Sensor Response time.

Steve

Both AI and Teledyne quote a response time of 6sec to reach 90% accuracy for their PSR-11-39 and the R22 sensors. Does anyone have a response graph for these sensors? Essentially I would like to know the accuracy as a function of time. How accurate are they expected to be after 0.5 of a sec, 1 sec etc?

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RobertSt

Typical such sensors can be modeled as a 1.order process:

y(t) = 1-e^(-t/T)

(Transfer function h(s) = 1 / (1+Ts) )

You need to find T for the cell:
90% (y(t)) of final output in 6 sec (t)

y(t) = 1-e^(-t/T)
gives you
T = - t / ln( 1-y(t) )

T = - 6 / ln( 1-0.9 ) = 2,6 sec

so plotting it gives:


But, I'm not an expert on O2-cells, so you may want to get this verified by someone who knows.

br
Robert

Lancer4545

Bored again Steve?

Lance

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Steve

nope.

Thanks Robert. That makes academic sense but I'd love some test data if it exists.

What I'm trying to understand is the efficacy of "instant" sensor validation. As far as I know this ability is purported by the manufactures of the Poseidon Cis Lunar VI and the Pelagian.

A few people have assured me it works but I'm looking for scientific data rather than anecdotes. My gut is telling me that a quick burst of gas is going to change the sensors but without doing it for at least 5 seconds you are getting an analysis with a high degree of error.

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deanc

Steve, a little funding if you're that way inclined have a play yourself :-

myPCLab Data Acquisition Module
Dual DC amplifier and Voltage Divider

Try a new cell, importantly a bunch of crappy old cells too !

Robert (Mr Laplace) you scared me .

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Rebreatherlab

A quick burst of gas will not tell you much. A slow inhalation over 10 seconds will put the Po2 in a ball park area, just as a couple of loop flushes will, but consume less DIL, so the idea is that you can afford to make the check a few times per dive. I personally think checking cells say three times per dive even if they all agree could be a life saver in a longer perspective.

Andy

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dave t

Andy can you expand on that? how does flushing with dil (which will have a po2 lower than setpoint...probably) prove the cells are ok?

thanks

Dave

Rebreatherlab

Dave,

There is no magic here. The only thing you can check with a dil flush or instant cell validation is that the cells makes sense at the depth and FgO2 you expose them to. The only difference is that our system requires less DIL and the point I am trying to make here is that instead of a button push and nozzles directed AGAINST the cell membrane, (Cis Lunar MK5P) that likely would expose the membranes to an elevated pressure, our way of controlling the speed of gas via inhalation control (nozzles at 20 degree from membranes and injected via ADV), is easier and with some practice tend to give a more accurate result.

For checking cell liniarity and ability to read correct above the set point you will need to go to a depth where the DIL Fg02 x ambient pressure should give 1,6 Po2 (or what ever you want to check them to), or expose the cells to pure O2 at 6m at the beginning of the dive.

That the cells CAN read higher than set point is tested with every manual addition as the Po2 typically over shoots for one or two breaths before settling at set point, but of course this does not prove they are accurate, just that they are not voltage limited at set point.

Andy

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dave t

Ok thats what I thought. Actually I really like the way the Pelagian flows dil over the sensors faces, to my mind its greatest atribute is to help keep the sensor membranes free of water droplets.

thanks

Dave

diverreb

Nice feature on the HammerHead Electronics.... They have an FO2 function that shows the correct FO2 for your Dil at current depth.

Do a good Dil Flush & the numbers should closely match your handsets. If not, you've got a problem.

I know you can figure out what it should be, but it's a whole lot easier to just push a button and see what it is.

Richie