User Guides > The IGI property model > Units in the IGI property model

Units in the IGI property model

Overview

This page explains the how units of measure and ratios are treated in the IGI property model.


Usage:  Throughout the system


Units and ratios - interpreting values

When working with data it is essential the user is aware of the unit of measure of the value. Without this information the numbers are essentially meaningless. All IGI software shares a common unit system which represents two key concepts:

Units are combined in unit groups, and conversion is possible within a unit group. Examples of unit groups in the software include Length (units in this include: metres, feet, centimetres, miles, micrometers etc), Temperature (Kelvin, Celsius, Fahrenheit), mass concentration (ppt (grams per kilogram), %wt (grams per 100 grams), ppm (grams per 1000 kilograms) , etc). Each unit group has a base unit which is generally the associated SI unit.  The user can select to show the values in any unit within the unit group associated with that property across all artefacts in the software. 

The concept of a ratio is only relevant to properties which are defined as a fraction "a/b", where the numerator "a" and the denominator "b"  might both be a combination of property measurements. It is common in geochemistry to show results in either un-normalised (a/b) or normalised (a/(a+b)) forms, so the ratio format is provided to allow users to easily switch between these. We also provide b/a and b/(a+b) forms for easy inversion (different data sources will often provide molecular ratios in all of these forms, so rather than have 4 separate properties we have a single equation derived form and then the user can select the ratio).

Note ratios are used alongside units of measure. So we can ask for e.g. the Pristane/Phytane ratio as a percentage of the normalised ratio (%, [a/(a+b)]) or as a non-dimensional natural number in un-normalised form (Euc, [a/b]). 

Molecular / Gas data and units

Molecular data can be measured in a number of indicators. Each indicator has a characteristic unit of measure:

Gas data is typically measured as a volume fraction. However if the complete mixture of the gas mixture is known, equations of state can be used to convert this to either mass or mole fractions. Each is measured in it's own unit, so conversion is not a simple unit conversion (it requires solving the equations of state!):


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