Hollis Explorer User Manual - page 53
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some of the limitations of the TPM are as follows.
The CO
2
absorbent produces heat when CO
2
is absorbed. However, there
is also a temperature rise even when incomplete absorption of the CO
2
in
the breathing gas is achieved. This is a potentially dangerous situation, as
the system appears to be working correctly as there is still a measurable
temperature rise and wavefront in the system. The human body is tolerant
to only approximately 5 to 10 mbar of CO
2
(ref. CE standards for a life
support system). A well-packed fresh Absorbent Scrubber absorbs all the
exhaled CO
2
for a period of time until an amount of CO
2
starts to creep
through. When this level reaches 5 mbar it is assumed there is no life left
in the scrubber. However even at 5 mbar there is still considerable thermal
activity within the scrubber.
!
DANGER: Be aware that a well packed and well maintained Scrub-
ber is key in achieving a life-support system. The CDM is not a sub-
stitute for good system maintenance and Pre-Dive checks. Always
use your training and discipline to ensure the sub-systems in the
EXPLORER are operating correctly.
Critical components and potential failures are:
• The scrubber seal around the scrubber
• A poorly packed (standard pack only) scrubber
• Used or out of date scrubber material
Although the algorithm that analyzes the thermistor curve is adequately
accurate during diving, if the scrubber is not being breathed on, the therm-
istor's curve will be abnormal. It also takes time for the absorbent to rise
to normal operating temperatures. Therefore for routine use, the addition
of the nitrox injection meter (described in the next subsection) is used to
report the filter (scrubber) percentage remaining to the user.
NITROX INJECTION METER
The CO
2
created by the diver is in direct proportion to the oxygen
breathed. The oxygen metabolized by the body is replaced by the oxygen
component of nitrox injected into the breathing loop. By knowing the vol-
ume and composition of gas injected, the amount of metabolized oxygen
and therefore the amount of CO
2
created can be estimated. From tests, the
duration of the scrubber types has been determined and the correspond-
ing volume of CO
2
absorbed before the absorbent begins to reduce its
effectiveness.
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