An accurate method of measuring medical humidifier output

Abstract

A low-cost capacitance hygrometer capable of measuring steady-state and some transient changes in the output of medical humidifiers is introduced and evaluated. The method of operation is presented in detail, as is a comparison with the widely used gravimetric standard. The hygrometer produces results within 7% of those given by the gravimetric method when using units of percentage relative humidity (which give the widest discrepancies). This is shown to be an acceptable figure for the proposed application. Problems encountered when converting units of humidity are discussed in detail and methods of conversion explained.

Changing a hydrophobic heat and moisture exchanger after 48 hours rather than 24 hours: a clinical and microbiological evaluation

“…It is important that this instrument is perfectly accurate when used to assess medical humidifiers, where humidity often approaches 100 % RH. The capacitance hygrometer has been shown to produce results very close (within 7 %) to those provided by the absolute gravimetric method over a wide range of RH (67±95.5 %) [23]. For the purpose of the present study and the proposed clinical application, this has been considered as an acceptable figure.…”

“…There is some indirect evidence of very little, if any, change in HME resistance over the 48 h study period since no modifications were observed in peak and mean airway pressures, with identical tidal and minute volumes in the study patients. Humidity measurements can be made using different methods and the mass transfer method (i. e., gravimetric method) is considered to be the gold standard [23]. This technique requires the humidifier to be weighed before and after a period of operation under strictly controlled conditions.…”

“…Respiratory rates, tidal volumes, FIO 2 and PEEP were adjusted to maintain PaO 2 at 10.5 kPa (80 mmHg) and PaCO 2 at 5.5 kPa (40 mmHg), and were not modified during the study period. Temperature, relative humidity (RH) (%) and absolute humidity (AH) (mgH 2 0/l) were obtained using the Gibeck Humidity Sensor System [23]. RH is the ratio of the AH to the saturated humidity at a given temperature ; saturated humidity is related to the maximum vapor capacity, whereas AH is the actual amount of water contained in a given volume of gas at a given temperature and pressure.…”

“…The system consists of an extremely fast reacting humidity sensor and a fast reacting temperature sensor, both integrated in an angled connector (15 M-15 F ISO Gibeck, Sweden) placed in the breathing circuit between the tracheostomy tube and the heat and moisture exchanger or the Ttube. The method used by the Humidity Sensor System is based on the capacitive sensor principle [23]. A very thin layer of hygroscopic polymer compound is placed in between two conductive layers to make up a condensator.…”

Independent testing of the Fisher & Paykel Healthcare MR860 Laparoscopic Humidification System

“…Several studies have been conducted to test humidifiers for breathing circuits (7,8,(11)(12)(13)(14)(15) However only a few of these utilised the gravimetric method to calculate absolute humidity, which is the technique generally regarded most accurate (16). A recent bench study by Schumann et al comparing a counter-flow humidifier and a conventional humidifier (of very similar design to the humidifier tested in our study) showed that the humidification capability of both humidifiers was independent of the flow rate (7).…”