4th October 2006 19:52
Homebuilt Meg Radial Scrubber
Homebuilt Radial Scrubber for the Megalodon
When my shiny new Meg was delivered in early September 2005 it came supplied with a perfectly good, well thought out axial scrubber. I worked hard to learn the unit specific drills and procedures in my cross over course, including the care and feeding of said stock scrubber. Radial scrubbers weren’t even on my radar screen. That caused me to miss a golden opportunity at DEMA 05 where ISC had a great display of the many different systems they’ve tested lined up for all the world to see. I looked them over with interest, especially their prototype radial and the Cyslunar unit, but did not give them the intense scrutiny I would have had I been planning to build my own. Oh well- so goes life. The seeds of an idea had been sown, however, and the damage had been done.
The stock Meg scrubber is very well constructed. It is easy and relatively fool proof to pack and has very low WOB for an axial unit, so why want something else? Several reasons: 1. Stock unit has 3 hour recommended burn time. That’s good for most single days of diving, but not for an intense entire weekend or an easy full week. 2. Low as the WOB is, a radial will be less. Yes, some folks debate this. They are surely entitled to their opinion, but I believe it. 3. Inside the can there’s a LOT of empty volume under the axial. Some is needed for the clever spring/ bar mechanism that keeps the ‘sorb well packed. Some is there to allow the breathing gas to flow around the end of the scrubber and change direction. Some is there just taken up with a spacer so the canister is also able to accommodate a Cyslunar scrubber. I wanted to fill it with ‘sorb! 4. Last but not least, I enjoy designing and building useful things.
Along about January 2006 I began design. The project was under way.
This is not what I’d call a first time homebuilder project. That said, if you have the shop skills, go for it! The essential tools needed are a lathe and a decent drill press. Not absolutely necessary but highly recommended is a third tool, a slip roll. The process can be broken down into seven basic sub groups plus a few odds and ends. They are:
- Inner Screen
- Outer screen
- Final Assembly
Build the scrubber parts in any order you choose, but the final assembly prior to building the spacer is important for optimum results. The working drawings are here for you to use as you see fit. Included pictures will hopefully make the descriptions more clear.
All materials and sources will be mentioned as we go along, plus be summarized at the end. Also at the end I’ll touch on how I go about packing and using my radial scrubber.
Some numbers: As of this writing, the two prototypes have somewhere over 100 hours of dive time between them. More than half of the dives were trimix dives, the deepest in the 250 ft/ 75 m range, all trouble free. The longest any single scrubber pack has been used is 9 ½ hours. Again, trouble free. Most dives have been in cold (15 deg C) to very cold (4 deg C) water. The design holds 7.5 lb/ 3.4 kg of 6-12 or 8.1 lb/ 3.7 kg of 8-12.
A few words about layout, tolerances, and craftsmanship: Most of the parts here get drawn onto the raw stock prior to transforming it into dive gear. I often use drafting tools directly on the stock. Other times I’ll find a pattern made from stiff paper- like a file folder- useful. However you go about it do it with care and be as precise as humanly possible. The old carpenters’ adage of “measure twice, cut once” is well employed. Don’t permit yourself to build junk. As far as fit goes, keep tolerances as tight as you can without going over board. More decimal places on the drawings usually implies more precision is required. If you have a finite amount of patience for being precise, spend it where O rings must fit and something being too tall will prevent assembly.
A few words about bonding: The adhesive used throughout this project is 3M brand Scotch Weld #2216.
I’ve had excellent results with it in several projects. Guys: read and follow ALL package directions. Anything you expect it to stick to MUST be clean, dry, and free of any and all contaminates from dust to your own oily fingerprints. Also, any glaze or shine on metal or PVC must be thoroughly scuffed and removed with 100 to 200 grit sandpaper. An initial cleanup with soap and water and a final cleanup with acetone have so far worked well for me. Wherever I say “prep for bonding” this entire process is what I’m referring to.
The first part I designed and built was the top. It is constructed from 1 inch thick gray PVC sheet stock.
This is the working drawing:
To get started, mark a center and draw a 6” circle on your flat stock. Do ALL of your layout for the inside of the top now before the physical center is machined away.
