Three drums sit in the same storeroom, all sold as coolant. One is a soluble oil, one a semi-synthetic, one a full synthetic. None of those three terms carries a binding definition, and the same concentrate can appear in three suppliers' catalogues under three different headings. A buyer who believes he "switched from semi-synthetic to synthetic" may simply have bought a lower-oil version of what he already had.
That matters because most water-miscible fluid failures are not chemistry failures. They are dosing failures, and dosing is checkable in thirty seconds with a hand refractometer — provided the operator knows the factor. This article covers that factor, the pH window the workpiece metal imposes, the water hardness that quietly ends emulsion life, and the concentrate arithmetic that biocide labels assume every formulator already knows.
Three families, one continuous axis
The industry divides water-miscible fluids by mineral oil content, and the divisions are conventions rather than standards. What actually differs is how the oil and the additives are held in water: a soluble oil uses an emulsifier package to carry a large oil phase, a semi-synthetic carries a smaller one plus synthetic lubricity additives, and a full synthetic carries no mineral oil at all and relies on polyalkylene glycols, esters and carboxylates for lubricity.
| Property | Soluble oil (emulsion) | Semi-synthetic | Full synthetic |
|---|---|---|---|
| Mineral oil content | 50–85% | 10–40% | 0% |
| Refractometer factor | 1.0 (read directly) | 1.3–1.7 | 2.0–3.0 |
| Brix reading at 5% true concentration | 5.0 | 2.9–3.8 | 1.7–2.5 |
| Working concentration | 5–10% | 4–8% | 3–8% |
| Cooling | Good | Very good | Best |
| Lubricity under low-speed, high-load cutting | Best | Good | Moderate |
| Foam tendency in soft water | Low | Moderate | Highest |
| Tramp oil rejection | Poor (emulsifies it) | Moderate | Good (releases it) |
| Typical service life, well maintained | 3–6 months | 6–12 months | 12 months or more |
| Sump hygiene demand | High | High | Highest (no oil to buffer) |
Read the last two rows together and the commercial logic appears. The full synthetic is the most expensive per litre of concentrate and the cheapest per litre of working fluid, because it runs at a lower concentration and lasts longer — but only if the sump is actually managed. Put a full synthetic into a shop that tops up with tap water and never skims, and it will fail faster and more expensively than the soluble oil it replaced.
The refractometer reads sucrose, not coolant
A hand refractometer measures refractive index and prints the result on a Brix scale calibrated against sucrose solutions. For a straight soluble oil, the sucrose calibration happens to land close enough to the truth that the number can be taken at face value. For anything with a synthetic component it does not, and the error is large enough to double or halve the dose.
The conversion is one multiplication:
Actual concentration (%) = Brix reading × refractometer factor
Run the arithmetic backwards and the two classic failures appear immediately. A full synthetic with a factor of 3.0 showing 2.5 Brix is already running at 7.5%. An operator trained on soluble oil sees 2.5, decides the sump is weak, and tops up to a "correct" 5 Brix — which puts the fluid at 15%, twice the intended dose, at twice the concentrate cost, with foam and skin complaints to follow.
The opposite error is quieter and more common. A semi-synthetic with a factor of 1.6 reading 5 Brix sits at 8%, the top of its range. Diluting it back to a "correct" 3 Brix drops it to 4.8% and cuts corrosion protection below the level the DIN 51360/2 result was established at.
Tramp oil pushes the reading the wrong way
Hydraulic oil, way oil and spindle oil leaking into a sump raise the apparent Brix by roughly 1–3 points and smear the boundary into a 2–4 point gradient instead of a sharp line. Two habits follow. Read the lower edge of the colour change, because the upper edge includes emulsified tramp oil. And treat any reading taken through a visible oil layer as an estimate rather than a measurement — skim first, then sample from mid-depth, never from the surface foam.
