Apparatus for accurately measuring the water content of gas

Abstract: An apparatus for accurately measuring the water content of a moist gas containing less than 50 parts per million (ppm) of water comprises an isothermal heat-sink (14), a radiator (20), a Peltier heater-cooler (24) thermally arranged between the isothermal heat-sink (14) and the radiator (20). The heater-cooler (24) is transferring heat from the isothermal heat-sink (14) to the radiator (20) and vice versa. A phosphoric acid moisture cell (32) is in thermal contact with the isothermal heat-sink (14). Upon passing the moist gas through the cell (32) the amount of moisture in the moist gas is measured as variation of electrical current throughout the cell.

Description: 1) Field of the invention The invention relates to an apparatus for accurately measuring the water content of a moist gas containing less than 50 ppm or even less than 10 ppb of water. 2) The Prior Art Apparatuses for accurately measuring the water content of gases are well known and are frequently referred to as moisture analyzers. One important class of moisture analyzers are those which rely on moisture-induced changes in the electrical resistance of phosphoric acid (P 2 O 5 ) which is contained within a cell of the analyzer. Such moisture analyzers are described for example in the following U.S.A. Patents: Serial Number Hereinafter occasionally defined as: 2,830,945 "Keidel '945" 2,900,317 "Keidel '317" 2,993,853 "Berry" 3,146,181 "Bell" 3,147,202 "Johnson" 3,244,602 "Glass" 3,630,875 "Kuffer" 3,799,846 "Capuano" 3,926,745 "Czuha" 4,280,885 "Savery" 4,800,000 "Zatko" There is no agreement, in the prior art, as to the operating temperature of the cell. Johnson discloses the use of a cooling coil and states that the "...temperature...preferably is in the range from about 0 to about 50°F (-18 to 10°C)". Czuha disagrees and claims a method wherein his "phosphoric acid solution is maintained at a temperature of at least about 50°C". In order to maintain the cell, according to Johnson, below room temperature, let us say 20°C, it is necessary to remove heat from the cell. Most of the readily available cooling means however suffer from a lot of drawbacks; in particular the use of tap water requires a source and excludes the use of the analyzer in the field and the use of phase changing refrigerants, like the CFC, suffer from a multitude of drawbacks. All the phase changing products are requiring a compressor in order to change their phase back to liquid for continued use, and the chloro-fluorinated hydrocarbons (CFC) damage the ozone layer of the atmosphere if they escape. Many jurisdictions are considering passing laws restricting their use for cooling purposes. As is well known in the art the phosphoric acid must be completely dry when first contacted with the moist gas to be measured. It is therefore common practice to cycle the cell by periodically increasing the current through the cell. Examples of this cycling are shown in Bell (Figure 2), Glass (Figure 4), Kuffer (Figures 1 and 2) and Capuano (Figure 3). However what the prior art in general and these references in particular have not identified are the problems which occur when a large amount of moist gas is in the system upstream of the cell. These problems include slow cell response times, possible inaccuracies and an inability to measure small moisture concentrations equal to or lower than one part per million. Accordingly it is an object of the present invention to provide an apparatus for accurately measuring the water content of a moist gas, which apparatus is substantially free from one or more of the problems of the prior apparatuses. Another object is to provide an improved apparatus which does not employ tap water, nor a phase changing refrigerant, nor a chlorinated hydrocarbon, nor a fluorinated hydrocarbon. Yet another object is to provide an improved analyzer which has a fast cell response time. Still another object is to provide an improved analyzer which can measure the moisture content of a gas even down to less than one part per million. DISCLOSURE The above and other objects are accomplished, according to the present invention, by providing an improved apparatus designed by the Applicant. More precisely, the invention relates to an apparatus for accurately measuring the water content of a moist gas containing less than 50 parts per million of water. The apparatus comprises: an isothermal heat-sink; a radiator; a Peltier heater-cooler, thermally arranged between the isothermal heat-sink and the radiator (the heater-cooler is transferring heat from the isothermal heat sink to the radiator and vice versa). The apparatus further comprises: a phosphoric acid moisture cell in thermal contact with