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Ballast tank with water pumps instead of compressed air

I am trying to create a variable ballast tank system using 4 inch pvc sch40 pipe. However, instead of using compressed air, I want to use water pumps instead. Ideally I would like to use one pump per tank, to do the pumping out or removing of the water, and have some kind of vent mechanism to fill the tanks. What needs to happen in order for this to work? I also, thought about some type of check valve to allow water out of the tank, but prevent from refilling through the pump. I have been struggling with this idea and am not getting anywhere in my tests.

I put one hole in the 13’’ pipe and fitted my small 92 GPH fish tank pump securely into the hole inside the pipe, so that it would “pump” out the water. And another hole to “vent”, but instead what happens is no water is removed from the pipe, but instead flows through the pipe and out of the second hole. I feel like it shouldn’t be this complicated, however, I am struggling a bit trying to figure it out. This test as well as putting the pump on the outside and pumping water in works better when its not submerged? Only when everything is full submerged do I have this issue. Instead of the air in the pipe being vented through the second hole, the water just comes racing in instead. Is the pump to small, do I need one more powerful, or is there something else important that I am missing? Should I possible implement some control valves, or something of that nature? I appreciate all the feedback hints and tips, and sorry I am clearly not all that experienced with this type of system.

Also, the tanks are 13 inch length by 4 inch diameter and capped with rubber qwik caps like these at the link below.

https://www.menards.com/main/plumbing/p … 434796.htm

The ROV will have the ability to go to 100’ however realistically I would probably only use roughly 50 as most lakes near me are about max depth of 50 or so. And I know someone is sure to recommend just adding weight to reach neutral or slightly positive buoyancy as a much simpler alternative and you would be correct, however, I really wanted to add this feature for possible helping with weight lifting and to simply solve a problem that I see no one else doing. (I am starting to see why LOL). I may have to just end up going with the added weight to achieve the neutral buoyancy, but it would be really cool if I could find a solution. Thanks.

I also seen in another forum post someone suggesting the use of a 12v peristaltic pump? Would this allow me to fill the entire ballast, or does this as well only allow 75% or so of the tank to be filled without using valves? Thanks for any help.

I think I’m missing something, but I don’t understand how you are adding air to the tanks when you are trying to drain the water out. Won’t you just be trying to create a vacuum in the tanks if you fill them with water and then pump the water out without adding something to replace the water?
What is the problem that you are trying to solve that no one else is doing?

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I wanted to use a 2 way pump in order to pump water in and out, but the way I currently have it was to open vents to flood and use the pump to remove the water? Maybe I am missing the way it works because I have no way of adding air, I thought with pumping it out I would create an air pocked, but maybe thats not the case? Thanks for any help and understanding.

The problem I was trying to solve that no one else was doing is ballast system using strickly pumps, no piston style, or compressed air versions with compressed air hoses running along the tether. I know this is possible, but I am struggling to find information on it as it seems most people are using weights to achieve neutral bouyancy or using alternative ballast systems instead of pumps. Im sure there are people using pump method, but I can’t find any. Below was the closest I could find, and where I was refering to when I said someone recommended in another post to use peristaltic pumps.

I’m just an amateur so I might be incorrect about the physics. All of the air is removed from the tanks when they get filled with water and there is no air underwater so there is nothing to displace the water as it gets removed from the tanks. That should cause a vacuum in the tanks and I assume that it would be impossible for a single stage pump to remove the water from it due to an extreme amount of suction needed. There can’t be an air pocket in the tanks without the air being added somehow.
The tanks were full of air, you removed the air when water was added, and when you remove the water there will be no air or water and that will cause a vacuum because there’s literally nothing in the tanks.

It seems like both the positive buoyancy and compressed air ballast are easier systems to use. There’s probably a reason why no one does it the way that you want to. I’m sure the military submarines would use electric water pumps if they could rather than the inconvenience of having to keep recharging their high pressure compressed air.

