Pint Units

Pint Units Skill

Overview

Pint is a Python package for handling physical quantities with units. It provides:

• Automatic unit conversions
• Dimensional analysis and consistency checking
• Unit-aware arithmetic operations
• Integration with NumPy arrays
• Support for custom unit definitions
• Temperature conversions (including offset units)
• Scientific notation and formatting

Pint eliminates unit conversion errors and ensures dimensional consistency in engineering calculations, making it essential for any technical work involving physical quantities.

Installation

pip install pint

For integration with NumPy and Pandas:

pip install pint[numpy]

Basic Usage

Creating Quantities

A quantity in Pint consists of a magnitude (number) and a unit.

from pint import UnitRegistry

# Create unit registry (do this once per module)
ureg = UnitRegistry()

# Define quantities with units
distance = 100 * ureg.meter
time = 5 * ureg.second
pressure = 50 * ureg.psi

# Alternative syntax using string parsing
flow_rate = ureg('250 gallons/minute')
temperature = ureg('75 degF')
viscosity = ureg('1.5 centipoise')

# Access magnitude and units
print(f"Distance magnitude: {distance.magnitude}")  # 100
print(f"Distance units: {distance.units}")  # meter

Unit Conversions

Convert between compatible units automatically:

# Length conversions
distance = 100 * ureg.meter
print(distance.to('feet'))  # 328.084 foot
print(distance.to('kilometer'))  # 0.1 kilometer

# Pressure conversions
pressure = 100 * ureg.psi
print(pressure.to('bar'))  # 6.895 bar
print(pressure.to('pascal'))  # 689475.7 pascal

# Flow rate conversions
flow = 500 * ureg.gallons / ureg.minute
print(flow.to('liter/second'))  # 31.546 liter/second
print(flow.to('m**3/hour'))  # 113.562 meter**3/hour

# Temperature conversions (handles offset units)
temp = 75 * ureg.degF
print(temp.to('degC'))  # 23.889 degree_Celsius
print(temp.to('kelvin'))  # 297.039 kelvin

Dimensional Analysis

Pint ensures dimensional consistency in calculations:

# Velocity = Distance / Time
distance = 100 * ureg.meter
time = 5 * ureg.second
velocity = distance / time
print(velocity)  # 20.0 meter/second

# Force = Mass × Acceleration
mass = 10 * ureg.kg
acceleration = 9.81 * ureg.meter / ureg.second**2
force = mass * acceleration
print(force)  # 98.1 kilogram·meter/second²
print(force.to('newton'))  # 98.1 newton

# Dimensional consistency check
try:
    invalid = (100 * ureg.meter) + (50 * ureg.second)  # ❌ ERROR
except Exception as e:
    print(f"Error: Cannot add length and time")

# Correct operation
total_distance = (100 * ureg.meter) + (50 * ureg.feet)  # ✓ OK
print(total_distance)  # 115.24 meter

Unit-Aware Calculations

Fluid Flow Calculations

# Calculate volumetric flow rate from velocity and area
velocity = 3.5 * ureg.meter / ureg.second
diameter = 150 * ureg.millimeter
area = 3.14159 * (diameter/2)**2

flow_rate = velocity * area
print(flow_rate.to('liter/second'))  # 61.86 liter/second
print(flow_rate.to('gpm'))  # 1638.1 gallon/minute

# Reynolds number calculation
density = 998 * ureg.kg / ureg.meter**3
viscosity = 1.0 * ureg.centipoise  # Common unit in industry
Re = (density * velocity * diameter) / viscosity
print(Re.to_base_units())  # Dimensionless: 523170.0

Pump Power Calculations

# Hydraulic power: P = ρ × g × Q × H
flow_rate = 100 * ureg.meter**3 / ureg.hour
head = 50 * ureg.meter
density = 1000 * ureg.kg / ureg.meter**3
gravity = 9.81 * ureg.meter / ureg.second**2

hydraulic_power = density * gravity * flow_rate * head
print(hydraulic_power.to('kilowatt'))  # 13.625 kilowatt
print(hydraulic_power.to('horsepower'))  # 18.27 horsepower

