Coolant concentration across shifts: keep the mix without guesswork

Why the mix drifts between shifts

If the coolant concentration was fine in the morning but is higher or lower by evening, it's rarely random. The mixture changes throughout the day even if no one intentionally touches it.

Most often water leaves faster than oil. The machine heats up, some moisture evaporates, some leaves with mist, and the refractometer reading rises. It looks like the emulsion got "stronger", but in the tank there is simply less water.

Then another shift adds water by eye to restore the level. After that the mixture can easily fall below the target. Over 24 hours the shop can swing both ways: first up because of evaporation, then down because of an unmeasured refill.

Another reason: different machines lose coolant differently. One lathe carries away more fluid with chips, another produces more mist, and a machining center may lose volume after a long run. With several machines the overall picture quickly becomes confused. One machine’s concentration goes up while the neighbor’s goes down.

Missed records almost always make this worse. One shift added 20 liters of water, another removed chips along with wet emulsion, a third saw a low level and added ready mix. If no one recorded what and how much was added, the next shift won't understand where the new numbers came from.

Shifts typically combine several sources of change: evaporation during operation, emulsion carried away with chips and mist, and finally unmeasured refilling. So relying only on tank level is useless. The same level can hide very different mixtures.

To keep concentration steady you need one procedure for all shifts: who measures, when they measure, what they add and what they record.

What to keep by the sampling point

If the refractometer sits in the office and the logbook is in the supervisor's drawer, measurements will almost always be postponed. On the shop floor this usually ends the same way: the mixture starts to "float" and the cause is lost by the next shift.

At the sampling point you need a simple kit that can be grabbed in a minute and put back. The core is a handheld refractometer with a scale suited to your specific coolant. A universal device without the right correction often gives a number that looks normal but leads refills the wrong way.

Keep a clean pipette or a dedicated syringe for sampling. Don't take fluid with whatever is at hand: oil from gloves, dirt from the tank rim and fine chips quickly spoil a sample. Also have soft wipes for the prism and cover glass. If you wipe the lens with rags used around the machine, a film stays on the glass and the reading shifts by a few tenths. For emulsion that’s enough for a shift to add extra product.

Another often-forgotten item is the conversion table for the specific fluid. The refractometer gives a number, not the final percent; you need to multiply by your mixture coefficient. If the shop uses two coolants, label the tables by name or color to avoid confusion.

The measurement log can be paper or a simple form on a tablet. Usually a few fields are enough: date, shift, machine, instrument reading, final percentage after conversion, and what was added after the measurement.

Store everything in one closed box near the sampling point. Each item should have its place. If the pipette or wipes get contaminated, replace them immediately. Fasten the conversion table inside the lid or put it in a clear sleeve so people don't search for it across the shop.

On a lathe shop this kit yields a simple result: people measure following the same routine instead of relying on memory.

How to take a measurement, step by step

A precise measurement takes a couple of minutes, but it must be done the same way every shift. If one operator samples the surface and another samples the flow, the log may look consistent while the tank mixture has already drifted.

First, let the pump run for 5–10 minutes so the fluid mixes and oil and water don’t sit in layers. After a long idle period a sample almost always lies.

Take the sample from where the mixture is circulating, not from the surface. Foam, stray oil and dirt often collect on top and can make the instrument read high.

The procedure is simple:

1. Wipe the prism and cover with a clean soft wipe.
2. Apply 2–3 drops of sample to the prism.
3. Close the cover gently so the liquid spreads evenly.
4. Take the reading in normal light.
5. Immediately convert the result using the table for your coolant.

Here the most frequent error appears. The number in the eyepiece is not yet the real concentration. Each coolant has its own coefficient, and without conversion you can miss by 1–2 percent. For a machine that matters: lubrication falls off, tools wear faster and surface finish worsens.

Record the result right away, not at the end of the shift. Otherwise the numbers get mixed. Usually date, shift, machine number, instrument reading and converted percent are enough.

If a result is outside the usual range, repeat the measurement immediately. For example, if it was 7.5% in the morning and an hour later you see 6.0%, take another sample from the flow and check that the prism is clean. That prevents refilling based on an erroneous number.

How to refill without causing imbalance

The most common mistake is refilling "by eye." After that concentration jumps and the next shift doesn't know what changed.

First look at two things: tank level and the last measured concentration. If the level fell but the reading is higher than usual, most likely water left. If level and concentration both fell, the system lost part of the working mix through chips, mist or leaks.

Do not pour concentrate directly into the tank. It doesn’t mix immediately, creates a locally too-strong zone and spoils the next measurement. After that an operator will see a wrong number and try to correct a problem they created.