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Hi @btrue

Yes, you are correct with the physics involved with that setup- with the pump pulling a vacuum given no other air source. However, there are setups where they have a compressed air bottle attached to the ballast to avoid pulling a vacuum as the pump removes water from the ballast. Have a look at this paper A Ballast System for Automated Deep-Sea Ascents | Request PDF (the authors are nice and will send it to you for free if you can’t find it.)

People in the RC sub-community also use something called a Semi Aspirated System which stores air internally to alleviate the vacuum pressure.

Also, military submarines use a combination of pumps and compressed air systems for their ballasts systems. There are different tanks, like trim tanks, that are pumped, rather than blown like when they blow ballast tanks in an emergency.

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A peristaltic pump is absolutely your ticket. It is a positive displacement pump so it will give you the pressure you need to pump your ballast without the potential for back flow. From what I’ve seen, if you use a peristaltic pump the holding torque of the motor will prevent it from turning backwards and allow for reverse flow- so you don’t technically need a check valve.

This type of ballast is popular in the RC submarine community btw. Might want to check out some info on this page, this guy’s a legend. https://www.rc-submarine.com/

Also, I mentioned the use of NPT fittings. That was specifically in reference to doing stuff with PVC. The Blue Robotics caps won’t accept NPT fittings. IF you use NPT fittings anywhere in your design then you must be liberal with thread sealant (use aerobic resin if you do this). Check out this Parker info on sealing solutions https://www.parker.com/literature/Tube%20Fittings%20Division/Dry-Technology-The-Guide-to-Leak-Free-Connections.pdf.

Parker recommends double ferrule compression lock fittings or fittings with O-rings for use in ROVs for stuff with hydraulics.

Ah, I see what you’re trying to do. As I said in another post, this is a common setup in RC subs. You cannot vent the tank underwater or it will flood. This specific webpage should answer all of your questions. https://www.rc-submarine.com/rc-submarine-technology I’d be glad to help you if you still have any questions. Your limiting factor is going to be finding either:

A) A pump rated for the pressure required for your depth that is submersible, that isn’t oversized for your application and that can handle the depth.
B) A pump rated for the pressure required for your depth that is small enough to fit inside a watertight enclosure.

You might have trouble finding a 12V peristaltic pump that meets your needs. That’s why pumping isn’t a popular method unless it’s on a large ROV. It works fine on RC subs though where you don’t need to dive more than several feet.

Hi @tylerssims,

I appreciate your feedback. Could I use an emtpy small fire extinguisher tank to hold the compressed air and have a y splitter so when I need to blow the water out of the 2 ballast tanks it ideally does so at the same time? Does the fire extinguisher need to have a shutoff valve, or ball valve to open closes when needed to push water out of tanks? Also, when water is in the ballast tank and I push it out with the compressed air, is the air pushed back to the tank when water comes in, or do I eventually run out of air in the tank from ballasting and de-ballasting? Reason I ask is because I’m obviously not very familiar with this and am wondering if I lose air and it’s not essentially recycled, moved from one tank to another how would I be able to tell on the surface when the fire extinguisher holding the compressed-air is empty? I seen a similiar idea somebody used with a compass being in the view of the camera feed, could I do that so i can see the air tanks pressure level from the guage attached to the tank, and if so what fittings could I use to allow completely air tightness and the ability to disconnect and reconnect for adding compressed air at the surface before the dive if I do lose air from de-ballasting? Thanks again and I really appreciate all the feedback, help, tips, and tricks!