Rough cut the disc keeping well outside the 6 inch line. A band saw is nice, but careful hand work will suffice. Drill a small hole at the center mark of your disc then bring it up to a ¾” hole using a Forstner type bit. This type drill bit is far superior to an ordinary bit for this application. It will not wander and produces the precise hole necessary for the next step.
To get from a rough circle of plastic to something round in the lathe is not as difficult as one might think if you first build a small tool some would call a mandrel. I made mine from a bit of 1-3/8” brass hex stock that was 3 ¼’ long. 2” of it is turned to as close to 0.750 inch diameter as you can get and then the first 1 ½” of that has ¾-10 TPI threads cut onto it. This simple tool will mount in the lathe’s 3 jaw and allow you to bolt up you rough cut PVC circle and machine it into a precise 6.000 inch disc.
I’ll not try to teach basic lathe operations here, but a few shop notes are in order. Once removed from the mandrel the disc was mounted in the 3 jaw with the pre drawn layout lines safely hidden on the back side. All turning operations for the outside of the top were then completed including bringing up the center hole to the required diameter. The top was then reversed in the jaws and the deep, narrow groove for the outside screen was cut.
Special note: I found I had to custom grind a lathe bit from blank HSS stock to properly cut this groove. I first tried to use a narrower tool and plunge cut from either side of the groove to bring it up to full width. It looked and measured fine on the surface but the tool flexed deep inside the part and resulted in a groove narrower inside than at the surface. That made final assembly difficult to impossible. When I finally figured out what was going on I ground the long narrow .100” wide tool seen here.
I remounted the top in the 3 jaw and cleaned out the groove. Assembly went fine after that.
The three 3/8”- 16 TPI holes are now drilled and tapped. Note: these holes must be both precisely located and as close to perpendicular as possible to prevent problems getting the scrubber's lid on and off.
Now create the 3 kidney shaped holes that allow the filling of the scrubber. The rough stock removal is done with a 7/8” Forstner bit. Be VERY careful locating the pilot holes. The balance of the stock is removed with a 3/8”carbide burr mounted in the drill press and the top slid by hand on a smooth board clamped to the drill press table. This is the trickiest handwork involved in the entire project: The lip around the fill holes is small and you are cutting from the back side where you cannot see it. Grasp the top FIRMLY. Remove small amounts of stock as you work your way to the layout lines. Check your progress frequently.
The last step is the mounting of the 3 stainless steel bolts into the top. They are ordinary 3/8”- 16 TPI stainless bolts, 1” long. Prep the bolt heads, the ¼” of bolt threads closest to the heads, the bolt holes, and a ½ inch area around the bolt holes on the inside of the top for bonding (see above notes on bonding). Mix a small amount of adhesive. Carefully apply some to JUST the ¼” of bolt threads nearest the head. Snug the bolts into the holes but don’t over torque them. Stainless steel ALWAYS wins in a contest with PVC threads! Apply a decent bead of adhesive around each bolt head to lock it permanently in place.
Clean up NOW: This stuff is nearly impossible to remove once cured. The scrubber’s top is finished.
The bottom is a lot easier to build than the top and might be a good place to start if you have not been in the shop for a while. It is constructed from ¾” thick gray PVC sheet stock.
Here is the working drawing:
The bottom is laid out and sawn similar to the top. A ¾” center hole is again drilled. The mandrel is again used to help true up the disc to precisely 6.000 inches diameter. The disc is then removed from the mandrel and mounted directly in the 3 jaw but this time all turning operations are done from one side. The same tool ground to cut the deep narrow groove on the top is now used on the bottom as well. Be careful with the depth of the hole that receives the inner screen. It affects proper fit of the Meg’s head into the top of the scrubber- really!
The final step is to plug the hole made for the mandrel. The plug is a simple affair made from 1” diameter PVC rod. It almost does not need a drawing, but here is one anyway:
Prep the ¾” center hole and its plug for bonding. Bond the plug into the hole from the inside of the bottom. Thoroughly clean up any and all excess adhesive that squeezes out from between the two parts both on the outside of the bottom- just for neatness sake- and on the inside.
Any, repeat, any excess adhesive that cures in the hole that receives the bottom of the inner screen will most likely interfere with the screen’s fit and prevent use of the scrubber. The scrubber’s bottom is now finished.
