Calibration
Content
CALIBRATION Calibration Is the key requirement in ensuring proper control accuracy Is the process of adjusting an instrument or compiling a deviation chart so that its reading can be corrected to the actual value being measured
The process of Calibrating an instrument
Is also primary objective for all work performed in the field of instrumentation
A correctly calibrated instrument ensures the safety and proper operation of the controlled process.
Bench Calibration
Simulates and process the instruments will measure and determines if corrective action is needed
It is essential to understand how to calibrate an instrument by using the appropriate input and output standards rather than observing the instrument’s output with respect to an actual process input
To properly perform an instrumentation calibration, it is necessary that you understand the concepts of range and span of an instrument’s input and output signals
A pressure transmitter can be calibrated in pounds per square inch(PSI) and adjusted to output a corresponding signal of 4-20 mA
When an instrument is properly calibrated, its input is a direct representation of its output %input = %output
Instrument Error
Are discovered by comparing the measured output to the expected output given a specific input
Calibration Errors
Zero Shift Span Error Zero Shift and Span Error Nonlinearity
Zero Shift
Is the term for an instrument whose output is consistently higher or lower than the
expected value This Shift is consistent throughout the output signal span Solution: turning the zero adjustment screw can correct the problem
Span Error
It is an error in an instrument’s output signal that do not reflect 100% of the output signal or do not follow the input span Solution: the span adjustment can be tuned to eliminate span error
To properly calibrate an instrument with span errors requires a five-point check (0%, 25%, 50%, 75%, 100% range) to ensure that the values of the output are accurate over the working range of the transmitter
Zero shift and Span error
Are the most common errors found in an instrument’s calibration Both zero and span errors exist simultaneously Solution: the instrument must first be zeroed
and then the span should be adjusted, the zero setting needs to be checked several times in order to calibrate again
Nonlinearity
An instrument’s output will be linear plot that may or not be parallel to its expected output Nonlinearity will produce an output that, when measured and plotted, will not have any consistent error shift between the upper and lower limits of the ranges
A large linearity problem will not be correctable and the instrument must be repaired. In some cases, nonlinearity error are small enough that their effect is scarcely noticeable
Accuracy Percentage Accuracy % = (deviation/span) x100
Accuracy
Can be determined if an instrument must be recalibrated to meet specification Is usually expressed as a percentage of the transmitter span
Is equal to the difference between true and measured, divided by the instrument’ span and multiplied by 100 Is used to specify the maximum overall error to be expected from a device
In order that calibration of an instrument may be corrected, two basic adjustment are usually essential. It is necessary to be able to adjust the instrument ZERO and to adjust the scale SPAN.
Span
Is the difference between the highest scale value and the zero value
Range
Specifies the start point and the end point for measurement
Temperatures between -20 degrees C and 100 degress C Instrument zero: - 20 degrees C Instrument span: 100 – (-20) = 120 degrees C Instrument range: -20 to 100 degrees C
Calibration
The process where measuring instruments are periodically compared with standards which are known to be constant
The accuracy (usually expressed as the inaccuracy) of an instrument may be expressed in a number of ways:
Measured Variable Percentage of the instrument full scale(FS) reading of full scale deflection (FSD) Percentage of instrument span Percentage of the actual reading
The process of Calibrating an instrument
Is also primary objective for all work performed in the field of instrumentation
A correctly calibrated instrument ensures the safety and proper operation of the controlled process.
Bench Calibration
Simulates and process the instruments will measure and determines if corrective action is needed
It is essential to understand how to calibrate an instrument by using the appropriate input and output standards rather than observing the instrument’s output with respect to an actual process input
To properly perform an instrumentation calibration, it is necessary that you understand the concepts of range and span of an instrument’s input and output signals
A pressure transmitter can be calibrated in pounds per square inch(PSI) and adjusted to output a corresponding signal of 4-20 mA
When an instrument is properly calibrated, its input is a direct representation of its output %input = %output
Instrument Error
Are discovered by comparing the measured output to the expected output given a specific input
Calibration Errors
Zero Shift Span Error Zero Shift and Span Error Nonlinearity
Zero Shift
Is the term for an instrument whose output is consistently higher or lower than the
expected value This Shift is consistent throughout the output signal span Solution: turning the zero adjustment screw can correct the problem
Span Error
It is an error in an instrument’s output signal that do not reflect 100% of the output signal or do not follow the input span Solution: the span adjustment can be tuned to eliminate span error
To properly calibrate an instrument with span errors requires a five-point check (0%, 25%, 50%, 75%, 100% range) to ensure that the values of the output are accurate over the working range of the transmitter
Zero shift and Span error
Are the most common errors found in an instrument’s calibration Both zero and span errors exist simultaneously Solution: the instrument must first be zeroed
and then the span should be adjusted, the zero setting needs to be checked several times in order to calibrate again
Nonlinearity
An instrument’s output will be linear plot that may or not be parallel to its expected output Nonlinearity will produce an output that, when measured and plotted, will not have any consistent error shift between the upper and lower limits of the ranges
A large linearity problem will not be correctable and the instrument must be repaired. In some cases, nonlinearity error are small enough that their effect is scarcely noticeable
Accuracy Percentage Accuracy % = (deviation/span) x100
Accuracy
Can be determined if an instrument must be recalibrated to meet specification Is usually expressed as a percentage of the transmitter span
Is equal to the difference between true and measured, divided by the instrument’ span and multiplied by 100 Is used to specify the maximum overall error to be expected from a device
In order that calibration of an instrument may be corrected, two basic adjustment are usually essential. It is necessary to be able to adjust the instrument ZERO and to adjust the scale SPAN.
Span
Is the difference between the highest scale value and the zero value
Range
Specifies the start point and the end point for measurement
Temperatures between -20 degrees C and 100 degress C Instrument zero: - 20 degrees C Instrument span: 100 – (-20) = 120 degrees C Instrument range: -20 to 100 degrees C
Calibration
The process where measuring instruments are periodically compared with standards which are known to be constant
The accuracy (usually expressed as the inaccuracy) of an instrument may be expressed in a number of ways:
Measured Variable Percentage of the instrument full scale(FS) reading of full scale deflection (FSD) Percentage of instrument span Percentage of the actual reading
