Patent Application
20060120920
1. Field of the Invention
This invention relates to a method and apparatus for measuring the concentration of hydrogen gas in any gas mixture in which no helium gas and neon gas are present. This invention relates to a method and apparatus for measuring the concentration of helium gas in any gas mixture in which no hydrogen gas and neon gas are present. And, finally, this invention relates to a method and apparatus for measuring the concentration of neon gas in any gas mixture in which no helium gas and hydrogen gas are present.
The measurement of hydrogen concentrations in a gas mixture, particularly at low levels, is subject to numerous difficulties including interference from other gases present in the mixture, deterioration of the sensor by exposure to certain gases, and short sensor lifetimes caused by physical or chemical deterioration of the sensor components. Known methods and devices for determining the concentration of hydrogen in a gas mixture include laboratory techniques such as gas chromatography and mass spectroscopy. Other approaches include metal hydride based techniques and electrochemical techniques in which hydrogen is dissociated in passing through a transport membrane. However, such techniques rely upon the use of costly materials and sensor devices.
Accordingly, it is one object of this invention to provide a method and apparatus for measuring hydrogen concentration in a gas mixture that is reliable and low-cost.
It is one object of this invention to provide a method and apparatus for measuring hydrogen concentration in a gas mixture suitable for making quantitative determinations of any hydrogen concentration from 1 ppm up to 100%.
It is still another object of this invention to provide a method and apparatus for measuring hydrogen concentration in a gas mixture which is not subject to interference from other gases present in the gas mixture.
It is still a further object of this invention to provide a method and apparatus for measuring hydrogen concentration in a gas mixture which addresses the issue of sensor deterioration when exposed to certain gases.
It is yet a further object of this invention to provide a method and apparatus for measuring hydrogen concentration in a gas mixture which overcomes the short sensor lifetimes caused by physical or chemical deterioration of conventional sensor components.
These and other objects of this invention are addressed by an apparatus comprising a membrane-substrate assembly comprising a porous, chemically inert substrate material and a chemically inert permeable membrane having a hydrogen gas diffusion rate higher than the diffusion rate of the remaining bulk gas mixture components, which membrane is secured to the porous substrate material. A housing constructed of at least one non-permeable wall is disposed around the membrane-substrate assembly such that only an active portion of the chemically inert permeable membrane is exposed directly to the bulk gas mixture during use of the apparatus to measure hydrogen concentration. The apparatus further comprises evacuation means for substantially evacuating the housing, pressure means for measuring the pressure of the hydrogen gas diffusing through the membrane-substrate assembly into the housing, temperature control means for controlling temperature within the housing, bulk pressure means for measuring the pressure of the bulk gas mixture and bulk temperature means for measuring the temperature of the bulk gas mixture.
The method and apparatus of this invention are able to measure the concentration of hydrogen gas in any gas mixture except mixtures containing helium and mixtures containing neon gas. This is due to the fact that both helium and neon are very close in size to hydrogen and, thus, have membrane diffusion characteristics comparable to the membrane diffusion characteristics of hydrogen. Thus, it is also the case that the method and apparatus of this invention are suitable for measuring the concentration of helium in any gas mixture which does not include hydrogen and neon gases and for measuring the concentration of neon in a gas mixture which does not include hydrogen and helium, and such embodiments are deemed to be within the scope of this invention. For the purpose of simplicity, this invention will be described in terms of applicability to hydrogen gas, but it will be understood that wherever mention of hydrogen is made, helium or neon could be substituted therefor.
These and other objects and features of this invention will be better understood from the following detailed description taken in conjunction with the drawings wherein:
a is a graphical representation of the diffusion of a single gas through a chemically inert membrane;
b is a graphical representation of the diffusion of a two component gas mixture in which one of the components is hydrogen;
c is an enlargement of the circled portions of
The invention claimed herein is a method and apparatus for measuring the concentration of hydrogen in a gas mixture that does not contain either helium or neon gas. The invention relies upon the permeability of gases by diffusion through inert membranes, which may be made of glass or many other materials. Suitable materials include, but are not limited to, VYCOR, NANOSIL and silica. Hydrogen has a higher permeability rate through such membranes compared to other gases (except helium and neon) and hydrogen permeates through such membranes faster than all other gases (except helium and neon). As used herein, the term “inert membranes” means membranes that are chemically inert. Permeation of gases through inert membranes is a highly predictable function of the thickness and properties of the membrane, the temperature of the membrane, the temperature and pressure of the bulk gas mixture, and the partial pressure difference of the diffusing gas between the two sides of the membrane.
