breathing / metabolic simulator

Gene_Hobbs

Sorry for the delay in responding to this thread. This is not the paper I wanted to post but it is a pretty good starting point

Performance Characteristics and Basic Design Features of a Breathing Machine for Use to Depths of up to 3000 Feet of Sea Water.
Reimers, SD 1973
RRR ID: 4100
NEDU: 1973_20
DoD: AD0776208

I have been looking for the more recent NEDU report and the Duke UBATS system report but not having much luck. I will update if I find them. In the mean time, I hope this helps some.

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AD_ward9

Sorry, I saw this thread only now.

The answers are:
1. We used to do it by injecting an inert gas to flush the O2 out of the system (via a calibrated mass flow valve), and inject CO2 in a span gas. However, this uses huge amounts of gas if you test at seriously deep depths and causes havock with the temperature control and humidity control of the loop. Just think about it: if your loop has a FO2 of 2% at a PPO2 of 1.0, with a 10l loop volume, then to flush out a 1.78l of oxygen for a PPO2 control test you need to use about 50 times that much helium at surface pressure. Imagine that much gas going into your loop every second or so and trying to take meaningful measurements at the same time, with controlled temperature and humidity: you see why the gas flush method was doomed.

2. We now use a calorifier burning ethanol. This is much more accurate overall. It burns ethanol (consuming O2 in the loop), to produce CO2 and water vapour, just like a person. The RQ in practice from an ethanol burner is around 0.67, compared to around 0.8 for a human: the theoretical level of an ethanol burner is higher than it is in practice. The water vapour helps keep the loop at the right humidity (above 80%). CO2 injection rate is measured very accurately by sampling the gas out of the breathing machine, and automatically by measuring the drop in O2 over time (using a simple bang-bang controller, but with PPO2 varying over a range 1.3 to 0.7 before the breathing machine O2 injector kicks in hard). A final check on the CO2 is that the right amount of ethanol is used. PPCO2 tests are then a piece of cake, and for PPO2 control tests, one simply measures the CO2 going out on the exhale.

Anyone else with a calorifier, we would like to bounce off a few items as we are planning a Mark 2, so please PM me. The main problem is the heat the Mark 1 generates: 680W for a 1.6l/min CO2 rate. The tests are done in a lot of cold water, but we test at CO2 rates of up to 3.6l/min, so this keeps the refigerator going on the chamber at max. Mark 1 uses a flame to do the burning, Mark 2 will use a catalyst to burn the ethanol. Anyone else working on these, we would be most happy to collaborate (so long as it is not all one way).

Calorifiers use very little alcohol (a few milli litres per minute), but the flame size is quite large if you use the burner method. One has to have two radiators in the pipe to the breathing machine to cool the gas down, instead of heating it as it does when using the gas flush method.

Alex

AD_ward9

Just a follow up, we have now gone over to Mark 2, i.e. burning ethanol using a catalyst instead of in a flame calorifier.

The flame calorifier had several problems: all the heat it generated was in the gas, the FO2 it needed to keep the flame alive caused a depth limit using the Rebreather O2 injector or very high local PPO2s using the burner injector, and some safety worries with all the O2 around under pressure. The Mark 2 does not suffer these problems.

The catalyst (Chromium oxide) means that the excess heat is easily conducted away, and there is no FO2 minimum. Chromium oxide works at much lower temperatures than palladium and platinium catalysts, which is very good in keeping down the power consumption of the heaters.

There is a solid fuel option (the common name of which is, solid alcohol), but ethanol vapour does not seem to be a problem.

Alex