Apparatus for accurately measuring the water content of gases

An apparatus for accurately measuring, at a given measuring temperature within the range of about 0.degree. to about 20.degree. C., the water content of a moist gas containing less than 50 parts per million of water 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 transfers 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 (33, 34, 36) for passing the moist gas through cell (32);
• F. means (46) for measuring an electrical current in the cell (32);
• and thereby determining the amount of moisture in the moist gas;
• and
• G. means for passing electrical current through the Peltier heater-cooler in a cell-heat-removing direction thereby removing heat from the heat sink and thereby from the cell when the cell has a temperature above the measuring temperature;
• and
• H. means for passing electrical current through the Peltier heater-cooler in a cell-heat-adding direction, opposite to the cell-heat-removing direction, thereby adding heat to the heat sink and thereby to the cell when the cell has a temperature below the measuring temperature;
• and
• I. means for maintaining the temperature of the cell at the given measuring temperature;
• and
• J. a source (12) of moist gas at superatmospheric pressure;
• K. a conduit (36) in fluid communication with the moist gas source (12) and the inlet of the cell (32);
• L. a purge line (38) in fluid communication with the conduit (36) having its junction (39) with the conduit upstream of the cell (32);
• M. a valve (42) in the purge line (38) immediately adjacent to the junction (39);
• N. an isolation valve (44) in the conduit (36) downstream of the junction (39) and immediately adjacent to the junction (39);
• and
• O. an outlet isolation valve (53) in a conduit (36) downstream of the cell (32) wherein:
• 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 a first and second juxtaposed parallel heat transfer surfaces (26, 27) 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 heat transfer surface being in contact with the radiator (20);
• wherein the electrical current can be passed through the Peltier heater-cooler (24) in either of 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 between 0.degree. C. and 15.degree. C.;
• said phosphoric acid moisture cell (32) is carried in said cylindrical recess (16) of the isothermal heat-sink (14), has a moist gas inlet (33) and a moist gas outlet (34) and is kept at substantially the same temperature as the isothermal heat sink.