Coolant Concentration: Refractometer Readings, Correction Factors and Titration

Coolant concentration is the percentage by volume of concentrate in a water-mix metalworking fluid, and at the machine it equals the refractometer's Brix reading multiplied by the correction factor on the product data sheet. This page covers that fluid in machine sumps and central systems: how to measure it, at what interval to check it, which range to hold and how to bring a drifting sump back into range.

Concentration applies to the water-mix metalworking fluids, not to straight oils used neat. UK guidance (HSE COSHH essentials MW5, published 03/21) asks for a refractometer check at least once a week together with pH, and 5 to 10 % concentrate by volume is the in-use range for general machining in both an STLE webinar (April 2022) and one supplier's 2025 brochure. The four key figures of this page are listed below.

• Concentration = refractometer Brix reading x the product's refractometer factor (UKLA/HSE Good Practice Guide v2.5, February 2025)
• Factor: from the product data sheet, never assumed
• Check: at least weekly with a refractometer, together with pH (HSE MW5, 03/21)
• In-use range: the product data sheet first; 5 to 10 % by volume for most machining (STLE webinar, April 2022; one supplier's 2025 brochure)

A lean sump takes a premix stronger than the target, a rich sump takes water or a very weak premix, and routine top-ups use a premix weaker than the target (HSE MW5, 03/21), because evaporation leaves the concentrate behind.

Standards, regulations and exposure limits are checked against their issuing bodies, product values against the named data sheet, and every figure carries its source and date; see our methodology and fact-checking.

What Is Coolant Concentration and Why Does It Matter?#

Coolant concentration is the share of concentrate in the working fluid, stated in percent by volume, and it decides whether a water-mix fluid lubricates the cut, protects parts against rust and resists bacteria as formulated. A sump that drifts outside the data-sheet range loses those properties, which is why every coolant management routine starts with this one number.

Concentration is the first of the monitoring tasks in coolant management programs. The OSHA Metalworking Fluids: Safety and Health Best Practices Manual lists monitoring of concentration, pH, microbial levels, tramp oil and suspended solids, with off-shift additions logged. That manual is guidance: OSHA has no standard specific to metalworking fluids.

Percent, ratio and Brix: how concentration is stated#

Coolant concentration is stated in percent by volume of concentrate in the use dilution; a ratio such as 1:20 means 5 % when it counts total parts, and a Brix value is only a raw refractometer reading. Color is no guide: the intensity of a fluid's color does not reliably show its concentration. Resolution sets the last limit, and one supplier's technical bulletin puts tank-side measurement at about ±0.25 % absolute. The three ways shop documents state concentration are listed below.

• Percent by volume: the share of concentrate in the use dilution
• Ratio with its basis: 1:20 in total parts and 19:1 water to concentrate both mean 5 %
• Brix: a raw refractometer reading that becomes percent only after multiplying by the product's factor

What happens when coolant runs lean or rich#

A lean coolant shortens tool life, invites bacteria and fungus and risks corrosion and eventual downtime; a rich coolant brings dermatitis, foaming and heavy residues (OSHA best practices manual).

Bacteria and fungus problems appear among the effects of low concentration in the OSHA list, so a lean reading and an odor complaint belong in the same check. Causes, dip-slide counts and control measures are on bacteria, fungi and odor in coolant.

A rich mix also costs money. Cost in use per liter of working fluid equals the concentrate price multiplied by the concentration, which is simple arithmetic rather than a published formula. Running at 7 % instead of 5 % therefore uses 40 % more concentrate (7 divided by 5 is 1.4) at the same concentrate price.

Foam and heavy residues on parts and machines are the visible signs of a rich sump in the OSHA list, and one supplier's bulletin adds excess fluid use. A refractometer check confirms the cause before any water goes in.

Dermatitis heads the OSHA list of high-concentration effects. A stronger mix puts more concentrate in contact with the skin; symptoms, causes and controls are under coolant dermatitis.

How Do You Measure Coolant Concentration?#

Coolant concentration is measured with three methods: a refractometer reading multiplied by the product's correction factor at the machine, an alkalinity titration as the cross-check, and an acid split for soluble oils in the laboratory. The table below compares the three methods by what they measure, where they work, what biases them and how long they take.

Refractometer readings: Brix times the correction factor#

Concentration in percent equals the Brix reading multiplied by the product's refractometer factor (UKLA/HSE Good Practice Guide for Safe Handling and Disposal of Metalworking Fluids, version 2.5, February 2025). The refractometer itself reports only how strongly the fluid bends light, on the Brix scale; the factor turns that reading into the share of concentrate for one specific product.

