Laser Cooling Water. What you should know, and what you need to know!

Bacterial growth in a laser cooling system

Introduction

The cooling system is the most critical component in a laser cutter, a well maintained system will last many years if not indefinitely, also a well maintained system will increase the cooling efficiency and life span of the laser tube.

We know that the primary cooling fluid is water, but what is the difference between distilled, deionized and tap water?
Some people recommend the use of biocides and algicides to prevent bacterial growth. But why do we disagree?

We have written this article to help explain our reasoning and methodology for recommending the use of pure distilled water as a cooling medium with no added impurities.

Distilled water has low conductivity due to the absence of mineral content. This is important due to the high voltages present in a dc excited laser. By adding impurities and additives to the system you are not only increasing the fluids conductivity but also its ability to absorb minerals and oxygen present in the surrounding environment further increasing the conductivity.

Typically a DC excited laser operates with voltages in the range of 15-65kV thats 65,000V DC, at these voltages you have to take into consideration the air gap, conductivity of surrounding materials and the capacitive coupling effect of the glass laser tube. Its hard to comprehend but at these voltage levels electricity can travel through air, glass and water. By raising the conductivity of the cooling water, you are potentially creating a shorter (less resistant) path to ground for the electricity to travel. The end result of this can be

  • Output power loss
  • Damage to sensitive electronic equipment
  • Positively charge cooling fluid, this can cause a range of issues including galvanic corrosion, this issue is further compounded when adding sodium based impurities such as bleach.
  • A ringing noise coming from the power supply.

conductivity of water
Image from https://www.apswater.com/article.asp?id=302&title=Conductivity+Vs+Resistivity+Vs+PPM+Quick+Chart

We are not going to attempt to calculate the conductivity of the cooling medium as this value will not be of any use to anyone.

What we will try to do is educate on why we believe pure distilled water to be the ultimate cooling medium.

Cooling fluids

  • Tap water – Not recommended due to the presence of  naturally occurring  minerals, mineral content increases the conductivity of water.
  • Engineered cooling fluid – specific fluids are now available for similar applications. At this present moment we haven’t found anything that is as practical as distilled water.
  • Deionized water – made by running water though electrically charged resin. If you have deionized water made from distilled water that has been sitting out exposed to air, it becomes ordinary distilled water. It’s fine to use this type of leftover deionized water in place of distilled water. Unless you’re certain it won’t affect the outcome, do not substitute one type of water for another for any application that specifies which type to use.Helmenstine, Anne Marie, Ph.D. “The Difference Between Distilled and Deionized Water.” ThoughtCo, Dec. 28, 2018, thoughtco.com/distilled-versus-deionized-water-609435.
  • Distilled water –  a process that involves boiling water and condensing the steam to achieve a solution with a very low to no mineral content. The quality of distilled water is affected by the process and stages used during distillation.

Bacterial growth

Bacterial growth in cooling systems is a common issue we must overcome. Bacterial growth will reduce the cooling efficiency of the fluid, and in severe cases will reduce its ability to flow. Once a bacterial free system is achieved it can be maintained by regular water changes and the use of a silver coil.
Bacteria like most living organisms need light, food, water and suitable temperature to bread and survive. Unlike humans when conditions become unfavorable bacteria will spread spore that will remain dormant until the living conditions become unfavorable again.
This is why we recommended shocking the systems to kill off all spores and bacteria before attempting to maintain a bacteria free cooling system.

Preventing bacterial growth in a cooling system

  • Seal the system, ensure minimal air can circulate the system, ensure air tight seals on hoses, ensure lids are always correctly seated etc. Ensure water return hose is always below the water level to prevent the return flow from aerating the water reservoir.
  • Remove light exposure, replace the clear silicone hoses for black silicon hoses. This will stop the bacteria from receiving light exposure needed to aid growth.
  • Heat, even when the laser is not in use it is advisable to keep the chiller running. Lower temperatures will slow down bacterial growth. This is not always possible or practical.
  • Regular water changes, we recommend changing water once every 3 months. But for a well maintained system we have seen water free of bacteria after several years.
  • Insert a silver coil into your water reservoir, the oligodynamic effects will cause damage and death to cells present in the cooling water.

Algaecides and Biocide treatments

We do not recommend the usage of algaecides or biocidal treatments due to the corrosiveness of some of these solutions, not to mention the effects on the ph instability that they can caused. They are simply not required as a preventative method of maintaining a bacterial free fluid.

The only method we recommended to prevent growth is a single silver coil in the reservoir.

