Mastering the Basic Principles of Instrument Calibration



Instrument calibration is an integral part of maintaining the reliability and accuracy of any measurement system. It makes assurance that a system’s instruments are generating results that are in line with accepted norms. Calibration in the domains of instrumentation and control engineering involves more than just preserving instrument accuracy; it also involves guaranteeing the security, effectiveness, and profitability of industrial processes. In order to help people studying instrumentation and control engineering, this article looks into the fundamentals of instrument calibration.


Basic principles of instrument calibration

Below are some of the basic principles of instrument calibration that you may want to know.


1. Definition of Calibration:

At its core, calibration is the process of adjusting an instrument so that its output accurately corresponds to a given input as per the standard. Essentially, it’s a comparison between measurements – one of known magnitude or correctness (from a standard) and one from the instrument being tested.


2. Necessity of Calibration:

Why is calibration necessary? Over time, every instrument tends to drift away from its standard measurement due to factors such as wear and tear, environmental changes, or electrical disturbances. Calibration ensures that these instruments remain within the desired accuracy range, guaranteeing reliable and safe operations.


3. Traceability:

A crucial concept in calibration is traceability. This means that the standard used for calibration can be directly related to national or international standards, which are universally accepted as being accurate. This chain of comparisons ensures that an instrument’s calibration is not only accurate but also universally recognized.


4. Frequency of Calibration:

Instruments don’t need to be calibrated constantly. Instead, each instrument has a recommended calibration interval. This period is determined based on the instrument’s stability, its importance in the process, the conditions in which it operates, and past calibration records. Regularly scheduled calibrations can prevent unexpected inaccuracies or failures.


5. Calibration Range:

An instrument shouldn’t just be calibrated for a single point. It should be calibrated over the entire range of its operation. For example, a temperature sensor that works from 0°C to 100°C should be calibrated across this range to ensure its accuracy at any given point.


6. As-found and As-left Data:

During the calibration process, it’s important to note the ‘as-found’ and ‘as-left’ readings. ‘As-found’ refers to the initial readings of the instrument before any adjustments, indicating its performance in its operational environment. ‘As-left’ readings are taken after adjustments are made, ensuring the instrument is now within its desired accuracy range.


7. Tolerance:

Tolerance refers to the allowable deviation from a standard value. It’s a predefined value and represents the maximum error that can be accepted. If an instrument’s reading falls outside this range, it will need adjustment or repair.


8. Adjustments vs. Calibration:

While both terms are often used interchangeably, they are distinct. Calibration is the act of measuring the instrument’s performance and comparing it to a standard. Adjustment, on the other hand, is the act of aligning the instrument’s output to match the standard.


9. Documentation and Certification:

Every calibration process must be documented meticulously. This documentation serves as proof of calibration and provides a record of the instrument’s performance over time. Often, for critical processes, calibration certificates provided by accredited bodies are mandatory.


10. The Role of Environment:

The environmental conditions in which an instrument operates (and is calibrated) can significantly influence its readings. Temperature, humidity, and pressure can all impact measurement accuracy. Hence, it’s essential to calibrate instruments under conditions that closely replicate their operational environment or account for the environmental influences.


11. In-house vs. External Calibration:

Companies can either perform calibrations in-house or send their instruments to external, specialized calibration labs. The decision depends on factors like the complexity of instruments, frequency of calibration, availability of standards, and the cost implications.


12. Use of Technology:

With advancements in technology, many modern calibration processes are now automated. Automated calibration systems can enhance accuracy, improve efficiency, and maintain a more detailed record of the calibration process.



Understanding the basic principles of instrument calibration is foundational for anyone delving into the fields of instrumentation and control engineering. Properly calibrated instruments not only ensure accurate measurements but also play a pivotal role in the safe and efficient operation of various industrial processes.


Remember, calibration is not just a routine task; it’s a guarantee of quality, safety, and reliability. As technology and industries evolve, the principles of calibration remain critical, ensuring that we can trust the instruments and systems that drive our modern world.

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