Working procedure

First compare the current level with the last recorded level. Then decide what to add: water or a pre-mixed portion of the correct strength. Better mix the portion in a separate clean container, after refilling run the pump for about 10 minutes and then check composition again. Record the result immediately.

A separate container is not just for show. It makes it easier to measure volume, mix calmly and avoid pouring blindly. On a typical lathe shop a single accurate refill is usually better than three quick corrections in a row.

Simple example: at the end of the night shift the level dropped and the measured concentration rose from 7% to 8.5%. This usually means water left faster than the emulsion. In that case don’t add pure concentrate. Prepare a portion that returns the mix to the working range. Sometimes you add water, sometimes a weaker mix than what’s currently in the tank.

After circulation check again. If the number didn't return to normal, check the prism, the surface for stray oil, and whether the pump ran long enough. It’s useful to record both values: before and after refill. After a few days the log shows where the mixture consistently leaks and which shift keeps it steadier.

How to manage control across shifts

When concentration jumps from shift to shift, the argument is usually not about the fluid itself but about the procedure. One operator measures right after start-up, another measures before stopping, a third records from memory. After that the log quickly loses meaning.

Set two identical control points for everyone: at the start and before handing over the shift. Prefer the same time window for each machine. For example, measure after circulation starts and again 20–30 minutes before the shift ends. Then numbers can be compared without guessing.

Each machine should have its own sheet or form. Not one common log for the whole area, but a clear card for each machine. Note the target, allowable deviation and the usual refill procedure. If one machine is kept at 7% and another at 9%, the operator shouldn’t have to remember that from memory during the shift.

Record more than just percent. Without the refill volume a log entry is almost empty. In a week nobody will know whether the mix changed because of evaporation, carry-off on parts or because someone added too much. Usually four fields are enough: measurement time, actual concentration, what and how much was added, and a short note about why the deviation occurred.

That last note often saves the most. Handover should be simple: "6.2% (norm 7%), added 15 L ready mix, carry-off increased due to a long batch, smell normal." Entries like that help the next operator understand the situation immediately. Phrases like "mix dropped again" or "check the tank" only add confusion.

If someone notices foam, sour smell, slime on the tank wall or darkening, record it and pass it on verbally. Then bacterial control doesn't become a complaint at the end of the week when the problem has spread across the area.

On a shop with three shifts this routine quickly pays off. Within a few days you can see which machine is refilled more often, which shift lets the mix fall, and where the cause is process-related or simply a habit of pouring by eye.

How to spot bacteria before a strong smell appears

Bacteria rarely ruin the mix in one day. The fluid usually changes gradually: first behavior alters, then appearance, and only later does a strong odor spread. If you watch a short set of signs, you can catch the problem earlier and avoid a full replacement.

A check at the start of each shift takes a couple of minutes and saves downtime and surprises by week's end.

What to check daily

First, smell the tank. A normal emulsion smells faintly or nearly neutral. A sour, musty or rotten odor is a clear signal.

Then look at the fluid. Watch for persistent foam, slime or sticky deposits on the tank walls, a darker or dirtier color, surface film, clumps or deposits in corners and calm zones.

One sign alone doesn’t always mean trouble. But if two or three signs appear together, don’t delay. Check concentration, remove visible contamination from the tank and see whether extra chips, oil or old mix got into the system.

Don't ignore operator complaints. Operators often notice problems before the log. If someone reports itching, dryness or skin irritation after work, check the coolant that same day. The cause isn't always bacteria, but it's a good early warning.

If test strips are available, use them at least weekly. That’s enough to spot a shift before a strong odor or whole-shift complaints. Do the test the same day each week and record the result next to regular measurements.

The most common mistake is checking the tank only when the whole area smells. Much better is catching small changes: dark film, sticky walls and odd foam usually appear before a strong odor.

Where mistakes happen most often

Mistakes are usually simple. They make the mixture jump even where there’s an instrument and people do measurements. The problem is rarely the refractometer itself. More often the procedure is wrong.

First mistake — sampling immediately after idle time. After a night, lunch or long pause the tank is stratified: foreign oil and foam collect on top, solids settle, temperature changes. A sample then gives a random reading. Always run the pump for a few minutes first.

Second mistake — relying on one number. Concentration is important but doesn’t explain everything. If the fluid smells sour, foams heavily, forms a sticky film or the machine soils more, numbers alone aren’t enough.

Another common error — adding water whenever the fluid darkens. Dark color doesn’t always mean the emulsion has changed strength. It can darken from fine chips, heating, dirt or stray oil. Measure first, then add based on facts, not color.

