About High Pressure Cylinders DID YOU KNOW

Written March 2005

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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