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Finally after getting the end caps and their penetrations sealed up and thoroughly leak tested, I was able to finally do the first complete assemble WTC test tank dunk today. Feels like it has been a million years since I started this project.
The first test was simple. Just flooding the main ballast tank and manually trimming the two piston tanks. Well dag nabbit, look ma it hovers at mid depth!
First time I’ve had a WTC that can hover statically. It a novelty for me.
Once the LiPos are charged fully, I plan to do more testing using the gyro trim functions.
All was not totally perfect with the test. I did experience one small issue with venting the main ballast tank that I will need to find a solution for. On the second dive the main tank would only vent about 2/3rds then air exiting through the vent valve stop flowing. It seems to be a airflow/hydraulic issue. The water column above the vent valve opening (which is remotely mounted lower than the top of the tank) appears to create an equalized pressure with the air trying to escape. I added a tube extension to the vent to bring the outlet up higher. That helped a bit but was not the silver bullet.
Anyways, I’ll need to solve this issue.
Nick
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Six months of hard dedicated development ending in 15 minutes of free time to do these tests and documentation that yields 1+ minute worth of video..... Yea it’s a labor of love for the craft but sobering at times for the work put in.Comment
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Looks really good Nick! That is quite an accomplishment.Comment
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Great job on your Hybrid ballast system Nick! I really like how well that system operated! You put a lot of time in planning, designing, and development. And it was very much successful!Finally after getting the end caps and their penetrations sealed up and thoroughly leak tested, I was able to finally do the first complete assemble WTC test tank dunk today. Feels like it has been a million years since I started this project.
The first test was simple. Just flooding the main ballast tank and manually trimming the two piston tanks. Well dag nabbit, look ma it hovers at mid depth!
First time I’ve had a WTC that can hover statically. It a novelty for me.
Once the LiPos are charged fully, I plan to do more testing using the gyro trim functions.
All was not totally perfect with the test. I did experience one small issue with venting the main ballast tank that I will need to find a solution for. On the second dive the main tank would only vent about 2/3rds then air exiting through the vent valve stop flowing. It seems to be a airflow/hydraulic issue. The water column above the vent valve opening (which is remotely mounted lower than the top of the tank) appears to create an equalized pressure with the air trying to escape. I added a tube extension to the vent to bring the outlet up higher. That helped a bit but was not the silver bullet.
Anyways, I’ll need to solve this issue.
Nick
Rob
"Firemen can stand the heat"Comment
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What diameter is your vent valve? If it's it's less than 3mm/1/8" then the vent maybe skinning over with surface tension after the first ballast vent/blow cycle.Comment
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The ID of the vent is 1/8”/3mm.
Small steam boilers for RC boats will sometimes get bubbles trapped in their vertical water sight glasses that result in a false water level reading. A simple trick to break the bubbles tension and prevent this from happening is to insert a thin diameter length of wire inside of the sight glass. I tried this yesterday with the vent valve outlet but it did not have the same effect.
Admittedly, I was concerned at the beginning of this project during the early design phase about mounting the vent valve lower than the vent at the top of the tank for this exact reason. There might be a simple fix.
I will need to take a closer look at what may actually be occurring to cause this. Then I’ll know if there is a solution or not. If it is obvious to anyone, please point it out. I’ve been staring at this project for far too long. It’s kind of like you can’t see the forest when you’re in the trees.
NickComment
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What vent valve are you using?Comment
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Steve,
The vent valve is a Clippard Mav-2.
The connection from the vent at the top of the ballast tank connects to the stainless steel barb in the above photo. The brass barb is where the tank is vented when the servo depresses the valve.
NickComment
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Maybe I have created a pee trap like the U-bend on a sink drain by plumbing it the way I have? I might try swapping the tube coming from the main tank with the barb sticking straight out of the back of the valve instead of the barb pointing vertically.Comment
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I am just spit wadding here, but I agree with your assessment. The resistance of air (surface tension) means we need an opening of about 1/8”(minimum). Even with David’s excellent Sub-Driver, there is a fine adjustment of the flap and opening. Not enough opening and the tank does not fully expel all the air. Too much and it will not seal. The simplistic path for air to escape, the better.Last edited by trout; 05-26-2021, 12:15 PM.If you can cut, drill, saw, hit things and swear a lot, you're well on the way to building a working model sub.Comment
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I didn't realise that was your vent. No wonder you're getting trouble. You want the vent directly on top of the tank or higher.Comment
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Is the Mav-2 giving you 1/8 venting internally? many 1/2" water valves don't really open internally the .19635 sq" that gives you 1/2" worth of flow.
