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TABLE OF CONTENTS Chapter 6 - Field Calibration
Calibration can be defined as the comparison of a measuring instrument, with
specified tolerances but an undetermined accuracy, to a measurement standard of
known accuracy for the purpose of determining and/or eliminating by adjustment
any out-of-tolerance condition.
| The use of an uncalibrated instrument creates the potential for incorrect measurement and resultant erroneous conclusions and decisions. It is calibration that provides confidence in a measurement and assurance that an instrument has the accuracy required to maintain a product or process within specification. | Precise measurement depends on the use of properly calibrated instruments. |
Calibrations can be performed in any location. In general, this chapter will discuss
those calibrations performed by the user, rather than those performed by the
equipment manufacturer or an independent laboratory. The proper overall
approach towards calibration applies, however, no matter where the procedure is
physically performed.
General Calibration Requirements
Scope
A specific calibration can range from a simple dimensional check to extensive
measurements of multiple variables. Accordingly, before the details of a calibration
can be planned, an initial step is to determine the appropriate scope. The first
decisions that need to be made are (1) which variables should be measured and (2)
what accuracy must be maintained. These decisions need to be based not only on
the accuracy desired from an application standpoint, but also upon the capabilities
of the device being calibrated.
The device to be calibrated must exhibit sufficient range, resolution, and freedom
from drift to enable transfer of the desired level of accuracy from the calibration
standard used. It is normally a futile effort to try to make a highly accurate
instrument out of a low-precision device through calibration.
Types of Calibration Errors
Some element of error exists in all measurements, no matter how carefully they
are conducted. The magnitude of the error can never be exactly determined, but
the experimental uncertainty - the possible value of the error - can be estimated.
The possible error in a measurement or calibration is referred to as the
measurement uncertainty. In practice, however, it is often referred to simply as the
error.