b shows that for a two component gas mixture in the bulk gas, when one gas is hydrogen, hydrogen, H2, will permeate through the membrane faster than the other gas, X. Both the single gas and two-component gas example assume there is no gas initially in the membrane or on the substrate side of the membrane-substrate assembly. The time in which hydrogen is the only gas diffusing through the membrane (time t1 to t2 in
As previously indicated, the method and apparatus of this invention use the unique property of hydrogen diffusion through an inert membrane at both a higher rate and faster than all other gases (with the noted exceptions of helium and neon). To accomplish this, the inert membrane must be thin, preferably in the range of about 0.1 microns to about 100 microns thick and most preferably in the range of about 1.0 microns to about 10 microns thick, and the diffusion properties of the membrane must be known for hydrogen and the other gases likely to be encountered in a bulk gas mixture. In addition, the membrane must be maintained at a constant temperature, typically by means of a suitable controlled heating source, and the temperature and pressure of the bulk gas mixture must be known. Yet a further requirement is that the membrane, in addition to being chemically inert, be stable. Stabilization is achieved in accordance with one embodiment of this invention by securing the membrane to a porous substrate of inert metal or ceramic material, forming a membrane-substrate assembly. The membrane is preferably bonded to the substrate and the two materials, i.e. the inert membrane and the substrate material, must have similar thermal expansion properties so that the membrane-substrate interface remains stable. Also required is a suitable means for accurately and quickly measuring either pressure or hydrogen partial pressure on the substrate side of the membrane.
As previously indicated, knowledge of the pressure and temperature of both the hydrogen gas diffusing through the membrane and the bulk gas mixture is required in order to carry out the method of this invention. Accordingly, sensor 10 further comprises a heater/temperature controller 15 adapted to control the membrane and substrate temperatures, at least one rapid hydrogen pressure indicator 18 adapted to measure the pressure of hydrogen diffused through membrane 12, at least one temperature indicator adapted to measure the temperature within housing 14, at least one bulk gas pressure indicator 21 adapted to measure the pressure of the bulk gas mixture, and at least one bulk gas temperature indicator 22 adapted to measure the temperature of the bulk gas mixture. Also as previously indicated, it is necessary to evacuate the housing between uses of the sensor. Accordingly, the sensor further comprises at least one vacuum pump 17 adapted to evacuate the housing 14 between uses of the sensor. Thus, when the membrane-substrate assembly is sealed from the bulk gas mixture by impermeable sensor seal plate 20, the vacuum pump 17 is used to reduce the pressure in the membrane-substrate assembly within the housing to a small absolute value relative to the bulk gas pressure, preferably in the range of about 0.001 to about 0.01 of the bulk gas pressure. When a bulk hydrogen concentration measurement is desired, the pump is shut off, and the sensor seal plate slid back to expose the membrane to the bulk gas.
In operation, when a hydrogen concentration determination is desired, the sensor seal plate covering the active area of the membrane is removed. Hydrogen partial pressure as a function of time data is collected from the time after hydrogen passes completely through the membrane and before other gases in the bulk gas begin to pass all the way through the membrane into the substrate. That is, after a time lag, t1, for hydrogen to pass all the way through the membrane, pressure in the membrane-substrate assembly is measured several times before other gases diffuse completely through the membrane, t2. This transient hydrogen partial pressure data (collected between time t1 and t2 as shown in
After determination of the bulk gas mixture hydrogen partial pressure, the membrane-substrate assembly is isolated from the bulk gas, the pump (or other means) used to lower the membrane-substrate assembly pressure is turned on, and the cycle is repeated. To make regular and fast readings of bulk gas hydrogen partial pressure (or concentration), a plurality of membrane-substrate assemblies can be used. All other assemblies are evacuated while only one assembly is used to make hydrogen pressure determinations.
When measuring particularly low hydrogen concentrations, the sensor is dependent on a fast-responding and accurate pressure indicator. Thus, times t1 and t2 will typically both be less than one second with a difference of not more than half of a second. To obtain accurate determination of hydrogen concentrations as low as 1 ppm in the bulk gas, a pressure sensor must accurately read pressure to five orders of magnitude, at absolute pressures between 10−5 and 1 mTorr, up to five times, at intervals of 0.05 to 0.1 seconds. Several pressure sensors now exist that are capable of satisfying this demanding requirement. One such sensor is an absolute capacitive pressure sensor available from Integrated Sensing Systems, Inc. of Ypsilanti, Mich.
While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for the purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of this invention.