Thanks again for the response. I did some research on peristaltic pumps and surprisingly wasn’t able to find a whole lot for 12v peristaltic pumps that had decent flow rates. I found a few in the ballpark of $30-$50 and only had flow rates of 400ml per minute. You said it will be absolutely key for a positive displacement pump and give me the pressure I need, however, according to an online flow rate calculator I found, based on the volume of my ballast tanks with 400ml per minute it would take roughly 6 in a half minutes to go from empty to “full” with just one tank, I have 2? Realistically I probably really wount ever need to pump it all the way full, but seems a bit slow? Any recommendations on this? Also, one last question, I have been looking over the ballast systems over on the rc submarine site you referred me to
https://www.rc-submarine.com/rc-submarine-technology and the “Pressure Ballast System” seems the most practical for me. It only utilizes 70-80% of the tank as the rest is full of air. With this system would I have to leave the tank at atmospheric pressure and no water in, and will the peristaltic pump with low ml per minute give enough pressure to force the water in and compress the air until it can’t anymore? Sorry again about this, I am sure your are busy, but I really do appreciate the help. Thanks,

@tylerssims @btrue

Here is the design of the ROV I am working on, it is mostly built, but the ballast system is not figured out yet. The two PVC pipes on the outer sides are the ballast tanks. Also, because they are PVC

sch40 pipe, am I even able to use with the Pressure ballast system as the pvc says not to be used with compressed air/etc. Or is that if I were to directly connect an air compressor to a valve to pump air that way. The atmospheric pressure debalasting the water shouldn’t be a problem, and as well as the pressure pump pumping water into it or is that not the case? If not, what tank material alternative would you recommend. Thanks again!

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That looks like a cool project and I will be interested to see how it works. Are you going to mount thrusters to it or is like a drop camera with adjustable buoyancy? It seems like it would be better to glue PVC end caps on the ballast tanks to eliminate those possible failure points of the hose clamped end caps.

One possible disadvantage of your buoyancy system compared to the positive buoyancy system of the BlueROV2 is that you won’t be able to just hit the thruster disarm button and float up. It’s extremely useful for me when I get too close to the mud bottom. I will “blast off” and make a huge silt cloud if I use the up thrusters, so I just disarm the system and let it float up about 3 or 4 feet before re-arming and trying again. It will also float to the surface on its own if it loses power.

You can see me having to do it during the very fist weekend that used my ROV at 41:44:


@btrue Very cool video by the way. It will have 3 thrusters on it, 2 on the sides and one for “diving” and “surfacing”. The motors have not been mounted yet, so I could add more and or change the placement still. I was actually really shocked at how powerful the 1100gph bilge pump and 3d printed propellers I printed were. This is the second version (3d model above), the first one was very similar to this design (These attached pictures are the original attempt); however, getting the electronics tube watertight was an absolute nightmare and I never ended up achieving that. Due to a poor design on my part with designing the pipes on the outside, I decided it would be a good idea to 3d print “pipes” to connect each ballast pipe to the electronics tube for concealing motor wires and so the motors could be connected to the motor drivers easier. I tested for leaks and it did not leak terrible, but there was a decent amount of water in ALL 3 tubes, so it was extremely hard to pinpoint which one leaked or if all 3 did because they were all connected together anyway. I decided to go with the 4" watertight enclosure from bluerobotics instead of trying to waterproof myself.

Also, the first one did not have a buoyancy system, rather added weight to achieve neutral or slightly positive buoyancy. With the second version I wanted to take on the challenge of a working ballast, not necessary solely for diving/surfacing, but more so to help with picking heavier objects up as I do not have a claw. But fully operational and have the ability to dive and surface. Also, I planned on programming in a depth hold of some sort, to counteract the ballast tanks and to never add so much water that it’s like a brick is attached and dives like a rocket uncontrollably to the bottom. Just have enough water for neutral buoyancy and have the ability to remove the water to come back to the surface much more efficiently without drawing unnecessary amounts of current from the vertical thruster as it struggles to surface with the payload. Hope that makes sense. I appreciate the feedback and enjoy these kinds of projects, just struggling a bit, so any help is greatly appreciated.

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Hi @travy641,

If you use compressed air in your system, then you shouldn’t use a fire extinguisher to refill with compressed air for several reasons. Practically speaking, you would have to modify it somehow, which could be dangerous. The best option would be to use a small pony bottle designed for scuba diving- they come in small sizes, are less than a few liters, and are designed for diving- which obviously has a similar environment to that of an ROV. They use standard DIN ports, so you can easily purchase a hose and fitting for it.

