Coolant concentration out of range: what should operators do?

This guide covers water-miscible metalworking fluids in machine sumps or small central systems, checked by operators with a handheld refractometer. It does not cover straight oils, pH or bacteria problems, or a sump that smells, separates or shows slime; those wait for the supplier. Target ranges and factors come from the product data sheet.

What do operators need before correcting coolant concentration?

Before correcting coolant concentration, operators need a zeroed refractometer, the product's refractometer factor and target range, the sump capacity, a premix or proportioner, the protective equipment named in the coolant's safety data sheet (SDS), and a log sheet. Without the factor and range at the machine, an operator cannot tell whether a reading is out of range at all.

Take the factor and range from the current product data sheet. Haas Tooling notes that the conversion value "can vary by coolant brand and type, so be sure to refer to the coolant's product data sheet" (Haas Tooling, accessed 2026). Post the factor, the range and the sump capacity on the machine.

Agree in writing how far operators may correct on their own and when they wait for the supplier. ASTM E1497-23, the active ASTM practice for selecting and safely using metal removal fluids, covers storage, dispensing and maintenance (ASTM E1497-23), and HSE guidance says people doing fluid checks should be trained in the procedure, how to record and interpret results, and "the remedial actions to take" (HSE MW5, 2021).

How do you confirm a coolant refractometer reading before correcting it?

Confirm a coolant refractometer reading before correcting it by zeroing the refractometer with clean water, taking a fresh sample from circulating fluid, applying the product factor and checking that the boundary line is sharp. Correct only when a second reading agrees with the first.

1. Zero the refractometer. Place a few drops of water on a clean prism; the line should sit on zero, and if it does not, turn the adjustment screw (Haas Tooling, accessed 2026).

2. Sample moving fluid, such as the nozzle, not the still surface. Quaker Houghton notes the refractometer is thrown off by tramp oil, which should be removed from the sample before testing (Quaker Houghton, 2020).

3. Apply the factor. Multiply the Brix reading by the product factor; a reading of 5 means 5.0% only if the product is 100% active (Quaker Houghton, 2020).

4. Check the line. A diffused reading means the emulsion is either unstable or heavily loaded with tramp oil (Blaser Swisslube, accessed 2026). Do not correct on a blurred line; report it.

5. Repeat with a second sample and compare.

Tramp oil also pushes readings up over time, because tramp oils in the fluid also refract light (Master Fluid Solutions, 2019).

What should operators do first when coolant concentration is low?

When coolant concentration is confirmed low, operators should first check the sump level and look for water getting in, then raise the concentration with a richer premix, not neat concentrate poured into the sump. The STLE article advises checking for water inflows when concentration is decreasing (STLE, 2022).

The STLE article gives a correction for low concentration: remove and save about 10% of the system volume, then add prediluted fluid at a higher concentration until the fluid is in range (STLE, 2022). The saved fluid, if properly aerated and not contaminated with excessive microbes, can be added back gradually over time (STLE, 2022).

Mix the premix in a clean container, adding concentrate to water, not water to concentrate; the article calls this "oil in last" and notes that diluting concentrate in the sump is not recommended (STLE, 2022). Wear the gloves and eye protection named in the concentrate's SDS and follow the plant's chemical handling procedure. For a routine level top-up when the concentration is already correct, OSHA's manual says water and concentrate can be added if prediluted fluid is not available (OSHA, accessed 2026); a low reading still needs premix.

Act on a low reading the same shift. Haas notes that lean coolant can reduce tool life, cause corrosion on parts and inside the machine, and may lead to bacterial growth in the tank (Haas Tooling, accessed 2026).

What should operators do when coolant concentration is too high?

When coolant concentration is too high, operators should add water or a leaner premix to bring it down, and check whether evaporation or a rich makeup mix caused it. HSE notes that water-mix fluids evaporate and become more concentrated in use (HSE MW5, 2021), and the STLE article advises confirming that makeup fluid is dilute enough to compensate for evaporation (STLE, 2022).

