Coolant Management

The refractometer (Brix meter) scale is Brix

A handheld refractometer is originally a tool for measuring the concentration of sugar solutions (sucrose Brix %), so its scale shows the Brix value, not the actual coolant concentration. Scales of 0–10 and 0–32 are common; for the same reading, the 0–32 scale is coarser and less precise to read.^[1][2][3]

Formula for concentration from the refractometer reading

C = R × F

C

Actual concentration (%)

R

Refractometer reading (Brix %)

F

Refractometer factor (a conversion factor specific to each product; also called Brix factor or RI factor)

Check the factor in the product's documentation (product data sheet or single-product catalog). For a product with a factor of 1.0, the reading is the concentration.^{[4][3][5][2][1][6][7]}

How to think about the refractometer factor

A refractometer picks up only the components of the concentrate that refract light (the active ingredients), and the factor is determined by that proportion. If the concentrate contains 20% water, a 10% dilution reads at most 8.0. The factor is a number used for multiplication; it is not a measure of performance or water content. A reading of 5 means 5% only when the concentrate is entirely active ingredients.^[1][4]

Typical factors by type (guideline from Master Fluid Solutions)

The company's material gives the range of factors as "from 0.9 for emulsions to 3.4 for synthetics". The bottom row of the table is headed Semisynthetics, but the text gives 3.4 as the maximum for synthetics. This is a guideline for the company's own product lines and cannot be used for other makers' products.^[4](1 source, for reference)

Examples of product refractometer factors (manufacturer-published values)

Even among products called "soluble" or "synthetic", factors differ greatly, from 1.4 to 2.9. Always use the factor of the product you actually use. Back-calculating from MORESCO's examples, the factor changes slightly with concentration.^{[6][8][9][7][11][12]}

Zeroing before measuring

Before measuring, set the scale to 0 with water (Blaser: good-quality water; Quaker Houghton: distilled water; Haas: distilled or tap water). A refractometer with automatic temperature compensation (ATC) does not need calibration every time, but check zero with water from time to time.^[3][1][2]

How to measure with a refractometer

^[5][2]

Tramp oil shifts the reading

Tramp oil such as way oil and hydraulic oil also refracts light, so in aged fluid the refractometer reading tends to be higher than the actual value. If the fluid is very dirty, the boundary line blurs and becomes hard to read. Fresh, well-mixed fluid gives a sharp boundary line. Remove tramp oil before measuring.^{[13][5][3][1]}

Ways to measure concentration other than a refractometer

These include alkaline titration (suited to synthetics and semisynthetics; in aged fluid it tends to read lower than actual) and acid splitting (suited to soluble oils; the oil is separated and measured, taking several hours to a day). Comparing the results of several methods lets you notice abnormalities early.^[13][14][1]

Converting concentration (%) to dilution ratio

Dilution factor = 100 ÷ C / water : concentrate = (100 − C) : C

C

Concentration (volume %)

The dilution factor is "how many times the concentrate was diluted"; the concentration is "how much concentrate is in the diluted fluid". Adding water to 2 L of concentrate to make 100 L gives a 50× dilution and 2%. 5% is often called "20:1", but the actual ratio of water to concentrate is 19:1. Stating it as a ratio easily causes misunderstanding, so it is better to communicate in %.^[15][1][16]

Guideline for the amount of concentrate to add to raise concentration

Concentrate to add ≈ tank volume × desired increase in concentration (%) ÷ 100

Example: raising 123 gallons by 1% takes 1.23 gallons of concentrate. Strictly, the volume grows by the amount added, so this falls slightly short (about 1.24 gallons), but shop concentration measurements are only accurate to about 1/4%, so this calculation is sufficient in practice. Measure the volume with the pump stopped and the fluid back in the tank.^[16](1 source, for reference)

Makeup amount to return to target concentration (volume balance)

Vc = (Ct × Vf − Cn × Vn) ÷ 100 / Vw = Vf − Vn − Vc / makeup fluid concentration Cm = 100 × Vc ÷ (Vf − Vn)

Ct

Target concentration (%)

Cn

Current concentration (%, refractometer reading × factor)

Vf

Full tank volume (L)

Vn

Current volume (L)

Vc

Concentrate to add (L)

Vw

Water to add (L)

Cm

Makeup fluid concentration (%)

A formula solved by treating concentrate as 100% and concentration as volume %, so that the amount of concentrate after makeup matches the target. When Vc is negative, the current fluid is too concentrated, so bring it back with water (or a dilute makeup fluid) alone. Master Fluid Solutions says four values are needed to work out the makeup amount (current concentration, target concentration, current volume, and full-tank volume) and points to an online calculator, but the formula itself was not found in published sources, so it was derived from the definition of volume %.(formula derived by Kezuriba)^[4][16](1 source, for reference)

How to make up

Do not add only concentrate or only water; add makeup fluid mixed at a set ratio. Blaser Swisslube (US) says it is common to make the makeup fluid about 1/5 to 1/3 of the target concentration. Yushiro recommends making up daily at a constant dilution ratio and recording the amounts of water and concentrate added. Keep the tank level at 85–100% (as it drops, temperature, contamination, and foam increase).^{[17][15][18][4]}