Designing a compressed air system has its advantages, like reliability, but the relationships between temperature, volume, and pressure can be a headache if you are not familiar with it. Best to avoid it if possible.

PVC shouldn’t be used with pressurized air because of the explosive potential of compressed gas. A failure of the tanks will result in an explosion rather than a leak because PVC tends to shatter. ABS would be a better choice if you use compressed air.

As for the pump flow rate, peristaltic pumps are designed for low flow, high precision applications. This works well for RC subs because they are small and need precise depth control to stay just beneath the surface.

My biggest advice here is to minimize the volume of your ballast, so you have less water to pump. Your ROV should have the least amount of floats possible, so you need less water to bring it to neutral buoyancy.

There are other types of positive displacement pumps. For larger systems, I have seen something like a radial piston pump, which has high flow rates but is larger. Something like a diaphragm pump might work too; you can find small 12V ones that get up to 60psi like this one https://rb.gy/lwhg7d . Anything sufficiently small for your application that has to pump at 60psi (pressure required for 100’) will probably have a low flow rate, though.

As for the Pressure Ballast System, which leaves about 20% headspace (unfilled space) in the ballast tank, I think that will be the best option for your case. It’s simple, and a good starting point if you want to change your design to something more advanced later after proving the efficacy of the current version.

Let’s run through the operation of that system real quick. It looks like your ballast is about 3L, so we’ll use that number. Let’s say you assemble your tanks, hook up a pump to the bottom where you have an outlet (perhaps both tanks have lines running out their bottoms that connect to a common tee fitting which then connects to the pump since you said your using two tanks).

So now your tank is on the surface, full of air at atmospheric pressure (14.7psi). You turn on the pump. The pump is going to bring in water, remaining on until filling 80% of the tank.

The 3L of air in the ballast tank must now compress into 20% of that space to make room for the water. 20% of 3L is 0.6L. So the math says that P2=(V1)(P1)/V2. Thus, (3L)(14.7psi)/(0.6L)= 73.5psi. That means that your pumps need to be rated to at least 74psi to compress the air into that headspace. If you want to reduce that required pressure, you can increase the headspace (use more than 20% space for the air) and/or decrease the size of your ballast to less than 3L.

Hope this helps!

Hi @tylerssims

That actually makes a lot of sense. With the high pressure system using pumps to compress the atmospheric pressure into 20% of the tank, are there any pumps of the diaphragm, peristaltic nature that can be submerged? The current design just doesn’t account for a dry compartment outside of the electronics enclosure, and that will be almost entirely occupied with electronics. So, stuffing the pumps in there just wount be possible. There might be a very slight chance one could fit, but I feel like thats a recipe for disaster if the pump or one of the fittings were to fail.

Is there any specific material that is easily accesible for creating the ballast tank, because like you said pvc can’t be used with pressure applications, and at the moment thats what they were going to be before I realized there would be some significant pressure involved? Also, two separate pumps with two separate connections into two separate tanks (pump per tank)? Or one pump with two tanks connected together with the T configuration? If that is the case wouldn’t the 73.5 PSI double to 147psi? Each tank requires 73.5 right? Thanks again sorry for the headache but I really appreciate it.

Hey @travy641,

It might be tricky finding a small submersible pump rated to that depth. You could definitely find one rated for 10-20’, but 100’ is not common. If I were you, I would purchase a submersible one, and then take it apart and fill it with oil.

As for materials, either ABS, PE, or HDPE are ok for use with compressed air systems (The Best Piping System for Your Compressed Air System). Just make sure to check the correct pipe schedule for your given diameter and material (there are different types of ABS, for example). This is important because not all pipe is made for pressure applications. I know Grainger has PE pipe, but it is $$$. You might also want to check the McMaster Carr website.

If you hook up both tanks to a T junction, and then connect it to the pump (so hooked up in parallel), then the pressure requirement stays the same, but the flow requirement doubles. In other words, you will still only be dealing with the 73.5psi of pressure, but you will pump it twice as slow. If you hook up a pump to each tank, then you will pump it twice as fast.

