. In general, not
FIG. 1 ASTM Cone-Shaped Centrifuge Tube more than four whirlings will be required for oils having a low
precipitation number.
TABLE 1 Calibration Tolerances for 200 mm Centrifuge Tube
Range, mL Subdivision, mL Volume Tolerance, mL
9. Calculation and Report
0 to 0.1 0.05 ±0.02 9.1 Read the volume of the solid sediment at the bottom of
Above 0.1 to 0.3 0.05 ±0.03 each centrifuge tube, estimating to 0.1 mL or closer if possible.
Above 0.3 to 0.5 0.05 ±0.05
Above 0.5 to 1.0 0.10 ±0.05 If the two readings differ by not more than 0.1 mL, report the
Above 1.0 to 2.0 0.10 ±0.10 mean of the two as the ASTM Precipitation Number. If the two
Above 2.0 to 3.0 0.20 ±0.10 readings differ by more than 0.1 mL, make two more determi-
Above 3.0 to 5.0 0.5 ±0.20
Above 5.0 to 10 1.0 ±0.50 nations and report the average of the four determinations.
Above 10 to 25 5.0 ±1.00
Above 25 to 100 25 ±1.00 10. Precision and Bias
10.1 The precision of this test method as determined by
statistical examination of interlaboratory results is as follows:
where:
10.2 Repeatability—The difference between two test results,
rcf = relative centrifugal force, and
obtained by the same operator with the same apparatus under
d = diameter of swing, in mm, measured between tips of
constant operating conditions on identical test material, would
opposite tubes when in rotating position.
in the long run, in the normal and correct operation of the test
Table 2 shows the relationship between diameter swing, rcf, method, exceed the following values only in one case in
and revolutions per minute. twenty:
Precipitation Number Repeatability
6. Reagent 0.00 to 1.20 10 % of Mean
6.1 Hexanes, Reagent Grade. (Warning—Extremely 10.3 Reproducibility—The difference between two single
flammable, harmful if inhaled.) and independent results obtained by different operators work-
7. Sampling ing in different laboratories on identical test material would, in
the long run, in the normal and correct operation of the test
7.1 For sampling techniques, see Practice D4057 or Practice method, exceed the following values only in one case in
D4177. twenty:
Precipitation Number Reproducibility
TABLE 2 Rotation Speeds for Centrifuges of Various Diameters 0.00 to 1.20 30 % of Mean
Diameter of Swing, mmA r/min at 600 rcf r/min at 700 rcf 10.4 Bias—This procedure is empirical, precipitation num-
483 1490 1610 ber is defined solely by this procedure, therefore, no statement
508 1450 1570 of bias can be made.
533 1420 1530
559 1390 1500
A
11. Keywords
Measured in millimetres between tips of opposite tubes when in rotating position.
11.1 asphaltenes; lubricating oils; precipitation number
2
D91 − 24
SUMMARY OF CHANGES
Subcommittee D02.06 has identified the location of selected changes to this standard since the last issue
(D91 – 02 (2019)) that may impact the use of this standard. (Approved Nov. 1, 2024.)
(1) Added Terminology D4175 to Sections 2 and 3.
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