In order to have some measure of the degree of Suspended solids and colloidal materials in feed water, a concept called Silt Density Index is used.  Here a 0.45-micron filter is exposed to the feed water under pressure and filtration rates are calculated.An SDI of less than 5 is considered acceptable for the reverse osmosis systems. 

RizonSDI^® Portable Automatic SDI Monitor provides following features:

• Quick, easy silt density readings that reduce costly and time-consuming manual methods. Push-button calibrated equipment monitors membranes and water quality.
• Automatic flow, Time record, No need for stopwatches, graduated cylinders, calculators and tedious labor
• With configuration of rechargeable batteries, superb stand-by time can last up to 25 hours, easily solving field test without Power supply problems.
• Booster pump added-on can solve the problem of low site inlet-water pressure or zero pressure.
• Power-down protection calendar provides corresponding testing time for each result.
• 200 sets of data storage capabilities
• Date/Time stamping on every test
• Non-volatile memory when the power is down