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<p>We describe the model and construction of a two-flow (or divided-flow) humidity generator, developed at LNE-Cnam, that uses mass flow controllers to mix a stream of dry gas with a stream of humid gas saturated at 28 <span class="inline-formula">^∘</span>C. It can generate a wide range of humidity, with mole fractions in the range of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M2" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">0.7</mn><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">6</mn></mrow></msup><mo>&lt;</mo><mi>x</mi><mo>&lt;</mo><mn mathvariant="normal">9000</mn><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">6</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="141pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="fa27aae7003840793ebb951bb98d046c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-15-819-2022-ie00001.svg" width="141pt" height="14pt" src="amt-15-819-2022-ie00001.png"/></svg:svg></span></span>, without using low temperature or high pressure. This range is suitable for calibrating balloon-borne instruments that measure humidity in the stratosphere, where <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi>x</mi><mo>∼</mo><mspace linebreak="nobreak" width="0.125em"/><mn mathvariant="normal">5</mn><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">6</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="62pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="799605ddb9ad3c56efd3c82ca4a3264a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-15-819-2022-ie00002.svg" width="62pt" height="14pt" src="amt-15-819-2022-ie00002.png"/></svg:svg></span></span>. The generator's novel feature is a saturator that comprises 5 m of silicone tubing immersed in water. Water enters the humid gas stream by diffusing through the wall of the tubing until the gas stream flowing through the tubing is saturated. This design provides a simple, low-cost humidity generator with an accuracy that is acceptable for many applications. The key requirement is that the tubing be long enough to ensure saturation so that the saturator's output is independent of the dimensions and permeability of the tube. A length of only a few meters was sufficient because the tube was made of silicone; other common polymers have permeabilities that are 1000 times smaller. We verified the model of the transition from unsaturated flow to saturated flow by measuring the humidity while using three tube lengths, two of which were too short for saturation. As a more complete test, we used the generator as a primary device after correcting the calibrations of the mass flow controllers that determined the mixing ratio. At mole fractions of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">50</mn><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">6</mn></mrow></msup><mo>&lt;</mo><mi>x</mi><mo>&lt;</mo><mn mathvariant="normal">5000</mn><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">6</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="138pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="87d698af343768bd59932158b478de6f"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-15-819-2022-ie00003.svg" width="138pt" height="14pt" src="amt-15-819-2022-ie00003.png"/></svg:svg></span></span>, the generator's output mole fraction <span class="inline-formula"><i>x</i>_gen</span> agreed to within 1 % with the value <span class="inline-formula"><i>x</i>_cm</span> measured by a calibrated chilled-mirror hygrometer; in other words, their ratio fell in the range <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>x</mi><mi mathvariant="normal">gen</mi></msub><mo>/</mo><msub><mi>x</mi><mi mathvariant="normal">cm</mi></msub><mo>=</mo><mn mathvariant="normal">1.00</mn><mo>±</mo><mn mathvariant="normal">0.01</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="112pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="0e1687d4f94bec9b3ed608323980b005"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-15-819-2022-ie00004.svg" width="112pt" height="16pt" src="amt-15-819-2022-ie00004.png"/></svg:svg></span></span>. At smaller mole fractions, their differences fell in the range of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>x</mi><mi mathvariant="normal">gen</mi></msub><mo>-</mo><msub><mi>x</mi><mi mathvariant="normal">cm</mi></msub><mo>=</mo><mo>±</mo><mn mathvariant="normal">1</mn><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">6</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="111pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="e5766c18eeb83270bdcc239f54ed8628"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-15-819-2022-ie00005.svg" width="111pt" height="17pt" src="amt-15-819-2022-ie00005.png"/></svg:svg></span></span>.</p>