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Morehouse Proving Rings 


The calibration standard against which all others are measured

The Morehouse Proving Ring is recognized wherever highly accurate force measurement calibrations are preformed as the premier standard. This reputation has been earned by years of established and maintained by the National Institute of Standards and Technology. Morehouse is dedicated to the design, manufacture and application of proving rings, which can be used with confidence in standards laboratories and throughout industry.  The reliability, repeatability and accuracy of Morehouse Proving Rings are unequalled and unchallenged by any other force measurement device. Only direct loads of qualified deadweights are more precise. 

The Ring 

    It has been proven that a steel ring made of correct steel alloy and properly manufactured will perform as a nearly perfect elastic member. When forces are applied along the diameter of the ring, the diameter will change, or deflection, the amount of the applied load can be determined. To properly utilize this characteristic, Morehouse machines proving rings from special alloy steel forgings. The external bosses, as well as the internal to which the deflection measuring apparatus is attached, are machined as an integral part of the ring. This costly but essential method of making the ring is the only known way to assure long term reliability. 


The Deflection-Measuring Apparatus

    A precision micrometer measures the deflection, or change of diameter of the ring under load, and a vibrating reed mounted diametrically within the ring. In Practice, the reed is vibrated and the dial of the micrometer is turned until it advances into the path of the vibrating reed, dampening out the vibrations. At this point, a reading is taken. This procedure is repeated throughout the calibration. The sensitivity of the vibrating reed and the micrometer is equivalent to 0.00005 of an inch. The micrometer mechanism is carefully machined and assembled to deliver extreme and lasting accuracy. It even has an automatic longitudinal adjustment to compensate for a minute thread wear. The number of divisions the micrometer dial and the graduation of the dial will vary by type of proving ring. For example, a Series 1000 Proving Ring will have about 2000 major divisions at full load. When combined with a vernier index for readings to one-tenth of division and estimating to one-twentieth of a division the
resolution is one part in 40,000. Of all the deflection measuring devices developed, none has ever matched the micrometer / reed type for lasting precision and rugged dependability.

The Ultimate Instrument

    After assembly, the proving rings are calibrated against known standards to ASTM specification E-74, either at the National Institute of Standards and Technology or in our laboratory using standards directly traceable to the institute. The characteristics of each proving ring depend on the requirements of the intended application. For this reason, Morehouse produces a series of proving rings offering a range of performance specifications varying primarily in degrees of accuracy. This permits you to select the proving rings your calibrations require without paying for unnecessary reserve accuracy. 

The Immutable Importance of ASTM Standard E-74

    While Morehouse proving Rings are still manufactured according to the design  specifications originally established by the National Institute of Standards and Technology, they are now calibrated according to a more exacting standard established by the American Society for Testing Materials. The standard, designated ASTM E-74, clearly defines the methods to be used in the calibration of all force-measuring instruments. More importantly it specifically states the requirements force standards must meet and specifies the procedure for determining the accuracy of each instrument calibrated according to the ASTM standard E-74. Accuracy in proving rings and all force measuring instruments calibrated according to this method is specified in terms of uncertainty. The definition of “uncertainty” in ASTM E-74, is “a statistical estimate of the limits of error in forces computed from the calibration equation of a force measurement instrument.”  “Calibration,” is the comparison of an unknown instrument or gauge with a known standard to determine its probable uncertainties or deviation from reading that can be expected in its use.

    ASTM E-74 also establishes two different loading ranges. Class AA and Class A, for force measuring instruments that are in turn used to calibrate materials testing machines with the lower limit of use being 2000 times the uncertainty.

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