Easy Meg Calibration Revisited

Easy Meg Calibration Revisited
or
Even EASIER Meg Calibration on One Liter of O2


Most divers seem to be always changing their rig's set up in a never-ending quest to make the overall package lighter, tighter, and more reliable. For me, I confess it is almost an addiction. It's been close to 2 years now since I posted the calibration plug article, and all who have used one in the field seem quite pleased with it. The following short article shows how to make this small bit of the system even easier to use and more bullet proof in transport.

The polycarbonate flow orifice is far from fragile but it can be broken by baggage gorillas or falling tanks. I was sitting at the workbench a few months ago, sketching up various ideas for protective housings, when I dawned on me I'd already built one. The flow orifice can be protected by the body of the plug itself. This set up uses the original plug with NO changes whatsoever and reduces the parts count on the remaining bits. Beyond the original plug body and its O rings, all that's needed is the flow orifice, ½ inch or so of ordinary clear vinyl tubing (1/4 inch OD x .17 ID), and a 1/8 inch MNPT-to-BC conversion fitting. Finally, a small dab of 3M 5200 sealant will be needed as well.



Let's get started

First step is to make a slight modification to the flow orifice. Several small holes need to be drilled in the side of the OUTPUT barb (down stream of the arrow) since the end of the barb becomes blocked once installed. I drilled four holes. Idea is to have enough area on the sides of the barb to let the low flow of O2 proceed unimpeded while retaining most of the strength of the part. Here is what the barb looks like once modified:



The BC Coupler

Your new 1/8 inch MNPT BC connector should be thoroughly cleaned. Ultrasonics are not absolutely necessary, but that is what I used. I then cleaned the last inch or so of my vinyl tubing with acetone. When both parts are clean and dry, a SMALL amount of 3M 5200 sealant (I used the fast cure) is put on the last ½ inch of the outside of the tubing and it is slipped into the inside of the threaded end of the BC fitting.


After the sealant completely cures (overnight at a minimum), the tubing is cut off flush with the end of the fitting using a NEW razor blade.


Look carefully down the inside of the tube to be certain there is no extra sealant oozing into the gas passage. Remove any you see with a suitable pick.


Final assembly is simple. Install the INPUT barb of the flow orifice into the tubing bit that is now safely sealed inside the BC coupler. Be certain to completely seat the orifice because overall length of the device is a factor. The fit is TIGHT- slightly wetting the barbs helps ease installation.

Flow Testing

Simple good sense dictates that you flow test your new device before final assembly into the plug. Connect your rig's O2 regulator to your O2 tank. Connect the regulator's O2 manual add BC coupler to your new flow device and then connect the device to a flow meter with a short length of the same vinyl tubing you used inside the BC fitting. The output barb of the device can still have vinyl tubing slipped over it; be certain the tubing covers all the holes you drilled. Pressurize the regulator and check the reading on the flow meter. A brown (.003) orifice should read in the 0.9 lpm range. A purple (.004) orifice should read in the 1.2 lpm range. It need not be exact- ball park is just fine for this application.

If you cannot get your hands on a flow meter, the old trick of timing bubbles will also work. Just fill a one liter bottle with water. Turn it upside down in a sink filled with water. Connect one end of some vinyl tubing to you new flow device and put the other end in the neck of the upturned water filled bottle. Pressurize the system and time how long it takes to displace all the water in the bottle with bubbles. It should take a little less than one minute for a purple orifice and a little more if you used a brown one. That's it.

Final Assembly

Now it's time to put it all together. Screw the new flow device into your PVC plug. As you snug up the BC connector with a wrench, don't go crazy with too much torque. About ¼ to ½ turn past finger tight is about right. The goal is to have the end of the output barb just contacting the bottom of the hole as the device is torqued just tight enough to prevent it backing out of the threaded hole in the PVC plug.


If your parts are made like mine, that's all there is to it; otherwise make small adjustments until they fit as noted. In the unlikely case the vinyl tubing eventually takes a set and tries to loosen, the orifice is prevented from blowing out by the bottom of the hole and gas leakage between the tubing and the inside of the BC coupler is prevented by the 5200 sealant.

Random Neural Firings

This article is intended to be an addendum to the original calibration plug article and all caveats there still apply. I still use my plug with the brass barb input around the shop when I'm tinkering with something and the dive regulator is stowed away, not assembled to its O2 tank. I never seem to put away the medical O2 tank; the reg stays attached. The new orifice integrated plug lives in the kit of "essentials" that goes with me whenever I go diving.

Dive safe!

Ken

You can leave comments and feedback on the thread.