Mixing order (water first, concentrate last)

Put the water in the container (or tank) first and add the concentrate while stirring (in English, "Oil In Last" = O.I.L.). Doing it the other way round can produce a water-in-oil emulsion or leave the mix uneven. For emulsion types, an emulsion that has once formed cannot be fixed afterwards. It is better to mix the makeup fluid outside the tank before adding it.^{[4][18][17][15]}

Recommended concentration guide (by type and operation)

A guideline compiled from the recommended concentrations in Yushiro's product list (Yushiroken Ver.20), grouped by type. In practice, follow the recommended range of the product you use. Blaser Swisslube (US) says "in many cases, 3–5% above the minimum in the documentation gives better results", and recommends keeping the fluid at 8% (by volume) or more while the machine is stopped, such as during shutdowns, to maintain pH buffering.^{[19][4][17][5]}

What happens when it is too concentrated or too dilute

^[13][2][15]

pH control range

What they share is that "it is weakly alkaline (roughly 8–10.5), and a drop from the fresh-fluid value is a sign of deterioration". Check the proper value for each product in its documentation.^{[15][20][21][5]}

What pH changes indicate

As fluid is used, pH gradually drifts toward neutral, and falls further as spoilage progresses. A drop leads to poorer machining quality, rust, and a spoilage odor. Conversely, if the pH of in-use fluid is higher than that of fresh fluid, look into the cause. Blaser lists skin irritation, aluminum staining, foam, and attack on paint and resin parts for pH too high, and bad odor, rust, unstable emulsion, and more bacteria and biofilm for pH too low.^{[15][20][22][5]}

How to measure pH

The test method in JIS K 2241:2017 (cutting fluids) uses a glass-electrode pH meter. To improve accuracy on the alkaline side, calibrate the pH meter with pH 6.86 and 9.18 standard solutions, and draw the sample from a place with few metal chips (HORIBA). In the shop, pH test paper is generally sufficient, but dyes and tramp oil can throw off the reading (Master Fluid Solutions).^{[20][23][22][5]}

Signs of spoilage (bacteria and mold)

A rotten-egg smell when the machine is started after a break (hydrogen sulfide produced by anaerobic bacteria; it builds up easily when a film of tramp oil keeps the fluid from air), brown or black stains on workpieces, rust, separation of the emulsion (soluble oil), clogged filters or poor flow in piping from mats of mold, and a drop in pH.^{[24][25][15][5]}

Characteristics of coolant bacteria

Aerobic bacteria divide about every 20–30 minutes and multiply, eating emulsifiers and rust inhibitors and destroying the fluid itself. Anaerobic bacteria (sulfate-reducing bacteria) multiply slowly, about once every 4 hours; they do not damage the fluid much but are the main cause of bad odor. Most bacteria cling to sludge on the tank walls and bottom, and sampling the fluid picks up less than 1% of them. Bacteria counts are expressed as the number per mL in exponent form such as 10⁶. Even tap water contains about 10²–10⁴.^{[26][24][25][27]}(1 source, for reference)

Control to prevent spoilage

When odor appears, Yushiro lists strict concentration control, adding a biocide, and using a fluid with high spoilage resistance, and advises avoiding aeration (blowing in air) as an odor measure because it promotes aerobic bacteria growth and a pH drop. Blaser, on the other hand, recommends circulating the fluid for at least 30 minutes a day even during shutdowns.^{[25][28][27][17][5][15]}

Inspection frequency

^{[3][25][15][20][29]}

Quality of dilution water

More than 90% of the working fluid is water, so water quality matters. Hardness components such as calcium and magnesium react with the fluid's emulsifiers and rust inhibitors and reduce their effect, breaking the emulsion or forming tofu-like metal soap (scum) on the tank surface. Hardness does not evaporate, so it builds up with repeated makeup. If tap water is too hard, deionized or RO water is also an option.^[27][15][17]

Types of water-soluble coolant

In Yushiro's explanation, a fluid that turns milky white when the concentrate is dissolved in water is an emulsion, one that looks translucent is a soluble, and one whose main components are inorganic and organic rust inhibitors and that dissolves clear in water is specifically called a solution. Master Fluid Solutions classifies by oil content: emulsion (40% or more oil), microemulsion / high-oil semisynthetic (20–50%), low-oil semisynthetic (0–20% mineral oil), and synthetic (no mineral oil, or 0–10%).^[15][4]

JIS K 2241 (cutting fluids)

The Japanese Industrial Standard for cutting fluids is JIS K 2241:2017 (revised 2017-10-20; draft prepared by the National Metalworking Fluids Industry Association (Japan)). It specifies the types and properties of water-insoluble and water-soluble fluids, and test methods such as pH.^[23][20]

Definitions of the water-soluble types in JIS K 2241 (Type A1, A2, etc.)

The standard text is paid, and the JSA preview goes only as far as the table of contents. MORESCO's product documents describe both its soluble and synthetic products as "equivalent to JIS Type A2 No. 1", but the definitions of the types themselves could not be confirmed in published sources.^[23][7][12](could not be confirmed in published sources)