the isothermal heat-sink; means for passing the moist gas through the cell; and means for measuring an electrical current in the cell and thereby determining the amount of moisture in the moist gas. The apparatus of the present invention can be used to analyze the water content of any gas which does not react with phosphoric acid. Examples of suitable gasses include: air, nitrogen, oxygen, carbon dioxide, carbon monoxide, hydrocarbons, such as methane, ethane and benzene, and noble gases such as argon, helium and neon. The invention may be better understood by reference to the following drawings which are supplied for illustrating purposes but do not limit in any way the scope of the invention. Figure 1 is an elevation view of an apparatus of the present invention; Figure 2 is a plan view taken along line 2-2 of Figure 1; Figure 3 is a graph showing the impact of thermoregulation, hence of the temperature changes, on the performance of the phosphoric acid moisture cell along the time; Figure 4 is a graph showing how critical is the control of the temperature, in order to get satisfactory performance of the moisture cell; Figure 5 is a graph showing the relationship of current over time, when measuring the moisture content of a typical gas; and Figure 6 shows alternative self explanatory details for the valving system. Referring now to the drawings in general and in particular to Figures 1 and 2 there is shown an apparatus 10 for accurately measuring the water content of a moist gas source 12. The moist gas source 12 is preferably maintained at superatmospheric pressure. However, in the broadest aspects of the present invention, the pressure of the moist gas can be subatmospheric as long as there is a pressure difference sufficient for the moist gas to flow through the apparatus 10. The moist gas can contain widely varying amounts of water. However the apparatus is especially useful for measuring the water content of moist gases having less than one part per million of water. Because of the strong improvement, represented by this invention, the new apparatus can analyze moisture contents of less than 50 and preferably 30 parts per billion (ppb) and even 10 parts per billion (ppb). The apparatus 10 has a metal isothermal heat-sink 14. Within the isothermal heat sink 14 is a cylindrical recess 16. The heat sink 14 is also provided with a planar heat conducting surface 18. The apparatus 10 has a metal radiator 20 which has a planar heat conducting surface 22, which is parallel to and juxtaposed a distance from the planar heat-conducting surface 18 of the isothermal heat sink 14. The apparatus 10 is provided with a Peltier heater-cooler 24 which has a first heat transfer surface 26 parallel to the heat transfer surface 18 of the heat sink 14; and has a second heat transfer surface 27 parallel to the heat transfer surface 22 of the radiator 20. Thus the heat transfer surfaces 26, 27 of the heater-cooler 24 are thermally disposed between the heat transfer surface 18 of the isothermal heat-sink 14 and the heat transfer surface 22 of the radiator 20. The first heat transfer surface 26 is in contact with the isothermal heat sink 14. The second heat transfer surface 27 is in contact with the radiator 20. Electrical direct current can be passed through the Peltier heater-cooler 24 in either of the two directions by means of the conductors 29, 30. In one direction heat is transferred from the surface 26 to the surface 27. In the other direction heat is transferred from the surface 27 to the surface 26. By the choice of current direction it is possible to heat one surface 26 or 27 and cool the other surface 27 or 26. Passing the current in either direction through the heater-cooler 24 serves to maintain the isothermal heat-sink 14 at its desired temperature, preferably at less than 20°C and even better between 0° and 15°C. Heat is transferred to or from the ambient by conduction, convection and radiation, all of which is facilitated by fins on the radiator such as the fins 28, 28', 28". The apparatus 10 is also provided with a phosphoric acid moisture cell 32 within the cylindrical recess 16 of the isothermal heat-sink 14. The cell 32 is in thermal contact with the isothermal heat-sink 14, such that the cell 32 and the heat sink 14 always have the same temperature. The cell 32 has a moist gas inlet 33, and a gas outlet 34. Cells useful in the present invention are well known in the art and their description can be retrieved in Keidel '945, Keidel '317 and Zatko. The apparatus 10 has a conduit 36 in fluid communication with the moist gas source 12 and the inlet 33 of the cell 32. A purge line 38 is in fluid communication with the conduit 36. The junction 39 of the purge line 38 with the conduit 36 is upstream