SDI Index Calculation
The SDI test is based on measuring the change in water flow rate through a 0.45 micron filter over time. The manual test requires first measuring the time for 500 mL of water to pass through a 0.45 micron filter at a regulated pressure of 30 psi, continuing the stopwatch for an interval of 5, 10, or 15 minutes, and again measuring the time for the 500 mL after each interval.
The SDI index is calculated for each of these intervals by first finding the filter %plugging with a ratio of that measured time over the initial time. Subtract the %plugging from one-hundred and divide by that time interval in minutes. The most representative SDI result is the one that uses the longest time interval with a %plugging of less than 50%.
The SDI results are heavily dependent on the relative permeability of the filter and the water temperature. Use our SDInormalizer™ tool to normalize for their effect. Better yet, automate the entire test measurement procedure with the SDIcube™. You will quickly see how easy SDI testing can be while also obtaining far more repeatable results.
SDIcube™ Accessory to a Manual SDI Device
Automates the SDI measurement for significant time savings
Normalizes the SDI results for temperature and filter permeability
Mobile - battery included capable of operating the (optional) SDIpump™ for boosting pressure, or for batch SDI or suspended iron measurement
Click to download the SDIcube™ specs/options/instructions
Millipore SDI Replacement Filters
The original Silt Density Index test procedure was developed using a Millipore™ 0.45 micron, 47 mm nitrocellulose filter pad. SDI results can vary dramatically based on the particular filter. The Millipore 0.45 micron filter is the standard for SDI measurement.