I test coolant regularly because I learned the hard way what happens when you do not. A batch of hydraulic cylinders came out with stained bores because the coolant pH had dropped too low. The coolant had gone acidic and was etching the steel surface. The customer rejected the whole batch — 45 parts scrap at $80 each.
That loss was $3,600 from a $50 testing kit I had on the shelf but was not using. I test religiously now. Here is exactly what I use and how often.
The Four Tests I Run
Concentration (Refractometer)
A refractometer measures coolant concentration by reading the refractive index of the coolant sample. The number on the scale corresponds to the percentage of coolant concentrate in the water.
I target 8-12% for emulsion coolant in deep hole drilling. Below 8%, the coolant does not provide enough lubrication and corrosion protection. Above 12%, the coolant becomes tacky and leaves residue on the parts and machine.
The refractometer costs about $30-$80. I use a digital model that costs $120 — it eliminates the guesswork of reading a manual scale. Calibration is simple: I check against distilled water before each use.
pH (Test Strips)
pH strips confirm that the coolant pH stays between 9.0 and 10.0. Below 8.5, the coolant starts to go acidic and can cause corrosion. Above 10.5, the coolant can cause skin irritation and may attack aluminum.
I use pH test strips with a range of 7.0 to 14.0 and 0.5 increments. A pack of 100 strips costs $10-$15. I dip a strip in the coolant, wait 15 seconds, and compare the color to the chart on the bottle.
When the pH drops below 9.0, I add a pH booster to bring it back up. If the pH drops below 8.5, the coolant is likely depleted and needs replacement. I do not try to save coolant at that point.
Bacteria (Dip Slides)
Bacteria contamination causes rancid coolant, foul odors, and operator skin irritation. It also breaks down the coolant chemistry, causing pH drift and corrosion.
Dip slides have an agar surface that grows bacteria colonies. I dip the slide in the coolant sample, let the excess drain off, and incubate it at room temperature for 48 hours. The colony count tells me the bacteria level.
The acceptable level is under 10,000 CFU/mL. Above that, I add a biocide treatment. Above 100,000 CFU/mL, I change the coolant entirely. A box of 10 dip slides costs about $40.
Water Hardness (Test Strips)
Water hardness affects coolant performance. Hard water (over 200 ppm calcium carbonate) reacts with coolant emulsifiers and causes the coolant to separate. Soft water (under 50 ppm) can cause foaming.
I test the water I use for mixing, not the coolant in the machine. Hardness strips cost $10 for 100 strips. If the water is hard, I use deionized water for mixing coolant. The cost of a small deionizer is about $200 and pays for itself in extended coolant life.
Testing Schedule
| Test | Frequency | Cost per Test | What I Look For |
|---|---|---|---|
| Concentration | Weekly | $0.30 ($30 refractometer / 100 uses) | 8-12% |
| pH | Weekly | $0.15 | 9.0-10.0 |
| Bacteria | Monthly | $4.00 | <10,000 CFU/mL |
| Water hardness | Quarterly | $0.10 | 50-200 ppm |
I do the weekly tests at the start of the Monday shift. It takes about five minutes total. The monthly bacteria test takes a few minutes to set up the dip slide and 48 hours to incubate.
Keeping Records
I keep a log of all coolant test results for every machine. The log shows trends over time:
- If pH drops from 9.5 to 8.8 over three weeks, I know the coolant is degrading
- If bacteria count increases from 1,000 to 8,000 over a month, I treat with biocide before it hits the danger level
- If concentration drops from 10% to 7% over a month, I look for leaks or excessive make-up water
The log takes two minutes to update per machine per week. It has saved me from scrapping parts more times than I can count.
Visual Checks: The First Line of Defense
I also look at the coolant. The test strips give me numbers, but my eyes tell me if something is wrong:
- Milky appearance. Normal for emulsion coolant.
- Clear separation. Oil or water has separated out — indicates emulsifier failure.
- Dark or gray color. Fine metal particles in suspension — indicates filtration problem.
- Rancid smell. Bacteria growth — confirmed by dip slide.
- Foam on surface. Soap-like bubbles — indicates concentration issue or water hardness problem.
A visual check is still the first thing I do when I approach a machine. If the coolant looks or smells wrong, I investigate regardless of what the test strips say.
When to Change Coolant
I change coolant when any of these conditions are met:
- pH below 8.5 and does not respond to pH booster
- Bacteria count above 100,000 CFU/mL
- Coolant is visibly contaminated with tramp oil
- Coolant causes skin irritation for operators
- Concentration cannot be maintained within range
A coolant change costs about $200-$500 per machine in materials and takes 2-4 hours for draining, cleaning, and refilling. That is much cheaper than scrapping a batch of parts because the coolant chemistry failed.
Key Takeaways
- Test coolant concentration and pH weekly — takes 5 minutes and costs less than $1
- Bacteria testing monthly catches contamination before it causes odor and corrosion problems
- A simple log shows trends that predict coolant failure before it happens
- Visual checks are the first line of defense — look at the coolant every time you approach the machine
- A coolant change costs $200-$500 but a scrapped batch of parts can cost thousands