What is calibration?
The process of adjusting test and product equipment to confirm the standards for the instrument and verify their compliance with the standards is the process of calibration.
What is the range of an instrument?
Every instrument is designed to operate between a lower and upper limit. These limits express the range of an instrument.
The range of an instrument is determined by the physical limitation of the instrument itself, particularly its transducer.
What is the span?
The absolute difference between the highest and lowest range-limit is the span of an instrument. For example, a pressure sensor with a range of 100psi to 300psi has a span of 200psi.
What is accuracy?
The overall degree to which the measurement system can determine the value of the variable measured is referred to as the system’s accuracy. Accuracy is usually expressed as a percent uncertainty with reference to some part of the measurement.
What is precision?
The precision refers to the ability of the measuring instrument to agree with itself repeatedly. The more precise the instrument the closer repeated measurement will be o a common value.
What is an error?
The error is the difference between the actual value of some variable and what the measurement indicates the value to be. The value of an absolute error is determined by a measurement made with a calibrated instrument.
What are the different types of error?
Zero error: When an instrument is calibrated so that its reads 0%. When the measured value is at the lower limit of the range. If this adjustment is not made properly, all values from 0% to 100% be shown incorrectly.
Span error: An instrument must also be calibrated so that it goes from 0 to 100% as the measured variable goes from its lowest operating value to its highest.
Hysteresis: This type of error comes from the way an instrument operates. Many instruments indicate a different value depending on whether the measured variable is increasing or decreasing.
For example, suppose a variable increases from a value of 48% to a value of 50%. The indication of this instrument at 50% may be different from the indication it would give if he measured value had decreased from 52% to 50%. Hysteresis can be caused by a sloppy or dragging bearing surface.
Dead band: That of range through which an input change will cause no output change is called the dead band. The major cause is insensitivity of transducer or primary element.
Linear and Non-linearity: In a linear instrument, equal increment of change in the measured variable produce equal increments of change in the indicated value. But many instruments are non linear. Over part of their range, the increments of indicated value become smaller or larger than in other parts of the range.
What is resolution?
The resolution of a measurement system refers to the minimum detectable change in the measured variable. Resolution contributes to overall accuracy of the system.
What is NIST?
NIST stands for National Institute of Standards and Technology, provides a common standard for the instrument to be calibrated thus accomplishing a general standard unit.
Why standards are needed?
Standards are necessery in order to engage in commerce of any type. The component parts of different instruments are made at different locations and those parts must fit and work together even though they are manufactured at different physical location. Without common agreement as to the units of measurement and a common reference to check for instrument accuracy, such a procedure would not be possible.