Alternate Cylinder Fill Technologies

The traditional method of filling high pressure medical gas cylinders by pressure/temperature charts is being challenged by newer, faster and more accurate technologies. This article examines the compliance issues around the established and the newer ways of filling high pressure cylinders. Please keep in mind that there are some potential problems if the new processes are not done well.

Pressure/Temperature Filling
This procedure uses a pressure/temperature chart, gauge and thermometer to determine when the cylinders are full. This is still the most popular process.

Compliance issues:
• The gauges and thermometers must be calibrated according to your SOPs.
• The fill charts assume you know the true temperature of the gas inside the cylinder. Temperature readings only measure the surface of the cylinder wall-the inside gas could be as much as 30 degrees warmer, creating inaccuracies in the settled pressure. Experienced operators know this and compensate for the target pressure.
• Settled pressures should be periodically verified to assure the compensations are accurate. Adjustments should be made as needed.
• Slightly under-filled or slightly over-filled cylinders are more likely with this technology.

Gravimetric Filling
This procedure uses a sensitive scale to determine with high accuracy when the cylinders are full. The labor can be lower since the temperature comparison steps are eliminated. Final pressure/temperature readings should be taken to assure DOT regulations are being observed.

Even with the gravimetric filling method, there are some compliance issues to observe:
• The scale must be calibrated according to your SOPs.
• Be careful that you know the exact internal volume of the cylinder you are weighing.
• The fill leads and ambient air currents can contribute to weighing errors.
• Even with the gravimetric fill method, you should periodically verify that the cylinder is actually properly filled.
• If the cylinder will be repeatedly filled, be sure the cylinder is rated for continuous filling/venting.

Automatic Filling
Automatic cylinder filling processes usually use a gravimetric scale or pressure and temperature transducers to determine when to stop filling. The above compliance issues apply to automatic systems. Advantages are the consistency of the fill process and potentially lower labor costs.

In addition, there are some special compliance issues with automatic fill systems:
• Be sure the system keeps records of the actual results of the batch: pressures, temperatures, weights, etc.
• The computer system and records of the fill results are subject to the requirements of 21 CFR Part 11, Electronic Records. In some cases, you can create a hybrid electronic record and significantly lower your compliance exposure.
Software and entire automated system must be validated.
• Recent validation guidance requires the automated system to be verified periodically in order to maintain its validation status. Be sure your automatic fill system validation meets the latest interpretations of the FDA.

Many decisions about your cylinder fill plant and operations will have FDA compliance implications. If you have questions about this or other FDA issues, please contact me.

Gases and Welding Distributors Association
Meet the Author

GAWDA’s FDA & Medical Gases Consultant Thomas L. Badstubner is president of AsteRisk LLC in Blackstone, Massachusetts. Members can reach him at 508-883-0927 and tom.badstubner@asterisk-inc.com.