Systems and methods for humidity control

1. A method for humidifying a breathing gas using a vapor transfer unit, the method comprising the steps of:

delivering gas to a vapor transfer unit having a vapor transfer device and a bypass gas passage having a constriction positioned therein, the constriction comprising a ring-shaped protrusion that narrows a portion of the bypass gas passage, wherein the gas is delivered at a gas flow rate;

passing a fraction of the gas through the constriction in the bypass gas passage; and

automatically altering the fraction of the gas passed through the bypass gas passage inversely with a change in the gas flow rate.

2. The method of claim 1 , further comprising the step of:

maintaining fixed internal dimensions of the vapor transfer unit.

3. The method of claim 2 , wherein the constriction is sized so that the fraction of gas received by the bypass gas passage decreases as the gas flow rate increases.

4. The method of claim 3 , further comprising the step of:

delivering liquid to the vapor transfer device.

5. The method of claim 4 , wherein the vapor transfer device includes a first gas passage and a liquid passage, and wherein the gas is delivered to the first gas passage and the liquid is delivered to the liquid passage.

6. The method of claim 5 , wherein the vapor transfer device comprises a plurality of hollow fiber membranes.

7. The method of claim 6 wherein delivering the liquid comprises directing liquid toward outer surfaces of the plurality of hollow fiber membranes.

8. The method of claim 1 , wherein the gas is delivered at a flow rate of greater than or equal to about 8 liters per minute.

9. The method of claim 1 , wherein the gas is delivered at a flow rate of greater than or equal to about 30 liters per minute.

10. The method of claim 1 , further comprising the steps of:

exiting output gas from the vapor transfer unit at an initial relative humidity and at an initial flow rate;

increasing the gas flow rate from the initial flow rate to a second flow rate;

automatically decreasing the fraction of the gas passed through the bypass gas passage at the second flow rate;

exiting output gas at the second flow rate and at a second relative humidity.

11. The method of claim 10 , wherein the second relative humidity is within 10% relative humidity of the first relative humidity.

12. The method of claim 10 , wherein the initial flow rate differs from the second flow rate by about 10 L/min or more.

13. The method of claim 1 , further comprising the steps of:

exiting output gas from the vapor transfer unit at an initial relative humidity and at an initial flow rate;

decreasing the gas flow rate from the initial flow rate to a second flow rate;

automatically increasing the fraction of the gas passed through the bypass gas passage at the second flow rate;

exiting output gas at the second flow rate and at a second relative humidity.

14. A method for humidifying a breathing gas using a vapor transfer unit, the method comprising the steps of:

providing a first vapor transfer unit having a gas passage and a liquid passage;

delivering a liquid to the liquid passage;

delivering a gas to the gas passage;

passing a fraction of the gas through a constriction positioned in a bypass gas passage formed in the first vapor transfer unit, the constriction comprising a ring-shaped protrusion that narrows a portion of the bypass gas passage;

humidifying the gas in the gas passage by delivering vapor from the liquid in the liquid passage to the gas in the gas passage;

exiting the humidified gas outside the vapor transfer unit at a first relative humidity and at a gas flow rate greater than about 35 liters per minute; and

reducing the gas flow rate through the first vapor transfer unit to less than about 20 liters per minute, while preventing the relative humidity of the humidified gas exiting the vapor transfer unit from exceeding the first relative humidity by more than a margin;

wherein the margin is about 10% relative humidity or less.

15. The method of claim 14 , wherein the bypass gas passage runs parallel to the gas passage.

16. The method of claim 15 , wherein the fraction of the gas passed through the bypass gas passage automatically varies inversely with a change in the gas flow rate.

17. The method of claim 14 , further comprising the steps of:

obstructing gas flow through a portion of the gas passage; and

adjusting the relative humidity by changing the amount of the gas passage that is obstructed.

18. The method of claim 14 , wherein delivering the gas to the gas passage further comprises delivering gas to a plurality of hollow fiber membranes disposed within the gas passage.

19. The method of claim 14 , wherein delivering the liquid to the liquid passage further comprises directing liquid toward outer surfaces of the plurality of hollow fiber membranes.

20. The method of claim 14 , wherein the margin is about 8% relative humidity.

21. The method of claim 14 , wherein the margin is about 6% relative humidity.

22. The method of claim 14 , wherein the margin is about 4% relative humidity.

23. The method of claim 14 , wherein the first relative humidity is less than about 95% relative humidity.

24. The method of claim 14 , wherein the first relative humidity is less than about 85% relative humidity.

25. The method of claim 14 , further comprising the step of:

automatically increasing the fraction of the gas passed through the bypass gas passage in response to the reduction in the gas flow rate.