The inner screen is built from two identical off the shelf parts and one custom PVC part built to mate them. My stumbling upon this stock part in a local farm supply store was, in fact, the catalyst that took me from the “thinking about” stage to serious design work on the entire project.
The two identical parts are replacement filter screens for what is called a “Y-Line Strainer”. These strainers are intended for use with all manner of noxious and corrosive farm chemicals sprayed in the field. They are glass filled injection molded plastic with a stainless screen and are close to indestructible. Perfect for the tough environment inside a scrubber and they only cost $6.85 each at my local farm supply store
Now for the bad news: Since I have a local retail source for these, I’ve not had the need to buy them on line. The good news is that a quick Googol search brings up the usual flood of links that, with a bit of work, eventually produces several sources for on line purchase. Here are the keys to what you will search for: The screen is distributed by the Banjo Corporation and goes by their part number LS-140. It is used in their polypropylene Y Line Strainer, sizes 1 inch and 1 ¼ inch. (The size refers to the size of the pipe thread that connects the strainer housing.) The Y Line Strainers can use any one of several size mesh filters, but the LS-140 is a 40 mesh. That sizing is important because there are numerous other elements for several style strainers out there in all different sizes, materials, and meshes. It can get downright confusing when wading through replacement parts lists.
The filter element itself is nominally 5 ¼” long and 1.7 inches in diameter. The screen is type 304 stainless. It comes with an EDPM O ring already installed on each end and get this- it fits in the exact, and I do mean EXACT to the 1/1000th of an inch- same size hole as is used by the Meg’s connection to the original scrubber. Coincidence? At any rate that happy fact allows me to pull out my center screen for cleaning with ease. I even get 2 spare O rings since the 2 at the middle are removed.
Once you get your hands on your two filter elements you will note that there is a lip inside one end but none inside the other.
The mating piece will connect the lip-less ends. Look inside them and you will see the lengthwise rib that gives the part its strength. The mating piece has slots cut into each end to clear these ribs.
The piece itself is made from PVC rod. Since the injection molded plastic does not bond well I turned to the screen itself to give the adhesive something to “grip”. This is the working drawing for the connector piece:
As I said above, the mating part is first turned on the lathe and the slots are then cut. I used a Dremel to cut the slots, but a coping saw and round file would suffice. Assemble all parts dry to insure a snug and correct fit. The two elements should line up into one straight piece that will become your scrubber’s center screen. It takes each filter screen seating squarely onto the lip of the mating piece for this to happen.
All parts are then prepped for bonding. Even though the filter element’s plastic does not bond well, I cleaned and scuffed it anyway. I figured it couldn’t hurt. The outside of one end of the mating piece is coated with adhesive as is the first ½” of the inside of one filter element. Slide the mating piece inside the element and then carefully fill the mesh of the screen with adhesive- from the OUTSIDE of the screen- everywhere the screen is supported by the connector piece. Repeat the process with the other filter element on the other end of the connector piece.
Clean up any mess or drips, double check that the filters now again align into one straight part, and let it cure over night. Your scrubber’s inner screen is now complete.
The outer screen assembly consists of two stainless steel parts working together, a structural outer shell lined by a screen inner portion. Since both start out as just rectangles no drawing is necessary.
The outer shell is constructed from what McMaster-Carr calls “Type 304 Stainless Steel Perforated Sheets”. The size used is the 20 gauge (.036” thick) 3/16” staggered hole pattern with ¼” hole spacing. For those who care the web catalog says it has 51% open area. The latest McMaster part number is 9358T151. The bad news is they only sell it in sheets 36” x 40”, but look at the bright side: You and a friend can go in on a sheet together, each screw up once, and still have enough material to each build a scrubber! Note that if you can find a source of 304 stainless steel perforated sheet with similar thickness and physicals, it should make a suitable substitute. Just keep in mind that you are entering uncharted territory and minor dimensional adjustments might become necessary.
