def convert_scale(quantity)
      if Quantity===quantity
        assert_same_dimension(quantity.unit)
        v = quantity.value * quantity.unit.conversion_factor
        v = v / self.conversion_factor
      else
        quantity / to_numeric
      end
    end
def convert_scale(quantity)
      if Quantity===quantity
        assert_same_dimension(quantity.unit)
        v = quantity.value * quantity.unit.conversion_factor
        v = v / self.conversion_factor
      else
        quantity / to_numeric
      end
    end
Convert a quantity to this unit only in scale.
 @param [Phys::Quantity] quantity to be converted.
 @return [Phys::Quantity]
 @raise [UnitError] if unit conversion is failed.