Never use acetic acid cure room temperature vulcanizing (RTV) silicones in your well logging tools, or on any electronics for that matter. Common RTV silicone made for automotive and general purpose use, the stuff that smells like vinegar, is corrosive to electronics, and its use is a death sentence to reliability. The use of acetic acid cure RTV silicone is forbidden in many government labs. (Acetic acid is the ingredient in vinegar that makes it smell like vinegar and taste sour; it is also used in photographic developer stop bath. The pure material is called glacial acetic acid and has a nasty habit of freezing if your air conditioner is turned down too low.)
Perhaps one of the best examples of the evils of acetic acid cure RTV silicone is what Oilex did to their customers. Oilex used acetic acid cure RTV silicone in the potted photomultiplier tube (PMT) voltage divider area of their compensated density logging tools. The PMTs had plated iron flying leads; the result is that all these tools were ticking time bombs that self destructed, destroying the PMT's when the leads were eaten from the base of the tube. AnaLog Services, Inc. has serviced many of these tools repairing or replacing the damaged PMTs.
RTV silicone made for non-corrosive electronics applications uses an alcohol curing system rather than an acetic acid curing system. The alcohol cure products have a slightly sweet or ethereal odor as opposed to the distinctive vinegar odor of the common acetic acid cure products. Both are one part curing systems that depend on air moisture to cure into silicone rubber. Examples of non-corrosive RTV silicone pastes are: clear or gray Dow Corning 3145; white General Electric RTV 162 or gray high strength RTV 167; and clear Loctite 5140. All these items are ridiculously expensive, no doubt the reason Oilex and others have been tempted to use the less expensive dime store variety acetic acid cure RTV silicone products. The General Cement (GC) Electronics 10-150 Silicone Rubber Adhesive Sealant is an acetic acid cure product and should not be used for electronics work, despite GC's representations to the contrary.
But help has arrived! Presumably in response to consumer complaints about odor, the major manufacturers have introduced non-acetic cure silicone products for the consumer market. General Electric Silicone II is the best we have found. It comes in handy 2.8 fluid ounce tubes in clear and white (the exact same tube they now pack their high priced material in), and is available from mass merchandisers like Wal-Mart at perhaps one-tenth the cost of the above mentioned products.
The belief that fully cured acetic acid based RTV silicone is harmless is incorrect; even fully cured, there may be enough acetic acid residue to cause a disaster. Stated otherwise, even waiting a week to seal up a tool containing acetic acid cure RTV silicone is a dangerous proposition. The non-acetic based RTVs can safely be sealed up in a tool at any time.
Deep sections should not be attempted with the one part RTV silicones. These potting and encapsulation applications are better served with two part cure systems. General Electric RTV-11 is a white two part pourable RTV silicone rubber that is readily available from electronics distributors. Dow Corning 3110 is roughly equivalent to GE RTV-11, and cheaper if you can find it. These products are also handy for casting custom shock absorbers / spacers for downhole tools. Clear two part curing silicones have been used as optical couplants in logging tool scintillation detectors, but we prefer non-curing silicones for this application. Dow Corning Sylgard 184 and the more expensive GE RTV-615 are candidates as curing optical couplants.
RTV silicone is referred to as "pookey" or "pucky" by many older logging electronics technicians. The origin is not clear, but it has been suggested it derives from an early television or old radio comedy routine. The use of the term must have been fairly common at one time since some automotive mechanics refer to gasket compound as pookey or pucky. When inquiries were made on the Wireline Mailing List on the subject, one correspondent mentioned that the ultimate glue was called "uckum pucky" at Basin Surveys many years ago.
Thanks to Robert Baer and Paul Knight, our two bestest gurus, for their contributions to this page.
FTC Disclosure: Neither AnaLog Services, Inc. nor the author has an economic interest in any of the companies or products discussed above, and no monetary compensation was received. Free samples were received from various manufacturers. None of the manufacturers / distributors was aware this page would be written.
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