An automatic titration system for oil concentration measurement in metalworking fluids

Most metalworking fluids (MWFs), widely used for cooling and lubrication, consist of oils diluted in water in concentration ranging from 1% to 10%. To guarantee good performance and reliability of both machines and processes, oil concentration should be maintained (approximately) constant, hence also measured at regular intervals to compensate for significant deviations from the optimal value. The official technique to measure oil concentration in MWFs is titration with 0.5 M HCl solution and endpoint detection obtained measuring the sample pH. Such a method is accurate and substantially insensitive to sample contamination, but must be carried out in a laboratory, while fast, inexpensive and in-situ measurements would be desirable for effective process and machine control. This paper presents a system to measure oil concentration in MWFs that is new in that: (a) titration is made automatic using a stepper motor; (b) the end-point is detected by means of electrical conductance measurements. Experimental results show a very good correlation (R2 = 0.9793) between the oil concentrations of prepared solutions and those measured with the proposed technique, while the presented system is suitable for in-situ operation within working environment.