Beyond the reading, tramp oil is the single largest cause of shortened sump life. It blankets the surface, cuts the fluid off from oxygen, and creates the anaerobic conditions in which Pseudomonas and sulphate-reducing bacteria thrive — which is where the rotten-egg smell and the pH collapse come from.
pH is set by the workpiece metal, not by the concentrate
Aluminium is amphoteric. Its protective oxide layer dissolves in both acid and alkali, which means the alkaline margin that protects steel is the same margin that attacks aluminium.
| Workpiece | Working pH window | Failure above the window | Failure below the window |
|---|---|---|---|
| Carbon steel, cast iron | 8.5–9.5 | Skin irritation, attack on aluminium fixtures, EP additive hydrolysis | Flash rust within 48 h, bacterial bloom |
| Aluminium alloys | 8.2–9.2 | Oxide dissolution, grey-black staining, pitting | White hydroxide deposits, microbial corrosion |
| Copper alloys | 8.0–9.0 | Dezincification of brass, staining | Tarnish, amine attack |
Freshly mixed fluid reads high, and this trips up more buyers than any other single number. A 5% dilution of a typical semi-synthetic can sit at pH 10.1–10.4 for the first day or two and then settle to 9.0–9.5 as the amine system equilibrates with atmospheric carbon dioxide. Two predictable complaints follow: a strong amine odour for 48 hours, and darkening of aluminium parts during the first one to three days of a fresh charge. Neither means the concentrate is wrong. It means the charge is young.
The corrective action is correspondingly narrow. Do not add caustic soda or ammonia to raise a drifting pH — you will overshoot, and you will add an aggressive alkali that the corrosion inhibitor package was never designed around. Add make-up fluid at the correct concentration from the same product, and if pH has fallen below 8.0 with an odour, the sump has a microbial problem that a pH adjuster cannot fix.
Hardness ends more emulsions than any other single variable
Calcium and magnesium do not attack the fluid. They attack the emulsifier. An anionic emulsifier prefers calcium to the oil droplet it was assigned to, forms an insoluble soap, and leaves the droplet without a stabilising film. The visible consequences are a scum ring on the machine, oil floating free on the sump, and a fluid that stops wetting the workpiece.
| Water hardness | Equivalent | What to expect |
|---|---|---|
| Below 100 mg/L as CaCO3 | below 5.6 °dH | Very soft: stable emulsion, but foam becomes the limiting problem |
| 100–200 mg/L | 5.6–11.2 °dH | Optimum for most concentrates |
| 200–300 mg/L | 11.2–16.9 °dH | Specify a hard-water-tolerant grade; expect shorter sump life on standard products |
| Above 300 mg/L | above 16.9 °dH | Soften the make-up water or accept calcium soap formation |
| Above 400 mg/L | above 22.5 °dH | Reverse osmosis is the practical answer, not a bigger emulsifier dose |
Hard-water tolerance is a formulation property that varies enormously between products and is almost never quoted in a one-page flyer. Some concentrates are specified stable from 5 to 30 °dH at make-up (roughly 90–535 mg/L) and tolerate up to 60 °dH in service as dissolved salts accumulate through evaporation. That tolerance is worth paying for in a plant on well water, and it is the first question to ask before quoting into a hard-water region.
Bacteria and fungi have thresholds four orders of magnitude apart
The working limits quoted for sump control are usually 106–107 CFU/mL for bacteria and 103 CFU/mL for fungi. The gap is not an arbitrary convention; it reflects how poorly the actives perform against each class.
For benzisothiazolinone (BIT, 2634-33-5) in metalworking fluids, the minimum inhibitory concentration is 13–18 ppm against bacteria and 25–45 ppm against fungi. In-use trials at 100–350 ppm delivered a 4–5 log reduction against bacteria over two to six weeks, but only a 1–2 log reduction against fungi at every dose tested. A dose that clears a bacterial bloom leaves the mould standing. That is why the fungal threshold sits four orders of magnitude lower: it is a threshold for a control agent that is already working at the edge of its spectrum.
