What is tank gauging per API MPMS Chapter 2?

Tank gauging is the measurement of liquid level in storage tanks to determine the quantity of product. API MPMS Chapter 2 covers manual gauging methods including innage (measuring from the bottom) and outage/ullage (measuring from the top). The gauge reading is converted to volume using a strapping table (capacity table) specific to the tank dimensions.

What is a strapping table?

A strapping table (or tank capacity table) correlates liquid level heights to volumes for a specific tank. It is developed by physically measuring the tank circumference at multiple heights (strapping) or by calculating from as-built dimensions. API 2550 covers the procedure for calibrating upright cylindrical tanks.

What are CTL and CPL corrections?

CTL (Correction for Temperature of Liquid) adjusts the observed volume to a standard temperature (typically 60 deg F / 15 deg C) to account for thermal expansion. CPL (Correction for Pressure of Liquid) adjusts for compressibility effects at elevated pressures. These corrections are defined in API MPMS Chapter 11 (Table 6A/6B for CTL, Table 5A/5B for CPL).

What is the difference between innage and outage?

Innage is the measurement of liquid depth from the tank bottom (datum plate) to the liquid surface. Outage (or ullage) is the measurement of empty space from a reference point at the top of the tank to the liquid surface. Innage = Reference Height - Outage. Most custody transfer measurements use innage gauging.

Tank Gauging / Strapping Calculator | API MPMS Chapter 2 (API 2550)

Tank Configuration

Tank Type

Tank Diameter (ID)

Tank Height / Length

Shell Thickness

Roof Type

Gauge Reading

Gauge Method

Gauge Reading — Feet

Gauge Reading — Inches

Reference Height (for outage)

Temperature & Pressure

Liquid Temperature

°F

Shell Temperature (if different)

°F

If blank, defaults to liquid temperature.

Tank Pressure (gauge)

psig

API Gravity of Liquid

°API

Standard Temperature

Deductions

Floating Roof Displacement

bbl

Volume displaced by floating roof (from tank table). Enter 0 for fixed roof.

Free Water & Sediment (BS&W)

Understanding Tank Gauging

Strapping Table Method
A strapping table converts liquid level to volume based on tank geometry. For vertical cylindrical tanks, volume is proportional to height. For horizontal tanks, the relationship is non-linear due to the circular cross-section.

Key Corrections:
CTSh: Shell correction for temperature expansion
CTL: Liquid temperature to 60 °F
CPL: Pressure compressibility correction

Volume Units: 1 barrel (bbl) = 42 US gallons = 5.6146 ft³. Standard conditions: 60 °F and atmospheric pressure.

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Results

Net Standard Volume

barrels (bbl) at standard conditions

Liquid Level (innage) -

Outage / Ullage -

Gross Observed Volume -

Shell Correction (CTSh) -

Gross Standard Volume -

CTL Factor -

CPL Factor -

Combined CTL × CPL -

Roof Displacement -

BS&W Deduction -

Tank Fill % -

Formula

NSV = (GOV × CTSh × CTL × CPL) − Roof − BSW

NSV = Net Standard Volume (bbl)

GOV = Gross Observed Volume (bbl)

CTSh = Shell temperature correction

CTL = Liquid temperature correction to 60 °F

CPL = Pressure compressibility correction

Standards & References

API MPMS Ch. 2
Tank Gauging — Manual and automatic tank gauging
API 2550
Measurement and Calibration of Upright Cylindrical Tanks
API 2551
Measurement and Calibration of Horizontal Tanks
API MPMS Ch. 11
Physical Properties Data — CTL and CPL tables
API MPMS Ch. 12
Calculation of Petroleum Quantities
API MPMS Ch. 17
Marine Measurement — shore tank gauging

Engineering Notes

Innage vs Outage: Innage measures from tank bottom to liquid surface. Outage measures from reference point to liquid surface.
Shell correction: Steel shell expands/contracts with temperature. CTSh adjusts the strapping table volume.
Standard conditions: 60 °F and 0 psig (atmospheric) for US petroleum measurement.
Floating roof: Deduct roof displacement volume from gross volume for roof weight submerged in product.
BS&W: Free water and sediment at tank bottom is deducted from net volume.
Horizontal tanks: Volume vs. level is non-linear due to circular cross-section geometry.

Tank Gauging / Strapping Calculator

Tank Configuration

Gauge Reading

Temperature & Pressure

Deductions

Understanding Tank Gauging

📚 Learn the Theory

Results

Formula

Standards & References

Engineering Notes

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