How does diluent affect rebreather scrubber duration?

It has been observed that depth has little to no effect on scrubber duration but Diluent can have a significant effect on scrubber duration in controlled objective testing.

In 2019 we tested a scrubber to the EN14143 standard. The results of 6 tests, 3 at 40 m, 3 at 100 m showed an extended time to breakthrough for the tests done at 100 m using 7/90 diluent as opposed to the 40 m tests using air diluent. This was contrary to the expected results.

Dr. John Clarke offered to do a quick look at the test data and offered this report with the caveat “Lacking critical data on gas and granule bed temperatures, this remains a preliminary analysis of a potential explanation of your data” Dr. Clarke advised, “Instrumented canisters are needed to settle this issue, similar to the way in 2018 Heine and Bozanic instrumented canisters in Antarctica.”

We hope to be able to properly instrument a canister to be able to record “granule bed temperatures” and provide more complete data for analysis.

Cause analysis was provided by

Testing was done at QinetiQ in the UK using EN14143 standard testing.

b) with oxygen and nitrogen gas mixtures at the maximum dive profile (pressure and bottom time) specified by the manufacturer, if not specified at a constant pressure of 5 bar;
c) if required, with oxygen and helium or oxygen, nitrogen and helium gas mixtures at the maximum dive
profile (pressure and bottom time) specified by the manufacturer.

For each test, the absorbent canister shall be prepared in accordance with the information supplied by the manufacturer.
Maintain the exhaled gas at (32 ± 4) °C and relative humidity greater than 80 %.
Test the apparatus with the breathing simulator ventilating at 40 I/min and carbon dioxide injected into the exhaled gas from the breathing simulator at a rate of 1,60 I/min.
Record the time taken for the end inspired partial pressure of carbon dioxide to reach 5 mbar and 10 mbar.