The inner liner is made from type 304 stainless steel screen, mesh 60 x 40. McMaster-Carr calls it “Strainer Grade Woven Wire Cloth”. It is their part number 9241T34, and has an open area of 49.4%. I bought and tested samples of several candidate wire screens in various alloys and meshes, and this one became a winner hands down for several reasons. First and foremost it is the same alloy as the outer portion. In a corrosive environment even dissimilar grades of stainless in intimate contact can be prone to promoting corrosion if left damp for a long enough time. “Same” alloys from different sources can also, but the risk is minimized as much as can be. This particular screen is also more flexible, so it is easier to force into the shapes needed. Best of all it is by far the least expensive of the various type 304’s available- and you even get more square feet to work with so you and a friend again have some screw up insurance. Should you choose to substitute a different stainless screen keep in mind that this screen has a wire thickness of 0.0060”. If what you choose has a significantly different thickness it will affect the overall height of the scrubber (see assembly section below).
An easy step: Layout a rectangle on your “Woven wire cloth” that measures 19” x 11-3/8”. Cut it out with a sturdy pair of scissors. This part is finished.
The initial cutting of the outer shell rectangle is most easily done on a metal cutting band saw, but hand methods will again suffice. Layout a rectangle 18.25” x 10.94”. Use a square to insure the corners are truly 90 degrees since the perforated metal as delivered is cut by a sheer and may be off just a bit. A trivial note: If you want your scrubber to look like the ISC prototype, the holes line up top to bottom. If you prefer a Cys look, the holes line up around the circumference. I went for the Cys look. Clean up and true up all cut edges with a sanding block, belt sander, or file. It’s now time to literally rock and roll.
The flat rectangle that becomes the outer shell needs to be rolled into a cylinder. As with many tasks there is a quick & easy way that requires a special tool- called a slip roll- or a time consuming hard way that can be done without special tools. I naturally did it the hard way the first time. I recommend easy. Here’s why: When done with the slip roll the cylinder took less than 5 minutes to produce and it fit PERFECTLY the very first dry fit attempt at final assembly. There was ZERO fitting required. Read the directions that come with your slip roll and crank away. Keep readjusting the tool until you get a nice 6 inch cylinder. I found a satisfactory slip roll with a 12” capacity for $115 at Harbor Freight, part number 36698-3VGA . You could also find a friend or sheet metal shop with one, but I’ll use any excuse to buy a new tool!
When done by hand it takes 20 to 30 minutes to form over successively smaller scuba tanks. The cylinder formed is perfectly functional, but if looked at very closely after final assembly you can see it remains a bit “wavy”. The waviness needs the deep narrow grooves of the scrubber’s top and bottom to force it into final alignment, resulting in a bit more effort required at final assembly. It also requires a LOT of final fitting to get the cut edges that meet each other (the edges that run from top to bottom) to match up precisely and look good. It took several hours of tedious assemble, mark, disassemble, file, assemble, check, remark, disassemble…….. You get the idea.
The by hand method is pretty simple but if you do not have strong hands it might not work for you. Gather several scuba tanks or pipes with various diameters.
The smallest needs to be in the 3” range. Go to a flat, sturdy work surface and put on a pair of leather gloves. Grip the perforated sheet FIRMLY and roll it around the largest cylinder. Pay special attention to the edges or they will remain somewhat flat. Repeat over successively smaller tanks until your rolled cylinder springs open to about a 6 inch diameter when released.
Choose the easy or hard way. The outer screen assembly is now finished.
The last section consists of the lid proper and its associated parts. The lid is built from ¼” thick PVC sheet. The other parts are the closure wing nuts, the D rings and their attachment hardware, and finally the foam packing pads. I’ll talk about each as I go along.
The layout and build sequence for the lid proper is similar to the top and bottom. Here is the working drawing:
The disc is rough sawn out. A center pilot hole is drilled and then brought up to ¾” with the Forstner bit. The disc is mounted on the mandrel and brought to the required outer diameter (see note below). The disc is removed from the mandrel, remounted in the 3 jaw, and the inner hole is brought up to the required diameter. The lid is now removed from the lathe and the three ½” holes and two #8 holes are now drilled where indicated. Countersink the two #8 holes at 60 degrees just deep enough to prevent the heads of your machine screws from protruding (see D ring attachment below).
Special note: The dimensions on the working drawing will work for the rough cut but your actual goal is for the lid to have just 0.001” to 0.002” clearance with the scrubber’s top on both the lid’s inner and outer diameter. Measure your finished top and adjust your dimensions accordingly. This fit reduces gas leakage to effectively zero at the differential pressures we are working with here and eliminates any need for the hassle of O rings.