# With pump efficiency
efficiency = 0.75 * ureg.dimensionless
shaft_power = hydraulic_power / efficiency
print(shaft_power.to('kilowatt'))  # 18.17 kilowatt

Pressure Drop Calculations

# Darcy-Weisbach equation: ΔP = f × (L/D) × (ρV²/2)
friction_factor = 0.018 * ureg.dimensionless
length = 100 * ureg.meter
diameter = 0.15 * ureg.meter
velocity = 2.5 * ureg.meter / ureg.second
density = 1000 * ureg.kg / ureg.meter**3

pressure_drop = friction_factor * (length/diameter) * (density * velocity**2 / 2)
print(pressure_drop.to('pascal'))  # 37500.0 pascal
print(pressure_drop.to('psi'))  # 5.44 pound_force_per_square_inch
print(pressure_drop.to('bar'))  # 0.375 bar

# Convert to head loss
head_loss = pressure_drop / (density * gravity)
print(head_loss.to('meter'))  # 3.822 meter
print(head_loss.to('feet'))  # 12.54 foot

Working with Arrays

Pint integrates seamlessly with NumPy for array operations:

import numpy as np
from pint import UnitRegistry

ureg = UnitRegistry()

# Create array with units
flow_rates = np.array([50, 75, 100, 125, 150]) * ureg.gpm
heads = np.array([80, 75, 65, 50, 30]) * ureg.meter

# Array operations preserve units
powers = (flow_rates * heads * ureg.kg/ureg.meter**3 * 9.81*ureg.meter/ureg.second**2)
print(powers.to('kilowatt'))
# [2.48 3.72 4.95 6.19 7.43] kilowatt

# Statistical operations
mean_flow = np.mean(flow_rates)
std_flow = np.std(flow_rates)
print(f"Mean flow: {mean_flow.to('liter/minute'):.1f}")
print(f"Std dev: {std_flow.to('liter/minute'):.1f}")

# Unit-aware interpolation
target_head = 70 * ureg.meter
target_flow = np.interp(target_head.magnitude, heads[::-1].magnitude,
                        flow_rates[::-1].magnitude) * ureg.gpm
print(f"Flow at {target_head}: {target_flow.to('m**3/hour'):.1f}")

Custom Unit Definitions

Define domain-specific units:

# Add custom units to registry
ureg.define('barrel_oil = 42 * gallon = bbl')
ureg.define('standard_cubic_foot = foot**3 = scf')
ureg.define('darcy = centipoise * centimeter**2 / (second * atmosphere) = D')

# Use custom units
oil_volume = 1000 * ureg.barrel_oil
print(oil_volume.to('gallon'))  # 42000.0 gallon
print(oil_volume.to('liter'))  # 158987.3 liter

gas_flow = 5000 * ureg.standard_cubic_foot / ureg.day
print(gas_flow.to('m**3/hour'))  # 5.90 meter**3/hour

permeability = 100 * ureg.darcy
print(permeability.to_base_units())  # Base SI units

Context Managers for Unit Systems

Switch between unit systems easily:

from pint import UnitRegistry

ureg = UnitRegistry()

length = 100 * ureg.meter
mass = 50 * ureg.kg

# Default SI units
print(f"Length: {length}")  # 100 meter
print(f"Mass: {mass}")  # 50 kilogram

# Use US customary units
with ureg.context('US'):
    print(f"Length: {length.to('feet')}")  # 328.084 foot
    print(f"Mass: {mass.to('pound')}")  # 110.231 pound

# Use imperial units
with ureg.context('imperial'):
    print(f"Length: {length.to('yard')}")  # 109.361 yard

Temperature Conversions

Handle absolute and relative temperature correctly:

