This guide explains the correct way to calculate top up concentration for water miscible metalworking fluids. The principles below apply to all types of water miscible coolants: soluble oils, semi synthetics, synthetics and ester based fluids. They do not apply to neat cutting oils, because neat oils contain no water and are not mixed as emulsions.
The initial fill is simple. You know the tank volume and the recommended concentration from the coolant supplier. For example, a 1000 liter tank at 8 percent requires 1000 L × 0.08 = 80 L of concentrate.
After this first fill you do not need to mix every bucket precisely. You can rely on the mixer or a rough ratio because the entire tank is being mixed from zero.
The situation changes once the machine begins running.
A water miscible coolant is a blend of oil concentrate and water. The water portion is what normally changes. Over time the coolant becomes more concentrated due to:
Because water is lost faster than concentrate, the emulsion becomes richer in oil. This means the refractometer reading increases even if you never add extra concentrate.
Most machinists assume that if the coolant was mixed at 8 percent originally, the safest top up is also 8 percent. This is a common mistake.
The concentration during operation depends on the actual amount of concentrate in the system, not the top up ratio used on day one.
Example
The tank contains:
700 L × 10 percent = 70 L of concentrate.
If you top up with 300 liters of fresh emulsion at 8 percent, you add:
300 L × 8 percent = 24 L of concentrate.
Total concentrate becomes 70 L + 24 L = 94 L. Final concentration becomes 94 L ÷ 1000 L = 9.4 percent.
This is still above the target of 8 percent. The top up was too rich.
The correct method uses a simple principle: The final amount of concentrate in the tank must match the target percentage.
To reach 8 percent in a 1000 liter tank, you need exactly:
1000 L × 0.08 = 80 L of concentrate.
You already have 70 L in the system, so you only need:
80 L − 70 L = 10 L of concentrate in the top up.
You add this 10 L of concentrate to the 300 L of topping up liquid, meaning the correct top up concentration is:
10 L ÷ 300 L = 3.3 percent
This is why most real world top ups are much lower than the original mixing ratio. It is not an opinion or a special method. It is a function of simple mass balance.
Neat cutting oils are not mixed with water, so their concentration never changes with evaporation. You only add more oil when the level drops. The calculations in this guide apply only to all type of soluble oils for metallworking incl.:
Most manufacturers provide a refractometer correction factor, also called an R factor. This is required because some coolants refract light more or less strongly than water. A reading of 4 Brix does not always mean 4 percent. Some coolants require multiplying the reading by 1.2, 1.5 or even 2.0 depending on formulation.
This is important for accurate top up calculations because an error in the measured concentration always leads to an incorrect top up ratio.
If you prefer to avoid manual mass balance calculations you can use our automated tool. It applies the same physics described above and supports refractometer correction factors.
Message our technical support directly on WhatsApp. Real person, fast reply.
✓ Chat on WhatsApp