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API MPMS 7.3:2011 – Temperature Determination by Fixed Automatic Tank Temperature Systems
A comprehensive technical guide to the Manual of Petroleum Measurement Standards Chapter 7.3 for fixed automatic tank temperature measurement
Scope and Field of Application
API Manual of Petroleum Measurement Standards (MPMS) Chapter 7.3, Second Edition (2011), formally titled Temperature Determination – Fixed Automatic Tank Temperature Systems, provides the industry standard for the design, installation, commissioning, calibration, and verification of automatic temperature measurement systems used in fixed storage tanks. The standard applies to systems that provide a continuous or intermittent electronic temperature signal representing the average temperature of the stored liquid, primarily for custody transfer, inventory control, and loss prevention.
This standard covers both single-element and multi-element temperature sensing assemblies, including resistance temperature detectors (RTDs) and thermocouples, when deployed in fixed positions within the tank. It addresses the entire measurement chain from the sensing probe through the transmitter to the indication or recording device. The document is part of the broader API MPMS series and supersedes the first edition (1985), incorporating modern electronic technology and refined accuracy requirements.
Key Application: API MPMS 7.3:2011 is mandatory for lease automatic custody transfer (LACT) units and other fiscal metering applications where automated tank temperature averaging replaces manual readings. It is also widely referenced in international contracts and national regulatory frameworks.
Core Technical Requirements
System Components and Accuracy Classes
The standard defines three accuracy classes for fixed automatic tank temperature systems (FATTS) based on the overall system uncertainty under field conditions:
| Class | Maximum Permissible Error | Typical Application |
|---|---|---|
| Class A | ±0.15 °C | Custody transfer, high-value hydrocarbons |
| Class B | ±0.25 °C | Inventory control and general process monitoring |
| Class C | ±0.50 °C | Low-accuracy applications, crude oil with high water cut |
Each class imposes specific requirements on sensor accuracy, transmitter drift, and calibration traceability. For example, Class A systems must use calibrated platinum RTDs (e.g., Pt100, Class A or better) with a documented accuracy of ±0.10 °C before transmitter uncertainty is added. Transmitters must be temperature-compensated and have a stability better than 0.05 °C per year.
Installation and Configurations
API MPMS 7.3:2011 identifies six standard installation configurations:
- Type 1 – Single spot sensor at a fixed immersion depth.
- Type 2 – Multi-spot sensor consisting of several discrete elements in the same thermowell.
- Type 3 – Averaging sensor with a continuous resistive element spanning the liquid height.
- Type 4 – Thermocouple chain with multiple junctions at defined intervals.
- Type 5 – Electronic averaging system combining multiple individual probes.
- Type 6 – Hybrid systems mixing two or more of the above methods.
The standard provides detailed guidance on thermowell design, material selection (316SS, Hastelloy), immersion length (minimum 300 mm for spot sensors), and the distance from tank walls and heating coils to minimize heat conduction and stratification errors. For floating roof tanks, the sensor must be attached to a stilling well or appropriate support.
Warning: Using a single spot sensor in a tank with significant vertical thermal gradients can introduce errors exceeding ±1 °C, even if the sensor itself is highly accurate. For non-homogeneous temperatures, use a multi-point or averaging system and verify via manual profile measurements per API MPMS Chapter 7.2.
Implementation in Tank Farm Operations
Adopting API MPMS 7.3:2011 requires a structured implementation plan. The following highlights help operators achieve compliance and optimize measurement reliability.
Commissioning and Initial Calibration
All FATTS components must be calibrated individually before installation. The standard recommends calibration over three temperatures spanning the expected operating range (e.g., 0 °C, 20 °C, and 40 °C for crude oil applications). System calibration is performed by comparing the entire chain (sensor + transmitter + display) against a certified reference thermometer in a stirred liquid bath. Acceptance criteria follow the accuracy class of the system.
Documentation for each system must include:
- Sensor type, certificate, and traceability
- Wiring diagram and shield grounding
- Calibration records with as-left and as-found data
- Software/firmware version and configuration settings
Tip: Implement a digital calibration management system that stores all FATTS records with expiration alerts. Many operators use a quarterly calibration cycle for Class A and semi-annual for Class B systems, but always check local regulatory requirements.
