A Word About O-Rings

O-ring, n. an elastomeric seal of homogenous composition molded in one piece in the shape of a torus with circular cross-section (think doughnut) used as a dynamic or static seal, usually installed in a machined groove.

O-Ring History

In the early days of well logging, there were no O-rings; tool housings were sealed with copper gaskets.  It is not surprising that downhole electronics often took a bath.

Resilient round sealing rings were used at least as early as the mid 1800's with mixed success.  Thomas A. Edison's 1882 light bulb patent shows a round rubber ring at the neck of the glass bulb as a seal.  The O-ring we know and love was developed by Niels A. Christensen.  He began looking at hydraulic system seals in 1933, and finally filed his first O-ring patent in 1937, at age 72.  Christensen eventually found a licensee (United Aircraft) in 1941, but the government "bought out" all key military patents and gave them away to manufacturers.  Christensen received a lump-sum payment of $75,000.00, and the O-ring belonged to the government.  Christensen was a litigious sort; his last court battle ended in 1971, 19 years after his death, with a $100,000.00 payment to his heirs.

In the early 1940's, the O-ring became the standard seal for US Air Force hydraulic systems.  Widespread acceptance of O-rings came in the 1950's, and today billions of O-rings seal everything conceivable.  Practically every well logging tool made in the last half-century has been sealed with O-rings.  The O-ring owes its success to its simplicity, low cost, ease of installation, and small space requirements without supporting structures.

O-Ring Sizing

O-ring size is designated by a three digit dash number in the form -xxx.  Two dimensions define the size of an O-ring: its inside diameter (ID), and its cross-sectional diameter (CS).  Aerospace Standard AS568A , Aerospace Size Standard for O-rings, published by SAE, groups most standard sizes under one of five cross-sectional diameters (or series), as follows:

                    -000 Series   0.070" 0.003"
                    -100 Series   0.103" 0.003"
                    -200 Series   0.139" 0.004"                    See allorings.com's O-Ring Sizing Chart.
                    -300 Series   0.210" 0.005"
                    -400 Series   0.275" 0.006"

An O-ring seals by distortion of its resilient elastic compound to fill the leakage path.  Successful use depends on proper groove dimensions and selection of the right elastomeric compound.  For O-rings to function properly there must be some initial "diametral squeeze" or there will be no sealing at very low pressures.  Stretch of O-rings is usually limited to less than 5% once in the groove.  Knowing these squeeze and stretch guidelines can help determine if the correct replacement O-ring has been selected.

See also EPM's O-Ring Shadow Graphs (must be printed out), EPM's O-Ring Groove Dimensions, and the Parker O-Ring Handbook (2001 Edition, 3.5 MB pdf file).

O-Ring Compounds

There are dozens of compounds from which O-rings are manufactured.  Early O-rings were made of natural rubber, but this limited usage to water and other fluids that do not deteriorate rubber.  DuPont introduced Neoprene or CR (polychloroprene) in the 1930's; Neoprene was the earliest of the volume synthetic rubbers.  Neoprene O-rings still find wide application, especially in the refrigeration industry.  The balance of this discussion covers the two elastomers most commonly used in logging tools.

Buna-N or Nitrile or NBR (acrylonitrile-butadiene copolymers) compounds are the most widely used industrial seal elastomer; with more nitrile rubber used for seals than all other elastomers combined.  Nitrile compounds are used for general purpose applications; they form the backbone of the O-ring industry.  Buna-S or SBR, a relative, was developed by both US and German scientists at the beginning of WW II as a substitute for natural rubber, while Buna-N was developed by the Germans during the war (an earlier synthetic rubber developed by the Germans during WW I was not satisfactory).  Within nitrile's temperature and fluid compatibility limits, no other rubber material performs as well.  Resistance to petroleum oils and saltwater is excellent, but ozone and weather resistance is generally poor.  Standard service Buna-N O-rings are rated for -40F to 250F (specialty formulations are rated from -65F to 300F, but not necessarily in one compound).  Most "off the shelf" Buna-N O-rings are 70 durometer, but the harder 90 durometer grade is readily available.  Shelf life is limited to one (1) to five (5) years for Buna-N O-rings (ten years for specially modified material), but can be much shorter if exposed to sunlight, ozone, etc.  Buna-N O-rings are inexpensive (the commonly used -211 size should be less than a dime in quantities of 100).  Buna-N is a good choice for logging tool O-rings if you do not run deep and/or hot.  Buna-N O-rings are usually black.