of the cell 32. There is a valve 42 in the purge line 38 immediately adjacent to the junction 39. There is isolation valve 44 in the conduit 36 downstream of junction 39 and immediately adjacent to the same junction and there is an outlet isolation valve 53 downstream of the cell. The apparatus 10 is provided with an analyzer 46 for measuring the electrical current in the cell 32 and thereby determining the amount of moisture in the moist gas from the source 12. As is well known in the art, the analyzer 46 impresses a potential across the phosphoric acid cell 32 by means of conductor 48, 50. The desired temperature of the cell 32 is set on the knob 52. The analyzer 46 compares this desired temperature with the actual temperature of the cell 32 and causes current to flow through the conductors 29, 30 to either heat or cool the cell 32 in order to maintain it at the desired temperature. The heater-coolers are described for instance in the treatise "Solid State Cooling with Thermoelectrics" edited by MELCOR (Trenton J.J., USA), particularly in the chapter "Frigichip Miniature Ceramic Modules-Series FC". A particularly useful heater-cooler is the one traded as Frigichip "CP 1.4-71-06L". In operation, the temperature is set by Knob 52; moist gas flows from the source 12 through the conduit 36 and into the cell 32. The moisture in the moist gas changes the electrical resistance of the phosphoric acid in the cell, making the cell a variable resistor. The current flowing through this variable resistor is measured by the analyzer 46 as the percentage of moisture in the moist gas. Cooling by Peltier effect in general, and by the particular structure described herein, avoids the use of tap water and of phase changing refrigerants. Furthermore the particular structure, including the valves 42, 44 and 53, grouped about junction 39 minimizes the amount of gas in the system from the last test and allows the testing of different sampling lines to be carried out in a very short time. Referring now to Figure 3, there is reported the water content, along the time, of two streams of dry argon measured with a thermoregulated H 3 PO 4 cell (line W: T = 15°C) and through a not thermoregulated cell (line Z). The wider excursion of line Z shows the impact of the temperature changes on the cell performance and clearly proves how it is necessary to thermoregulate the cell. Referring now to Figure 4, there is reported the water content, along with the temperature, after the passage through a moisture analyzer, traded by the Applicant as MICRODOWSER and containing a thermoregulated cell (MICRODOWSER CELL 47A), of four different streams of argon, containing the following different amount of moisture stream moisture (ppb) I practically absent II 120 ppb III 250 ppb IV 780 ppb From the lines II-IV on the figure, it is clear that in the case of a wet gas the final water content, in the range from 11° to 23° C, is indicatively constant. On the contrary, when the gas is dry (ZERO GAS; line I) temperature exerts a remarkable impact on the background noise; in fact, consequently to a temperature decrease from 23°C to 15°C, the background noise is reduced to less than 50% of the original value. This graph, in other words, is showing that the optimum temperature is lying between 10° and 15°C; there is no need therefore for the expensive cryogenic devices required by the low temperatures of Johnson (down to -18°C). Referring now to Figure 5, a first stream, consisting of dry argon , is passed through a moisture analyzer according to the invention (thermoregulated at 15°C and supplied with MICRODOWSER CELL 47A); the water content, at the cell outlet, is 18 ppb, as recorded on the figure. Valves 42, 44 and 53 (see Fig. 1) are then closed and the isolated cell remains consequently separated from the air moisture. The analyzer is subsequently disconnected and connected to a new gas source, supplying a new gas. By always keeping the cell isolated (namely by keeping isolation valves 44 and 53 closed), valve 42 is opened and the inner pipes of the analyzer (outside the cell) are vented with a stream of purge gas, thus removing the air and the moisture penetrated during the transfer operation). Eventually, the isolation valves (44 and 53) are re-opened and a new (second) analysis can be carried out. By this method, the time required by each of several analysis tests is extremely reduced (from 2 to 3 hours, against the 24 hours required by the analyzers of the prior art). Although the invention has been described in considerable detail with respect to preferred embodiments thereof, it will be understood that variations and modifications can be made without departing from the scope of the invention as described above and in the appended claims.