Nice lab/ workshop you got there btw. I’m jealous of your long table; mine’s a big square!


Thanks. The workbench/table top is not as clean at the moment as I would like, but it is nice to have an area to work on things.

Ok, I feel like my questions just keep coming, but ill try to finish them in this post. I would have to get a submersible pump of the paristaltic, or diaphragm category in order to use outside of a water tight enclosure? Or could I get a regular submerisble pump that is not of the positive displacement pump category? The regular submersible probably wouldn’t work as it is not a positive displacement pump and therefor does not list any pressure stats, as it does not produce any, is that correct? And would I need a solenoid valve or electronic control the valve of some sort to prevent backflow? And what aboit waterproofing of that as well? I also noticed that a lot of the peristaltic pumps do not list any pressures as well? The diaphragm pumps all seem to list working pressures though? Also, Is it possible to waterproof a diaphragm pump, and or fill it with oil to not only waterproof it but allow it to run at depths outside of a water tight enclosure? If it is possible and you know of any resources on that, that would be greatly appreciated because I always hear about filling with oil for Deepwater operations but I’m not familiar with how they do it. The hopefully last and final thing is I noticed the diaphragm pumps are not reversible so how exactly would I release the water when I want to empty the tanks, I’m under the impression that normally I would reverse the motors to pump out the water and the atmospheric pressure in the tank would obviously help along with that process? Sorry again for all of the questions, as you can tell I’m not very familiar with this type of thing but want to take on the challenge… Thanks again for all your help!!!

Also, could I use the elastic blatter method placed within those tanks?

I found this quoted " The elastic ballast tank can be made out of many things, balloons (only latex ones are reliable however), emptied medical drip bags, or drainer bags for urine (best in my opinion). The biggest advantage of this system is the ease of gathering all the elements and it’s price. The water still compresses the air here, but in the whole WTC – which means more space – less compression (smaller pressure) and less problems. The pump does not need to be a high pressure one, all of the model fuel pumps will manage. However as the pressure of the air surrounding the elastic ballast tank is much weaker then in a system with a closed tank, we need to use a two way pump to empty the ballast tank actively. Again, if we’re using a pump other the peristaltic, we’ll need a valve, but again as the pressure here is low, it’s extremely easy to make one ourself." From here Technology. – RC Submarine Shipyard – model submarines, parts and knowledge base

Would this setup work with what I have, as far as putting the ballons or heavier dutier style elastic bags within the tanks just on a slightly bigger scale. Osee that they say with this method I dont need a high pressure pump? How do the physics work with this as it tends to be a bit confusing because its the same pipe, but yet an expandable tank within it??? Sorry again for all of the questions, as you can tell I’m not very familiar with this type of thing but want to take on the challenge… Thanks again for all your help!!!

@travy641 no sweat, glad to help. You could always PM me down the line.

Here’s why a positive displacement pump is important. With centrifugal pumps (not postive displacement), the flow varies with changing pressure. Since positive displacement pumps work the other way round, their flow is consistent with changing pressure. The flow rate of a rotating positive displacement pump (like peristaltic pumps) is almost unaffected by the differential pressure, so you get consistent performance across a wide pressure range. (Positive Displacement Pump versus Centrifugal Pump.).

Given that your ROV needs to operate its ballast at unknown pressures (anywhere between surface level and 100’), something like a peristaltic pump will provide consistent performance anywhere in between.

Furthermore, centrifugal pumps usually move water across some elevation change, so the pressure is rated in head (vertical feet). To even find such a pump capable of delivering something like 75psi, you would need a centrifugal pump rated for over 173 feet of head. Most submersible pumps are used for sump pumps, ponds, or boat bilges and thus typically are rated for around 10-20 head feet. They have ones rated to 173 feet but are very big and would be would require a somewhat complex control system to deliver consistent flow across your pressure range. It would be very inefficient.