The D rings used are ¾” rings. Stainless steel is preferred over plated, of course, but can be very hard to find in the ¾” size. I finally found an on line source for stainless at Seattle Fabrics. The rings are attached to the lid using nylon cable clamps in the 1/8” size, available at any electronics supply store. The clamps are secured to the lid using 5/8” long #8-32 stainless countersunk head machine screws secured with stainless fiber locknuts.
I spent a lot of time considering just how to secure the lid to the scrubber’s top. Ordinary wing nuts were too wide. Regular hex nuts in either stainless or plastic would require a wrench be handy any time the scrubber was repacked. What I needed was some sort of wing nut with a narrow profile that also would strip before damaging the stud in the top of the scrubber but still be strong enough for the job. What I finally settled on jumped out at me one day as I wandered through the plumbing section of a local hardware store.
The wing nuts are nylon and are intended to be used to secure replacement toilet seats to the bowl. (OK, you can stop you sniggering now….) They are tough, the correct thread, and by necessity totally impervious to corrosion. They just needed one minor modification to make them work. The cone on the bottom (intended to be kind to the china bowl) needed to be machined away. Once the 3 jaw was swapped out for the 4 jaw chuck it was a quick and easy job. Since they come in a package of 2, I went ahead and modified the fourth nut as a spare.
The last scrubber parts may or may not be absolutely necessary, but I have not used my unit without them. They are simply 3 kidney shaped pads cut from ½” thick closed cell foam of the type back packers use for a sleeping cushion. The pads are cut with a new razor blade to a size just a small bit larger than the kidney shaped holes in the scrubber top.
They get compressed into the holes at the completion of the scrubber packing process for the twofold purpose of being a secondary gas seal and to take up SMALL amounts of volume due to ‘sorb settling.
The lid and its associated bits are now done.
The final assembly begins with a dry test fit of the various parts without the “woven wire cloth”. A small bit of O2 lube on the inner screen O rings is fine, but be VERY careful not to contaminate ANYTHING you expect the adhesive bond to later. That means clean your hands or, better, change your latex gloves after lubing the O rings and before touching anything else. The object of the assembly is to double check that the combined height of all the parts is correct. The design intends the inner screen to project 5/16” into the scrubber’s top and bottom. Since the top is 1” thick, there should be 11/16” measurable between the top of the installed inner screen and the top of the scrubber. Check that there is. You might detect a scant bit less than 11/16” since the scrubber will be 0.010” to 0.020” taller after final assembly due to the thickness of the woven wire cloth wrapping over one end of the outer shell, see below. If there is a gross deviation, say bigger than 0.035” or so, find out why and adjust it as best you can before proceeding. If the dimension is too small- less than 5/8”- the head of the Meg will not be able to fully seat its second O ring. If the dimension is too large, the inner screen’s top O ring will pop out into the inside of the scrubber, a situation which should be immediately obvious. Also measure and record the overall height of the assembly.
Once you are satisfied with the preliminary dry fit it is time to do a second dry fit that this time includes the woven wire cloth. This fitting serves several purposes. First it gives you a chance to see the fit of the outer shell’s edges and, if you hand rolled it, adjust the fit. Second it gives you a little practice for assembly prior to being on the clock during the pot life of your adhesive. Third it gets the wire cloth rolled over the edge for the first time and puts a fold in the screen. Last it gives you some practice getting the large worm clamps (that keep the shell edges in position) in place. I would suggest doing all of this while wearing disposable latex or nitrile gloves both to keep your oily fingerprints off of the parts now and to give you a feel for your lessened dexterity since you will need to wear gloves to protect your hands during the actual bonding process.
The final dry fit begins with installing the screen/ wire cloth into the deep narrow groove of the bottom. (You could also start with the top, but if things get ruined a second bottom is a lot easier to build than a second top. I’ve done the first bonding operation both ways. It makes no real difference.) The ends of the screen will overlap about ¾” or so inside the groove. Overlap the ends of the top a bit more than needed, to ease installation of the outer shell, and clip them together with a clothespin or similar. See picture. Now install your outer shell with its seam 180 degrees opposite to the seam in the wire cloth. Slip a #104 worm clamp over the assembly and gently snug it down, about 1” above the bottom, until the edges of the outer shell just come together. Do the same with a second #104 clamp an inch or so below the top.
