CourseWiki/Machining/Cutting Fluids

Coolant Concentration and Mixing

Read Brix, apply the product factor, and balance concentrate and water when mixing a fresh charge or correcting a partly full tank.

Zero the refractometer, then multiply Brix by the product factor

A refractometer measures how a sample bends light. On an analog Brix instrument, read where the light and dark areas meet. Brix is a scale reading; it is not automatically coolant percent.

Find the product's refractometer factor and working concentration range in its technical data sheet. Use the specified method for that product.
Clean the prism. Calibrate to zero with water, using the water and temperature procedure specified for the instrument and fluid. Fluid suppliers commonly specify the water used for mixing.
Remove the calibration water. Apply a representative, well-mixed coolant sample, close the cover and read the light/dark boundary. Avoid sampling a floating oil layer.
Calculate concentration (%) = Brix reading × product factor. Record the result and compare it with the product's range for the operation.
Clean the prism after use. Recheck a doubtful reading; contamination can blur the boundary or bias the result. Use the supplier's alternative test if the sample cannot be read reliably.

Illustrative Brix scale from 0 to 6 in 0.2 divisions. The dark upper field meets the light lower field at 3.2 Brix. — Read the boundary: **3.2 Brix**. With an invented teaching factor of 1.5, concentration is 3.2 × 1.5 = **4.8%**. That factor is not a shop recommendation.

There is no universal factor or target range. The SDS explains hazards and handling; the product's technical data sheet supplies the mixing and measurement information. See Chemicals and Safety Data Sheets.

Measurement references

Cimcool's refractometer FAQ covers zeroing, the boundary and factor conversion. Master Fluids' concentration guide discusses product factors and contamination. A refractometer is a concentration estimate, not a bacteria test or proof that the fluid is fit for reuse.

Add concentrate to water, and state what the ratio means

Water first; add concentrate while mixing. Follow the product's mixing instructions in a clean container or approved mixing system. For emulsions, this lets the emulsifier disperse oil into the water. Adding water to a body of concentrate can form an unstable or inverted mixture that does not disperse properly.

Write the order of a ratio. 24 parts water to 1 part concentrate contains 25 total parts, so its concentrate percentage is 100 × 1 ÷ (24 + 1) = 4.00%. It is not 100 ÷ 24. For water:concentrate = W:1, use C = 100 ÷ (W + 1).

To make 12 L of that 4.00% premix, measure 0.48 L concentrate and 11.52 L water. Put the water in first. This is a volume-based teaching example with additive volumes, not a product recommendation.

Why mixing order matters

The aim is small oil droplets dispersed throughout the water, rather than water trapped inside an oil-rich mixture. Cimcool's mixing explanation describes the emulsifier's role. Mixing order is separate from the arithmetic: the right measured amounts still need the right preparation method.

A falling level does not tell you how strong the coolant is

Measure both level and concentration before topping up. Evaporation mainly removes water, raising concentration. Coolant carried out on parts and chips removes both water and concentrate. Adding plain water lowers concentration.

A tank below the target is lean; one above it is rich. Compare with the actual permitted range before deciding it needs adjustment. A tank can be low in volume and rich in concentrate at the same time.

Top up fluid that is still serviceable. Persistent odor, separation, sludge or contamination calls for the evaluation and cleaning decisions in Coolant Condition and Tank Maintenance. New premix does not make spoiled fluid sound.

Calculate the premix needed to reach both target level and concentration; Interactive

Calculate the premix needed to reach both target level and concentration

Use the same volume unit throughout. Let V be the current tank volume, C its measured concentration, F the intended final volume, and T the target concentration. Concentrations below are entered as percent numbers, such as 6 for 6%.

Premix volume A = F − V

Premix concentration P = (F × T − V × C) ÷ A

Concentrate to measure = A × P ÷ 100.
Water to measure = A − concentrate volume.

Example: a hypothetical tank has 36 L at 4.0%. Bring it to 48 L at an assumed target of 5.5%. Add A = 48 − 36 = 12 L. The premix must be (48 × 5.5 − 36 × 4.0) ÷ 12 = 10.0%: 1.2 L concentrate added to 10.8 L water.

Check the balance: the tank contains 1.44 L concentrate, the addition supplies 1.20 L, and 2.64 ÷ 48 × 100 = 5.5%. These assumed values teach the calculation; the product must permit the proposed makeup mix.

Stop when the result cannot be made. A must be positive. A negative P means even plain water cannot achieve the target at that final level; P above 100% asks for more concentrate than the addition can contain. A full tank has no top-up space. Seek an approved partial replacement or other correction instead of overfilling. Even a mathematical result between 0 and 100% may be outside the product's permitted mixing method.

Mix the approved addition separately, add it, circulate as specified, then measure again. The calculation assumes accurate volumes, uniform mixing and no losses during the adjustment.

20:1 is not 5%

20:1 water : concentrate
1 ÷ (20 + 1) × 100 = 4.76%

5.00% concentrate
5 parts concentrate in 100 total parts.
95:5 = 19:1 water : concentrate.

Brix needs the product factor

6 Brix × an invented teaching factor of 1.0 = 6.00%. Use the actual product data sheet factor.

Correct a lean sump: account for the added volume

Teaching example: 100 L at 3%, target 5%, capacity 120 L. With x litres of neat concentrate, the balance is 3 × 100 + 100x = 5(100 + x).

x = 100(5 − 3) ÷ (100 − 5) = 2.1053 L. Final volume = 102.1053 L, within capacity. Check: (3 + 2.105263…) ÷ 102.105263… × 100 = 5.00%.

This is a concentrate balance, not an instruction to pour neat concentrate into a sump. Use the product-approved makeup method described on this page.

Predict what water replaces

Start with 100 L at 5%. Losing 20 L of well-mixed coolant leaves 80 L at 5%. Replace it with water: 4 L concentrate ÷ 100 L = 4%. Replace it with 5% premix: (4 + 1) ÷ 100 × 100 = 5%.

If instead only 20 L of water evaporates, all 5 L of concentrate remains: 5 ÷ 80 × 100 = 6.25%. Replacing that water restores 5%; adding 5% premix raises it to 6%. The loss mechanism matters.

For coolant condition and contact guidance, see Coolant Condition and Tank Maintenance and Safety collection.

Derive the balance, or solve for an unknown addition volume

Concentrate already in the tank plus concentrate added equals concentrate in the final tank: VC/100 + AP/100 = FT/100. Multiply through by 100 and solve for P. This tracks concentrate, not the Brix number.

If a premix concentration P is already chosen, solve for its addition volume: A = V(T − C) ÷ (P − T). Check that A is positive and that V + A fits within the allowed fill level. If P = T and C differs from T, no finite addition reaches T exactly. If C = T already, adding premix at T preserves it.

Practice the reading, the ratio and the top-up balance; Practice

Practice the reading, the ratio and the top-up balance

Use fresh hypothetical values to check the method. Every question states its rounding and assumptions; none supplies a shop concentration recommendation.

Practice Brix conversion Practice lean or rich Practice top-up concentration Practice mixing ratios

The practice catalog contains the full Coolant Concentration and Mixing bank.

Appears in

Cutting Fluids and Coolant Maintenance DET 330 DET