Calibration Back-Pressure and the Megalodon CCR

Louby Lou

Hi Everyone

Just to let you know that uwsojourner has written an article for the library on Calibration Back-Pressure and the Megalodon CCR which can be found HERE

Please feel free to use this thread for any comments, feedback or questions you may have regarding the article and with the new library software you can now leave uwsojourner reputation via the article

Thanks again to Kevin

Cheers

Miss Lou

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Danil

Great work Kevin, thanks.

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Dan Nilsson, Meg 67

jradomski

Hey Kevin,
Good writeup..

The cap on the Oxycheq analyzer is much better designed (it has 4 venting holes) than many others around.. conversely The OMS one I tried only had one.. When I was testing the various ones I had access to it (oxycheq) performed exceptional compared to some of the others..

The hoses used in the meg cal kit also allow a good volume to pass unrestricted as long as they dont get kinked..

Just be aware the perfromance is only as good as the weakest link.. It cant be said that there wount be BP, it depends on ALL the components.

On one anyzer I tested, I cant remember which one, the cap did well but the hose that was supplied was very restrictive and caused almost 1 psi of back pressure, changed to a better hose and there was almost no back pressure..

ANother example is the analyzer that I use, The alpha 1.. The cap is great, there is a larger outflow diameter than an inflow so BP is definately not a problem, but... the stock hose supplied with it causes about .3 to .4 psi of BP! When I disconnect the meter (pull the cap) no change, if I disconnect the source hose BIG change...

ANyone can test their setup to see if their anlyzer or hoses causes a change, while its being flushed, disconnect the anayzer while looking at the mv readings, if there is a change then the analyzer is effecting the measurements, next disconnect the hose and check the same.. no change great, if there is a change you now know what to do..

If you don;t know if there is BP its safest to assume there is...

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

Steve

Excellent work Kevin. Great to see some facts instead of the BS that has been sprouted here by certain members over the past few months . Please say Hi in this thread so I can give you some green.

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Louby Lou

Hi Steve

The new library software that Stu has installed now lets you give green via the article. On the right-hand side should be information about the author and also allow you to give rep as you would do for normal posts.

Hope that makes sense

Lou

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Miss Lou x

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RonMicjan

Excellent job Kevin, thanks for clearing up the confusion. I was sure I wasnt getting any back pressure, as readings didnt change when the analyzer was removed or gas turned off, but its great to see measurable data. Well done, have some green!!

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UWSojourner

It's funny you should mention that. I know the article was too long, but you should see the stuff I measured that didn't seem to really answer the question "Is there back-pressure if you follow the head-only calibration procedure." I can come up with a lot of random ways to get back pressure. The most fun is to block the outflow entirely and see how far you can shoot the end cap . Did you know that if the end cap is only placed over 1 of the sealing o-rings, it takes about 120ish millibar to blow it off?

Anyway, another of the tests I did was to cover-up each of the holes in the Oxycheq analyzer. There are 4 holes with the 3 side holes being a little over 1 mm in diameter and the top hole being about 1/2mm in diameter.

At a flow rate of 1.3 liters/min I get

• 1 large hole covered = 0.3 millibar (0.00027 ATM) of back-pressure
• 2 large holes covered = 0.6 millibar (0.00059 ATM) of back-pressure
• 2 large holes and smaller hole covered = 0.8 millibar (0.00079 ATM) of back-pressure
• 3 large holes covered = 5.0 millibar (0.0049 ATM) of back-pressure

At a flow rate of 3.7 liters/min ( almost twice the max recommended) I get

• 1 large hole covered = 2.4 millibar (0.0024 ATM) of back-pressure
• 2 large holes covered = 3.4 millibar (0.0034 ATM) of back-pressure
• 2 large holes and smaller hole covered = 4.5 millibar (0.0044 ATM) of back-pressure
• 3 large holes covered = 33.4 millibar (0.033 ATM) of back-pressure

From my vantage point 3.3% error is FAR too much. But if a diver is running 3.7 liters/min through the head and allowing the head to vent through only a 0.5 mm hole, they just aren't paying attention.

Bottom line - use the kit as directed and make sure by confirming your calibration that your setup is allowing proper venting from the head.

You can ensure your setup works well by

• Confirming your end cap holes are 1/4" in diameter (all the testing had holes with that diameter)
• Buying a Dwyer Rate-Master flow meter (about $30 I think) and verifying your restrictor flowrate. Personally, I like a rate of about 1 liter/min. It keeps the O2 consumption down because even with a higher flow rate you want to expose the O2 sensors to the gas long enough to stablize.
• Allowing proper venting of the head (the tests had about 0.17" restriction on the outflow end cap due to the inserted hose wall width)
• Or, if you choose to stop the flow of gas just before calibration, a perfectly acceptable good practice, be careful not to allow air back inside the chamber. A hose placed in the outflow end cap would help this situation as more gas would have to be displaced before air would come into contact with the sensors.

Anyway, my conclusion is the head-only calibration is safe and accurate if executed with some thought, is pretty robust to deviations from ideal standards (but not infinitely so), and VERY slick.

caveseeker7

Thanks Kevin, you just earned yourself a blob from me for your article.
Quite an effort which is much appreciated, well thought out and well documented.

Now we would like you to run some comparative WOB tests.

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Cheers
Stefan



"Political Correctness is a doctrine, fostered by a delusional, illogical minority,
and rabidly promoted by an unscrupulous mainstream media,
which holds forth the proposition that it is entirely possible to pick up a turd by the clean end.!"

schford

Blob from me as well - its great to see the time taken over this and the fact that we have some documented results.

Stuart

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UWSojourner

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