Remove the clothespin from the screen and smooth the screen out evenly inside the shell. Start folding the screen over the top edge of the shell beginning over the shell’s seam. Work your way in both directions from the shell’s seam to the screen’s seam folding and smoothing as you go. When done it should look like this:
Now temporarily loosen the top clamp and install the scrubber top. If you used a slip roll, the top should install with minimal effort. If you hand rolled your shell somewhat more persuasion may be necessary. Resnug the worm clamp and give the entire assembly a few raps on top with a rubber hammer to be sure all is seated. Look inside and check that the wire cloth is still smooth against the outer shell and not puckered anywhere. Again measure the overall height of the assembly and compare it to your earlier measurement. It should be at most 0.020” taller. If more, find out why and correct the problem. If you hand rolled your outer shell, now is the time to work out the final fitting of the shell’s edges.
When you are satisfied with the fit and finish of all the parts disassemble them and prep the deep grooves in the top and bottom, the outer ends of the shell, and the wire cloth for bonding. The wire cloth cannot be sanded, of course, but it can be cleaned. Also, its edge will need to be folded back up temporally to allow disassembly and subsequent reassembly. Take a break- preferably over night- and be absolutely certain all is clean and dry before proceeding. The next process goes nonstop and is a race with the pot life of the adhesive.
It’s time to make the first major bond. Prepare yourself and a work area. Get whatever tools you may want ready. I just use several wood tongue depressors with their ends cut square and some more split to have a narrow end. Put on the protective gloves of your choice. Mix a moderate quantity of adhesive. How much is that? 2216 mixes at a 3:2 ratio. Squirt 3 lines of the gray part about 3” long onto your mixing surface. Squirt 2 lines 3” long of the white part between them. Thoroughly blend the two. The pot life clock has started!
Fill the deep narrow groove of the bottom at least ½ and preferably 2/3 full of adhesive. This is a messy process that is more difficult than it sounds because the stuff traps air below itself. Keep working out the bubbles and adding more adhesive until you’re satisfied. Be certain all adhesive drips and smears are removed from anywhere that will become the inside of the scrubber. Now install the wire cloth as before including the clothespin. Install the outer shell with the same seam orientation as before. Install the worm clamps as before. Remove the clothespin and smooth out the wire cloth. Refold the top of the wire cloth over the edge. Check that it is still smooth over the inside of the shell. Again temporarily loosen the top clamp and reinstall the top DRY- no adhesive. Resnug the top clamp.
The most tedious part of the bonding process comes next. It could be argued that it is what separates the craftsman from the hack. Do no forget you are in a race with the pot life of the adhesive!
There will almost certainly be some gaps around the outside of the bottom that are not completely filled with adhesive. Now is the time to apply more in very small dabs to fill the gaps. Also at this time it will be necessary to be sure each and every hole in the outer shell that is at the mating line with the bottom is filled with adhesive. Most will be filled with an air bubble. You need to pop the bubble and fill the hole with a tiny dab of adhesive. Use a toothpick or a long thin sliver of wood and have “fun”. When you’re finally done clean up all drips, smears, gooey fingerprints, and adhesive “hairs”. Acetone and paper towels do a decent job. Remember, this stuff is nearly impossible to remove once cured. Put a flat board onto the top and 20 to 30 lb of lead weight on the board to insure all is and remains seated. Let cure over night.
Bonding the top comes next. Remove the bottom worm clamp. Slightly loosen the top worm clamp and pop the top off of the now cured bottom/ shell assembly. Prepare yourself, your tools, and your work area as before. All set? Mix more adhesive and the pot life race begins again. Fill the top’s groove as you did the groove on the bottom. A special word of caution is in order here. Be VERY careful to never touch or grab the top by the center hole. The reasons are twofold. First there is some lube residue inside left from the first trial assemblies. It will contaminate your gloves and they the surfaces you hope to bond. Second your gloves will leave some gooey fingerprints that will cure into ridges which will cause much grief with your O rings. Be careful!
