The symbol 
on a glass stopper used in laboratory equipment stands for “standard taper,” which has been
standardized as a 1 in 10 taper. Originally applied only to glassware, this system is now also used to describe
stoppers made of Teflon and other materials. A
size is the diameter in millimeters of the large end of the
ground area. Typical stopper sizes are
8,
9,
13,
16,
19,
22,
27,
32, and
38.
The system
is used to describe joints connecting glassware such as condensers and reaction vessels,
but the size of such joints is given as two numbers separated by a slash, e.g., 24/40. The taper is
1 in 10, the same
as that of glass stoppers.
Four different sets of lengths have been used. The standardized joint size designations were chosen so that each designation appears in only one series.
| Joint size designation |
Approx. diameter at small end, millimeters |
Approx. length of ground area, millimeters |
Computed diameter at large end, millimeters |
|---|---|---|---|
| 7/25 | 5.0 | 25 | 7.5 |
| 10/30 | 7.0 | 30 | 10.0 |
| 12/30 | 9.5 | 30 | 12.5 |
| 14/35 | 11.0 | 35 | 14.5 |
| 19/38 | 15.0 | 38 | 18.8 |
| 24/40 | 20.0 | 40 | 24.0 |
| 29/42 | 25.0 | 42 | 29.2 |
| 34/45 | 30.0 | 45 | 34.5 |
| 40/50 | 35.0 | 50 | 40.0 |
| 45/50 | 40.0 | 50 | 45.0 |
| 50/50 | 45.0 | 50 | 50.0 |
| 55/50 | 50.0 | 50 | 55.0 |
| 60/50 | 55.0 | 50 | 60.0 |
| 71/60 | 65.0 | 60 | 71.0 |
| 103/60 | 97.0 | 60 | 103.0 |
| Joint size designation |
Approx. diameter at small end, millimeters |
Approx. length of ground area, millimeters |
Computed diameter at large end, millimeters |
|---|---|---|---|
| 5/12 | 3.8 | 12 | 5.0 |
| 7/15 | 6.0 | 15 | 7.5 |
| 10/18 | 8.2 | 18 | 10.0 |
| 12/18 | 10.7 | 18 | 12.5 |
| 14/20 | 12.5 | 20 | 14.5 |
| 19/22 | 16.6 | 22 | 18.8 |
| 24/25 | 21.5 | 25 | 24.0 |
| 29/26 | 26.6 | 26 | 29.2 |
| 34/28 | 31.7 | 28 | 34.5 |
| 40/35 | 36.5 | 35 | 40.0 |
| Joint size designation |
Approx. diameter at small end, millimeters |
Approx. length of ground area, millimeters |
Computed diameter at large end, millimeters |
|---|---|---|---|
| 5/8 | 4.2 | 8 | 5.0 |
| 7/10 | 6.5 | 10 | 7.5 |
| 10/7 | 9.3 | 7 | 10.0 |
| 10/10 | 9.0 | 10 | 10.0 |
| 12/10 | 11.5 | 10 | 12.5 |
| 14/10 | 13.5 | 10 | 14.5 |
| 19/10 | 17.8 | 10 | 18.8 |
| 24/12 | 22.8 | 12 | 24.0 |
| 29/12 | 28.0 | 12 | 29.2 |
| 34/12 | 33.3 | 12 | 34.5 |
| 40/12 | 38.8 | 12 | 40.0 |
| 45/12 | 43.8 | 12 | 45.0 |
| 50/12 | 48.8 | 12 | 50.0 |
| 55/12 | 53.8 | 12 | 55.0 |
| 60/12 | 58.8 | 12 | 60.0 |
| 71/15 | 69.5 | 15 | 71.0 |
A tolerance of plus 0.15 mm is allowed on inner members (such as stoppers) and minus 0.15 on outer members. As might be expected for devices intended to make a hermetic seal, a large part of the standards is devoted to gages and gauging.
The most common joint is 24/40.
There is a fourth, "international" length. Discrepancies between the ISO Recommendation and the ASTM standards are currently being ironed out.
ISO 383:1976. Interchangeable Conical Ground Glass Joints.
ASTM International.
ASTM E675-02 Standard Specification for Interchangeable
Taper-Ground Stopcocks and Stoppers.
Annual Book of ASTM Standards volume 14.04
Replaced ASTM E675-94(1998). Visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org.
ASTM International.
ASTM E676-02 Standard Specification for Interchangeable
Taper-Ground Joints.
Annual Book of ASTM Standards volume 14.04
ASTM International.
ASTM E677-02 Standard Specification for Interchangeable Spherical-Ground Joints.
Annual Book of ASTM Standards volume 14.04
U.S. Dept. of Commerce, National Bureau of Standards.
Commercial Standard CS21-58. Interchangeable Taper-Ground Joints, Stopcocks,
Stoppers, and Spherical-Ground Joints.
Washington: U.S.G.P.O., 1958.
Withdrawn in August 1980.
It was replaced by ASTM E675-79, ASTM E676-79 and ASTM E677-79. CS21-58 is the fifth edition. The first edition appeared in
1930, with new editions in 1934, 1936, 1939 and 1958, in each case adding additional sizes.
CS21 is the original source of the dimensions in the ASTM standards.
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Last revised: 8 June 2007.