Plenty of peristaltic pumps list their pressure rating; sometimes it’s given in bar, so you might have to convert to psi or whatever.

And yes, you would need to use two diaphragm pumps if you use them, one to fill the ballast and the other to empty it; sorry, I forgot to mention that. You would also need to run a check valve on each of these pumps to prevent backflow. If you use a peristaltic pump, you probably don’t need any valves.

There is no advantage to that elastic ballast system if you use the same volume tank. They just give more headspace to the water, so the air has to compress less by placing it in their main electronics compartment. You will need a two-way pump either way, because of the differential pressure involved with your depth of 100’.

To reiterate, the elastic balloon is used so you can fill your primary electronics enclosure with water without getting everything wet. Given that in your system, you already need to pack a ton of stuff into the said enclosure in your system, there is no appreciable benefit to this.

An RC sub dives maybe 10’, where the absolute pressure is only 19psi. That means that given the original pressure ballast system we discussed (which had about 74psi of air pressure in the headspace), it would easily be able to push most of the water out against the 19psi of backpressure.

At 100’, the backpressure is 60psi absolute, so the 74psi of compressed air in your ballast will only push a little bit of water back out. The air pressure in your ballast is inversely proportional to the volume, meaning that if you had a 3L ballast with 0.6L of air, after purging 0.6L of water your headspace will double in size, the pressure will halve, and you will only have 37psi of air in your headspace to push the water out. If you only have 37 psi of air, the ballast won’t overcome the pressure differential until the ROV is above 50’. This is why you need to be able to pump it in and out.

If you end up going with a design requiring valves, I recommend using compressionless fittings and 1/4" stainless tubing. It is a very reliable connection, and with tube benders, you can fit the system into a tight area.

Example of a small, very high quality, check valve, Parker 4A-C4L-1-SS, 1/4 Compression Check Valve. Ebay is a good spot for Parker and Swagelock valves if you want high quality hardware.

Let me know if I can clarify, lots to digest there.


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You said if I used diaphragm pumps I would need two, one to pump it in and one to pump it out. I can find diaphragm pumps that work up to 80psi all day long for relatively inexpensive $30 and up like this one for example that is good for 80psi. Amazon.com: bayite 12V DC Fresh Water Pump with 2 Hose Clamps 12 Volt Diaphragm Pump Self Priming Sprayer Pump with Pressure Switch 4.5 L/Min 1.2 GPM 80 PSI Adjustable for RV Camper Marine Boat Lawn: Industrial & Scientific ? encoding=UTF8&aaxitk=c8fb0594ef471a8d14a900a4c22ece4f&hsa_cr_id=9509654330501&pd_rd_plhdr=t&pd_rd_r=61c84f59-c714-4c5e-b8b2-af25e36b94b6&pd_rd_w=6Ia0m&pd_rd_wg=dbEN3&ref =sbx_be_s_sparkle_mcd_asin_0_img

I see the max lift height is only 6.6 feet, would this need to be the 100 feet, or does this not matter as long as it can do at least the 73.5 psi required to condense the tank into 20%? If this will work, the only other issue would be its obviously not waterproof, so would you recommend a way to waterproof/oil fill it or build another water tight container strictly for it (I would prefer to waterproof it, but I’m open to options).

The other thing is I did finally find some peristaltic pumps with some decent flow rates and the 75 psi, however, I almost fell out of my chair when I seen the price lol the one I see was well over a $1000. Also, one last question, me and my dad are having a debate about the “condensing 20% of the tank” he seems to think that by forcing the water in with the pumps rated at, at least the 75 psi that I could still use the PVC as it was not technically adding air pressure, and I beg to differ,. I think it is just like adding an air compressor to it that’s why I needed the ABS, or PE pipe which I can’t find by the way to handle the pressure?? So is condensing the tank the same as adding an air compressor to it in the sense that pressure builds up? Or no because the 14.5 atmospheric pressure stays the same just gets forced into a smaller space? Maybe I did not explain that right. I appreciate all the help, and let me know if that last bit made no sense. Thanks!!!