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Fibonacci
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First Name: Graeme
Location: Melbourne, Australia

Magnehelic techniques?

Mon Jun 05, 2017 5:22 am

Spurred on by my modest success in restoring my USD Aquarius and the Scubapro 108 I've been thinking about getting a Magnehelic!
I have four Atomic Aquatics regs with octos that need maintaining, so been working my way toward that goal...

I know I can do a rough check in a basin of water, but I'd like to see actual data for each reg I'm rebuilding... and have a known point to return to if needed after each service.

From what I can determine Dwyer have the market pretty much sewn up, and the Model 2304 (Differential pressure gauge, range 2"-0-2" wc minor divisions .10) seems most suited to SCUBA use.

So far so good...

However, I'm having a bit of trouble working out the correct way of attaching the Magnehelic to the regulator... I've seen a couple of what looked like commercial accessories involving a mouthpiece mounted to an oval tube with a serrated spigot, but can't seem to locate one to buy online. Also several home made versions which just seem to be cobbled together with whatever spare bits were lying around the workshop :wink:

Is the attachment point of the test gauge intake spigot in relation to the mouthpiece critical to prevent excessive fluctuations in reading or not really a worry?

When taking readings what is the best procedure?
Let the LP seat bed in for a certain amount of time to stabilise before checking?
A series of checks then average them?
Take a soft gentle intake of breath for a double hose and a sharper one for singles to mimic their usual operation?
When exhaling I assume the 2"-0-2" gauge can just be left connected, but a 0-2" gauge would need to be reversed to read pressure instead of vacuum?

Be great to see one of Bryan's videos concentrate on using a Magnehelic properly, and what it can uncover as a diagnostic tool... but in the meantime I'd be very grateful for some pointers :)
'A man can never have too much red wine, too many books or too much ammunition' Rudyard Kipling

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Herman
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Re: Magnehelic techniques?

Fri Jun 09, 2017 7:40 am

I have several mags but I think for most people, the 3-0-3 is the best suited for scuba work but some might argue the 2-0-2 is a better choice. The normal values you will be working in are in the +/- 0.75 to 1.75 IWC range which pushes the 2" one up close to it's top range. That said, I use 2-0-2 for most work but I have a 0-1 beside it for use when I am really tweaking a reg to as low as it will go, it's not unusual for me to be working on a reg set to 1/2 to 3/4 IWC which is really too low for most regs. This is more for the experimenter than the normal reg servicer. I prefer the +/- range ones over the single range ones for most work since I don't have to move hoses to switch from exhaust to inhale but there is another reason you may not have considered. Any reg with a ventui can and often times will swing from a vacuum to a positive pressure when set up properly. If you are using a 0-X type gauge, it will register the vacuum part of the inhale properly then peg the needle on 0 when the venturi kicks in. I don't like pegging gauges plus you can't see what the effect of the venturi is. A swing from -1.5 to +1 is not uncommon on a reg with an aggressive venturi. Using the X-0-X types you can look at the entire inhale cycle which is useful when adjusting a venturi to your preference. Some like them to go somewhat positive and others don't, the X-0-X gauge allows you to see both sides of the curve in one inhale. While on the subject of the gauges, another choice is the simple U tube manometer. You can build one out of a few feet of clear plastic tubing, scrap wood and a ruler. They work great, are simple to build and are by default accurate. I have some photos somewhere of one I made years ago if you are interested.

For connecting the mag or manometer to the reg, I use an old mouthpiece with a hole drilled in it and a hose barb inserted into the hole. I have one for single hose regs and another for DH regs. But for quick and dirty checks I just slide the hose into my mouth beside the mouthpiece.

The technique I use depends on what I am trying to accomplish/ see. If I am looking for the exact cracking pressure of a second stage, then I use my highest sensitivity mag and a very gentle, slow inhale to see where the air just starts to flow. I will use this technique when I am tweaking a reg to it's max or if I am looking for small changes. For example, if I am comparing 2 exhaust valves that are very close to the same and I want to find out which is the better of the 2. But again, this is more for the experimenter than a normal service tech since the variations are so small no one would "feel" them in normal diving. For a normal service type check I would do a gentle inhale to see where cracking starts, a gentle exhale to see the opening of the exhaust valve, a hard inhale to see what the full effect of the venturi is , a hard exhale to see what the value of a hard exhale is(basically pushing the reg to it's limits) then some normal inhales to view the overall inhale and exhale cycle. I breath off a DH and a single hose the same way so I don't change the way I check either reg. All the values you get will be somewhat subjective since you are relying on your ability to keep your inhales and exhales constant. This is more of an art than an exact science (unless you use some form of machine to do the breathing) and really, most divers can't tell the difference in a reg unless it's way off.

Unless you are really pushing the limits of the reg, I don't see a need to worry about the exact numbers or to worry about special break in procedures. Just set it up like you prefer and go dive, you can always retest it later if you like....you do have the equipment and skills after all.
Herman

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luis
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Re: Magnehelic techniques?

Fri Jun 09, 2017 8:38 am

What Herman said... plus don't forget to read the manual for the Magnehelic. If you don't have a paper copy, you can get it on line.
On the first page of the manual it tells you that you need to mount your Magnehelic vertically, not laying it down. I have seen that mistake before.

Often, you can find Magnehelics for sale on eBay. I have seen many for about $20 (new, never used). They are a very common instrument use in HVAC and other ventilation and gas processes. Many companies just seem buy too many extra ones.
Luis

Buceador con escafandra autónoma clásica.

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Fibonacci
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First Name: Graeme
Location: Melbourne, Australia

Re: Magnehelic techniques?

Sat Jun 10, 2017 1:49 am

Thanks very much Herman and Luis... great stuff, just the sort of detailed info I was after :)

Looked far and wide for a 3-0-3" Magnehelic but Dwyer's catalogue now just lists a 2-0-2" then jumps to a 5-0-5":
http://www.dwyer-inst.com.au/Product/Pr ... 0#ordering

2302 Differential pressure gage, range 1-0-1" w.c., minor divisions .05
2304 Differential pressure gage, range 2-0-2" w.c., minor divisions .10
2310 Differential pressure gage, range 5-0-5" w.c., minor divisions .20
2320 Differential pressure gage, range 10-0-10" w.c., minor divisions .50
2330 Differential pressure gage, range 15-0-15" w.c., minor divisions 1.0

Found a #2304 on eBay so should arrive in a week or so...
Also found mouthpiece connectors at Scuba Tools
http://www.scubatools.com/p-591-mouthpi ... cetal.aspx

Now waiting, waiting...
'A man can never have too much red wine, too many books or too much ammunition' Rudyard Kipling

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couv
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Joined: Tue Jul 18, 2006 10:26 pm

Re: Magnehelic techniques?

Sun Jun 11, 2017 11:53 pm

As always, great information from Herman and Luis.

Don't forget to mount your IP gauge and Mag in a way so you can see both at the same time. As soon as the IP drops-even a little, that is point of cracking.
A sincere THANK YOU to all at VDH who make this wonderful resource available and to all the thoughtful contributors.

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