Model 938F water trap produced by U.S. CSI is the most popular water trap in the world, and it is widely used in ETCO2 monitoring occasions. The water trap looks fine, and has reliable performance. It can not only remove the majority of water vapor during human beings breath, also has filtering capabilities, to prevent impurities going into the monitor to contaminate gas measurement sensor . The water trap which applys to the moisture content in higher level is more effectively.In accordance with the original request, one water trap must be used in a patient, not in different patients, in order to avoid cross-infection. In addition, once water trap will be filled with water, a new one must be replaced immediately to avoid the water taking in the monitor to destroy CO2 module.