Concentration and pH

Jul 1, 2019

The loss of sump volume by evaporation will tend to increase the concentration of the coolant overtime. Many companies have stated they do one of the following to compensate for the coolant loss:

  • Add straight water
  • Add a preset concentration as if they are filling the tank from empty
  • Add a low concentration of coolant to reduce the current concentration.

While each of these methods can easily meet the volume requirement, they can also adversely affect the concentration or pH level. The best way to verify the concentration is within parameters is through testing, recording data over time, and analyzing the results. With enough data you can extrapolate the expected concentration change of an individual machine doing the same processes with the same metal each day. The key to the is “an individual machine doing the same processes with the same metal each day.” Not all machines do the same process or the same metals every day. Even the same metal can have different characteristics depending on the manufacturing process. An example is one batch being oily and the next is not. There are many variables that can affect the metal working fluid and that’s why it is recommended to test, record, and analyze findings on a scheduled basis. The more variables that are removed from this equation will reduce the frequency of tests that need to be performed.
Experts in the metal working fluid industry also state that it is good practice to do an analysis of your tap water on a routine basis. Many coolant manufactures offer this service at no charge. Samples should be taken from all water sources used to mix with coolant concentrate. Recording the test results gives the opportunity to develop trends. Tap water composition can be in a constant state of change based on external factors. (Run off from salt on the roads, fertilizers, city water treatment changes, etc.) If plants are using Deionized (DI) or Reverse Osmosis (RO) water for their main or supplemental water supply, testing these sources is recommended as well. How do you know if your DI/RO systems are working properly without testing them.
A basic water analysis should show, at minimum, the pH level, the amount/type of bacteria, any suspended materials, mineral composition, and other chemicals in your water. This information will be helpful if a problem develops with in the coolant’s longevity or effectiveness.


The most widely used tools for internal testing for pH and concentration are pH strips and a refractometer respectively. Both tools are easy to learn and use. Purchasing of pH strips can be done through local retailers as well as online. Coolant manufactures have suggested a digital refractometer is best for accuracy. Ask the plant coolant representative which digital refractometer they would recommend. The coolant representative can also provide you with the coolants refraction index which will be required to determine the sumps coolant concentration.
Fluid used for testing is a potential variable. Consistent testing locations ensure accuracy of test results when comparing to previous tests. Industry experts state that it’s best to sample the fluid from somewhere around the machine tool spindle. Fill a small clear container with fluid from the spindle area and let it sit for 30 minutes to allow the solids to move to the bottom and any free oil to move to the top. After the fluid separation, use an eye dropper to extract fluid from the center of the container and perform the refraction and the pH tests. With some coolants, having the container sit a little longer can give an idea as to the amount of suspended solids and non-emulsified oils that are in your sump.
Many coolant manufacturers recommend using tap water when the sump is completely refreshed; cleaned out and all the metal working fluid is replaced. At the same time, they will recommend RO or DI water each time the coolant tank is topped-off. RO or DI water reduces the amount of additional chemicals, minerals, etc. you are adding to your sump.
There is another important factor here that even the most experienced industry professional can overlook and that is the mixing of coolant. When using a mixer, check the concentration periodically to verify the new coolant mix is what you are expecting.
If mixing by hand, it is highly recommended to add the coolant concentrate to the water. It is best to not adjust the sumps concentration by adding only water or only coolant concentrate directly to the sump. The preferred method would be to add pre-mixed fluid. Even if the concentration is exceedingly high, adding a very light dilution of pre-mixed coolant is the best way to bring the concentration into range.

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