code: public static function getConversionFactor(Uom $from, Uom $to)
      {
          // Check to see if we need to do a conversion
          if ($from->isSameValueAs($to)) {
              return new Fraction(1);
          }
  
          if (!isset(static::$conversions)) {
              static::$conversions = json_decode(
                  utf8_encode(
                      file_get_contents(__DIR__.'/conversions.json')
                  ),
                  true
              );
          }
  
          // First lets see if we have a conversion for the from to to
          if (isset(static::$conversions[$from->getName()][$to->getName()])) {
              $numeratorDenominatorPair = static::$conversions[$from->getName()][$to->getName()];
          // I guess we didn't find one, try the inverse
          } elseif (isset(static::$conversions[$to->getName()][$from->getName()])) {
              // We found the inverse, set the conversion values appropriately
              $numeratorDenominatorPair = array_reverse(static::$conversions[$to->getName()][$from->getName()]);
          } else {
              // no conversion found. throw an exception
              throw new ConversionNotSetException($from->getName(), $to->getName());
          }
  
          // Is the conversion set up correctly
          if (count($numeratorDenominatorPair) == 2) {
              return new Fraction(
                  $numeratorDenominatorPair[0],
                  $numeratorDenominatorPair[1]
              );
          } else {
              // Guess it wasn't
              throw new BadConversionException();
          }
      }
docs: Get the conversion factor between two Units of Weight
  
  e.g. from LB to OZ = 16
  
  @author Tom Haskins-Vaughan <tom@tomhv.uk>
  @since  0.3.0
  
  @param Uom $from
  @param Uom $to
  
  @throws ConversionNotSetException Thown when a conversion is tried
  that has not been set in the conversions.json file
  @throws BadConversionException Thrown when a conversion is not set
  properly, e.g. [1] should be [1, 16]
  @return Fraction