PC Water Coolant Chemistry – Part I

Summary

Water is unique. Over two-thirds of the Earth’s surface is covered by water and life as we know it could not exist without it. Depending on its temperature, water can exist as a solid, liquid, or gas. Water has unique physical, thermal and chemical properties that also make it an excellent coolant for use in PC water-cooling systems. No other substance can provide all of the advantages and benefits offered by plain old water. Here is a brief outline of the topics discussed in this two-part article about water coolant chemistry.

By far, the majority of PC cooling applications rely on air-cooling. But water is much better at transferring and transporting heat than air is. This means a relatively small amount of water can transport a large amount of heat.

Advantages of using water as a coolant:

• Plentiful and inexpensive
• Liquid at room temperature
• Low viscosity
• Non-toxic and odder less
• Good thermal conductivity (for a liquid)
• Unusually high specific heat (for any material)

The main components used in a basic PC water-cooling system include a pump, water block, and radiator. Various lengths of tubing and fittings connect these components together and one or more fans may be used to blow air thru the radiator. Water is the fluid medium used to transport heat from one location to another and is the most commonly used coolant.

The pump circulates water thru the system and in the process moves heat from one location to another. The waterblock’s job is to transfer heat out of the processor and into the water while the radiator transfers heat out of the water and into the air. Water is particularly well suited for the task of transporting heat. Additional benefits of a water-cooling system include the large heat transfer surface area provided by the radiator and the potential for more effective cooling with less noise than a comparable air-cooled system.

The following picture illustrates a basic PC water-cooling system (compliments of SCompRacer).

Physical, Thermal, and Chemical Properties of Water

Many of water’s common attributes we take for granted, like availability and user-friendliness. However, it is water’s low viscosity and unique thermal abilities to conduct and absorb heat that makes it such a good heat transfer fluid for use in PC water-cooling systems. In addition, the pH and conductivity of water are also important. Let’s take a closer look at these properties and compare them with other common materials used in cooling.

Viscosity

Viscosity is a physical property that defines a fluid’s resistance to flow. Fluids with high viscosity have a high resistance to flow – like honey or mineral oil. Water has a relatively low viscosity so it flows easily, even when cold. This means the pump in a PC water-cooling system will have a much easier time pumping low viscosity water than a thicker, viscous fluid, which translates to higher flow rates and potentially better cooling. Coolant viscosity also affects convective heat transfer (boundary layer thickness and flow velocity) in the waterblock and radiator.

Viscosity is inversely proportional to temperature. This means as the temperature of most materials goes down, the viscosity will go up (become thicker). Adding a very small amount of surfactant (soap or detergent) to the water can also decrease viscosity. This happens because surfactants lower the surface tension of water, which slightly weakens the intermolecular bonding between water molecules.

The following table lists the dynamic viscosity of several common fluids (measured in Centipoise) used in PC cooling systems.

Automotive antifreeze (ethylene glycol) is a lot more viscous than water. While there may be reasons to add a small amount of antifreeze to a PC water-cooling system, one disadvantage is that it will increase the coolant’s viscosity, resulting in lower overall system flow rates.

Adding Methanol on the other hand will actually decrease the water based coolant’s viscosity. Methanol is sometimes used in systems employing active cooling to keep the water from freezing when temperatures fall below 0 C (32 F).

Thermal Conductivity

Thermal conductivity is a measure of how well a certain material can conduct heat. The conduction of heat occurs inside a material when kinetic energy is transferred from one molecule to another by numerous collisions. Metals are especially good conductors of heat due to an abundance of free electrons (these same free electrons are also responsible for conducting electric currents).

Thermal conductivity is important in a PC water-cooling system because it defines the various heat conducting materials (copper, aluminum, water, air) ability to transfer heat from one adjacent area to another. Conduction requires direct physical contact between molecules. As heat flows thru the various materials a slight differential temperature will always exist from one side (heat in) to the other (heat out). The higher the thermal conductivity is for a particular material, the lower the differential temperature will be across that material as heat flows thru it. Minimizing these differential temperatures related to thermal conductivity will help maximize cooling.

As we can see in the following table, water may not be as good at conducting heat as are metals, but it is better than most liquids and more than 22 times better at conducting heat than air.

Some day in the not too distant future, I expect to see two-dimensional silicon based semiconductors give way to three-dimensional diamond hybrid semiconductors, due to diamonds high thermal conductivity and optical properties… 🙂

Specific Heat

The specific heat (or heat capacity) of a material describes that material’s ability to absorb heat. Heat capacity defines the amount of heat required to raise a certain mass of material one degree in temperature. Water has one of the highest heat capacities of any known material. A kilogram of water can hold 4 times more heat than an equivalent mass of air and almost 11 times more heat than an equivalent mass of copper!

