public static function getConversionGroupUnits($group = null)
    {
        $conversionGroups = [];
        foreach (self::$conversionUnits as $conversionUnit => $conversionGroup) {
            if (($group === null) || ($conversionGroup['Group'] == $group)) {
                $conversionGroups[$conversionGroup['Group']][] = $conversionUnit;
            }
        }

        return $conversionGroups;
    }