Mixing different coolants in one tank is a separate problem. Even if both are suitable for metalworking, their additives differ. After mixing the emulsion can separate, foam or start to smell. When changing brand or type, drain and rinse the tank.

One persistent mistake — too-infrequent tank cleaning. People adjust concentration, hide the smell for a day or two, and the problem returns. That happens when residue in the tank, hoses and hard-to-reach corners still contains bacteria. Until you remove that deposit, the log can show normal numbers while the shop keeps returning to the same issue.

A good rule: don't trust a single sign. Check measurement, smell, foam, film and refill history together.

Example for a three-shift shop

In a three-shift shop concentration often drifts because of several small actions during the day rather than one big mistake. This is clear on a lathe where the working target is 8%.

In the morning the first-shift operator measures 9%. He doesn’t jump to conclusions. He repeats the sample, wipes the prism and checks again. The number stays the same. The log shows an unrecorded concentrate refill overnight. The problem is procedural, not "bad chemicals."

The first shift records time, result, tank level and a note about the unrecorded refill. They don't stop the machine if the emulsion looks normal and there's no sharp smell. Their job is not to swing the mixture further but to hand a clear picture to the next shift.

The day shift doesn't try to force 9% back to 8% with one big water addition. That usually creates a swing the other way. Instead they prepare a weaker mix than the tank and add it in small portions. After each refill they let the system run 10–15 minutes and measure again. By shift end the composition gently returns close to the target.

Daily log might look like:

• 07:00 — 9.0%, found an unrecorded overnight concentrate refill.
• 14:00 — 8.4%, added small portions of a weaker mix.
• 21:30 — 8.1%, foam normal, no sour smell.

In the evening the supervisor checks not only numbers but smell, foam, surface film and wall condition. If there’s a sharp odor or persistent foam, they add a note for early bacterial testing and cleaning rather than masking the problem with another refill.

After a few days that log shows a real picture: which machine loses water more often, which shift refills without recording, and where the mixture floats more than usual. Then it’s easier to change the work procedure: stop unapproved refills, fix measurement times and keep the target without arguments.

Quick check at shift handover

Three to four minutes before handing over the machine make a short check. This answers half of the morning questions: why the mix shifted, who refilled, where the foam came from and whether you can start the next shift without extra delay.

First, take the instrument reading and compare it to the working range for that coolant. If it’s out of range the shift should not just go home. Record the deviation and note what was added before handover.

For handover five actions are enough: take a measurement, record refill volumes during the shift, quickly smell the tank and work area, check foam and surface film, then hand a short clear note to the next shift.

Write refill volume in the log immediately rather than keeping it in memory. Even one missing line breaks the whole picture. The day shift might have added 15 liters of water after a long run and the night shift will see only a drop in concentration and start correcting it blindly.

Smell also says a lot. If the emulsion was neutral yesterday and by shift end a sour or musty note appears, that’s cause for attention. The same applies to foam and slime: even if the instrument still shows normal numbers, note those changes right away.

The handover should be short and precise. One line is enough: what was measured, what was added, tank condition and any comments. For example: "Concentration normal, added 10 L ready mix, smell unchanged, no foam." Such a note saves time and leaves no room for guessing.

If you do this check consistently the mixture stops "living its own life." The shop sees the real picture by shift instead of guessing from the morning tank state.

Where to start next week

Don't try to change all shop habits at once. In the first week pick one goal: make concentration stop being an argument and become a routine number in the log.

First set a target for each machine separately. Not "about 7–8% across the shop," but a specific working range for each machine and operation if they differ. Keep those numbers at the machine: target, tolerance, who measures, what they use and what to add when out of range.

Then start one 14-day log. Not several notebooks by shift and not notes "somewhere with the supervisor." One format for everyone quickly shows whether the mix problem is chemical or a discipline issue. Usually five fields are enough: date and shift, machine, instrument reading, what and how much was added, and a short note about smell, foam, stray oil or dirt.

After 5–7 days the pattern starts to appear. If the mix is almost always weaker in the morning, look for water-only refills, carry-off on parts, leaks, evaporation or different shift habits. If after weekends the mix is stronger, water is leaving faster than emulsion. If one machine swings every day, check the tank, a leak or the habit of refilling by eye.

Analyze deviations based on log entries. "It was fine yesterday" tells almost nothing. You need numbers by shift, refill volumes and repeatability.

If you’re starting a new area, agree this routine in advance. At EAST CNC it’s worth discussing during machine selection and then fixing it at commissioning and service. That way shifts have a clear routine from day one instead of three different habits.