Because water has such a high specific heat value, a relatively small amount of water can transport a large amount of heat. This helps keep the overall size of a water-cooling system small and allows for relatively low flow rates.

pH

The term pH comes from Pourvoir Hydrogene, which is French for “power of hydrogen”.

pH is a measure of how acidic or alkaline (basic) a solution is. The common pH scale ranges from 0 to 14 (0 being extremely acidic, 7 being neutral, and 14 being extremely alkaline). The pH value is based on measuring the relative hydrogen ion concentration (actually the hydrogen ion activity) in a solution. The greater the concentration/activity of hydrogen ions (H+) the more acidic the solution and the lower the pH will be.

Higher concentrations of hydrogen ions (acidic solutions) promote oxidation (corrosion). In a PC water-cooling system we generally want the pH to be on the alkaline side to help minimize corrosion. pH values between 8 and 10 are typically desirable. If aluminum components are being used then a pH not exceeding 8.5 should ideally be maintained to preserve the very thin protective oxide layer that naturally forms on aluminum surfaces.

Maintaining a mildly alkaline pH will also help retard the growth of biological organisms, as most bacteria, algae, and fungi prefer slightly acidic growing environments.

High-quality automotive antifreeze and corrosion inhibitors contain pH buffers, which will help maintain an alkaline pH and prevent the coolant from becoming acidic. Over time, these buffers may be used up resulting in the coolant water pH becoming acidic.

The pH value of a solution can be measured in several ways. In the lab, a pH meter can obtain very accurate readings. For our purposes though, simple pH indicator strips will provide useful readings at a minimal cost.

pH indicator strips are available at many pharmacies and stores selling swimming pool supplies. To measure the pH of the coolant in your PC water-cooling system, just dip a pH indicator strip into the reservoir or drain a few drops of water from the system to wet the strip. Specially treated blocks on the strip will change color according to the pH measured.

Here are a few real-world examples of the pH values for common water and additive combinations used in PC water-cooling systems. (Beckman F340 pH meter)

*Antifreeze – Prestone (Ethylene Glycol)
**Clorox liquid bleach – 2 Teaspoons bleach (6% Sodium Hypochlorite) in 1-quart water

Notice that distilled water is slightly acidic. This is because distilled water dissolves carbon dioxide from the air forming a weak carbonic acid solution.

Over time, the water in a typical water-cooled PC will slowly become acidic. The pH buffers in water additives (antifreeze, Water Wetter, etc.) will become depleted and periodically need to be replaced. We’ll talk about flushing and refilling later on in the Maintenance section.

Conductivity

Conductivity is the ability of a material (in our case water) to conduct an electric current. It is the inverse of resistivity. Conductivity is generally the most important criteria used to determine water quality or purity. It determines the total ionic concentration (dissolved solids) of a solution.

Ultrapure water is actually a poor electrical conductor. It’s all the substances (or salts) that are dissolved in water that determine how conductive the solution will be. Therefore, conductivity can be an excellent indicator of water quality. In general, the more impurities and contaminants dissolved in the water, the higher the conductivity.

The conductivity of a solution can be found by placing two electrodes of known size into a liquid and measuring the current flow with an applied voltage. Conductivity meters are routinely used to accurately measure the conductivity of solutions.

The basic unit of conductivity is the Siemens (S), previously called the mho, and is generally expressed in micro Siemens per centimeter (uS/cm). The resistance of pure water (the inverse of conductivity) is measured in Megohm-centimeter (MO.cm @ 25ºC). Very pure water will have relatively low conductivity and high resistivity. Conductivity meter probes or cells are standardized (using a cell constant) to compensate for variations in electrode dimensions.

We will use conductivity as a basic measure of water quality at the end of the next section when we compare the various types of water available and evaluate their suitability for use in PC water-cooling systems.

Types of Water

There are many different types of naturally occurring water (seawater, rainwater, groundwater) and also many different types of processed water (drinking water, bottled water, de-ionized water, etc). In this section, we will take a look at several different types of water and evaluate each for use in a PC water-cooling system.

• Tap water
• Distilled water
• Bottled water
• De-Ionized water
• Other weird stuff

Tap Water (drinking water, potable water)

Plain old tap water (also referred to as drinking water or potable water) is an obvious choice for filling a PC water-cooling system. It’s readily accessible (most likely to anyone with a computer) and essentially free. The downside of using tap water is that it may contain significant quantities of dissolved minerals, microorganisms, and other impurities.

