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Coolant, or Metalworking Fluid Measurement & Control

Concentration Measurement

Each brand and blend of coolant was formulated to work within a specific concentration range. If used outside that range, problems will occur, and usually hard and fast!

If using the coolant at too low of a concentration, not only does it not have enough of its constituent components required for maintaining the mixture and its ecology, but tooling will break prematurely or cause bad finishes.

If at a high concentration, not only is the expensive concentrate being wasted, but it may become a health & safety issue.

The easiest method to check for concentration is use of a refractometer. It does need to be calibrated before its initial use by using distilled water to zero the scale.

If you have a 5% solution, and your coolant has a one-to-one correspondence to the brix scale, then you should see a perfectly sharp line going right through 5 on the refractometer scale. If your coolant does not have a one-to-one correspondence to the brix scale, then make a note of the horizon line determined in step 3 above... some coolants have a “refractometer factor” on the drum label. Multiply the reading from the refractometer by this number to get the concentration.

If you have new clean coolant mix, then the line should be almost as sharp as if you were measuring pure distilled water (to calibrate the refractometer to zero). As the coolant ages, as more oil is beaten into it, then the line becomes less distinct. If you let your coolant go long enough, you will not be able to see any line at all, as there will be only one big gray area in the field of view. Please don’t let it go this far! Once the line becomes a little blurry, it may be time to think about changing your sump.

Before a sump refill, you must take into conderation your sump’s evaporation rate, as this will cause the concentraion of the remaining volume to increase from that of the origianl charge. To do this, follow the example below:

Example Known Factors
Desired Concentration: 5%
Total Sump Size, gallons: 50
Unknown Factors
Remaining Sump Concentration
Make-up Required, in gallons
Make-up Concentration Needed
Determine the volume of concentrate needed for the entire sump volume of 50 gallons at the desired concentration of 5%:
  Example: (.05 x 50 = 2.5 gallons of concentrate)
Check the concentration of the remaining sump volume with a refractometer (for this example calculation we will use 8%).
Figure the amount, in gallons, the remaining sump contains (in this example we will use 25 gallons). Subtract this amount from the original volume of 50 gallons:
  Example: (50 - 25 = 25 gallons)
Determine the volume of concentrate needed for a make-up batch if only 25 gallons are remaining in sump at 8% concentration:
  Example: (25 x .08 = 2.0 gallons of concentrate)
(2.5 gallons of concentrate originally in entire sump)
-2.0 gallons in remaining sump
.5 gallons of concentrate needed for make-up
Determine concentration for make-up batch:
Example: (.5 gal of conc. ÷ 25 gal fluid required = .02, or 2% conc.)
ANSWER: The sump now needs to be filled with a 25 gallon make-up batch at a concentration of 2%.

pH Measurement

pH is the measurement of hydrogen ions contained within a given solution. pH ranges of 0-6 indicate an acidic solution, whereas ranges of 8-14 indicate a base solution. A neutral solution has a pH of 7.

Coolant is formulated to work within a certain pH range, and this is usually a range of 8-10pH. Outside of this range, the coolant does not function as it should. Loss of pH control is the #1 reason why coolant stabilizers within the blend fail. The product literature or the MSDS for your coolant blend should have the pH range information for your reference.

When the pH of the solution falls below or raises above the working range, it indicates a more serious problem and should be dealt with as soon as possible.

Keeping your coolant at the high end of the range is the best practice, as it will help to inhibit rust and corrosion, and reject tramp oils. Do not increase above 10pH since this alkalinity can cause adverse health reactions.

A low pH indicates that the concentration is too low or there is bacterial contamination present. Both problems should be dealt with in a timely manner while the coolant still can recover. If left unattended, bacterial rafts (slime) will form and inhibit oil removal and possibly clog your flood lines. Your machine and parts may rust, and operators may develope dermatitus reactions. See the Failure & Prevention section for more details.

In order to know the pH of the coolant solution, it must be measured at least 3X per week. Many people use litmus paper strips since they are easy to carry and very simple to use. These strips, however, have an accuracy of only +/- 1 pH. By the time your coolant falls a full pH, there is little that can be done to get the coolant back to its effective operating level. Any problems should be dealt with when the change is +/- .1-.2 pH.

The most accurate device to check pH is a digital pH Tester, which has an accuracy of +/- .1 pH, or 10X that of litmus strips. With this, pH drops can be caught in time for corrective actions to be taken. This device is a little more expensive and needs to be calibrated every day of use, but is also easy to carry and use. If your coolant is important to you and you are implementing a program, using a digital unit is worthwhile.

Water Hardness Measurement

Water hardness is a measure of the dissolved minerals within the water solution. These minerals come mainly from the water source itself, and can occur via the reactions between the water and the metals being machined:

Calcium In water supply
Magnesium In water supply / Metal being machined
Iron, ferrous Metal being machined
Iron, Ferric Metal being machined
Aluminum Metal being machined

These minerals also accumulate via the evaporation process when the water leaves but the minerals stay. A sump that loses more than 5% of its total volume per day will eventually accumulate enough minerals daily to cause problems.

Coolant is formulated with pure water, but in reality, many of us do not use deionized or distilled water, and hardness is the most overlooked cuase of coolant failure. It is wise to measure for water hardness, and act quickly, or the following problems will occur:

  • Mineral soaps form on surface of sump making it hard to skim oils
  • Gummy residues are formed on the inside of machine tool surfaces, causing valve malfunction. Left untreated, an actual crust can form which is hard to remove
  • Split coolant emulsions render the MWF useless

Below are advantages of using pure water over that of city or well water. Pure water contains no minerals but is more expensive. Review of these advantages, which are proven by field trials, is still recommended to determine whether it would be right for your company:

  • Mixes easier with less concentrate use
  • Improves wetting and lubricity
  • Reduces corrosion and gummy residues
  • Provides greater resistance to bacteria
  • Improves filtration processes
  • Less carry-off
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Metalworking Fluid Maintenance