What is Calibration?

Whats is Calibration?

There are too many definitions of calibration and some are a lot more complicated than the process itself. One simplest definition is that, Calibration is: The process of comparing an instruments’s accuracy to known standards and adjusting it to meet those standards. Calibration of an instrument is checked at several points throughout its calibration range. The calibration range is the region between limits within which a quantity is measured. received or transmitted. The limits are defined by zero and span values specific to that instrument.

Why do we need to calibrate?

New instruments are calibrated to make sure that the instruments are providing accurate output signal. After a long time being installed in the field, instrument error can occur due to the following reasons: moisture, temperature changes, sensor drift, vibration, moisture, temperature changes, power fluctuations, power surges, process changes and degradation of electronic components.  The process in which the instruments are operating is dependent upon accurate, reliable inputs and outputs within the control system. Without accurate information the system performance will suffer.

Calibration Procedure

There are two general categories of calibration in the world of process controls:
Loop Calibration:  Loop Calibration is performed by disconnecting the transmitter and connecting a known standardized signal into the loop. All instruments in the loop are recorded and compared to the transmitted value for error and adjusted as necessary to within the specified tolerance.

Individual Instrument Calibration:  Individual instrument calibration is performed on a single instrument removed from the loop. The input and output are disconnected. A known standardized signal or source is applied to the input and the output is measured at various data points throughout the calibration range. If necessary, the unit is adjusted to meet the specified tolerance.

Accuracy and Tolerance

Accuracy:    The ration of error to the reading or the full scale output, expressed in the percent of reading or percent of full scale. For example; if the manufacturers stated accuracy is 1% of the full scale, then a 0-100psi pressure trasnmitter could have an error of 1psi at any points over its entire range. However, if the manufacturers stated accuracy is 1% of the reading. the same pressure transmitter could have an error of 1psi at 100% output. a 5psi error at 50% output and a 0.01psi error at 1% output. 

Tolerance:    The permissible deviation from a specific value, expressed in measurement units percent span or percent of reading.

Traceability

All calibrations should be performed traceable to a nationally or internationally recognized standard. For example, in the United States, the National Institute of Standards and Technology (NIST), formerly National Bureau of Standards (NBS), maintains the nationally recognized standards.

Traceability is accomplished by ensuring the test standards we use are routinely calibrated by “higher level” reference standards. Typically the standards we use from the shop are sent out periodically to a standards lab which has more accurate test equipment. The standards from the calibration lab are periodically checked for calibration by “higher level” standards, and so on until eventually the standards are tested against Primary Standards maintained by NIST or another internationally recognized standard.

The calibration technician’s role in maintaining traceability is to ensure the test standard is within its calibration interval and the unique identifier is recorded on the applicable calibration data sheet when the instrument calibration is performed. Additionally, when test standards are calibrated, the calibration documentation must be reviewed for accuracy and to ensure it was performed using NIST traceable equipment.

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Calibration and Maintenance

A program of routine calibration is important to ensure proper performance of the process control system. Each instrument should be documented including:  manufacturer, model number, serial number, specified accuracy, acceptable tolerance, date of installation, startup, calibration frequency, last calibration, calibration procedures, standards used, etc. A calibration report should be used when performing calibrations and should remain on the file for future reference.

Above all, the control system technician, performing the calibration must understand the process, pay attention to detail when following procedures and in preparing accurate concise and honest documentation.