We glued syringe in backwards. Now what?


#1

Hi Guys,

We glued our syringe piece into the 2.8 endcap backwards (i.e. plunger inside the main tube instead of outside.) I’ve already ordered a replacement 2.8 endcap kit, but I want to temporarily seal the hole so students can go ahead and test the ROV in the pool.

Is there a way to seal the syringe hole securely enough to waterproof the ROV to a 25’ depth? That would survive any pool test and even allow for one day of water sampling at a spot on the intercoastal waterway that is 23’ deep. (i.e. the kids could feel like it was a successful build, and I’d replace their endcap after the fact…


#2

Your main problem with this issue is closing tube. That is because without the syringe there will be a greater pressure within the main tube than it is outside. So it might be hard to properly close the main tube. The compressed air inside the main tubes behaves like a spring that wants to pop of the end cap. During a dive this is not so such a problem, as the water pressure acts against the air pressure. However, when you are close to the surface the pressure might cause the end cap to pop off.

Two ideas for using the ROV anyway: Apply the plunger from inside, and close the tube. Then use an additional strap to hold the end caps in place. Make sure this is really tight, so that it prevents the end cap from moving. Also you can try to minimize the pressure that results from closing the tube. For example heat the main tube and the air within, so once it cools down during a dive the pressure also drops (a little). For example you could use a hairdryer etc. If I guessed it correctly from your story, you could also make an additional tasks for your students to calculate the temperature difference in order to compensate the resulting pressure. (eg. warm hair in side tube --> close tube and create pressure --> let air cool down and minimize pressure) This can be easily calculated :slight_smile:


#3

Hm, not sure I would want to put the plunger on backwards, since the pressure at depth will push the plunger inwards.

For doing basic testing in a pool or similar you could just epoxy the whole endcap, and you should be good to go. As @Fe3C explains, you might have some more resistance when closing the endcaps, and you need to make sure the straps are properly tightened. Once I left the ROV running during a faire. It was sitting on a table, hooked up to a monitor so people would see themselves as they waked passed. After a few hours of continous running (no use of motors means you have quite a few hours of runtime), I noticed the endcaps had “popped” open, which is due to the heat generated by the electronics while running. As Lukas sais, it could be a good experiment for the kids to calculate how much the pressure would build up inside the tube at different temperatures. Maybe even have them publish the results on this forum.

Normally what we do in order to prevent the above problem is to apply a little bit of vacuum pre-dive, using a small handheld vacuum pump like the one recommended in the dozuki. This is where the plunger comes into play, as it allows you to connect the pump to the hole in the endcap, suck out some air, and then rapidly exchange the vacuum pump with the plunger.

You could off course also swap out the syring / plunger assembly with something a bit more sturdy, as the type of valve seats that are used in the first stage of scuba regulators, but this would require drilling a sufficiently big hole, and using a threading tool to make the threads.


#4

Thanks, guys,

I appreciate the responses.

Fe3C said
If I guessed it correctly from your story, you could also make an additional tasks for your students to calculate the temperature difference in order to compensate the resulting pressure. (eg. warm hair in side tube --> close tube and create pressure --> let air cool down and minimize pressure) This can be easily calculated

this is a great idea. They can certainly do the calculations, and it would be a great chance for them to see a real world application of some math

Roy_Petter_Dyrdahl_T said:
For doing basic testing in a pool or similar you could just epoxy the whole endcap, and you should be good to go.

Wonderful. That was our first idea… A variant on that I’ve thrown around is candle wax

Roy_Petter_Dyrdahl_T said:
You could off course also swap out the syring / plunger assembly with something a bit more sturdy, as the type of valve seats that are used in the first stage of scuba regulators, but this would require drilling a sufficiently big hole, and using a threading tool to make the threads.

OK, this is an AMAZING idea. We’ll be pool testing at the local dive shop, so I’ll be sure to discuss this with the owner while we are there.


#5

As @Roy_Petter_Dyrdahl_T noticed correctly make sure that water pressure does not remove the plunger if applied from inside (can be calculated too of course ;))

I hope that solution works for you. Make sure to keep us updated about the progress and the results. Maybe this can be helpful in future.


#6

You only need one of the plungers to work, so if only one is in backwards then epoxy the other.

If it is both, then I recommend filling both with epoxy, but do it the night before or earlier. Don’t wait until you are at the dive shop as the epoxy takes time to cure. Use black electrical tape on the bottom side to stop the epoxy running out.

Once the plungers are filled, it will be hard to keep the end caps on, so do the following. The night before, run the electronics for a while with the tube slightly open. This will generate lots of heat. Then turn off the electronics, fit the end cap and put in place in the ROV with the strap around it. As the tube cools it will pull the end cap into place. Wet the strap and tighten it some more as it comes loose when wet. When you dive, the water will keep the tube cooler and the pressure will keep the end cap on.

It’s good to add a packet of silica gel inside the tube too, so you won’t get fogging on your dive. Certainly do this in a non humid environment, not at the side of the pool!