Overview
Partial pressure blending
Constant flow blending
Membranes
IT College seminars

Membrane filling and constant flow blending have in common that the Nitrox is mixed before being brought on working pressure in a high pressure compressor system. Because oil free membrane compressors are not common in diving (very high investment), normally oil lubricated compressors are used. This requires that the compressor does not come in contact with oxygen percentages above 40% - so, the maximum Nitrox blends that can be created are EANx40.

Membranes are popular and suitable for Enriched Air filling stations with a high volume of fills and are a solution in areas where breathing grade oxygen is not available or expensive. The more the system is used, the lower the price of each individual fill. Unfortunately the opposite also holds true – the lower the number of fills, the higher the price for each individual fill. Installing the system is expensive, but once installed, the operation cost is limited to maintenance.

A rule of thumb is a weekly volume of 100 Enriched Air fills to come up with a cost per fill that is lower than with constant flow blending. Of course a more exact calculation is only possible when comparing the “true” cost in the region of operation.

The membrane requires air at a pressure between 3 and 20 bar (depending on the make). The membrane then “filters” part of the Nitrogen from the air, resulting in a blend with higher Oxygen.

There are two ways to “feed” the membrane with air at this pressure. The compressor can first be used to fill a bank and then, in a second filling phase, the air from the bank can be fed to the membrane via a regulator on the required pressure. Because the compressor is used twice and because of the draining of Nitrogen (and also some oxygen), the compressor needs to run about 3 times as long for a single fill, as when you were filling air.

The second method involves a low pressure compressor added to the system. The compressor feeds the membrane and the Nitrox out of the membrane then goes to the high pressure compressor. Again this means that two compressors are running to fill one cylinder and that the first (low pressure) compressor has to compress more air than actually gets in the cylinder. No matter which option you look at – the use of energy and the need for maintenance increase when comparing with filling air cylinders.

For the second method, the low pressure compressor also requires that the air is treated before entering the membrane. It requires extra filtering and cleaning of the air, because the membrane is rather sensitive. The quantity of Nitrox exiting the membrane must be adapted to the compressor intake – otherwise the membrane would interfere with compressor functioning.

Required training:
• Mathematics to fill a cylinder without draining residual gas.
• Compressor know-how to prepare and maintain the compressor to deliver oxygen compatible air and to assure the compressor inlet does not experience resistance.
• Unit specific knowledge and training from the manufacturer.

Some of these aspects are covered in the special IT College seminar, but not the unit specific knowledge and training.