Geographical location, source, and water treatment processes all play a big role in determining how pure your local tap water will be. Many areas are known to have hard water, which just means there is a higher than normal concentration of dissolved minerals (particularly calcium, magnesium, and iron). Most tap water contains small quantities of microorganisms (algae, fungi, and bacteria).

In the short term (days ~ weeks) using tap water will probably not cause any adverse effects on the components in your water-cooling system. But in the long term (weeks ~ months) you may see problems develop that will result in decreased cooling and in extreme cases, total cooling system failure.

Dissolved minerals may precipitate out of solution and/or combine with other materials in the system to coat wetted heat transfer surfaces. The pH of tap water is typically acidic and will accelerate corrosion processes. Trace quantities of various biological organisms may flourish inside the cooling system, eventually growing enough to foul internal surfaces and reduce heat transfer. In extreme cases, severe corrosion or massive microorganism growth can completely block off the flow and cause a total water-cooling system failure.

Distilled Water

Distilled water is water that has been heated to the boiling point in an apparatus called a still to produce water vapor, which is then condensed back into liquid water. Most of the impurities will remain in the boiling pot. The distillation process removes waterborne biological contaminants, organic and inorganic chemicals, heavy metals, volatile gases, and other contaminants. Some volatile contaminants may be carried over if they have a similar boiling point as water.

Distilled water is one of the purest forms of water. Distillation is frequently used in the final stages of producing ultrapure water for use by semiconductor and pharmaceutical industries.

One of the best sources for obtaining distilled water is the grocery store. Check out the bottled water aisle and look for water that is specifically labeled Distilled Water (steam distilled). Several other water treatment processes may be used to treat the water before and after distillation. These may include softening, pre-filtering, Reverse Osmosis (RO) filtration, activated carbon filtration, and exposure to UV light or ozone gas. Read the fine print on the container label – in general, the more water treatment processes listed, the better.

Distilled water is inexpensive, normally selling for less than $1.00 US for a gallon.

Bottled Water

There are many different types of bottled water available. Bottled water that has gone thru several different stages of water treatment and is essentially pure water is generally good for use in PC water-cooling systems. Some bottled waters have minerals and/or flavorings added and these are not well suited for use as a coolant.

Springwater should generally be avoided because it may contain high concentrations of minerals. Carbonated waters should also not be used as they contain carbonic acid and are therefore acidic.

One of the most popular brands of bottled water for use in PC water-cooling systems is Aquafina. As it turns out, Aquafina is an excellent source of relatively pure water. Pepsi uses both reverse osmosis and carbon filtration to produce its Aquafina brand of purified drinking water.

In general, most brands of purified drinking water will be OK to use in your PC water-cooling system. Just make sure it’s only purified water without any minerals, carbonation or flavorings added. Read the labels to be sure. Purchasing several bottles of purified drinking water will typically be more expensive than just buying a gallon of distilled water.

De-Ionized Water (Demineralized Water)

In the context of PC water-cooling, we can consider de-ionized water and demineralized water to be essentially the same. When minerals dissolve in water they produce ions – so removing ions removes minerals. De-ionized water (DI water) is water that has had most of the dissolved solids and minerals removed. In general, the more of these ionized substances that can be removed, the purer the water will be.

There are two principal processes used to de-ionize water: reverse osmosis and ion exchangers. They can be used individually or combined depending on the volume and purity of the water being produced. Either of these two processes may be followed by activated carbon filtration and exposure to high-intensity UV light. In some cases, ozone may be used instead of UV light.

• Reverse osmosis (RO) – utilizes a membrane under pressure to separate relatively pure water from a less pure solution
• Ion exchanger – process that incorporates specially designed ion exchange resins to remove ionized substances and some particulate matter from the water
• Activated carbon filtration – used to remove dissolved organics
• Ultra-Violet (UV) light – kill microorganisms and break down organic compounds

Reverse osmosis is a process that uses a special membrane (cellulose acetate) under pressure (50 to 1,000 PSI) to separate the relatively pure water being produced from less pure feed water. The RO process operates at comparatively low temperatures and is relatively energy efficient. The greater the charge and the larger the particles, the more likely the RO membrane will filter them out. Reverse osmosis is the finest filtration process known, functioning down to the sub-molecular level.

Normally when a semi-permeable membrane separates two fluids with different concentrations of dissolved solids, osmotic pressure will cause fluid to flow thru the membrane from the area of low concentration into the area of high concentration. However, if pressure is applied to overcome the osmotic pressure, the fluid flow can be reversed, causing a flow to occur from the area of high concentration into the area of low concentration – reverse osmosis! The RO membrane filters out dissolved solids, organic materials, oils, and fine particulates including bacteria and viruses.

