Finished acrylic reservoir after cementing
click photo to enlarge

This is my custom-made acrylic reservoir designed to sit in a 5.25 inch drive bay.

A reservoir is an optional component of a water cooled computer, but is often used because it makes bleeding out air bubbles much easier. A reservoir is essentially a tank with inlet and outlet hose couplings and a fill hole. Good reservoirs are designed so that the water flow dynamics allows enough time for trapped air bubbles to rise to the surface.

Reservoirs are also one of the most visible components, and perhaps the only place where you can actually see the movement of water. So if you're going to have a reservoir, it might as well look good too. There are a few prefabricated reservoirs available, but I decided I wanted to make a custom one. The key design points are:

My reservoir is constructed from clear acrylic and is designed to fit in one 5.25 inch drive bay. In this respect it is similar to some manufactured reservoirs. But mine has a few distinctions from those you can buy. I also chose to use ¼ inch acrylic which makes it much stronger and less likely to crack than the common thiner material. Perhaps the most distinctive feature is the hose couplings on mine are tilted downwards rather than horizontal. Side view of reservoir showing tilted hose connection That orientation allows for easier hose routing and helps prevent kinking as the photo to the left shows.

There are two inlet connectors and one outlet. The dual inlet allows a three-barb CPU waterblock to be used without having to use an inline Y-connector. There is a ceter divider running inside the chamber which forces the water to travel the entire length of the reservoir. By lengthening the travel distance any trapped air bubbles more easily rise to the surface and escape. There is also a small inlet near the front for filling the system and bleeding air.

Another interesting feature of my reservoir is the integrated water temperature probe. The probe is designed to provide very good thermal conductivity between the water and the thermistor, yet remains watertight. The probe also is placed directly in the path of inlet flow.

Fabricating the parts

All parts to the reservoir before fitting and assembly The most time consuming part of this build is shaping all the different acrylic pieces. The photo to the right shows all the parts prior to fitting and gluing. It is essentially a box, except for the rear which is made out of two pieces in order to provide the downward tilt to the barbs. To help insure that it will be watertight, all the pieces must be meticulously crafted so they fit with very tight tolerences. This involves a lot of hand filing and sanding.

An interesting part of the construction was making the threaded pipe holes where the various connectors would fit. I wanted the threads to be as tight as possible so they would be mostly self-selaing without having to rely on a bunch of silcon or other goop. At this point it is worth discussing the different type of pipe threads. I chose to use NPTF threads, which are tapered rather than straight, as well as being dryseal threads. I made the holes by starting with small drill bits and then used a round file to expand the hole to the approximate final size.

Next I needed to thread the holes. I happened to have a ¼ inch NPTF tap for the smaller fill hole, but not the larger ½ inch. Since those large-bore taps are rather expensive I decided to make my own tap by modifying an extra hose barb. Hand made pipe thread taps I carefully filed several deep and sharp grooves cross cutting the threads at an angle that would allow it would bite into the material when screwed inwards. Brass is a rather soft metal and not suitable for taps, but I could get by with this because the acrylic is even softer and I only needed to tap a few holes. Occasionally I would refile the grooves to keep them sharp and clean.

Temperature probe

Temperature probe The temperature probe is made with a small ½ inch threaded pipe nipple and corresponding end cap. The assembly then protrudes inside the reservoir. All parts use NPTF threads to insure watertight seals. The electical part itself is a bulb-type thermistor, which essentially is just a high-precision calibrated resistor which varies it's resistance value in relation to it's temperature. Both electrical leads where sealed with heatshrink tubing to prevent electical shorts with the brass parts.

To allow for the best heat transfer I carefully filed the end cap down to remove as much excess material as possible. I used a micrometer to measure the metal thickness, and continued filing until there was less than 1mm of metal left. The thermistor was then affixed to the inside of the cap at the point where the metal was thinest. I used Artic Alumina™ Thermal Adhesive to "glue" the thermistor to the cap. The thermal adhevise is a two-part epoxy, but unlike most epoxies provides very good thermal conductivity. Finally I had to screw everything together tightly before assembling the reservoir because this part would be inside and not reachable after assembly. (Do note that I tested the thermistor prior to permamently epoxing it in place.)

Dryfitting and gluing

Reservoir with pieces dry fit together before gluing Before final assembly it is important to dry fit all the pieces. To allow the cement to form a strong watertight bond the fit must be very precise. Generally you don't want to avoid any gaps larger than about half a millimeter.

After I was satisfied with all the pieces and their fit I was finally ready to assembly it for good. To join acrylic you want to use acrylic cement. Cement is different than glue in that it actually melts the pieces together to create a strong permament weld-like joint. For well-fitted pieces you should use IPS Weld-On #4, which is a water-thin cement. The pieces are first dry fit and held together and then the cement is applied along the joint. The capillary effect will draw the thin cement well into the joint. To make this easier you will want to get a cement application, which is a small squeeze bottle with a very thin needle. Acrylic cement and applicators If you happen to have any larger gaps (a millimeter or larger) you can get a thickened cement, #16, which you squeeze from a tube. You should however try to get tight fits because the results of using the water-thin cement will be much better.

If you have never cemented acrylic you should definitely practice first. The cement will set within just a few seconds and the bond will be as strong as the acrylic itself. Also it is very easy to damage the clarity or appearance of your pieces if you let any cement come in contact with any place except the joint. You can not just wipe it up quickly. If you do happen to make a mistake, you may be able to use super fine grit sandpaper and polishing cream.

Closeup of inlets after leak testing The cement will bond the pieces solidly within just a few seconds. However for the strongest permament bond you wait for one or two days. Finally you are ready to leak test the reservoir. Fill it with water (distilled preferably if you don't want water stains) and carefully check for small leaks.