When producing a specialty gas mixture, much of the time taken to manufacture it goes into cylinder preparation to ensure that the mixture is stable. This is one of the most important aspects in a quality gas mixture. If what comes out of the cylinder is not what is described on the label it will result in a poor calibration and inaccurate results. Much time, testing, and research is invested in the development of techniques related to the interaction with cylinders and valves, solely to ensure the stability of mixtures. In general, it includes:
- Evacuation/purging cycles.
Cylinders are then kept sealed and under vacuum to prevent contamination until the time of filling.
If such time and effort is spent preparing a cylinder to handle a specific gas mixture, it begs the question: Why aren’t the regulator and gas delivery system treated with the same importance?
Most users will be familiar with material compatibility. That is, choosing between brass, plated brass, stainless steel, or special alloy, to ensure there will be no undesirable effects to the regulator or delivery system. Material selection is important both for reasons of safety and for accuracy in calibration. Considering the requirements of cylinder preparation, it becomes clear that this is only part of the equation. Of course, we must choose the correct wetted materials but there are other important factors to consider.
Surfaces which have not been properly prepared will react with a gas mixture and change the composition. Imaging, by electron microscopy, of the surfaces of commonly used materials like brass or stainless steel illustrate how these surfaces are very uneven and have many crevices in which molecules can be lodged. These molecules can be exceedingly difficult to dislodge, which is a major factor contributing to the complexity of cylinder preparation.
Moisture is of particular concern as it is very prone to adhering to surfaces. It will react with nearly all commonly used reactive compounds including hydrogen sulphide, sulphur dioxide, and chlorine. Normal ambient air can contain up to 4% moisture content. If a regulator or delivery system has been exposed to ambient air, it is certain that there would be moisture contamination on all exposed surfaces.
Similarly, if your calibration mixture contains moisture it will adhere to any unprepared surfaces it meets. A similar result is known to occur with C6+ hydrocarbon compounds when they meet a surface while in the liquid phase.
Contamination of, reaction with, and surface adhesion of compounds in your gas mixture can be minimised by:
Reducing surface area. Reactions with wetted materials will always occur on a surface. Therefore, a reduction in surface area is also a reduction in potential adverse effects. This can be accomplished by:
- Reducing the internal volume of the regulator and cylinder connector
- Reducing the internal diameter of tubing from the regulator to instrument
- Reducing the length of tubing as much as is practical.
Preparing the wetted surfaces. Reactions on wetted surfaces can be reduced by:
- Effective purging. Any gas introduced into the regulator and supply system before the surfaces are prepared to handle it can be considered wasted gas. This gas must be purged out to ensure an unaffected sample with which to perform a calibration. Sometimes it is best practice to use a dry gas purge (nitrogen, typically) to reduce the amount of calibration gas wastage, especially when preparing a particularly contaminated delivery system.
- Special coatings. Special coatings can be applied to all wetted surfaces to prevent adhesion or direct contact with the underlying layer.
Some gas mixtures are very high-value so reducing the amount that is ‘wasted’ through purging saves money, especially over time. Even when using a dry gas purge, this gas still needs to be purged with the calibration gas. A reduction in delivery system volume is a reduction in cost.
CAC Gas, in partnership with Pressure-Tech UK Ltd, developed a specially designed regulator to address each of these areas. This regulator is designed with high-accuracy calibration in mind.
The MICRO300 Precision Regulator includes:
- An AISI 316/316L stainless steel body, bonnet, piston and main pin, compatible with nearly all calibration gases.
- The low internal volume of the regulator (less than 3cc) reduces surface area and volume while still safely controlling up to 300BAR inlet pressures.
- A special range of low-volume cylinder connectors with reduced length and a 0.5mm bore, reducing volume by a factor of 100, and surface area by a factor of 10 when compared to standard cylinder valve connectors.
- A range of special coatings from SilcoTek® Coatings
To accompany this regulator, a range of 1/8” and 1/16” stainless steel tubing and associated fittings are offered, also available with special coatings.