Leave the top on your bench upside down. Recheck one last time that the wire cloth is smooth against the inside of the outer shell and that it is folded snugly over the shell’s top edge. Now invert the bottom/ shell assembly and install it into the adhesive filled groove in the upside down top. Give it a few taps with the rubber hammer to be sure it’s seated. Resnug the top’s worm clamp and proceed to have “fun” again filling the gaps and the holes in the shell at the line where it mates with the top. When finished do another thorough clean up and again place the board and weights onto the inverted assembly. Again let it cure over night.
After a full night’s cure the weights, board, and clamp can be removed. You can now lube the O rings on your center screen and pop it into the scrubber through the center hole. Carefully work it into the center hole in the scrubber’s bottom and seat it. Step back and admire your new radial scrubber!
Most of the raw stock materials can be purchased on line from McMaster-Carr (M-C). Some like the PVC sheet or stainless screws and nuts almost certainly can be purchased less expensively from a local supply house or hardware store (LHS). The cost savings could be significant since you can sometimes buy small amounts or remnants at reduced cost.
For spacer materials see below.
The last part to build is the spacer that goes into the bottom of the can.
I built mine from a few short lengths of 6 inch PVC pipe but it could just as easily if somewhat more expensively be machined as a single piece from a 1 ¼ inch thick sheet of PVC. The center rod is just a length of 3/16” diameter stainless rod. A short length of any 3/16” or ¼” stainless rod or tube would work just fine. It is only a grab bar to allow easy spacer removal to facilitate cleaning of the can. Here is the working drawing:
The spacer looks deceptively simple but it serves several purposes The most obvious is to bring the scrubber to the proper height to mate with the Meg’s head. As you have most likely figured out by now this design’s height is more or less fixed by the height of the two mated filter element screens inside. Subtler is the fact that, unlike the absolute precision if ISC, we homebuilders are prone, let us say, to larger variations in output. The spacer height can be altered to accommodate such variations. The other purposes are to serve as a centering mechanism, avoiding the need for protrusions on the sides of the scrubber, and to create a small chamber in which to allow the absorbent pads of your choice to collect water.
Chuck up a 6 to 8 inch long scrap of 6” PVC pipe in the 3 jaw and get it running as true as possible. True up the end. Reverse ends of the pipe in the chuck and true up the other end. Take the outside diameter of the pipe down to just under 6.5”; call it 6.495” or so for about 2 or 3 inches of its length. Now clean up the inside diameter to 6.005” for an easy but snug fit on the scrubber bottom. Chamfer the inside corner and cut off a ring 1 ¼” long. Do a test fit into your can. This is the outer part of the spacer. Now cut off a ring 13/16”long. This will become the inner part of the spacer after a bit more work. (Note: If you desire a more precise vertical fit, see the method of determining the alternate dimension to the 13/16” below.) Cut a small wedge from the inner ring that’s just enough to allow it to be compressed and slid inside the outer ring.
Prep the outside of the inner ring and the lower ¾” of the inside of the outer ring for bonding. Bond the inner ring into the outer ring, letting it cure over night. Be sure to clean up ALL adhesive that squeezes out EVERYWHERE. It will cause all manner of grief with the fit otherwise. After cure you need to mill the half dozen shallow grooves pictured to let any water that finds its way into the can to easily flow around and under the scrubber. I used the same burr and board in the drill press that were used to remove the last of the stock from the kidney shaped holes in the top of the scrubber.
The 1/16” dimension on the drawing is only approximate. Eyeballing it is just fine. Install the rod and you’re done.
While the 13/16” height of the spacer should work fine, you can get a “custom” number easily. Place you new scrubber onto a clean flat surface. Place a 3/32” O ring- a #219 or a #220 would work fine- into the center hole of the scrubber to act as a spacer and mount your Meg head in place. It will most likely be sitting on the O ring. Measure the height of the underside of the Meg head’s top lip above the surface. Be sure to take a number of measurements all around the perimeter. If not all the same the head is not quite parallel with the work surface. Carefully move it until it is and record your measurement. Your measurement should be in the neighborhood of 16 ¾”. Subtract your number from 17-9/16”, the internal height of the Meg can. The difference between the internal can height and the height of you head-scrubber-O ring-spacer combination is what you may substitute for the approximate 13/16” spacer height.
