Solutions that contain dissolved solids may not appear clear. Dissolved solids disrupt the amount of light that passes through. The measure of the attenuation of light when passing through cloudy solutions is turbidity measurements. Instruments that measure the degree of turbidity of a solution record their results on a meter in terms of a user selected unit. The units used for these measurements are nephelometric turbidity units (NTU). The conversion between various scales requires calibration of the instrument. Each application of turbidity measurements is slightly different and instrument response will vary. The standardized solution used to calibrate the instrument is a Formazin solution. 1 NTU is defined as the response to a 1 mg/l solution of Formazin.
Turn on the turbidity instrument and allow to warm up. This time will allow the light source to reach a point of stable light output.
Select a series of standard solutions that match the range of NTU you expect to find. Many chemical catalog vendors have these standards available, such as one sold by Cole-Parmer that spans the range of 0.2 to 1 NTU and another that spans the range of 2 to 10 NTU.. Commercially available standard solutions are also available that use latex beads to simulate particles in solution at various concentrations.
Take measurements of the standards and draw a calibration curve of instrument response versus concentration (NTU).
Correlate the values of NTU to mg/l defined by the standard. Define a conversion factor between NTU reading and mg/l. For example, the turbidity meter may read 15 NTU and the concentration of the standard solution to yield this response may be 5 mg/l. The conversion factor would be 1 mg/l = 3 NTU = 1 ppm based in the definition that 1 mg/l = 1 ppm. Each application may have a different conversion factor due to the varying response of the turbidity instrument based on the particulars of the application and samples.
TL;DR (Too Long; Didn't Read)
Careful calibration is necessary because while you may measure turbidity in ppm, the particle sizes present in the water will change the response of the turbidity instrument.