Patent Application
20190316736
A calibration gas is a reference gas, or gas mixture, that is used as a standard in the calibration of certain analytical instruments. A calibration gas must be very precisely defined and prepared to maintain stability of concentration and composition over time. There are a number of possible influences that may cause this highly particular mixture to vary over time.
One of the most important factors affecting the shelf life of a gas mixture is the reactivity of the components of the mixture with any contaminants that may be present on the interior surface of the cylinder. However, the actual criteria for passing or failing may not be easily defined or identified. Hence, there exists within the industry a need for a well-defined cylinder cleanliness assessment test, with good repeatability and accuracy.
A method of gas cylinder interior cleanliness validation, including placing a volume of liquid on an interior surface of a component of a gas cylinder, measuring a contact angle of the liquid on the interior surface, and estimating from the contact angle the cleanliness of the interior surface with respect to at least one contaminant.
For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:
101=gas cylinder
102=interior surface (of gas cylinder)
103=coupon (to be removed from the gas cylinder)
104=light source
105=video processing device
106=syringe
107=control and processing system
108=filter
109=micromanipulator
110=drop to be tested
In a pressurized cylinder, the likelihood of the various components of the calibration gas mixture reacting increases with the cleanliness of the cylinder and the reactivity of the inner surface. The interior surface of the cylinder is often credited with affecting the stability of the calibration mixture. The more reactive the mixture, the more pronounced this effect is.
Often it is necessary to chemically alter, or inert, the inner surface of the cylinder in order to achieve stability. For example, when a 100 ppb H2S mixture is prepared in an aluminum alloy 6061 gas cylinder without surface treatment, the stability of the mixture is quickly compromised. Increasing the stability is realized with passivation and surface treatment followed by passivation.
One of the most important factors affecting the shelf life of a gas mixture is the reactivity of the components of the mixture with any contaminants that may be present on the interior surface of the cylinder. One of the main properties affecting the stability of a gas mixture is the chemical composition and concentration of impurities present on the interior surface. Even visible residues left on the surface during the cleaning process are known to cause mixture stability issues. In some cases, process chemicals used during the manufacture of the cylinders may remain on the interior of the cylinder after cleaning.
Compressed gas cylinders are typically made of either an aluminum alloy or steel. Steel cylinders are typically only used for non-reactive gas mixtures. The cylinders used to contain calibration gas are typically made from an aluminum alloy, such as 3003. As aluminum is a very ductile metal, cylinders are typically produced by an extrusion process, such as pressure extruding. Such a process requires a lubricant at the interface between the extrusion die and the aluminum, to ensure the extruded part release easily from the die. One of the most common such lubricant is zinc stearate.
Commercial zinc stearate is actually a mixture of zinc stearate, zinc palmitate and zinc oxide. While zinc stearate and zinc palmitate are insoluble in water, they are soluble in some organic solvents.
Typically, a dedicated cleaning process is used to remove the lubricant. If the cleaning baths are not properly maintained, filtered, and/or changed before becoming exhausted, the zinc compounds could redeposit on the cylinders. Such incomplete cleaning of the lubricant has bene found to occur on some cylinders. A manual inspection process often fails to identify all of the improperly cleaned cylinders at the manufacturing site.
This redeposition issue can really only be addressed by cylinder manufacturers by utilizing careful monitoring, filtering, and change out procedures for the cleaning baths. Visual inspection of the cylinders can be very tedious, requiring the assessment of large numbers of cylinders.
This may result in a concentration decrease, or an alteration of the mixture composition, of calibration gas mixtures in these cylinders. This may lead to costly failures at customer sites. And the actual criteria for passing or failing may not be easily defined or identified.
The instant method utilizes measuring the contact angle of a liquid, preferably water, on the surface in question, preferably the interior surface of a cylinder or cylinder component. This procedure is commonly called the sessile drop technique. The contact angle is the angle formed by a line tangent to the intersection of the droplet with the surface and the surface itself. A drop of water resting at equilibrium on a surface will intersect that surface at a measurable angle, called the contact angle
The contact angle of the liquid will indicate the wettability of this surface. Wetting describes the degree to which a liquid spreads onto a solid when it comes in contact with it. One measure of wettability is the contact angle. The wettability can be used to determine if the surface is hydrophobic or hydrophilic. The contact angle can therefore be used to validate the cleanliness of the surface.
In the case of complete wetting, the contact angle would theoretically be zero. Generally speaking, if the contact angle is less than 90, as represented in
A clean aluminum surface suitable for stable calibration standards should have a contact angle of less than 50 degrees, preferably less than 30 degrees. Measurement of the contact angle can be done utilizing commercially available equipment, for example by Kruss and Bolin Scientific.
Turning to
In one embodiment, as illustrated in
Turning to
Video processing device 105 may be a high-resolution camera, such as a digital or CCD camera. Included with video processing device 103 may be one or more filters 108 to suppress scattered light emanating from the substrate, or other local surfaces. Light source 104 should be of a type to provide as sharp an edge of droplet 106 as possible for the video processing device 105.
Device for supplying the droplet 106 may be a syringe such as a microliter syringe. The syringe may be connected to a micromanipulator 109, which may be controlled by control and processing system 107.
To place the volume of liquid 110 is place on coupon 103, control and processing system 107 controls syringe 106 which releases a drop of liquid. In some embodiments, the liquid is water. Drop 110 then expands into the equilibrium point between the cohesion within the liquid and the adhesion with the surface of the coupon. While being illuminated, typically back lit, by light source 104, video processing device 103 captures the shape, or profile, of drop 110 and sends this image, or images, to control and processing system 107. The contact angle is the angle between the surface of the liquid and the outline of the contact surface (see
In one embodiment, a contact angle that is less than or equal to 50 degrees indicates an adequately clean component, while a contact angle that is greater 50 degrees indicates an inadequately clean component. In another embodiment, a contact angle that is less than or equal to 30 degrees indicates an adequately clean component, while a contact angle that is greater 30 degrees indicates an inadequately clean component.
In addition to the above sessile drop test of contact angle, visually inspecting the interior surface for evidence of staining is also preferred. In one embodiment, after the cylinder, or interior surface, cleanliness has been validated, the interior surface may be subsequently exposed to a reactive gas or reactive gas mixture, which would have experienced accelerated decomposition or storage reaction rate if the measured contact angle were to exceed 50 degrees, or preferably 30 degrees. The reactive gas, or reactive gas mixture, may include hydrogen sulfide.
During the in situ test illustrated in
It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.
This application claims the benefit of priority under 35 U.S.C. § 119(a) and (b) to U.S. Provisional Application No. 62/655,941, filed Apr. 11, 2018, the entire contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 62655941 | Apr 2018 | US |