Concentration (% by volume) = Brix reading x product refractometer factor

Worked example from one supplier's guide: a reading of 4.5 Brix with a factor of 2.0 gives 9.0 % concentrate, while the same 4.5 Brix with a factor of 1.0 gives 4.5 %. The same reading therefore means twice the concentration in one product as in the other.

How to take a refractometer reading#

A refractometer reading takes six steps, and the first is zeroing the instrument on the same water used for mixing. The six steps are listed below.

1. Zero the refractometer on the same water used for mixing, at about 20 °C (68 °F)
2. Take the sample from the circulating fluid
3. Place a few drops of the sample on the prism
4. Read the clear boundary line at the bottom of any haze
5. Multiply the Brix reading by the factor on the product data sheet
6. Record the result with the date and machine

Zero water matters: one supplier's guide states that zeroing on distilled water when the sump is mixed with hard tap water introduces an error. Records from step 6 are kept for at least 5 years under HSE MW5 (03/21).

Why the correction factor differs between products#

The correction factor belongs to the product, not the fluid type: one supplier's data sheets list 1.0 for a soluble oil, 1.3 for one semi-synthetic and 1.5 for two others; another supplier reports about 0.9 to 3.4. That second range runs from about 0.9 for emulsions to 3.4 for some synthetics across that supplier's own products, and it notes 1.0 for many emulsions.

Two semi-synthetics from one maker already need different factors, so no fluid-type factor exists that a shop can apply blind. Brix-to-percent charts are drawn for one product and do not transfer to another. Use the factor printed on the data sheet of the product in the sump; a Brix reading equals the percent only when that factor is 1.0.

Titration: the alkalinity drop count#

Titration measures concentration by adding 1.0 N sulfuric acid drop by drop until the indicator changes color and multiplying the drop count by the product's titration factor. The method comes from supplier test kits, and one supplier's bulletin rates it best for synthetics and semi-synthetics. The kit's own SDS governs handling of its reagents.

Alkalinity is what the drops count. Monoethanolamine and triethanolamine make up 1 to 5 % of the concentrate in a review of 209 safety data sheets (Gerulova et al., 2017), and the acid neutralizes the alkalinity from alkanolamines and other bases in the fluid, so the drop count follows the alkaline reserve the concentrate brings into the sump.

Titration is independent of tramp oil, which makes it the natural cross-check for a refractometer in a dirty sump. Its own weakness is chemistry added later: tankside alkaline builders, acids and biocides shift the result, and one supplier's bulletin notes that titration tends to read lower than the actual concentration as a fluid ages.

Acid split for soluble oils#

An acid split breaks a soluble-oil emulsion with 50 % sulfuric or hydrochloric acid in a 100 mL graduated cylinder and reads the separated product oil and tramp oil after 6 to 18 hours. One supplier's bulletin limits it to soluble oils and high-oil semi-synthetics and reads the product oil, tramp oil and surfactant layers separately.

Acid splits belong in a laboratory or with trained staff. Sulfuric acid carries the harmonized CLP Annex VI classification from the original Annex VI list under index 016-020-00-8: Skin Corr. 1A H314 (specific concentration limit Skin Corr. 1A at 15 % or more). Hydrochloric acid solutions carry index 017-002-01-X: Skin Corr. 1B H314 and STOT SE 3 H335 (Skin Corr. 1B at 25 % or more), so both acids at 50 % are corrosive.

When the refractometer and titration disagree#

A large gap between refractometer and titration results calls for a laboratory method, the rule in German statutory accident insurance guidance for metalworking fluids (DGUV Regel 109-003).

Each method has its own bias, which is why a gap appears. Tramp oil falsifies the refractometer reading (DGUV Regel 109-003), while tankside additions and fluid age pull the titration down (one supplier's bulletin), so neither result alone settles a large gap.

How Often Should Coolant Concentration Be Checked?#

Coolant concentration is checked at least once a week with a refractometer, together with pH, under UK HSE guidance MW5 (published 03/21), and at the start of every shift in the practice described in an STLE webinar (April 2022). The table below compares the check frequencies of four sources and what each checks alongside concentration.

Weekly is the UK minimum, not a target, and HSE MW5 asks for the records of every fluid check to be kept for at least 5 years. The OSHA manual, which is guidance rather than a standard, names a sump 30 % below its full mark as the signal to check concentration.

