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Machine Coolant Testing Methods

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Machine Coolant Testing Methods

Run these tests daily at first.  Once you get the system under control you shouldn't see much change from day to day. Then run the tests weekly.

How and When to Test machine coolant - Outside Lab                            

1. Particle size and count

before machine coolant change

2. Turbidity


3. Viscosity


4. pH


5. Conductivity


6. Cobalt levels

before machine coolant change

7. Color

no need - covered in other tests

8. Amount of sludge

no need - covered in other tests

9.  Refractive index


I suggest quarterly.  I would recommend this as a minimum.  It is enough to look pretty good to the government.  If you do a lot of grinding and have a lot of exposure you might want to consider doing this monthly.  Take a sample.  Test half of it with your own tests.  Send half to the lab.  This will tell you how good your coolant testing instruments are.  These instruments go bad just like anything else.  Take a look at Machine Daily Tests and our Machine Coolant Index for more information on Testing Machine Coolant. 

Before you change the machine coolant get a small sample.  This is as bad as your machine coolant will get.  Send it to a lab and get an official analysis.  This will tell you several things.

1.  Was your machine coolant safe and clean when you changed it?

2.  You can use this to decide if you need to change more often or less often. 


1. Particle size and count

Outside lab - $60

This tells you how much of each size particle you have in your machine coolant.  This will help tell you how much danger you and your equipment are in.  Particles below 10 microns are inhaled very easily.   The smaller particles are more likely to be suspended in the atomized machine coolant that you breathe.  This test is essential at least once to tell you what size filters you need on your machine coolant cleaning system. 

You can use a clear glass  jar. 

A.  The more sludge you have on the bottom the more particles you have.

B.  If you swirl the bottle you will get some idea of the size of the particles by just looking at it.  

Official Lab test results           Dirty                     filtered                    Unused

Particle size and count      75 - 80 million       80,000 - 1 million            12,000 

2.  Turbidity 

Outside lab - Turbidity  $30

Turbidity is a measure of how much solid material is suspended in your machine coolant.   This is a less accurate version of the particle count.  The particle count uses a laser and a computer to measure the particles in a sample and divide them by size.  Turbidity measures how much light shines through the material.  The more particles, the less light that gets through.  Another way to get an idea is to fill a clear glass bottle and let the machine coolant settle down.  It will eventually (maybe an hour or two) settle down so there is a layer of particles on the bottom.  In really dirty machine coolant there may be a sludge layer about half the depth of the machine coolant.   The bottle of filtered machine coolant has a layer on the bottom but it is too small to measure and I can see through the layer in places.  It covers maybe 3/4 of the area of the bottom. 

In an official lab test we got the following numbers.

Dirty: 45,000     filtered: 15    unused:  7.02 

3. Viscosity

Outside lab - Viscosity  $30

This is a test of how slippery the machine coolant is.  It is a measure of the rate of flow of a liquid due to internal restraints on that flow.   Slippery machine coolant is good for cutting.  

Test 1

The easiest and cheapest way to do it is to use a special cup.  You dip the cup in the machine coolant and fill it.  Then you measure the amount of time from when it starts draining until it starts forming drops.  You use a stopwatch to do this. 

Test 2

You use a specially shaped tube and measure the amount of time it takes the liquid to flow through an area. 

Test 3

You get a tube with balls inside it.  You measure the amount of time it takes the balls to fall.   You sometimes see this sort of thing sold in automotive stores to measure antifreeze.  The scientific tube is expensive and doesn’t work very well  

4. pH 

Outside lab - $20

pH is based on complex chemical analysis.  It is important because almost anything you do to a liquid changes its pH in some way.  So pH is good because it is easy to measure.  It is important in this industrial process because it is pretty easy to control.  

The pH scale.  This is the way it works because scientists around the world agree on it.  The reason is really complicated so just remember 7 is neutral. 

7 is neutral pH.  Below 7 means it is acid.  The further below it is the stronger the acid is. 

Above 7 means it is basic.  The further above 7 it is the stronger the base is. 


Citric acid is Vitamin C and you drink it in orange juice.  It is an acid and just a little bit above 7.  You can drink it and it is good for you.  It can be concentrated to where it is harmful but diluted in orange juice it is good.  Sulfuric acid is a ways below 7 and it is battery acid.  It will eat holes in your clothes and your skin.  


Dishwasher soap is a basic material as are many soaps.  If you wash with them enough you will get dishpan hands although part of the drying effect is simply from the water removing oils from the skin.  Drano or other toilet bowl cleaners are above 7 and they will eat holes in your clothes and skin. 

The easiest way to measure pH is with little dip papers.  You dip the papers into the machine coolant and they change color.  You then compare the color change to the chart and that tells you what your pH is. 

Official Lab test results

Dirty 8.08     Filtered  8.04     Unused  8.02 

Papers run from $10 to $50 depending on how accurate you want them.  They can get pretty accurate, but probably not accurate enough for these small differences. These can be pretty accurate especially if you specify a narrow range.  There is a paper that has a set of different strips on it.  The range is from 7.2 to 8.8 and has about 10 divisions in it.  That should be pretty good here.

The pH of a metalworking fluid is readily determined using litmus paper (available through fluid suppliers or laboratory‑supply companies) or a handheld pH meter. Litmus paper provides a quick, low cost means of estimating fluid pH. Its accuracy is limited to plus or minus one full pH unit and is not particularly effective in predicting biocide failure. 

pH meters are more expensive, but provide more accurate readings.  Depending on the degree of accuracy and other desired options, pH meter kits may be purchased at a cost ranging from as little as fifty dollars to several hundred dollars. Low‑ to medium‑cost pH meters are accurate to plus or minus 0.2 pH units, an accuracy sufficient for monitoring biological degradation. Although high‑cost meters are accurate to hundredths of a pH unit, this degree of accuracy is of little benefit with regard to fluid management. 