Packing Your Scrubber
One significant downside of using this or any radial scrubber vs. the stock axial is that far more attention to detail is needed when packing one with fresh ‘sorb. It is far easier to screw up and permit channeling. There are various techniques. One good general method is described in the excellent new ANDI Megalodon training manual. The way I pack mine is similar, but takes into account the minor differences of my design.
To pack, the first order of business is to plug the center hole to keep the fresh ‘sorb out. Perhaps the easiest way to do it is with a simple rubber plug or bung, size 9 or 10 (yes, bungs really DO have sizes!) from your local hardware store or chemistry supply house. Another way is to put the cap from something the correct size over the center. The cap from an ordinary can of spray paint fits nicely. Another way is to get fancy like I did and machine a plug that looks like a scaled up version of the Meg dust plugs complete with O ring. You can take the dimensions from the Meg’s center or from the end of your center screen.
Pour in ‘sorb until the scrubber is 1/3 full. Tap, shake, and thump a while to settle it. Fill to 2/3 full and repeat the tapping. Now fill it all the way up to the very top of the scrubber, above the fill holes and level with the top rim. Keep tapping and shaking and the sorb will eventually settle down below the level of the fill holes. Put in a little MORE ‘sorb and keep tapping and shaking. Amazingly it too will settle into the scrubber eventually. Keep up the ‘just a little more’ routine until packing is really is complete. This is my rule of thumb: If you think it might be full, it isn’t. If you’re pretty sure it’s full, add some more ‘sorb and keep tapping and shaking. If you’re absolutely certain it’s full, it might be- but try to add some more anyway! Eventually it really won’t take any more no matter how much you tap and shake. Then you really are done. The ‘sorb should be level with the top of each fill hole but with NONE above- not one grain. This is the point where Cys users put their top on, but I go one step further.
I compress the fresh ‘sorb down into the hole with my thumb just a bit, about 1/8” to 3/16” and then put one of the closed cell foam pads into each hole. The pads are slightly larger around than the holes so the fit is snug. I put on the lid and start the wing nuts. I then press down on the lid by hand, compressing the foam pads, and spin the wing nuts down. Do not, repeat DO NOT, use the mechanical advantage of the wing nuts to compress the pads or you will eventually strip the nuts. Keep this routine up- pressing and spinning- until the lid bottoms on the scrubber’s top. Give the wing nuts a final snug and you’re done- maybe. If you can vigorously shake the freshly packed scrubber and hear no noise, you really are done.
One final clean up step I find necessary: After packing some ‘sorb dust will inevitably have found its way into the inside of the center screen. It needs to be removed. The easiest method I’ve found is a shot of compressed air into an inverted scrubber. Unless you’re willing to chance raw ‘sorb dust in your eyes or nose (OUCH!) I’d suggest not using direct lung power.
In use I find the WOB to be essentially nonexistent. I stopped my first test on an exercise bike about 1 minute into it because I thought I’d forgotten to hook something up- the WOB was that low. How long can this thing be used on one charge of ‘sorb? Without trying to be cute, my response is to use your own judgement. I consider 6 to 6 ½ hours to be my limit in 4 deg C water in order to leave a couple of hours reserve for contingencies. It has been taken to 9 1/2 hours with no problems.
The final test I did prior to deciding to write this report was a 250 ft deep trimix dive in 4 deg. C water. At the start of the dive the scrubber pack already had about 4 ½ hours of use. The 1:15 run time dive, in strong current, went fine. A week later that same pack got an additional 1:15 use in slightly less challenging conditions, again trouble free, before I decided to empty it.
One Final Caveat
I know I often address the reader as if he or she were in the process of building one of these. Despite that, this article is at its core a technical report on how I went about building several experimental homebuilt radial scrubbers for MY Meg. They have given service far beyond my most hopeful expectations so I have decided to publish my shop drawings in order that other homebuilders can improve on my work. Any and all are free to use whatever part of this article you desire without charge, but never forget that YOU are the sole judge as to whether the design, materials, and construction methods I found suitable for my use are suitable for your use as well.
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Last edited by Louby Lou; 26th February 2007 at 19:05.