What Is the Right Coolant Concentration?#

The right coolant concentration is the range on the product data sheet, and for most machining that range lies between 5 and 10 % concentrate by volume in 90 to 95 % water. That range comes from an STLE webinar (April 2022) and matches the general machining range in one supplier's 2025 brochure. HSE MW5 shows 4.5 to 5.5 % on its example control chart, but that band is an example set from a supplier's range, not an HSE limit.

Target ranges by fluid type and operation#

Target ranges differ by fluid type and operation, from about 2.4 % at the weakest NIOSH dilution to 10 to 12 % for tube and pipe threading in one supplier's 2025 brochure. The table below compares in-use ranges from public guidance, an emission scenario default table and one supplier's brochure, each with its source.

NIOSH states its dilutions as 1 part concentrate to a number of parts water, so 1 to 5 converts to 1 part in 6 total, or 16.7 %, and 1 to 40 converts to 1 part in 41, or 2.4 %. The OECD figures are defaults for estimating emissions, and the supplier rows describe that supplier's products only. None of the rows replaces the data sheet of the fluid in the sump.

Coolant mixing ratio: from percent to parts water#

A coolant mixing ratio converts to percent by dividing 100 by the total parts, so 1:20 in total parts, or 19:1 water to concentrate, is 5 %. The table below converts eight concentrations into both ratio forms.

Percent is by volume of concentrate in the use dilution; ratios are given as concentrate to total parts and as water to concentrate.

A new charge follows the same arithmetic: 400 L (105.7 US gal) at 6 % needs 24 L (6.34 US gal) of concentrate and 376 L (99.33 US gal) of water, with the concentrate poured into the water. Any ratio, new charge or top-up volume is computed in the coolant concentration calculator.

Why Does Coolant Concentration Drift?#

Coolant concentration rises when water evaporates from the sump, and it falls when the sump is topped up with water alone. UK guidance ties faster concentration rise to sumps above 30 °C (86 °F), and tramp oil, temperature and droplet-size changes can make a correctly mixed sump read wrong.

Evaporation and wrong top-ups#

Evaporation removes water and leaves the concentrate behind, which is why HSE MW5 (03/21) tells shops to top up with a premix weaker than the target concentration rather than with water or concentrate alone. The UKLA/HSE guide v2.5 (February 2025) gives the same rule and adds that above 30 °C (86 °F) evaporation raises concentration and microbial growth is optimal.

Wrong top-ups drive the other half of drift. Neat water added to replace losses pulls the sump lean, and neat concentrate pushes it rich. The OSHA manual names a sump 30 % below its full mark as a sign of fluid loss or evaporation and as the moment to check concentration before anything is added.

Tramp oil, temperature and droplet size: what distorts the reading#

Tramp oil makes a refractometer read higher than the actual concentration and turns the boundary line hazy, and heavy contamination can prevent a clear reading at all. One supplier's bulletin gives the rule for a hazy line: read the clear boundary at the bottom of the haze.

Temperature deviation and a change in the emulsion's droplet size distribution also shift the reading, according to the German rule BGR/GUV-R 143 (2009), the predecessor of DGUV Regel 109-003; the UKLA/HSE guide sets the zero at about 20 °C (68 °F). Titration avoids the tramp oil bias, since tramp oil does not change the drop count.

Tramp oil has its own limit: HSE MW5 keeps it below 2 % by volume, checked at least weekly. Sources, that 2 % limit and removal methods are on tramp oil in coolant.

How to Correct Coolant Concentration#

Correct coolant concentration in four steps: confirm the reading with a zeroed refractometer and a titration, decide whether the sump is lean or rich, adjust it with premix or water, and log the addition. The four steps are listed below.

1. Confirm the reading with a zeroed refractometer and a titration
2. Decide whether the sump is lean or rich, and why
3. Adjust with a premix or with water
4. Log the addition

Correcting a lean sump#

A lean sump is corrected with a premix stronger than the target, sized from the sump capacity, the current volume and concentration, and the target concentration. The worked example below comes from one supplier's technical bulletin.

A sump of 378.5 L (100 US gal) holds 283.9 L (75 US gal) at 4 %, and the target is 6 %. The full sump at 6 % needs 22.7 L (6 US gal) of concentrate and already holds 11.4 L (3 US gal), so the 94.6 L (25 US gal) of room takes 11.4 L (3 US gal) of concentrate in 83.3 L (22 US gal) of water, a 12 % premix.

The premix is made by pouring the concentrate into the water, never water into concentrate, which can form an inverse emulsion (cream). After the addition, the new reading and the volume added go into the log.

