What is calibration?
Calibration in measurement is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Calibration is the task associated with establishing an accurate correspondence between any instrument’s input and it’s output signal.
Calibration procedure for each type of meters and instruments differs from each other.
Different types of Calibration:
The purpose of a measurement system is to accurately measure the state of the unit under test. The transducer converts one form of energy to another form such as an electrical signal.
The engineer sets up the signal conditioner to receive a known input type and range and convert the signal to a standard data collection unit. Using information from the manufacturer of the transducer and signal conditioner, the engineer can calculate an output.
Calibration is basicaly divided into three, based on the system relationship under calibration.
Transducer calibration which focuses on the transducer input-output output relationship
Data system calibration which simulates or models the input of the entire measurement system
Physical end-to-end calibration
The transducer usually comes with a transfer characteristic, but the instrumentation engineer needs to know the final calibration through all components of the signal conditioning system.
An instrumentation engineer should be familiar with the techniques used by the manufacturer to calibrate individual transducers. The engineer perform an in-house calibration on the individual transducer to verify the accuracy of the manufacturer’s transfer function.
If there are deviations from the manufacturer’s transfer function, the engineer may define a new transfer function for that unit, or in some cases, reset the device to conform to the original transfer function. Successive calibrations may indicate upcoming failures.
Many engineers stop after performing a transducer calibration. They then combine the transducer’s transfer function mathematically with the data system signal conditioner’s transfer functions. This provides a calibration estimate under the assumption that the engineer precisely knows all the transfer characteristics of the wiring and other signal conditioning between the transducer and the data storage system.
Data system calibration:
For making valid engineering measurements, the important consideration is how the transducer operates in the actual test environment with all signal conditioning attached.
After mounting the transducer on the test article, perform a minimum of a data system calibration. This can be accomplished by simulating an excitation of the transducer such as is often accomplished for strain gauges by using a shunt calibration resistor to simulate a change in resistance of the strain gauge.
In addition, inserting a simulated transducer signal into the system verifies all signal conditioning transfer function predictions. This calibration simulates transducer excitation by its physical input.
A data system calibration is the best that an instrumentation engineer can do to ensure acquisition of valid data. Data system calibration simulates the desired measurand rather than physically stimulating the transducer’s sensing device.
Physical end-to-end calibration:
An end-to-end mechanical calibration means a full calibration of the instrumentation from the actual physical input to the transducer to the output where the analog or digital signal will normally be analyzed.
An end-to-end calibration verifies the measurement system characteristics. Engineers perform these calibrations after installing the measurement system in the test article. A calibration source stimulates the transducer. The instrumentation engineer monitors the signal entering the data collection unit to ensure the calculated value matches the actual system transfer function.
It is highly recommended that an end-to-end calibration must be performed before the experiment and after the experiment is completed but before the instrumentation system is removed. The end-to-end calibration checks the measurement system (including wiring, connectors, routing, etc) installed on the test article, so that the engineer can identify and correct many potential problems early
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