# Absolute temperatures
temp1 = 25 * ureg.degC
temp2 = temp1.to('degF')
print(temp2)  # 77.0 degree_Fahrenheit

temp3 = 300 * ureg.kelvin
print(temp3.to('degC'))  # 26.85 degree_Celsius

# Temperature differences (delta)
temp_rise = ureg.Quantity(20, ureg.delta_degC)
print(temp_rise.to(ureg.delta_degF))  # 36.0 delta_degree_Fahrenheit

# Heat transfer calculation
mass = 10 * ureg.kg
specific_heat = 4.18 * ureg.kJ / (ureg.kg * ureg.kelvin)
temp_change = 50 * ureg.delta_degC

heat = mass * specific_heat * temp_change
print(heat.to('kJ'))  # 2090.0 kilojoule
print(heat.to('BTU'))  # 1981.9 BTU

Formatting and Display

Control how quantities are displayed:

pressure = 150000 * ureg.pascal

# Default format
print(pressure)  # 150000 pascal

# Compact notation
print(f"{pressure:~}")  # 150000 Pa

# Specific precision
print(f"{pressure:.2f}")  # 150000.00 pascal

# Scientific notation
print(f"{pressure:~.2e}")  # 1.50e+05 Pa

# Pretty format
print(f"{pressure:~P}")  # 150000 Pa

# Custom format
print(f"{pressure.to('bar'):.3f~P}")  # 1.500 bar

Common Engineering Unit Conversions

Pressure

pressure = 100 * ureg.psi
# Common conversions
print(pressure.to('bar'))  # 6.895 bar
print(pressure.to('kPa'))  # 689.476 kilopascal
print(pressure.to('MPa'))  # 0.689 megapascal
print(pressure.to('atm'))  # 6.805 atmosphere
print(pressure.to('mmHg'))  # 5171.5 millimeter_Hg
print(pressure.to('inch_H2O'))  # 2767.7 inch_H2O

Flow Rate

flow = 100 * ureg.gpm  # gallons per minute
# Common conversions
print(flow.to('liter/minute'))  # 378.541 liter/minute
print(flow.to('m**3/hour'))  # 22.712 meter**3/hour
print(flow.to('ft**3/second'))  # 0.223 foot**3/second
print(flow.to('barrel_oil/day'))  # 3428.57 barrel_oil/day (if defined)

Viscosity

# Dynamic viscosity
mu = 1.0 * ureg.centipoise
print(mu.to('pascal*second'))  # 0.001 pascal·second
print(mu.to('lbf*second/ft**2'))  # 2.089e-05 pound_force·second/foot²

# Kinematic viscosity
nu = 1.0 * ureg.centistokes
print(nu.to('m**2/second'))  # 1e-06 meter²/second
print(nu.to('ft**2/second'))  # 1.076e-05 foot²/second

Energy and Power

energy = 100 * ureg.kWh
print(energy.to('MJ'))  # 360.0 megajoule
print(energy.to('BTU'))  # 341214.2 BTU
print(energy.to('therm'))  # 3.412 therm

power = 50 * ureg.horsepower
print(power.to('kilowatt'))  # 37.285 kilowatt
print(power.to('BTU/hour'))  # 127259.0 BTU/hour

Mass Flow Rate

mass_flow = 1000 * ureg.kg / ureg.hour
print(mass_flow.to('lb/minute'))  # 36.74 pound/minute
print(mass_flow.to('ton/day'))  # 0.024 metric_ton/day
print(mass_flow.to('g/second'))  # 277.78 gram/second

Best Practices

1. Create UnitRegistry Once: Define ureg = UnitRegistry() at module level, not inside functions
2. Use Base Units for Storage: Store values in consistent base units (SI) in databases
3. Validate Input Units: Always check that input quantities have expected dimensionality
4. Handle Dimensionless Quantities: Use ureg.dimensionless for unitless ratios
5. Convert at Boundaries: Convert to display units only when presenting results
6. Check Compatibility: Use quantity.check(dimension) to verify dimensional consistency
7. Be Explicit: Use explicit unit definitions rather than assuming defaults
8. Temperature Care: Use delta_ prefix for temperature differences vs absolute temperatures

Dimensional Consistency Checking

def calculate_reynolds_number(velocity, diameter, density, viscosity):
    """
    Calculate Reynolds number with automatic dimensional checking.