Consider a flow check-a timed leak down of a set volume thru an unrestricted 1/8" tube vs the same volume thru the mav-2 depressedLast edited by Bob Gato; 05-26-2021, 01:15 PM.Comment
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Tom, Andy, I agree with both of you.
I did some more tests this morning to see what was actually occurring. Made a couple small changes then did some more tests and observed. That's when I saw what was happening. It was so dumb.... I'll try my best to explain below. Long story short though, problem found, solution to problem tested and the venting works now every time. Still need to decide on how to go about adding a permanent fix.
And now for the long winded explanation:
First here's the changes I made this morning.
Above is the first change that was made. After witnessing short slugs of water inside the clear tubing blocking the venting air from escaping I decided to try stepping up the tube's ID to increase flow by replacing a section with a 3/16' ID piece of hard aluminum tube.
Next I swapped where the tube from the vent connects to the barbed Tee fitting on the back of the vent valve to eliminate the possible U-bend sink drain trap effect I mentioned earlier. I re-attached a short vertical length of clear tube to the brass barb on the vent valve to allow the vented air to be above water level while surfaced. These changes did make slight improvements of the venting but they were not the problem, nor did they solve the intermittent and variable filling of the ballast tank after the first dive.
A quick explanation of the connections to the TEE fitting attached to the back of the vent valve. Center connection is to the vent fitting at the top of the ballast tank. Small tube connection at bottom of TEE is the input from the emergency gas blow valve outlet. Upper tube connection is the input from the low pressure blowers outlet.
Okay now for the dumb...
Here's what I observed happening and what I did to stop it from happening. After doing this the main ballast tank vented perfectly each time. I'll try my best to explain.
While observing the WTC submerged with a completely full ballast tank and with the vent valve closed, I noticed water slowly entering the clear tube attached at the top of the ballast tank and traveling through it to the TEE connection. From there it traveled into the low pressure blower line. During the design process I had anticipated water getting into these lines. I debated whether or not to add a check valve to the blower outlet tube but decided it was not not necessary for two reasons. One any water entering would be blown out during the blowing of the main tank. Secondly I assumed the diaphragm pumps would not allow any flow when they were not being used..... Wrong bonehead.
Back to the water getting into the tubing. Yes it does get blown out during the blowing process of the tank but not all of it. A small amount of residual moisture is left inside of the vent tubing mostly on the inner walls after blowing. However, the more water that floods into the tubing the more residual moisture that will be present after blowing. This is a part of the problem. The next time the tank is vented the residual moisture is pushed through the tubing, collecting into small slugs of water while traveling towards the vent valve.
When the tank is vented from the completely empty state, most of this residual water is pushed out through the vertical vent tube outlet attached to the valve but not all of it. As the airflow being vented from the main tank becomes a less and a weaker flow as the tank fills, the small amount of residual moisture still present creates enough of a blockage in the system that air trying to escape is not enough to push the last of the moisture from the vent valve.
Here's what I did to stop this from happening.
Starting fresh with a dry system, I pinched closed the snorkel valve tube leading to the low pressure blower intake. Then the main tank was flooded completely. Once the tank was flooded, no more water would enter the tubing at the top of the ballast tank vent fitting even if the vent valve was left open and no water would flow into the vertical vent tube outlet attached to the vent valve. The system was then blown as I stopped pinching the snorkel tubing. I repeated this multiple times and the system dove consistently each time with not moisture in the tubing.
Possible longer term fixes could involve adding a check valve on the snorkel intake. Or possibly adding a float valve to the snorkel intake. A sledge hammer approach would be to not mess with the snorkel tubing at all and add another small low pressure blower to the vent valve outlet tube to pull the air out of the ballast tank but that is more complicated and not necessary.
Anyway this is what I observed happening with the system this morning and what I tried that worked. I'm definitely open to hearing suggestions from others.
I agree the best practice is to have a simple free flowing vent valve at the top of the tank but at this stage that would require a substantial redesign and now that I know what was causing the problem and so far what I quickly and crudely did to make the system function as planned.
Nick
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