The official technique to measure oil concentration in cutting fluid samples is manual titration: 100ml of the fluid under test is titrated with a 0.5M HCl solution to an endpoint of pH 4 and the volume of titrant used to reach the endpoint is used to calculate the oil concentration. This technique is accurate and not affected by fluid contamination, but needs to be performed by trained personnel in a laboratory environment. A hand-held refractometer is the industrial standard used to determine the mix ratio of water-soluble coolants that estimates oil concentration from the sample refractive index measured in the Brix scale. The refractometer allows for in situ measurements of oil concentration within industrial plants. However, contamination of the sample reduces the accuracy of the measure. Other techniques are used to measure the oil concentration in cutting fluids, such as measure of the fluid viscosity, density, and ultrasound speed. Other test equipment is used to determine such properties as acidity and conductivity.