Answers from the people who run the analysis

Both water and concentrate should be at room temperature; large differences lead to instability of the resulting mix. The coolant concentrate must be added to the water while continually stirring, never the other way around. An automatic mixer is strongly recommended.

Several causes are common: an unstable emulsion from incorrect mixing or bacterial attack; a sump that is too small or under-filled so entrained air cannot dissipate; a pump sucking air or cavitating; concentration that is too high; contamination with surfactants such as machine cleaners; or incorrect product selection for the water quality on site.

That odour comes from microbial attack by bacteria, yeast and fungi. Contributing factors include reduced bio-stability from low concentration, tramp oil on the surface with poor aeration promoting anaerobic growth, unstable emulsions with low bio-resistance, microbial contamination in the mixing water, and machines that were not cleaned before refill.

Concentration may be too low, salts can accumulate from top-up with hard water, the emulsion may be unstable from incorrect mixing, pH may have fallen due to biological attack, or tramp oil may be giving false concentration readings.

Emulsifiers are chemical molecules that allow the stable distribution of oil droplets in water. They fall into three groups by polarity: anionic (potassium or sodium soaps, amine compounds, sulphonates), nonionic (ethoxylated fatty alcohols, fatty acids and fatty amides) and cationic (quaternary ammonium compounds), which are rarely used in water soluble coolants.

Yes. All mineral, vegetable and ester oils have an explosion range, commonly between 0.6 and 6.5 vol.% for mineral oils. Within that range an ignition source such as a broken tool is dangerous. Keep the oil-air mix below 0.6 vol.% with local exhaust ventilation, or above the upper limit with heavy flooding. Flashpoint matters less because it usually sits far above 100 degrees.

Water soluble coolants have roughly four times the cooling power of neat oils and are used where cooling is the main demand. Neat oils offer higher lubrication power where that is the priority.

Multi-functional oils are coolant concentrates that are also suitable for hydraulic, spindle and slideway lubrication. They can be used undiluted or as a water soluble coolant, which removes tramp oil contamination as a problem.

Water quality; tramp oil; microbiological attack from bacteria, yeast and fungi; chemical reaction with the machined material; and temperature that is too high or too low.