OSHA's manual gives the simple case: when the sump level is low and the concentration is too strong, "add water to reach the proper concentration" (OSHA, accessed 2026). When there is no room to add water, use the STLE article's correction: remove about 10% of the volume, save it, and replace it with prediluted fluid at a lower concentration (STLE, 2022).

Do not leave a rich sump for the next visit. Rich coolant wastes concentrate and leaves a sticky, oily residue on parts, tools and inside the machine (Haas Tooling, accessed 2026), and the STLE article notes that a fluid that is too concentrated increases the risk of irritating workers' skin (STLE, 2022). Before the correction, check that tramp oil is not inflating the reading: skim the surface oil, then read again.

How do you top up a coolant sump without overshooting the range?

To top up a coolant sump without overshooting the range, work out the premix strength before adding it, add it in part, mix, and read again before adding more. The calculation is a volume balance: new concentration equals the kept volume times its concentration, plus the added volume times its concentration, divided by the total volume.

An illustrative example: a 400 L sump reads 4.5% against a 5% to 7% range. Removing 40 L (10%) and adding 40 L of 15% premix gives (360 x 4.5 + 40 x 15) / 400 = 5.55%, inside the range. The same 40 L at 25% would give 6.55%, near the top.

Water has a limit too: an illustrative 400 L sump holding 340 L at 7.5% and topped up with 60 L of water ends at (340 x 7.5) / 400, about 6.4%, still above a 6% target.

Retest only after full mixing. The STLE article advises retesting after the fluid has recirculated long enough to turn the system over at least twice (STLE, 2022), and OSHA's manual says the correct concentration should be verified when finished (OSHA, accessed 2026).

How do you check that the coolant concentration correction worked?

A coolant concentration correction worked when a reading taken after full recirculation, on a zeroed refractometer and with the current product factor, falls inside the target range and is logged with the amount added. The check table below decides whether the correction stands or needs another round.

The reading after mixing matters most: a reading taken right after adding premix describes the premix, not the sump. Work through the rows in order, because a failed zero or an old factor makes every later row meaningless. If two rounds of correction have not brought the reading into range, stop and report, because the cause is likely a leak, a proportioner fault or contamination that more fluid will not fix. HSE recommends keeping records in a table or graph "to make it easy to spot trends and act on the results to maintain fluid quality" (HSE MW5, 2021), so log each correction on the same sheet as the routine readings.

Which coolant values should operators send the supplier before the visit?

Operators should send the supplier the machine, product, date and time, raw Brix reading, factor used, corrected concentration, retest value, sump level, what was added with its volume and strength, and any notes on smell, foam, oil or a blurred line. Those values let the supplier arrive with the right test kit and premix.

OSHA's manual says operators can be trained to report anything unusual, and that employee observations should be documented and compared with laboratory data and chemical additions (OSHA, accessed 2026). Send pH too if the plant measures it, since the STLE article links sudden or gradual pH changes to changes in concentration (STLE, 2022).

Send the values the day they are taken, as numbers: "topped up, now fine" does not tell the supplier how far out the sump was.

For how suppliers can collect these readings without retyping them, see collecting site readings from customers without retyping. In Lubinpla, a supplier can share the response procedure document through Field Care, and the Field Care agent answers operator questions from that document and hands questions it cannot answer to the assigned staff member.

Common coolant concentration correction mistakes

The commonest coolant concentration correction mistakes are correcting on a single reading, pouring neat concentrate into the sump, topping up with water when the concentration was already right, using an old refractometer factor, and retesting before the fluid has mixed. Each one can push a sump from one side of the range to the other.

Neat concentrate poured into the sump mixes poorly, and the STLE article says diluting concentrate in the sump is not recommended (STLE, 2022). Water-only top-ups are wrong when the concentration was in range and the level fell: OSHA's manual says that in this case fluid was lost to dragout and fluid at an appropriate dilution should be added (OSHA, accessed 2026). An old factor after a product change shifts every reading, because each product has its own refractometer factor (Quaker Houghton, 2020). Retesting too early reads the premix; wait for two system turnovers (STLE, 2022).