In the case of certain types of filter sparging stones where the tubing is inserted into the stone, the ID of the stems fits the OD of the low pressure tubing snuggly.

The following schematic should clarify the above sentence. The ID of the sparging stone stem and the OD of the low pressure tubing are the same (red lines). The OD of the sparging stone stem (green) is larger than the ID of the low pressure tubing (blue).

In another type of sparging stone, the OD of the sparging stone stem and the ID of the low pressure tubing are the same (blue lines). The ID of the sparging stone stem (orange) is smaller than the OD of the low pressure tubing (red).

Click HERE for filter sparging stone ordering information and pictures.