The psychrometric method used in the present study may also be considered as a limitation since some researchers may consider the ISO 9360 as the “reference method.”52 The ISO 9360 using the gravimetric method is an attempt to homogenize the practices for HME evaluation and is used by a majority of manufacturers. However, the gravimetric method is difficult to set up, and weight-in as well as spirometric measurement errors make this technique difficult to use.11,22 Another limitation of this technique is that it was not described for evaluation of heated humidifiers. In addition, the gravimetric method cannot be used with patients, and the results thus cannot be directly compared with clinical data, unlike the psychrometric method.15,17,49 To date, there are no data that allow to state that one method is more accurate than another, and there is no “gold standard” for hygrometric measurements. The ISO 9360 standard was used by 55% of the manufacturers for HME or HMEF evaluation, and this technique does not seem to overestimate humidity measurements in comparison with the psychrometric method. Indeed, when comparing the performances of the same HMEs evaluated by the hygrometric or the gravimetric methods, the differences are small. In the study by Branson and Davis,11 Hygrobac, Hygrobac S, Hygroster, and Sims Portex 1200 provided 32.5, 29.6, 33.2, and 27.2 mg H2O/L, respectively, when tested with the gravimetric method. In the present study, with the psychrometric method, the same devices provided very close results: 31.7, 31.2, 30.7, and 27.8 mg H2O/L (Table 1). Thus, the differences we found in the present study between our measurements and manufacturer data are likely not related to the measurement technique.