These sell from maybe $60 to $1500.   There are a whole bunch under $100.  Stay away from the really cheap ones the way you would from a really cheap anything else.   $60 ought to get you a good, basic meter.  

Tips on using pH meters

1.  pH electrodes must be kept wet and clean. If one dries out, soak it in water or an appropriate buffer solution for 24 hours. 

2. pH meters and testers must be calibrated with buffer solutions. It is best to use two buffers such as pH 7 and pH 4 to make sure the meter is working properly. 

3.  Mix the solution and let the meter reading stabilize for 10 to 20 seconds. Take the measurement by immersing the tip of the electrode only I inch into the solution.   

4. Do not be alarmed when white crystals form on the electrode; just soak the electrode in buffer or water. 

5. Conductivity

Outside lab - Conductivity  $30

This measure how well an electric current travels through the liquid.  It helps tell you how much of what is suspended in that machine coolant.  Conductivity measures how well an electric current travels through the fluid.  The more metals in the fluid the better the current will travel sort of.  A little meter runs maybe $80.  They go up to $1500. 

Official Lab test results - conductivity

dirty    2,210       filtered   1,508     unused  1,683 

6.  Cobalt, Chromium, Cadmium and other metals

You can test cobalt yourself.  The rest need lab tests.  You can also get some idea of the metals in your machine coolant from a conductivity test. 

You can buy special dip papers.  You need to keep the papers refrigerated to help extend the life of them.  They work on the basis of ion exchange.   You need to have the machine coolant sample below 7 pH.  You did not need to know what pH was to measure it.  You do need to measure pH to use the dip papers.  To measure Cobalt with the dip papers you have to adjust the pH to 7 or below.  Most machine coolants seem to run on the basic side, which means above 7.  This means that you will have to add some acid to bring the pH to 7 or below.  

Official Lab test results

Cobalt levels                          3,210                       299                           0.138 

7. Color

Color is important because if you are using the right machine coolant you can tell by the color change to red or purple how much dissolved cobalt you have in it.  Color is pretty easy with the right machine coolant.  As the machine coolant dissolves more and more cobalt it gets redder and redder or maybe more and more purple.  There is definitely a change towards more redness in some form.  This is actually a pretty good rough test. 

All you do is put some machine coolant in the bottle and let it settle down.  As the crud settles to the bottom you will typically see green on top and a reddish color in the middle with the sludge on the bottom.  The redder the color the more cobalt there is. 

8. Amount of sludge

 This is a simple test you can do yourself to find out just how much particulate you do have in your machine coolant.  It is really a no cost test to get about the same results that $90 worth of lab tests gives you.  This gives you an idea how much solid material you are getting in your lungs.  

9.  Refractive index

Refractometers are an inexpensive tool ($200 to $250) capable of measuring fluid concentration. A refractometer is a portable, hand held optical device that reads a fluids' index of refraction. The term "index of refraction" refers to a measurement of how much light is bent as it passes through a liquid. A fluid's index of refraction changes with the density and chemical composition of the fluid. Therefore, refractometer readings obtained for a metalworking fluid correspond to its concentration (the higher the reading, the greater the fluid concentration). 

By measuring a metalworking fluid's index of refraction, the optimum fluid concentration can be maintained. 

Refractometers are typically available though machine coolant suppliers and provide fast, reliable results. Tramp oils, cleaners, hydraulic fluids and other contaminants reduce its accuracy. 

Manufacturers recommend dilutions and corresponding refractometer readings for specific operations. For example, some fluids use a 20:1 to 30:1 dilution ratio. As illustrated in Figure 4‑3, a shop would need to keep the fluid's refractometer reading between 1.8 and 3.0 to maintain the 20:1 to 30:1 concentration. 

How to measure concentration using a Refractometer

You need:

1 . Refractometer

2. Sample jar

3. Eye dropper ‑ if available

4. Distilled Water / a small container of

5. Refract Chart (for your particular machine coolant)

6. Evaluation Sheet (for record keeping) 


1.  Fill sample jar with machine coolant sample about half way. (Sample jar should be clean.)

2. Take out the refractometer and apply distilled water to the lens to calibrate, (be sure to cover entire lens with solution) now focus ‑ calibration should be zero.

3.  Clean & dry off solution from refract lens.

4.   Using the eyedropper, take some of the sample machine coolant from sample jar and apply liberally to refract lens. Hold up to light and adjust focus. Read where the white line is and record (i.e., 2.1). This is telling you the number of working solids in solution.

5.  Refer this number to your refract sheet. The number will correspond to the dilution ratio (i.e., dilution; 15:1 = 2.1 solids). These numbers are demo only.

6. *Adjust your machine coolant up or down as necessary 

Titration Methods

Refractometer measurements are fast but are less accurate when the fluid is contaminated with tramp oils. To overcome this problem, vendors of fluids have developed titration kits to determine fluid concentration. The titration measures a specific chemical or group of chemicals and is less affected by interferences from tramp oil or water quality. While titration is more accurate than refractometer readings, the procedure varies by machine coolant, and excess contaminants can affect accuracy. 

The titration is done by taking a measured volume of fluid, adding an indicator, and then adding the titrant drop by drop until a color change is noted. The machine coolant concentration is determined from the number of drops of titrant added. 

Industrial testing kits  -$599

These contain:

Four different good quality meters:  pH, conductivity, viscosity, refractometer 

Two sets of test strips, pH & cobalt

An industrial quality, waterproof stopwatch   

12 sampling bottles

Unlimited technical support on a toll free 800 number 

Total machine coolant Laboratory Analysis

$272.00 will get you an analysis of your grinding machine coolant.  This will show you what you have in your machine coolant.