Changing the units of returned values

[MarnoGrewar]

Background

During the use of H, S and G functions thermochemistry module, it was found that if one can change the units of the return values, for example from kWh to Joules (J), would be helpful.

Purpose

This change to auxi's returned values would enable users to choose the units the return value of a function must be displayed in.

Approach

A possible approach is to provide a list generally used units for a specific functions. Another approach is to provide a unit converter function or package to aid in converting return values from functions to required units. Any other suggestions to approach this issue will be greatly appreciated.

[johanzietsman-em]

Thanks for suggesting this Marno. It is an important aspect to address, and it would have a very wide impact within auxi.

I changed the purpose statement to be more clear.

I don't understand your approach statement. Please clarify.

[MarnoGrewar]

Johan thank for your feedback on my issue, and sorry for taking so long to respond. I edited my previous approach to be more clearly stated.

[johanzietsman-em]

Thanks Marno. My suggestion would be that we add another package/module to address the dimension and unit infrastructure required in auxi. Here are a few suggestions that have come to mind so far:

• Add a new units.py module into the auxi.tools package. I don't see that units will require a package of its own. A module should be sufficient.
• Introduce the concept of dimensions, and create the seven fundamental dimensions used for the base units in the SI system. These include length, mass, time, electric current, thermodynamic temperature, amount of substance and luminous intensity. (This should address most derived dimensions, but we may have to add something to address angles and money later on.)
• Create derived dimensions based on the fundamental dimensions, for example, mass flow rate, energy, power, velocity, etc.
• Introduce the concept of units for dimensions, and create the seven base units used by the SI system for its seven fundamental dimensions.
• Create units for the derived dimensions by using the base units.

I don't think we should build units directly into all auxi calculations, since this will impair computational performance. We should do this as simply as possible through multiplication factors for unit conversions, and allowing users to ask auxi which units it is using for a specific calculation. I am not certain how we should do this yet, but the following concepts may become part of the implementation:

• units set: A module like auxi.tools.chemistry.thermochemistry may have its own units set to specify in which units it should perform its calculations. Users can select different units into this unit set, or change to a different units set altogether, to modify the units used.
• dimension and units repository: This repository should reside in the units module. It should list all the available dimensions, and all the available units for each dimension. Users can then select from the repository into units sets.

Some more thinking is required, but this is a start. I am moving this issue to Ready status so that we can start tinkering with the ideas. I will create branches for you and me to work on.

[johanzietsman-em]

After poking around on the internet, I found a wealth of resources about this issue.

Standards:

• International System of Units (SI)
• Measurement units: the SI
• SI base unit
• The Unified Code for Units of Measure

Here are some Python projects that already address the units issue:

• units
• numericalunits
• Pint
• quantities
• Unum
• PhysicalQuantities
• SimTK
• Buckingham
• Brian
• natu
• Sympy
• Magnitude
• pyformex
• python-units
• DimPy
• astropy

Other languages:

• C++
  • Boost.Units
• C#
  • Data structure for accessing units of measure
• Fortran
  • Evolving Fortran types with inferred units-of-measure
  • Units-of-Measure Correctness in Fortran Programs
  • Units of Measure in Fortran (ftp)
  • UDUNITS
• F#
  • Units of Measure in F#

Articles and posts:

• Dimensions, Units and Conversions
• Quantities and Units in Python
• Unit Converter in Python
• Python units/quantities packages
• Physical quantities (numerical value with units) in python
• What are some example implementations of Units of Measure in programming languages?
• Types for Units-of-Measure: Theory and Practice
• Relational Parametricity and Units of Measure
• International System of Units

A good start would be to see the principles behind what these guys have done. We should also create a bit of a requirement specification for our implementation. Since auxi is a scientific and engineering tool, units of measure are fundamental to what we do. It should integrate well with the entire framework, especially the modelling and simulation parts.