In-Service Verification
Periodic verification is mandatory. API MPMS 7.3:2011 recommends a full system verification at least once per year, with spot checks (single temperature point) every three months for critical custody transfer installations. The verification must demonstrate that the system still meets the specified accuracy class. If the error exceeds the maximum permissible error, recalibration is required and a root-cause analysis must be performed.
The standard also addresses online verification using built-in or external reference references. For instance, a second independent temperature element can be installed in the same thermowell for cross-checking without tank entry.
Safety Note: When performing calibration or maintenance on tank temperature systems, follow the facility’s confined space entry procedures and use intrinsically safe tools. Do not remove thermowells containing sensors while the tank is in service unless the product is non-flammable and the thermowell is designed for hot-tap removal.
Compliance and Verification Notes
Compliance with API MPMS 7.3:2011 is typically enforced through contractual agreements, national metrology laws, and custody transfer audit frameworks. The following points are essential for maintaining compliance:
- Traceability: All calibration must be traceable to a national metrology institute (e.g., NIST in the USA, PTB in Germany). In-house laboratory standards must be recalibrated at regular intervals (annually recommended).
- Documentation Retention: Calibration and verification records must be kept for at least the lifetime of the system plus five years, as required by many regulatory bodies.
- Audit Readiness: Prepare a compliance folder for each FATTS point including: equipment specifications, installation drawings, calibration certificates, verification reports, and maintenance logs.
- Deviation Handling: If a system fails verification, stop using it for custody transfer immediately, flag the data from the previous period (apply correction factor if possible), and recalibrate before returning to service.
The standard also references API MPMS Chapter 12 (Calculation of Petroleum Quantities) and Chapter 13 (Statistical Methods) for handling temperature corrections and uncertainty analysis. Users should be aware that API MPMS 7.3 is periodically reviewed; the 2011 edition remains current as of 2026, but operators should monitor API for any addenda or revisions.
Tip: When procuring a new FATTS, specify that all components (sensor, transmitter, wiring, and controller) must be individually tested and documented per API MPMS 7.3:2011 Class A or B requirements. Ask the vendor for sample calibration reports and evidence of field performance.
Frequently Asked Questions
Q: What is the difference between API MPMS Chapter 7.3 and Chapter 7.2?
A: Chapter 7.2 deals with manual (non-automatic) temperature determination using portable thermometers, while Chapter 7.3 covers permanently installed automatic systems that provide continuous or periodic temperature data without requiring personnel to access the tank.
A: Chapter 7.2 deals with manual (non-automatic) temperature determination using portable thermometers, while Chapter 7.3 covers permanently installed automatic systems that provide continuous or periodic temperature data without requiring personnel to access the tank.
Q: Can I use a thermocouple for a Class A FATTS?
A: While thermocouples are permissible for some configurations (Type 4), achieving Class A accuracy (±0.15 °C) is very difficult with standard thermocouples due to their lower inherent accuracy and drift. For Class A, platinum RTDs with matched transmitters are strongly recommended.
A: While thermocouples are permissible for some configurations (Type 4), achieving Class A accuracy (±0.15 °C) is very difficult with standard thermocouples due to their lower inherent accuracy and drift. For Class A, platinum RTDs with matched transmitters are strongly recommended.
Q: How often should the entire system be recalibrated?
A: API MPMS 7.3:2011 recommends a full recalibration (or verification against a reference) at minimum intervals of one year for Class A, two years for Class B, and three years for Class C. However, site-specific conditions or regulatory requirements may dictate more frequent intervals.
A: API MPMS 7.3:2011 recommends a full recalibration (or verification against a reference) at minimum intervals of one year for Class A, two years for Class B, and three years for Class C. However, site-specific conditions or regulatory requirements may dictate more frequent intervals.
Q: What happens if my tank temperature system fails a periodic verification?
A: The system should be taken out of custody transfer service immediately. A root-cause investigation must be conducted, the system recalibrated, and any affected batches recalculated using alternative measurements (e.g., manual readings from Chapter 7.2). Records of the failure and corrective actions must be retained.
A: The system should be taken out of custody transfer service immediately. A root-cause investigation must be conducted, the system recalibrated, and any affected batches recalculated using alternative measurements (e.g., manual readings from Chapter 7.2). Records of the failure and corrective actions must be retained.
© 2026 API MPMS Implementation Guide. This article is intended for informational purposes. Always refer to the official API MPMS 7.3:2011 standard for complete requirements.