Viton® fluoroelastomer or FKM / FPM (vinylidene fluoride-hexafluoropropylene) was introduced by DuPont in 1957.  Fluorocarbon compounds exhibit excellent resistance to petroleum oils, low compression set, and high temperature resistance.  Standard service Viton® O-rings are rated for -20F to 400F, but may withstand 600F for short exposures.  Most "off the shelf" Viton® O-rings are 75 durometer, but the harder 90 durometer grade is readily available.  Viton® O-rings have a shelf life up to twenty (20) years.  Viton® O-rings are several times more expensive than those made from Buna-N.  Viton® is the best choice for logging tool O-rings if you run deep and/or hot.  Viton® O-rings are often brown, but may be black or green (the black version is usually a grey-black as opposed to the more black-black of Buna-N).  AnaLog Services, Inc. stocks and uses Viton® O-rings for the vast majority of the work we do (shelf life and crack resistance alone makes Viton® our favorite O-ring elastomer).

There is a remote risk of sustaining chemical burns from Viton® O-rings if they are burned or otherwise exposed to temperatures above 600F.  Viton® begins to decompose above 600F, yielding some nasty compounds including hydrogen fluoride.  In the presence of water vapor, this can produce hydrofluoric acid, an extremely corrosive mineral acid which causes severe burns to exposed skin.  Proper protective gear should be warn when dealing with burned Viton®.  There is no danger in dealing with cured Viton® used within the rated temperature range.

O-Ring Lubrication

Never use motor oil on logging tool O-rings.  Use Lubriplate 930-AA; it will work up to 500 F and is electrically non-conductive.  This is exactly the same grease that GO and MLS repackaged in squeeze bottles, and sold for many years as "Nogall" (at incredibly inflated prices).  If you cannot find Lubriplate 930-AA in a can, purchase a tube of it intended for use in a standard grease gun, then scoop it out into small containers for use on the bench or in the field.  We find the 1-3/4 ounce squeeze tubes convenient also (Lubriplate Part Number L0096-086).

Parker Super O-Lube is a premium O-ring lubricant.  It is a high viscosity silicone oil and not a silicone grease (the less expensive Parker O-Lube is a petroleum based grease).  Parker Super O-Lube is water clear and works well at any temperature encountered by a logging tool, even in geothermal wells.

O-Ring Purchasing

Locally, check rubber and gasket supply houses and bearing houses for price and availability.  But pricing of O-rings in such institutions seems to be more a matter of whim, than anything else.  Even in "onesies and twosies", a -211 Buna-N O-ring should be no more than 15 cents, and a -211 Viton® should be under 50 cents (much cheaper in quantity), if they ask more, they ain't your friends.  For online O-rings, try www.metrogasket.com, www.allorings.com, and www.mcmaster.com.  Do not even bother with any of the online O-ring purveyors lacking the nerve to post prices on their websites.

An O-Ring To Remember

The following list is horribly incomplete; as time permits we will add to it:

Tool Connections

GO/MLS 1-7/16" OD 1-3/16" 12 thread Type "A" Pin (wet thread) -211(2)
GO/MLS 1" OD       13/16" 16 thread Type "A" Pin (dry thread) -115(2)
SIE    1-7/16" OD  Jumbo Banana                  (dry thread) -120(2)


1-11/16"     Housing Subs             -217 & -219
             Top Sub Insulator        -211
             Metal Insulator Insert   -012
             CCL Subs                 -218
             Source Holder            -218

SIE Bond Tools

2"           Housing Subs             -218(2)
3"           Housing Subs             -231(2)
3"           Transmitter Can          -324 & -336
             Receiver Mandrel         -111(4)
             Oil Fill Screws          -006

Comprobe Tools

1-1/4"       Housing Subs             -212
1-5/8"       Housing Subs             -217
2-1/8"       Housing Subs             -223

FTC Disclosure: Neither AnaLog Services, Inc. nor the author has an economic interest in any of the companies discussed above.  There are no paid links on this page, though small gifts have been received from time to time from a few companies (advertising coffee mugs, advertising calendars, etc.).  No free samples or other compensation has been received in exchange for favorable mention herein.

Viton® is a registered trademark of DuPont Dow Elastomers.  Incredibly, they have nothing better to do than harass this website because we did not include this footnote.  The four page document they sent admonishes against calling them DuDow, but  we can think of a better name.

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Last 10-20-109