Building a Waterproof Case

Once you have your DIY Sensor system, you will need to find a way to encapsulate it so you can deploy it underwater. There are a variety of methods / ways this can be done, allowing for a ton of flexibility to fit your environment, power needs, and budget. Below are some different options for cases. Warning though, these are only intended for shallow water environments (30 ft and less). If you wish to go deeper, and have more pressure, you will need a much more robust case, and make sure that the sensors themselves can survive.

Case Option 1: Hardware store PVC

The cheapest and quickest option to make is using widely available parts from pretty much any well stocked hardware store. This case is fully customizable, in that you can pick any size or length of PVC to suit your needs.

To build, all you really need is a section of PVC big enough to fit your electronics and power source, and then 2 caps to seal it. One (or both) of the caps will need to be something that can be taken on and off; I'd suggesting using a rubber PVC flexible cap like this.

For securing your sensors outside of the cap you can either use a cable gland or you can also seal the sensors in place. Cable glands are a good option for shallow environments, where you also want the ability to remove the sensors from the case. They have a rubber bushing inside that will seal around your sensor as you tighten down the front of the cable gland. The main downside to them is that they can be prone to leaks if you don't get all the seals tightened down well, and they are a bit bulky.

Another option is to caulk the sensors in place. For this you will be permanently sealing the sensors to the case, but this does ensure a good waterproof seal. This can also be the most low profile option as well. For both options you'll need to drill out an appropriate sized hole from one end cap of the PVC case. Then either add a cable gland, or caulk in the sensor. 3M 5200 is a really good marine sealant which makes a pretty permanent waterproof bond around the sensors, if you choose to caulk in everything.

Case Option 1: Hardware store parts. The end caps (B) are made of rubber, and can easily be taken off by loosening the hose clamps. The length and width of pipe (A) used is totally up to your needs.

Case Option 2: Modified Waterproof Box

A more, 'clean' looking option is to modify a waterproof box. There are a wide variety of these out on the market, often they are intended to protect electronics while boating. Typically they are a simple plastic box with an O-ring or other rubber seal around the lid. Because of this seal, they can make for great sensor cases.

To build this, simply find an appropriately sized waterproof box to fit all your electronics and power supply. Then drill out holes on one end, large enough to either have a cable gland, or to caulk in your sensors. From there it is just about getting everything to fit inside the box. This option is great for simplicity (no need to really build anything) and for access (since you can open the lid and get to all your electronics fairly easily).

Boxes like these work decently, or if you want something more robust you can use something like a Pelican case.

Case Option 2: Modified waterproof box. These boxes allow for more access to all the internal electronics. Also they can be easier to waterproof, since the starting box is already designed to be submerged. One thing to note, drilling holes in hard plastic can lead to cracks, so be careful.

Some Waterproofing Ideas and Principals

Some things you may come across when testing and building your case is that: it's really hard to keep water out. Whenever you are adding holes, or modifying sensors, there is a risk for water to get in.

One way to help, is to seal any surface / hole with caulk (or marine sealant) that doesn't need to be accessed. This can help stop some water coming in.

It is also import to make sure you grease any O-ring or rubber seals. O-ring grease will help to keep the rubber from cracking, and the grease itself will also help to repel water and keep things dry.

If you're still worried about damaging electronics, you can also 'pot' some of the sensor components. Potting is a process by which you use epoxy (or hot glue) to coat exposed electronics. By encasing them, you prevent water from ever causing damage. Obviously, this makes those electronics inaccessible, so don't seal up any connections you need to access in the future (i.e. the USB connections on the Arduino's). Potting can be useful for protecting some of the exposed parts of sensors, or some of the other breakout boards being used for the loggers. It isn't a perfect solution, but it can buy you time if your case ever leaks.

PTFE sleeve that can be used to waterproof gas sensors.

Example of a sensor being caulked / sealed in place with marine sealant. Makes for a pretty good waterproof seal into the case.

Example of Gas sensor sealed with PTFE (A) and using a Cable gland (B) and caulk to create waterproof seal into case.

Sensors (A) are running through cable glands (B) to create a waterproof seal. The cable glands are then caulked in place around the plastic case to make fore pretty strong seal. The cable glands also provide some protection / rigidity to the sensors they are holding.

Backside of the sensor picture above. Marine sealant used inside and outside of the case to create a stronger barrier from leaks / water seepage.

Examples of the 2 main waterproofing methods talked about. Top (A): empty cable glands. Bottom (B): 3M 5200 Marine sealant.

Additional Things for Deeper Waters

All of the techniques and methods listed above work well for shallow water environments (less than 30 ft / 10 m). If, however, you are looking into going deeper, or you really want to make sure you don't have leaks there are a few additional things you can do. Keep in mind that you will need to use a very different case design depending on your depth. The PVC and waterproof boxes can work up to a certain depth, but you should do some pressure checks for leaks / strength first. Also if you decide to keep using PVC, DO NOT use the rubber end caps for anything at depth. You will want to use something more solid, so that it doesn't flex and break the seals with depth. A design like this would be better for withstanding pressure. Some of the waterproof boxes may also work depending on how deep you need things to go, and how strong the box seals are. Finally, you will also need to ensure that the sensor itself will survive the pressure.

Beyond a different case design, you will also want to change how you have sensors (or other external power) attached / going though the case. Cable glands aren't the best choice for deeper environments. The marine sealant may work, but again this is a permeant fixture and is messy. For something that can be removed, and hold a seal with pressure, try using a wet-mateable connector. These are paired connectors (you need a male and female end together), that allow you to have a waterproof interface to transfer power or data through. For these, you wire the sensor to the wet-mateable 'pig tail.' Then on your case, you would drill a hole and install the corresponding bulkhead connector. Inside of your waterproof box, you wire up the bulkhead connector to the Arduino (you wire it as if you were connecting the sensor / power directly to the Arduino board). Now when you are ready, simply plug in the sensor with the 'pig tail' to the bulkhead connection on your case, and you have a working sensor with a pretty good seal. These types of connections are pretty common in the Marine Science world, and in other marine commercial sensors / systems.

If you choose to do this, you will need to make sure you have a fully waterproof 'splice' where you connect your sensor (or any wires) to the pigtail. This is important because, if not done properly, you will have water get in to the wires and cause major problems. This guide here will walk you through a pretty simple and DIY waterproof splice. In general though, the basic idea is that you want to encapsulate your spliced wires in some sort of epoxy, sealant, glue, etc. to ensure no water gets in.

Another trick from the Marine world is using mineral oil. If you really want to keep your electronics from getting damaged by water or pressure, you can fill up your case with mineral oil. Obviously this is extremely messy and pretty much ensures your electronics will be covered in oil forever, but it does a great job of keeping water out. And, importantly for deeper waters, mineral oil is less compressive than air (or water). Meaning that with depth the oil won't compress (as much); keeping your case from getting 'squeezed' with increasing pressure. You will often see this technique used with ROV electronics. The oil is non-conductive too, meaning it won't interfere with the electronics (like water would). If this is something you choose to do, it might be worth changing your case design to be more modular, this way you are only filling up the parts that need it. Potting electronics in epoxy also can also give you some pressure protection too, but only for your electronics and not the case itself.

Subconn Wet-Mateable connector example

Example of water proof splice using epoxy. B - wires before being sealed in epoxy. A - Fully sealed splice. Credit: Robotic Ocean

ROV side electronics panel; full of oil for pressure compensation. Credit: SeaTools.