Pit depth measurement has been a requirement of corrosion inspectors for many years. Many different tools for the measurement of corrosion damage have been developed. The most successful of the pit depth tools was a lever type pit gauge used by Mr. William R. Thorpe, a former Chevron inspector in the Tulsa area. Mr. Thorpe had company craftsmen fashion a variety of configurations during his tenure there. As a retirement present, his coworkers presented him with a new copy of his latest configuration. (adapted from Pit Gauges – History and Development by: Mark Palynchuk, General Manager Western Instruments Inc.)
Unfortunately Mr. Thorpe passed on soon after his retirement, but his pit gauge lived on, through an accounting firm. Any further development ceased, but this incredibly simple product became an industry standard for 40 years. As a result, of corrosion inspectors complaining about the limitations of the Thorpe, the development of Western Instruments bridging pit gauge™ resulted.
As the operational envelopes of pressure equipment (pipelines, boilers, vessels, piping, and storage tanks), oil country tubular goods (tubing/casing, drill pipe, bottom hole assemblies), bridges/structures, concrete, and aircraft components were pushed, so were their corrosion allowances. Unfortunately tools were slow to evolve, simply due to cost, but as the cost of equipment (and failures) increased, so did the perceived need of equipment to measure corrosion increased. Inspectors were constantly required to perform more accurate pit depth measurements even if their tools were inadequate. Other tools were “adapted” for pit measurement, such as machinist’s depth gauges. While the dial indicators (and micrometer barrels) used in depth gauges were an improvement in accuracy, they were cumbersome to use for field inspection. Furthermore, dial indicators were expensive and any modification to them was cost prohibitive. With the advent of automatic machining, and computer aided designs, manufacturers of dial indicators were more readily able to perform short runs, with very specific modifications. This lead to the development of dial Indicator pit gauges.
Today we see the use of highly specialized computer controlled coordinate measuring machines mapping entire areas of weight-loss corrosion. A modern ultimate pit gauge costing over $50,000. While such specialized tools fulfill a niche in industry, they require a skilled technician to operate, a specialist to interpret, and skilled maintenance and repair personnel. When an inspector is not familiar with specialized measurement tools, such as dial or digital indicators, a dial indicator pit gauge can be an intimidating piece of equipment. It was not recognized until recently that the greatest demand for pit gauges was not for these specialized tools but still for the the simple lever type pit gauge, a tool that had not changed in 40 years.
Simple lever pit gauges have not evolved to meet modern industry requirements. For example, no gauge could be found with both metric and imperial scales. A development program was undertaken to test and identify the benefits and shortcomings, of lever type pit gauges, correct them, and manufacture them economically. The result of this program is a simple to use pit gauge, with a metric and two imperial scales, a unique pointer that aids the inspector to eliminate alignment errors. Since then the cost of the ‘task specific’ dial indicators has been coming down. Furthermore, with an aging infrastructure the demand for corrosion measurement and monitoring has increased substantially. Over the last 20 years we have seen this industry evolve, with companies specializing in Corrosion Measurement and Monitoring. Specialized manufactures have evolved, such as ourselves, manufacturing industry specific tools. As Business 101 has taught us, when there is a demand, a supply will soon follow. Today we see digital indicators, and specialized fixtures, with their increased cost, being offered and used. This same gauge has applications for weld Inspection, with such capability as measuring; undercut depth, weld crown height, a porosity comparitor, with metric and imperial units.