The concentrate arithmetic behind every biocide label
Biocide labels specify a dose in the end-use fluid, while the formulator has to dose the concentrate. The conversion is a division by the dilution fraction — and it produces numbers that surprise buyers comparing two products at the same label dose.
| Target in working fluid | Dilution | Required in concentrate |
|---|---|---|
| 1,250 ppm sodium pyrithione | 5% | 1,250 ÷ 0.05 = 25,000 ppm = 2.5% |
| 1,250 ppm sodium pyrithione | 8% | 1,250 ÷ 0.08 = 15,625 ppm = 1.56% |
| 250 ppm BIT | 5% | 250 ÷ 0.05 = 5,000 ppm = 0.5% |
| 250 ppm BIT | 8% | 250 ÷ 0.08 = 3,125 ppm = 0.31% |
Two concentrates that deliver identical protection in the sump can differ by a full percentage point of active in the drum. A purchase order that specifies "0.2% biocide" without naming the dilution is not a specification at all — the supplier has no way to comply and no way to be wrong.
What belongs in the specification sheet
- Family and mineral oil content, stated as a percentage rather than a category name.
- Refractometer factor. Mandatory. Without it the sump cannot be controlled, and the shop will invent its own number by trial and error.
- Recommended concentration by operation — grinding typically 4–6%, turning and milling 7–10% on steel and cast iron, 8–12% on aluminium.
- pH of a fresh 5% dilution and pH after 48 hours, so the buyer knows that the initial reading is transient.
- Hardness tolerance at make-up and in service, in °dH or mg/L, not as "suitable for most water".
- Corrosion result at the working concentration, e.g. DIN 51360/2 chip/filter paper test at 4% with corrosion degree 0.
- Biocide identity and dose in the end-use fluid, plus whether it is dosed in the concentrate or tankside.
- Foam behaviour at the recommended hardness. Soft water is where foam shows up, and it is the complaint that gets a product rejected.
- Tramp oil behaviour — whether the fluid emulsifies leaking lube oil or rejects it for skimming.
- Concentrate shelf life and freeze–thaw stability, since a 200 L drum may sit through a winter in an unheated warehouse.
Selecting by application
- General steel and cast iron turning, milling, drilling: soluble oil or semi-synthetic at 6–10%; soluble oil where low-speed heavy cuts need film strength, semi-synthetic where sump life and rancidity are the recurring cost.
- Grinding: 4–6% of a low-foam semi-synthetic or synthetic; the job is cooling and flushing, not lubricity, and foam is the enemy.
- Aluminium and its alloys: 8–12% of a dedicated aluminium grade held at pH 8.2–9.2, silicate-based corrosion inhibition, and no sulphur or chlorine EP additives — those turn the machined surface dark.
- Central systems and high-value transfer lines: full synthetic at 3–6% where the fluid must last a year and tramp oil must be skimmable rather than emulsified.
- Hard water above 300 mg/L: either a concentrate specified to 30 °dH or more, or softened make-up water. Increasing the dose to compensate accelerates calcium soap formation instead of fixing it.
For the oil-phase side of these formulations we supply white mineral oil as 5#, 22# and 32# industrial grades (CAS 8042-47-5) for the carrier phase of soluble oils and semi-synthetics, butyl oleate (CAS 142-77-8) and butyl stearate (CAS 123-95-5) as ester lubricity additives, stearamide (CAS 124-26-5) for boundary lubrication and corrosion co-protection, and pentaerythritol (CAS 115-77-5) and dipentaerythritol (CAS 126-58-9) as the polyol cores behind synthetic esters. For sump preservation we stock metalworking fluid preservative YS-MK based on benzisothiazolinone (CAS 2634-33-5) and YS-HEY based on sodium pyrithione (CAS 3811-73-2).
Biocide registration and product-type classification are a separate subject and are covered in our industrial preservatives guide; the oil-phase chemistry behind these fluids is covered in our polyol ester base stock guide and the ester slip and lubricity additives in our fatty amide and ester lubricants guide. For machines rather than fluids, the viscosity window is set out in our hydraulic oil ISO VG guide.
All grades are available for B2B supply with sample quantities shipped worldwide. Requests for specifications, refractometer factors, or formulation support are welcome through the enquiry form on any product page.