Storage Tank Calculations with Custom Quantity Types

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Overview

This example demonstrates how to create domain-specific quantity types with constrained quantity equations, modeling a practical engineering problem: calculating fill levels, capacities, and flow rates for storage tanks.

Key Concepts

Custom Quantity Specifications

The library allows defining specialized quantity types that are more specific than the base ISQ quantities:

--8<-- "example/storage_tank.cpp:52:57"

Why constrain quantity equations?

horizontal_length is explicitly a kind of isq::length, but it represents specifically horizontal measurements
horizontal_area must be calculated as horizontal_length * isq::width, not just any two lengths
This prevents mixing incompatible physical interpretations (e.g., vertical × vertical areas)

Engineering Calculations with Quantities

The StorageTank class demonstrates practical engineering calculations with full dimensional analysis:

--8<-- "example/storage_tank.cpp:60:94"

Notice how:

filled_weight() properly multiplies volume by density and gravitational acceleration
fill_level() inverts the calculation, and dimensional analysis automatically cancels g from both numerator and denominator: (measured_mass × g) / (density × volume × g) → measured_mass / (density × volume)
spare_capacity() computes remaining volume from geometric constraints

Polymorphic Tank Shapes

The example shows how object-oriented design works naturally with quantities:

--8<-- "example/storage_tank.cpp:97:112"

Different tank shapes can be modeled through inheritance while maintaining type safety and dimensional correctness.

Example Application

Monitoring a rectangular tank being filled with water:

--8<-- "example/storage_tank.cpp:118"

Sample Output:

fill height at 200 s = 0.04 m (20 % full)
fill weight at 200 s = 100 kg g₀ (980.665 N)
spare capacity at 200 s = 0.08 m³
input flow rate = 0.1 kg/s
float rise rate = 2e-04 m/s
tank full E.T.A. at current flow rate = 800 s

Why This Matters

Domain Modeling: Custom quantity types encode domain knowledge (horizontal vs vertical measurements)
Compile-Time Safety: Invalid quantity equations are caught at compile time
Engineering Accuracy: Complex formulas are automatically verified for dimensional correctness
Practical Applications: Tank monitoring, fluid management, industrial process control

This pattern is valuable for any domain where specialized quantity types improve clarity and safety.

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