A proportioner having a body portion that defines a fluid passage for transporting a fire protection fluid and a foam passage for transporting a foam concentrate. The foam concentrate mixes with the fire protection fluid to form a fire protection solution. The proportioner also includes a restrictor assembly having a restrictor disk and an orifice plate. The orifice plate has an opening for receiving the restrictor disk. The restrictor disk and opening are configured to form an annulus between an outer surface of the restrictor disk and an inner surface of the opening when at least a portion of the restrictor disk is disposed within the opening and the restrictor disk is spaced from the orifice plate. The restrictor assembly is configured to maintain the annulus for a full travel range of the restrictor disk.