measuring diff. pressure across the scrubber

Faceless

Reading through your FMECA Volume 8 report i've encountered very interesting idea you're going to implement for monitoring scrubber life: a measurement of differential pressire across the scrubber.
I wonder, how sensitive pressure sensor would be to measure such a small pressure diffs, and how this difference is changing during Sofnolime (or other "magic powder) exhaustion process ?

jradomski

I would think measuring the pressure differential across the scrubber is more to measure breathing rate.. since there is a small resistance in the scrubber its a good point to be able to see when a person inhales/exhales.. If co2 is rising, the persons breathing rate should be rising as well and give the diver a warning to slow down.. There are plenty of sensors that have the resolution to see this..

__________________
Joe Radomski
CCR Trimix Instructor Trainer
ANDI Instructor Trainer Director #10

All posts are personal opinions and DO NOT reflect any affiliated agency unless specifically stated.

Faceless

Sure, WOB measuring is critical requirement (at least when i've looked at video from David Shaw last dive with comments from his buddy i've been impressed by this fact), but change over scrubber resistance (if any) occuring during scrubber life cycle would be another safety factor, which can be taken into consideration.

AD_ward9

The scrubber life monitoring is quite different to the WOB monitoring, but WOB happens to use the same sensors.

To monitor scrubber life, our algorithm looks at scrubber temperature using 16 NTC resistors in a scrubber stick and compares that with the heat i should generate based on the gas composition, breathing rate (from which the rate at which CO2 is being generated can be estimated), and flow.
From this data we know what temperature the scrubber should be, and what it is. The difference is a measure of scrubber health. Taking lots of readings one gets a quite accurate result of the scrubber health, and integrating the data, scrubber life: the scrubber is analagous to a battery.

Moving to WOB: for the above scrubber life monitor, breathing rate and flow are measured using two pressure sensors, as that is more reliable than using a differential sensor (the sensors are more reliable). This means we have a source of data we use for monitoring breathing and WOB dynamically.

On the scrubber pressure questions:

1. The scrubbers will only work with Micropore cartridges. The price of these is coming down to equate to Sofnolime on a per hour basis. This decision has many benefits, including completely avoiding tunnelling, dust, all risks of caustic cocktails, as well as giving much better scrubber life and lower breathing resistance. They do require a water pump as the water that is generated by the reaction to absorb CO2 is not absorbed by the lime.

2. The pressure sensors have ample resolution to measure breathing cycles for CCR use: we use two sensors and subtract one from the other digitally. Their type is on the circuit diagrams. We are not trying to chart the exact breathing waveform, just get the tidal volume to within 20% and the breathing rate (accurately).

Hope that answers your query.

As Joe commented, there are many sensors with the resolution to do WOB monitoring, and it is fairly cheap.

Cheers,

Alex