Correcting a rich sump#

A rich sump is corrected with water or a very weak premix; the worked example needs a 1 % premix, and when the sump has no room for that volume, part of the fluid is drained first and the sump refilled. The volume to drain before adding water is computed in the concentration and mixing calculator.

In one supplier's bulletin, a 400 L (105.7 US gal) sump holds 300 L (79.25 US gal) at 9 %, and the target is 7 %. The 100 L (26.4 US gal) of room takes 1 L (0.26 US gal) of concentrate and 99 L (26.15 US gal) of water, a 1 % premix.

The OSHA manual gives the same direction for a low sump: when the sump is 30 % below its full mark and the fluid is too strong, add water.

Dilution fixes strength, not contamination. A rich sump that also smells or carries sludge needs sump cleaning and recharge, not dilution.

How to Keep Coolant Concentration in Range#

Coolant concentration stays in range when top-ups use a premix weaker than the target, concentrate always goes into the water, the sump runs at or below 30 °C (86 °F) and every check and addition is logged. The six controls are listed below.

• Top-ups: a premix weaker than the target, not water or concentrate alone (HSE MW5, 03/21)
• Mixing order: concentrate into water, never water into concentrate
• Sump temperature: at or below 30 °C (86 °F) (UKLA/HSE guide v2.5, Table 3)
• Concentrate storage: indoors at 5 to 40 °C (41 to 104 °F)
• Mix water: hardness 80 to 200 ppm as CaCO3, depending on the formulation
• Records: every check and addition logged and kept for at least 5 years in the UK

Automatic proportioners and mixers form the equipment class that makes a premix of fixed strength without hand measuring, and the same refractometer check confirms their output. Logged additions also belong to the OSHA monitoring program, which lists off-shift additions among the entries to record.

When to Call a Fluid Management Provider#

Call a fluid management provider when the refractometer and titration keep disagreeing, when a corrected sump drifts out of range again, or when a whole shop or central system needs a fluid audit with laboratory testing. The German guidance sends a large method gap to a laboratory method, and the UKLA/Pera disposal guide gives 9 to 12 months as the service life of a water-mix fluid in the UK.

Audits, testing and recharge are scoped under coolant management services, and the program elements OSHA lists (monitoring of concentration, pH, microbes, tramp oil and solids, logged additions) give the scope of such an audit.

What Else Shows Coolant Condition Besides Concentration?#

Concentration is one of five weekly fluid checks in UK guidance, and pH, dip-slide bacteria counts, tramp oil and sump temperature complete the picture (HSE MW5, 03/21). The same guidance adds a daily look at appearance and odor.

Does Concentration Cause Foam?#

Partly: foam rises with high concentration and with very soft mix water. The UKLA/HSE guide v2.5 sets mix-water hardness at 80 to 200 ppm as CaCO3, depending on the formulation, because soft water foams.

Does pH Move With Concentration?#

pH is checked at least weekly together with concentration, and UK guidance corrects a low pH with a higher-strength premix or an additive named by the supplier (UKLA/HSE guide v2.5, Table 5). Target ranges and causes of a falling reading are on CNC coolant pH.

Coolant Concentration FAQ#

These five answers cover the reader questions on measuring and mixing coolant that the sections above answer in detail.

What is used to measure coolant concentration?#

A refractometer measures coolant concentration at the machine, and the Brix reading becomes percent only after multiplying it by the factor on the product data sheet. An alkalinity titration with a supplier kit serves as the cross-check, and its drop count is multiplied by the product's titration factor.

How do you read a coolant refractometer?#

Read the light-dark boundary line, taking the clear boundary at the bottom of any haze, after zeroing the instrument on the mix water at about 20 °C (68 °F). Then multiply the Brix value by the product's refractometer factor and log the result.

What happens if coolant is too concentrated?#

Too concentrated coolant raises the risk of dermatitis, foams and leaves heavy residues, and 7 % instead of 5 % uses 40 % more concentrate. Causes of foam beyond concentration are on coolant foaming.

What is the best coolant ratio?#

The best coolant ratio is the value on the product data sheet, 5 to 10 % for most machining, which is 1:20 to 1:10 in total parts. In water-to-concentrate terms that range is 19:1 to 9:1.

What is a concentrated coolant?#

A concentrated coolant is what the German terminology standard DIN 51385 calls a water-miscible fluid: a concentrate that is diluted with water to its use concentration before use. The diluted fluid at use concentration is the water-mixed fluid that runs in the sump.