Reverse osmosis can remove up to ~98% of the dissolved solids in water and is frequently used as a pre-treatment process prior to ion exchange and/or steam distillation.

The ion exchange process incorporates different types of resin beads that selectively remove ionized impurities based on their charge. Resins are formulated with either a positive (+) or negative (-) charge to capture the ionized impurities in water. Some particulate matter may be filtered out in the resin bed but ion exchangers primarily utilize an electrochemical process rather than mechanical filtration to remove dissolved solids.

Many common, low volume ion exchangers use one or more columns of mixed resins, which must be replaced periodically as the resin beads become saturated with impurities. These are typically used in benchtop pure water systems along with UV light and activated carbon filters. The water purification system shown in the following picture can produce analytical grade, ultrapure water with a conductivity of 18.2 MO.cm.

Ion exchangers used in high volume industrial applications generally use a process called continuous de-ionization (CDI), which uses a high-voltage power supply to regenerate the resins to greatly extend their useful life.

Ion exchangers can remove >99% of the dissolved solids in water and are frequently used as a final treatment process after reverse osmosis in DI water systems.

Activated carbon filters utilize charcoal that has been activated to increase the exposed surface area of the charcoal particles. Activated carbon is ideal for adsorbing organic, non-polar, and halogenated substances that might not be filtered out by RO or the ion exchange processes. Charcoal filters remove oils, aromatic hydrocarbons, chlorine, and other substances that often give water a bad taste or odor.

Ultra-violet light is used not only to kill stray microorganisms that might be floating around in the water but also helps break down neutral organic compounds that can then be removed by de-ionization.

For all of the good things, de-ionized and demineralized water have to offer, they have a couple of drawbacks as well. Unless you happen to have access to a chemistry lab, DI water may be harder to find and is usually more expensive than distilled water.

Another potential concern is that DI water is more corrosive to metal parts than plain old ionized water. It takes a lot of effort and energy to remove the majority of ions from water. Water desires these ions back and will aggressively take them from any available source. Some people will tell you to never use DI water in a PC water-cooling system. Like most things in life, moderation is the key! If you use DI water to occasionally fill your cooling system, the water will immediately strip ions away from the surfaces of all wetted metals. But, no measurable damage will occur. You would have to continuously fill and drain your system 24/7 for weeks before you might see your water block or radiator starting to dissolve. Truly pure, DI water doesn’t stay de-ionized very long when exposed to air and poured thru various containers.

Other Weird Stuff (Windshield wiper fluid, Booze, etc.)

It’s amazing to me what some people will put into their PC water-cooling systems – if it’s a liquid, somebody, somewhere has probably tried it. Beer, whiskey, windshield wiper fluid, you name it! The saving grace for many of these fluids is they usually contain mostly water.

The downside is that they also contain other less desirable substances (sugars, proteins, dyes) that degrade water’s natural ability to conduct and transport heat. In some cases, they might even contain substances that can harm plastic components, seals and tubing.

Most commercial windshield wiper fluids are primarily water and methanol (33%). By itself, this could be OK, especially if you need freeze protection for a chill water system operating below the freezing point of water (but still not as good as straight water). However windshield wiper fluid also contains various solvents, surfactants, and dyes that don’t help in the cooling process and in fact may hamper it.

Various Types of Water Compared

Ok, that was a lot of theory and tech talk about the various types of water that can be used in a PC water-cooling system. Now, let’s take a practical look at how one type of water compares to another. As mentioned earlier, conductivity is the most widely used measurement of water purity.

I used a VWR (Control Company) model 61161-362 bench conductivity meter and a Beckman F340 pH meter to take measurements. The following table lists the conductivity (along with temperature and pH) for various water types.

As you can see, based solely on conductivity, plain old grocery store distilled water is an excellent source of pure water for filling PC water-cooling systems. For less than $1.00/Gal US, the purity comes close to DI water and is far superior to tap water and most bottled waters. Aquafina is one of the best bottled waters available – but still not quite as good as distilled and it costs more.

Notice the large variation between conductivity readings of the five different bottled waters sampled. As you might expect, most spring waters have a higher conductivity due to their natural mineral content. Some bottled waters intentionally add minerals for taste and/or health reasons.

This concludes the first part of this two-part article on PC water coolant chemistry. I hope you have found the information presented here interesting and useful. Please watch for Part II where we will take a closer look at corrosion and the wide variety of additives that are available to help minimize corrosion, retard microorganism growth, and promote more efficient cooling. We’ll finish up with some thoughts and recommendations for maintaining your water-cooling system.

Thank you,

RoboTech – aka Lee Garbutt