Tap Water vs. DI Water vs. RO Water

Tap Water vs. D I Water vs. R O Water  

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 Why Isn't My Coolant Emulsifying?

By  Dom Ruggeri

In any water-extendable metalworking fluid, the water used to dilute the concentrate will affect the fluid's performance.

Back in the late eighties, I was working for a major metalworking-formulating house.  Things at the time were moving along no major problems, and I was even able to work on a few pet formulating projects of mine.  As you all know just when you think everything is OK, a firestorm erupts and usually it's on a Friday afternoon.  I was winding down on this particular Friday afternoon when a major customer called to inform us that the last tank truck of fluid we sent them refused to emulsify in their tap water.  I pulled the batch retain and our sample of the customer's tap water.  Were they nuts?  The product emulsified perfectly.

I reported my findings, obviously something was wrong with our water sample or the retain sample.  I quickly contacted the salesman and informed him that I was going to contact the customer and request a sample from the tank truck and another sample of their water.  Further, I requested that QC recheck the specifications of our retain sample, another firestorm, but hey, I was burned already.  The customer was very cooperative and shipped the samples so that we would have them first thing Monday morning.

On Monday, I began to test the customer's sample.  All of the specifications checked out.  I also checked the customer's water and it was then that I was able to reproduce the customer's problem.  In distilled water, our laboratory water, and our original sample of the customer's water, the product emulsified perfectly.  Therefore something changed in the customer's water supply and once again I am in the position of having to tell a salesman that he had to go to his customer and find out how the water changed (if the customer knew).  A third firestorm, but I was getting used to them.

The salesman reported that this account had switched over from city water to well water.  Adding to this variable, it was a very dry summer.  A hardness titration of the new well water sample yielded a hardness of 1200 ppm as CaCo3.  I tested the fluid in 1500 ppm synthetic hard water.  This was by no means a normal test, but these were not normal circumstances. The product emulsified but there was always the question of synthetic vs. natural water.  Still, it was a start.

I submitted the sample of water to the analytical group and asked if they could test for all the elements.  They resisted because of all the work involved but my manager convinced them that it was a good idea to run the experiments.  When the results came back, we found that the Ca to Mg ratio was one to one.  Normally, the ratio of Ca to Mg in natural water is two to one.  Further testing revealed that this Ca to Mg ratio was causing our emulsification problem.  As a quick fix, we sent the customer a chelating agent.

Any engineer will tell you "if it is not broke do not fix it." Such is the case with city water and metalworking fluids.  As long as your systems are running well, why go through the expense of installing a DI or RO unit.  City or tap water is consistent and of reasonably good quality.  As always, if the systems are maintained properly, cleaned, and changed as your coolant supplier recommends, you should have no issues.

Now if you are in an area where water quality is questionable you may want to consider installing either a DI or RO unit. Both units will yield reasonable to good quality water, but bear in mind there are problems associated with either system.

Deionized (DI) Water:

These units range from household water softeners to large industrial units capable of producing water quality equivalent to distilled water.  This is accomplished by passing the water through a series of filtration units.  Each stage purifies the water until the desired quality is achieved. Remember, this is an ion exchange system.  You are exchanging the Ca and Mg for some other ion, normally Na.  If sodium is not going to cause you any problems, then this is the best and most cost effective system to use.

Reverse Osmosis (RO) Water:

These units are expensive to install and maintain.  However, the water quality is as good as distilled water.  The water passes through a membrane that removes all ions and impurities, yielding ultra pure water.  However, the membranes can be expensive and depending on the water quality, going into the RO unit these membranes may have to be changed frequently.


1. Foam - Calcium fatty acid salts contribute to foam control.  If the calcium ions are replaced with sodium, or otherwise removed, you may have a foam problem depending of course on the metalworking fluid.

2. Cost - As stated above, these units are expensive to maintain and repair.

3. Expertise - Someone has to insure the unit is always working properly (additional cost).

To conclude, if your systems are running fine with city water, they are not broke, and you might want to avoid fixing them.

Thank you all for reading METALWORKING FLUID MAGAZINE.

Good luck to all.