When diving with an open-circuit UBA, ppO2 increases with depth. With a closed-circuit UBA, ppO2 remains constant at a preset level regardless of depth. Therefore, standard U.S. Navy decompression tables cannot be used.
17-10.1 Use of Constant ppO2 Decompression Tables. Closed-circuit UBA users must use constant ppO2 decompression tables Oxygen in Nitrogen (air diluent), and Oxygen in Helium (Helium-Oxygen diluent). Closed-circuit, mixed-gas UBA decompression tables (Table 17-14 and Table 17-15) are included at the end of this chapter.
17-10.2 Monitoring ppO2. During decompression, it is very important to frequently monitor the secondary display and ensure a 0.7 ppO2 is maintained as closely as possible. Always use the appropriate decompression table when surfacing, even if UBA malfunction has significantly altered the ppO2.
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