The calibration standard against which all others are measured
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.