Units in the IGI property model

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Usage:  Throughout the system

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Units and ratios - interpreting values

When working with data, the user must be aware of the value's unit of measure. Without this information, the numbers are essentially meaningless. All IGI software share a common unit system, comprised of two key elements:

• Unit - a standard reference system used to quantify/provide meaning to a measurement, e.g. %wt, degrees centigrade, micrograms per gram.The unit Euc stands for Euclid, a natural non-dimensional unit.
• Ratio - used to define properties as the quotient of two other properties, e.g. Pr/Ph - Pristane/Phytane and to specify if the ratio is in its un-normalised (a/b) or normalised (a/(a+b)) form.

Units

Units are combined into unit groups, and conversions are possible within a unit group e.g.

• Length: (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)

Each unit group has a base unit, which is generally the associated SI unit. The user can select to view the values in a given artefact in any unit of choice from within the unit group associated with that property. 

Molecular / Gas units

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

• Height [h] and area [a] values have essentially arbitrary units, which we call (count) units. The values will depend on several factors, the derivation of the heights and areas, the amount of substance injected into the instrument, the column setup, etc. Consequently, care must be taken as these units cannot be directly compared across samples.
• 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). Data is typically reported in parts per million by mass (ppm (µg/g)), but is sometimes given in (ppt (mg/g)) or (ppb (ng/g)).
• Values with an unknown indicator [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 gas mixture is known, equations of state can be used to convert values 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 per cent volume (%vol) or (ppm (cm³/m³)), although other volume fractions (ppt (cm³/l)), (ppb(cm³/1000 m³)) and even (scf/bbl) are available.
• Mass/weight fractions [mass] are typically reported in either per cent 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.
• Values with an unknown indicator [un] cannot be assigned a meaningful unit, and are given the (unknown) unit.

Ratio

A ratio is only relevant to properties that are defined as a fraction "a/b", where the numerator "a" and the denominator "b" are numeric property measurements. As it is common in geochemistry to show ratio results in either un-normalised (a/b) or normalised (a/(a+b)) forms, the ratio format allows users to easily switch between these. We also provide b/a and b/(a+b) forms for inversion purposes. In contrast to data providers who provide separate properties for each ratio form, we use a single equation ratio property and allow the user to select the appropriate ratio.

Note: ratios are used alongside units of measure. For example

The pristane/phytane ratio as a percentage of the normalised ratio = (%, [a/(a+b)]) 

The Pristane/Phytane ratio as a non-dimensional natural number in un-normalised form = (Euc, [a/b])

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Video tutorials

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