Lecture 3: Calibration in Instrumentation

Study Notes

Measurement Errors

Static error: The difference between the measured value and the actual value of the quantity, caused by restrictions of measuring instruments or physical laws.
Dynamic error: The difference between the true value of a quantity varying with respect to time, and the reading indicated by measuring instrument without assuming any static error.

Static Characteristics

Range: The boundary of the values that identify the minimum and maximum limits of an element.
Span: The difference between the highest and lowest numbers in the range.
Operating range: A part of the total range, where an instrument or controller may be calibrated to only use part of the maximum range.

Bias, Accuracy, and Precision

Bias: A systematic error or offset introduced into a measurement system, often showing up as an error in measurement where the measurements are all on one side of the true value.
Accuracy: The degree to which an observed value matches the actual value of a measurement over a specified range, often stated as a percentage of the full-scale range or as a percentage of the reading.
Precision: The closeness to which elements provide agreement among measured values, only measuring agreement among the measured values, but not comparing to the true value.

Calibration and Instrument Loop

Calibration: Instruments are periodically calibrated to ensure accurate measurement of process conditions.
Diagnosis and troubleshooting: Problematic instruments must be investigated to determine the origins of poor or erratic operation.
Repair and Installation: Instruments may require specialty repair away from the process facility, involving removal, dismantling, servicing, reinstalling, calibrating, and returning to service.

Elements of the Instrument Loop

Process Operation: Physical or chemical change of matter, conversion of energy, state, composition, dimension, or other properties.
Measuring Element: Sensors, Transmitters, Transducers, Process Switches, Indicators, Recorders, Controllers, Alarm Units, Totalizers, and Computer-based instruments & systems.
Final Control Element: Control Valve, damper, variable-pitch blades, motor drives, feeders, relay/contactor, and thyristors.

Performance Characteristics

Static characteristics: Describe the operation of an element at steady-state conditions when the process is not changing.
Dynamic characteristics: Describe the operation of an element at unsteady-state conditions when the process is changing.

Description

Explore the importance of calibration in instrumentation, along with diagnosis, troubleshooting, repair, and installation of instruments. Learn how periodic calibration ensures accurate measurement of process conditions and how to restore reliable measurement through troubleshooting and repair.