Units in the IGI property model
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:
- Unit - a well defined system used to quantify / provide meaning to the result of a particular measurement, e.g. %wt, degrees centigrade, micrograms per gram. Every numeric value stored in the system must have a defined unit of measure.
- Ratio - for many equations, e.g. Pristane/Phytane, we need to specify whether the ratio is given in un-normalised (a/b) or normalised (a/(a+b)) form.
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:
- heights [h], areas [a] has essentially arbitrary units, which we call (count) units. Care must be taken here as these units cannot be directly compared across samples, as the values will depend on the details of the actual derivation of the heights and areas, and the amount of substance injected into the instrument.
- concentrations from height [ch] or from area [ca] are measured as a mass ratio (based on the injection of a known mass of a standard). The results are typically reported in parts per million by mass (ppm (µg/g)), but are sometimes given in (ppt (mg/g)) or (ppb (ng/g)).
- values from an unknown source [un] cannot be assigned a meaningful unit, and are given the (unknown) unit.
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!):
- volume fractions [vol] are typically reported in either percent volume (%vol) or (ppm (cm³/m³)), although other volume fractions of (ppt (cm³/l)) or (ppb(cm³/1000 m³)) and even (scf/bbl) are available.
- mass / weight fractions [mass] are typically reported in either percent weight (%wt) or parts per million by mass (ppm (µg/g)), but are sometimes given in (ppt (mg/g)) or (ppb (ng/g)).
- mole fractions [mol] are reported directly as (%mol) or (ppm (mol)) - again other units are supported.
- where the indicator of the type of measurement provided for the gas fraction is unknown [un], the unit is again defined as the (unknown) unit.