While auditing an introductory hyperbaric training course the other day I heard mention on several occasions that the humidity level within the monoplace was always zero. This statement served to justify a requirement that patients should remove their contact lenses before each treatment. The stated basis for zero humidity was that incoming LOX sourced oxygen was dry.
This statement failed to account for the patient’s ongoing respiratory and transcutaneous evaporative insensible moisture loss. Historically, our group had measured monoplace chamber relative humidity under varying conditions, namely changes in oxygen flow rates and differing patient body mass. RH was never zero, rather it was found to range from the low 20’s to the high 60’s. The lower the flow rate the higher the resulting RH. Experienced chamber operators may have observed RH levels so high on occasion that condensation occurred on the internal surfaces of the acrylic hull.
From a monoplace chamber fire safety perspective, one should aim for the higher of the above range. Dryer conditions tend to result in a higher risk of static electricity buildup, which can lead to dangerous electrostatic discharges if patients are not appropriately grounded. If patients complain of being cold while in the chamber this should prompt a reduction in oxygen flow, not provision of a (another) blanket! The 1996 monoplace chamber fire and explosion was an object lesson, in that combustible materials must be limited to the greatest extent possible. If patients complain of being too warm, titrate the oxygen flow higher. Do not simply default to the highest flow rate…comfort levels may be reached with only incremental increases.
I thought it useful to revisit this topic. Too commonly, oxygen flow is maintained at its highest rate, as a previous patient may have complained of overheating. When subsequent patients become too cold the default position may be to provide an additional blanket, or two. Consistent with the above, this would be considered F work! Don’t do it.
Dick Clarke, President
National Board of Diving & Hyperbaric Medical Technology
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Updated October 20, 2020