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About High Pressure Cylinders DID YOU KNOW by Ed Elms, PSI Instructor Written March 2005 |
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I
am surprised at how few dive shops that perform scuba repair
have torque wrenches. There are certain fasteners in scuba
equipment that absolutely require the use of a torque wrench for
installation. One of the most frequent uses of a torque wrench
is to install valve burst disc safety bodies (plugs). These
fasteners are hollow, and often made of chrome-plated brass,
which inherently do not posses high shear strength.
Additionally, the torque values are relatively light, in the
range of 50-125 in/lbs. for most burst disc safety bodies
(depending on the manufacturer). Over-tightening of these
fasteners can result in the failure of the fastener, which would
be considered a catastrophic failure of life-support equipment.
(Editor’s note: DOT requires burst discs to be set at hydro
test pressure with a tolerance of +0% to -10%. Torquing to the
manufacturer’s recommendations is your only practical way to
ensure this setting).
When purchasing a torque
wrench, there are several considerations to bear in mind. One
single torque wrench will not adequately cover all torque
specifications. Choose a torque wrench with the appropriate
scale for the specification. Torque wrenches are only accurate
from 20%-100% of their scale, in other words, the first 20% of
the scale is totally inaccurate. If the torque value is 50-55
in/lbs., do not chose a torque wrench with a 0-1000 in/lb scale,
since 0-200 in/lbs of the scale is unreliable. There are two
basic types of torque wrenches to choose from; dial and
click-stop (I am not considering beam-style wrenches, as they
are generally too inaccurate to consider reliable for
life-support equipment). My personal preference is the
direct-reading dial type with a maximum torque indicator needle
(similar to the maximum depth indicator needle on depth gauges),
which allows the technician to visually confirm the torque
applied on a dial gauge. The click-stop wrench will release a
few degrees of travel with an audible click when the correct
torque is reached. Whichever type you choose, select one with a
minimum accuracy of 2-3% of scale. Many torque wrench
manufacturers offer calibration service for their wrenches with
a certificate of accuracy, so you can periodically test the
accuracy of your wrench. This insures accurate torque
application, and is a good idea from the standpoint of
liability.
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