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Don't Get Caught Off Guard: A Guide to API 2.2A Annex A and Optimal AST Calibration Frequency

Updated: May 22

In the world of aboveground storage tanks (ASTs), accurate volume measurements and strapping charts are the cornerstone of efficient operations. The American Petroleum Institute (API) standard API 2.2A plays a crucial role by outlining procedures for tank strapping. In 2019, the U.S. Customs and Border Protection (CBP) implemented a grandfathering clause for certain above-ground storage tanks (ASTs) used for inventory control purposes. This clause essentially allowed existing ASTs to continue using their established calibration methods for the next 15 years, However, all new construction AST’s do not apply to this grandfathering provision and will need to have a Calibration at the time of going into service, and another external calibration completed within 5 years. These tanks DO NOT need to be taken out of service for the second external calibration to be performed. Petroleum companies choose Gauge Point Calibration for in-service tank calibration because it utilizes cutting-edge technology like 3D laser scanning for unmatched accuracy and minimal downtime. Here's where API 2.2A Annex A comes in, offering valuable guidance on determining the optimal calibration frequency and recalculation of calibration tables for your ASTs. 

Understanding the Importance of Calibration Frequency:

Several factors can affect the accuracy of an AST's volume measurements over time, highlighting the importance of regular calibration:

  • Settling: ASTs can settle slightly, impacting their shape and volume.

  • Deformation/Damage: Minor dents or bends and bulges in the tank walls can alter precise dimensions.

Many Factors at Play

It's crucial to remember that API 2.2A Annex A provides Frequency of Calibration guidelines but the optimal time frame depends on several factors outlined in the Annex:.

  • Outdated Calibration Methods: Relying on outdated calibration methods like Manual Strapping for ASTs can lead to inaccurate inventory data, which can have consequences like product loss, financial discrepancies, and potential issues with CBP during inspections. Transitioning to the latest calibration methods technology, like 3D laser scanning, ensures regulatory compliance and unlocks the benefits of superior accuracy in inventory management.

  • Custody Transfer: Accurate tank calibration ensures both buyers and sellers receive the correct amount of petroleum. This minimizes financial discrepancies and protects both parties from accidental overpayment or underpayment during transactions.

Beyond API 2.2A: Establishing a Customized Calibration Schedule Gauge Point Calibration offers a user-friendly Frequency of Calibration App that seamlessly integrates with any project. We provide two options:

  1. GPC Calibrated AST’s: This service is provided at no additional cost for every GPC 3d Calibrated tank.

  2. Non-GPC Calibrated Database Management: For a small chargeWe handle the creation and ongoing maintenance of your calibration frequency data within the app.

  3. Do-It-Yourself: Our clients are provided with free access to the app, allowing you to independently manage and maintain your own calibration frequency database.

While API 2.2A provides a framework, here are some additional considerations for customizing your AST calibration schedule:

  • Current and Historical Data: You need to have Current and past calibration data in order to Analyze and identify trends or inconsistencies and volume shift percentages - informing the need for adjustments to the calibration frequency.

  • Stock Loss: By minimizing underestimation of tank volume, undetected product loss, and enabling better inventory management decisions and improved profitability

The Consequences of Skipping Calibration:

The consequences of neglecting proper calibration frequency can be significant:

  • Grandfathered Tanks:  Without the proper archiving of historical capacity tables, your current capacity table shall require updating in order to be within compliance.

  • Inaccurate Inventory Data: Outdated tank strapping charts can lead to inaccurate volume calculations, impacting inventory management decisions.

  • Product Loss: Underestimating tank volume due to inaccurate calibration can result in undetected product loss.

  • Financial Losses: Inaccurate inventory data can lead to financial losses due to over or under-ordering of products.

  • Regulatory Non-Compliance: Failing to meet regulatory requirements regarding tank calibration frequency can result in fines or penalties.


Petroleum companies reap significant benefits from maintaining a regular AST calibration schedule. It ensures accurate inventory data, preventing financial losses from product loss or inaccurate sales. This, in turn, optimizes inventory management and minimizes the risk of product loss, ultimately improving financial performance. Additionally, adhering to best practices outlined in API 2.2A Annex A and establishing a customized schedule with experienced professionals further guarantees accurate volume measurements and minimizes the risk of non-compliance with regulations and potential fines. Consulting with GPC experts can also help determine the optimal calibration schedule for your specific needs. To Learn more about how Gauge Point Calibration can assist you with a Frequency of Calibration Plan, please contact us today! 

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