Calibration explained: principles, processes and modern reporting

Accurate calibration ensures reliable measurements, supports preventive maintenance, and guarantees measurement traceability.

Calibration is essential in industrial automation, ensuring that measurement instruments provide accurate readings. Today, IIoT platforms can simplify documentation, provide central access to calibration data, and enable efficient calibration planning.

What is calibration?

Calibration can be simply described as the process of comparing the measured value from an instrument under calibration with a reference standard of known and high accuracy. In essence, it establishes whether the instrument provides measurements within acceptable limits.

The International Bureau of Weights and Measures (BIPM) defines calibration as an “operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties (of the calibrated instrument or secondary standard) and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication”.

Performing calibration requires specific tools and instruments, which vary depending on the type of calibration. Common examples include calibrators with valid calibration certificates, standard devices, and calibration rigs.

Why is calibration important?

Calibration is essential to ensure accurate measurement. Measuring devices are installed in diverse industrial environments where they are exposed to challenges such as abrasion, vibration, sudden temperature changes, harsh conditions and mechanical shocks. These factors can affect device performance, making calibration necessary to verify accuracy and, if required, adjust the instrument to meet application specifications.

Accurate calibration positively impacts production processes by ensuring reliable measurements. It also reduces variation within technical specifications, supports preventive maintenance and guarantees measurement traceability.

Additionally, modern smart instruments can provide continuous health status information, offering a clearer picture of device condition and measurement reliability.

What do you need to know about the calibration certificate?

During calibration, all measurements must be recorded, either manually or through an automated system. Upon completion, a final document — known as the calibration certificate — is generated, containing all technical details of the procedure.

Typically, the certificate includes comparisons between the calibrated device and the traceable reference standard. It must also provide technical specifications of both instruments, procedural data, calibration uncertainty, calibration number and the signatures of authorised personnel.

Which instruments require calibration?

All measuring devices can be calibrated to ensure proper functionality and the level of accuracy required for their application. While the concept of calibration remains consistent, the procedure varies depending on the type of field instrument.

For example, calibrating a pressure transmitter may involve using a calibrated deadweight tester as a reference to generate pressure. Alternatively, another pressure device with higher accuracy than the instrument under calibration can be used.

All calibration standards must include a valid calibration certificate confirming compliance with applicable standards in the relevant region.

How does calibration work?

Calibration involves comparing the device under test with a reference standard, typically at multiple points across the measuring range — commonly 0%, 25%, 50%, 75% and 100% of range. Additional test points can be included if required, although this may increase time and costs.

The reference standard used depends on the type of device:

• Flow transmitters: Calibration may involve a master device, comparison with a weight scale or mobile prover calibration.
• Pressure transmitters: A higher-accuracy standard device, digital calibrator, or deadweight tester is typically used.
• Temperature transmitters: A calibrated reference such as an electronic temperature sensor simulator is applied.
   

Calibration procedures are guided by a Standard Operating Procedure (SOP) that outlines each step. The interval between calibrations is not universally defined but can be determined based on factors such as:

• device type and application
• manufacturer recommendations
• trend analysis from previous calibrations
• instrument historical data
• comparison with similar devices in the plant
• required measurement accuracy.

The difference between calibration and adjustment

Calibration is generally understood as the process of comparing a device with a reference standard of higher and known accuracy. Adjustment, if required, follows calibration to correct deviations identified during the comparison.

During calibration, the procedure involves verifying the measuring range against the reference standard. If an error greater than the acceptable limit is detected, the instrument must be adjusted.

For example, adjusting a pressure transmitter typically involves trimming the zero and then the span value. These parameters can be modified through mechanical or software settings, depending on the device’s age and manufacturer specifications. After adjustment, the measuring range must be rechecked against the standard to confirm that accuracy meets the required limits.

What’s the benefit of the onsite calibration?

Onsite calibration is a common practice in industrial environments, particularly during planned production shutdowns when multiple instruments require calibration. In such cases, external service providers are often engaged to calibrate pressure, temperature and flow instruments.

Field calibration, including flow calibration, is increasingly prevalent. Many companies now employ mobile calibration rigs to perform these services directly onsite.

The benefits of onsite calibration include eliminating the need for instrument transportation, enabling immediate adjustments and repairs, and facilitating quick instrument replacement — all performed by qualified experts. This approach reduces downtime and ensures compliance with calibration standards.

How often should you calibrate?

The frequency of calibration depends on several factors, as there is no universal standard. Best practices suggest considering the following points when defining calibration intervals:

• The criticality of the measurement to the process.
• Quality system requirements at the plant.
• Regulatory compliance.
• Manufacturer recommendations.
• Impact of failure due to lack of accuracy.
• Other technical requirements.
   

These factors help establish an appropriate calibration schedule, which can be adjusted as needed. Modern IIoT solutions further simplify calibration planning and execution by providing easy access to device data and scheduling tools.

What is calibration uncertainty?

Calibration uncertainty refers to the degree of doubt associated with the calibration process and is influenced by factors such as installation conditions, reference traceability and environmental variables. If calibration uncertainty exceeds the tolerance of the instrument being calibrated, the validity of the calibration must be questioned.

For example, using a clamp-on flowmeter to calibrate an in-line device may result in calibration uncertainty higher than the installed meter’s tolerance, making the process ineffective.

What should you know about pass and fail calibration?

A device under test can either pass or fail calibration based on its tolerance limits, which are defined by the manufacturer or specified in the initial calibration certificate. During calibration, if the measured error exceeds the tolerance limit, the calibration is considered to have failed. In such cases, the device should be adjusted and recalibrated. If the difference between the calibrated device and the reference standard falls within the tolerance limit after adjustment, the device passes.

How to manage calibration reports with IIoT

Proper storage and accessibility of calibration documents are essential. Modern IIoT services enable centralised cloud-based management of calibration reports, technical data and related documentation under each device tag. This approach ensures that all team members can access, share and update information efficiently, saving time during field verification or when retrieving historical calibration records.

When integrated with edge devices, IIoT platforms can automatically create digital twins of all instruments, making files accessible from smartphones, tablets and laptops. This simplifies collaboration and ensures that technical documentation and calibration reports are always available, improving efficiency and compliance.

Image credit: iStock.com/Smederevac