Claims: An apparatus for accurately measuring the water content of a moist gas containing less than 50 parts per million (ppm) and even less than 1 ppm of water, characterized by comprising: A. an isothermal heat-sink (14); B. a radiator (20); C. a Peltier heater-cooler (24) thermally arranged between the isothermal heat-sink (14) and the radiator (20), wherein said heater-cooler is transferring heat from the isothermal heat-sink (14) to the radiator (20) and vice versa; D. a phosphoric acid moisture cell (32) in thermal contact with the isothermal heat-sink (14); E. means (36, 33, 34) for passing the moist gas through the cell (32); and F. means (46) for measuring an electrical current in the cell (32) and thereby determining the amount of moisture in the moist gas. The apparatus of Claim 1 characterized in that the isothermal heat-sink (14) is made from metal. The apparatus of Claim 1 characterized in that the radiator (20) is made from metal. The apparatus of Claim 1 characterized by further comprising means (29, 30) for passing an electrical current through the Peltier heater-cooler (24) in a first direction in order to heat the cell (32). The apparatus of Claim 1 characterized by further comprising means (29, 30) for passing an electrical current through the Peltier heater-cooler (24) in a second direction in order to cool the cell (32). The apparatus of Claim 1 characterized by further comprising: G. a source (12) of moist gas at superatmospheric pressure; H. a conduit (36) in fluid communication with the moist gas source (12) and the inlet of the cell (32); I. a purge line (38) in fluid communication with the conduit (36) having its junction (39) with the conduit upstream of the cell (32); J. a valve (42) in the purge line (38) immediately adjacent to the junction (39); K. an isolation valve (44) in the conduit (36) downstream of the juction (39) and immediately adjacent to the junction; and L. an outlet isolation valve (53) in a conduit (36) downstream of the cell (32). An apparatus according to Claim 6, characterized in that: said isothermal heat sink (14) is made from metal, has a cylindrical recess therein and has a planar heat conducting surface (18); said radiator (20) is made of metal, has a planar heat conducting surface (22) parallel to and juxtaposed a distance from the planar heat conducting surface (18) of the isothermal heat-sink (14); said Peltier heater-cooler (24) has first (26) and second (27) juxtaposed parallel heat transfer surfaces thermally arranged between said isothermal heat-sink (14) and said radiator (20), with the first heat transfer surface (26) being in contact with the isothermal heat sink (14) and the second surface being in contact with the radiator (20); wherein the electrical current can be passed through the Peltier heat-cooler (24) in either of the two directions in order to heat one surface (26) of the heater-cooler and to cool the other surface (27) of the heater-cooler or vice versa; wherein means (29, 30) are provided for passing an electrical current in either direction through the heater cooler (24) in order to maintain the isothermal heat-sink (14) at a temperature equal to or lower than 20°C and preferably between 0°C and 15°C; said phosphoric acid moisture cell (32) is lodged in said cylindrical al recess (16) of the isothermal heat-sink (14), has a moist gas inlet (33) and a moist gas outlet (34) and is substantially kept at the same temperature as the isothermal heat-sink. Use of the apparatus according to claim 1 for accurately measuring the water content of gases containing as little as 50 and preferably 30 ppb of moisture or less. Use according to Claim 9 for accurately measuring the water content of gases containing as little as 10 ppb of moisture.

Inventors: COPPOLA ANTONIO, DONI FABRIZIO, SOLCIA CAROLINA, SUCCI MARCO, COPPOLA, ANTONIO, DONI, FABRIZIO, SOLCIA, CAROLINA, SUCCI, MARCO
Applicants: GETTERS SPA, SAES GETTERS S.P.A.
Application year: 1992
Main classification: G01N27
IPC codes: G01N33, G01N27, G01N25