    Re = ρVD/μ (dimensionless)
    """
    # Pint automatically checks dimensions
    Re = (density * velocity * diameter) / viscosity

    # Verify result is dimensionless
    assert Re.dimensionality == ureg.dimensionless.dimensionality

    # Return magnitude (pure number)
    return Re.to_base_units().magnitude

# Correct usage
rho = 1000 * ureg.kg / ureg.meter**3
V = 2.5 * ureg.meter / ureg.second
D = 0.15 * ureg.meter
mu = 1e-3 * ureg.pascal * ureg.second

Re = calculate_reynolds_number(V, D, rho, mu)
print(f"Reynolds number: {Re:.0f}")  # 375000

# Incorrect usage will raise error
try:
    Re_wrong = calculate_reynolds_number(V, D, rho, rho)  # Wrong dimension
except Exception as e:
    print("Error: Dimensional inconsistency detected")

Integration with Engineering Workflows

Example: Pump Performance Curve

import numpy as np
from pint import UnitRegistry

ureg = UnitRegistry()

def pump_curve(flow_rates, coefficients):
    """
    Calculate pump head from flow rate using curve fit.
    H = H0 - A*Q - B*Q²

    Parameters
    ----------
    flow_rates : Quantity array
        Flow rates with units
    coefficients : dict
        'H0', 'A', 'B' with appropriate units

    Returns
    -------
    heads : Quantity array
        Pump heads with units
    """
    H0 = coefficients['H0']
    A = coefficients['A']
    B = coefficients['B']

    heads = H0 - A * flow_rates - B * flow_rates**2
    return heads

# Define pump curve coefficients
coeffs = {
    'H0': 80 * ureg.meter,
    'A': 200 * ureg.meter / (ureg.meter**3/ureg.second),
    'B': 3000 * ureg.meter / (ureg.meter**3/ureg.second)**2
}

# Calculate performance at different flow rates
Q = np.linspace(0, 0.1, 11) * ureg.meter**3 / ureg.second
H = pump_curve(Q, coeffs)

# Display in different units
print("Flow (GPM)  Head (ft)  Head (m)")
print("-" * 40)
for q, h in zip(Q, H):
    print(f"{q.to('gpm'):8.0f~P}  {h.to('feet'):8.1f~P}  {h.to('meter'):7.1f~P}")

Troubleshooting

Issue: DimensionalityError

Cause: Attempting to add/compare quantities with incompatible units Solution: Check that all terms have the same dimensionality, or convert explicitly

Issue: UndefinedUnitError

Cause: Using a unit that's not defined in the registry Solution: Define custom units using ureg.define() or check spelling

Issue: Offset units (temperature) errors

Cause: Mixing absolute and relative temperature units Solution: Use delta_degC for temperature differences, plain degC for absolute

Issue: Lost units in calculations

Cause: Using .magnitude too early in calculations Solution: Keep quantities as Pint objects until final output

Quick Reference Card

Common Units

Quick Conversions

# Create quantity
Q = 100 * ureg.gpm

# Convert
Q.to('liter/minute')  # To specific unit
Q.to_base_units()     # To SI base units
Q.to_compact()        # To compact SI (kilo, mega, etc)

# Check dimensionality
Q.dimensionality      # [length]³/[time]
Q.check('[volume]/[time]')  # Verify dimension

# Access components
Q.magnitude          # Numeric value
Q.units              # Unit object

References

• Official documentation: https://pint.readthedocs.io/
• Source code: https://github.com/hgrecco/pint
• Unit definitions: https://github.com/hgrecco/pint/blob/master/pint/default_en.txt
• NIST Guide to SI: https://www.nist.gov/pml/special-publication-811

Further Reading

• NIST Special Publication 811: Guide for the Use of the International System of Units (SI)
• ISO 80000: Quantities and units
• API Standards for petroleum industry units
• ASME standards for engineering calculations