Known as the oligodynamic effect, silver ions bond to cellular proteins and enzymes causing damage to cells and death. Silver can be purchased inexpensively for this exact purchase in the form of a silver coil. When placed into the cooling  the silver oxidising releasing ions into the cooling medium. Do not be concerned about placing metal into the cooling water, it poses no risk of increasing the conductivity of the water, this is due to the fact that silver is insoluble in water.

anti bacterial silver coil for laser chiller

Antibacterial silver coil

An inexpensive way of preventing bacterial growth.

Biocide for laser

If absolutely necessary the only algaecide we would recommend is the Tetra brand Algae Control, its primary active chemical is Polyquaternium (WSCP) also known as Poly[Oxyethylene(Dimethyliminio)Ethylene (Dimethyliminio)Ethylene Dichloride].

View the material data safety sheet here.

using bleach in laser cooling system

We often see people advising bleach being adding to your cooling formula as a biocide, most specifically the Clorox brand. If you take a few moments to study the safety data sheet of this product you will notice the two primary ingredients.

  • Sodium hypochlorite – Its corrosive properties, common availability, and reaction products make it a significant safety risk. In particular, mixing liquid bleach with other cleaning products, such as acids or ammonia, may produce toxic fumes.Source wiki
  • Sodium hydroxide – is a highly caustic base and alkali that decomposes proteins at ordinary ambient temperatures and may cause severe chemical burns. It is highly soluble in water, and readily absorbs moisture and carbon dioxide from the air. Source wiki

The above properties make the product absolutely unsuitable for use in a cooling system with sensitive components as a long term solution.

Shocking a system with regular Chlorine based bleach and water mixture is ok as a short term solution to killing bacterial growth.

Mix chlorine based bleach with regular water at 1:4 ratio. Run this mixture around your system for 24 hours. Ensure this mixture is thoroughly flushed out the system. We recommend several flushing washes after shocking a system.

You can flush the system by draining all liquids, fill with regular tap water. Circulate for one hour, drain system and repeat.

For the final flush we recommend to flush with distilled water.

Bleeding air from your cooling loop

Pockets of localised air in the cooling loop of the laser tube will lead to a reduction in heat transfer, the larger the pocket of air the lower the efficiency of the loop to cool the specific area.

The laser tube is sensitive to temperatures greater than 25’c with long term  irreparable damage being possible. air pockets located in the cooling cavities of the tubes optics, both reflective final mirror and columinating lens can lead to superheating of the optical components. This is also why bacterial growth is common to be found at either end of the tube, where conditions are hotter and more favorable for bacterial growth.

We have written an article that covers this subject in depth, we recommend having a read for yourself.

Coolent additives

Please do not use cooling additives, they are simply not needed. If you are struggling to maintain stable temperatures in your cooling system your problem lies in your setup or chillers cooling ability, not the cooling waters ability to transfer heat.

Antifreeze

A popular subject, we all know glass laser tubes tend to crack when filled with water and frozen. This is never a desirable scenario to find yourself in. We recommend the use of storage heaters, or if budget allows a chiller with heating function.

Antifreeze can be introduced into the system, but we would never recommend to rely on it for a long term solution as it has its trade offs.

Any antifreeze that is suitable for our specific application will have reduced thermal transfer properties. For example propylene glycol (the base chemical of most “RV Antifreeze” tends to change in viscosity and gel up in cold conditions.

Automotive antifreeze is not suitable for our application due to the presence of silicon hoses and sensitive gaskets in the chiller / water pump being susceptible to degradation.

If you absolutely have to use an antifreeze we recommend the use of 99.9% proof propylene glycol. But expect to see an increase in bacterial activity. In this case you will have to add a biocidal treatment to your cooling fluid.

Mix PG with distilled water at a 1:1 ratio.

Surfactants

Surfactants – a compound that lowers the surface tension of water to help with the removal of air. Once again using surfactants has its own trade offs, we recommend the best method of removing  all air from a system is by using gravity and time. Its simply a waiting game.

“Reducing the surface tension of the cooling water is a last resort as it adds impurities to the cooling loop, but it is highly effective for removing smaller micro bubbles. Surface tension can be reduced by adding one or two drops of chemical safe dish soap. The concentration of the soap is very low, and can be reduced by dilution with fresh distilled water at a later point.”

Bubbles in the laser tube, should i be worried?

How to tell when the water needs replacing?

EC TDS METER

Using a device known as a EC meter or TDS meter (Electrical conductivity & Total dissolved solids) we can test the cooling fluids conductivity and the ppm (parts per million) of the dissolved solids.

By monitoring this value at the time of filling and the rate of change we can calculate when the water will require changing.  Fresh distilled water will have an ec close to 0

 

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