for determining Silt Density Index
- Millipore® Filter Pad Holder with a 0.45 micron
- 500 ml. Graduated cylinder
- Dull Tweezers
- Unbolt the filter holder, wet it, and place a 0.45 micron
filter on the back-up plate using the dull tweezers.
- Place the "O"-ring properly and then replace the
top half of the filter holder and bolt loosely.
- Connect apparatus to the feedwater line (minimum 30
PSIG) in a vertical, downflow position and open the needle valve a crack.
- Loosen two adjacent filter-holder bolts, tilt the apparatus
and bleed out all trapped air, then retighten bolts and adjust the pressure
to 30 PSI while starting the stopwatch.
- Immediately run the flow into the graduated cylinder and
measure the time required to collect 500 ml, maintaining a pressure of 30
PSIG during the run. Mark the time (t1) and keep the watch
- Repeat the previous step immediately after 5, 10, and/or 15
minutes of total elapsed time. Mark the collection times (t5,
- Close the sample valve and disconnect he apparatus after
the 15 minute reading. Remove the disk and place it in a plastic bag
for later examination.
- Calculate the ratios R = t1/t5 , t1/t10
- Estimate % P30 at the selected time(s) from the
% P30 = [1-R] × 100.
- Estimate the Silt Density Index, SDI, from the following
- SDI = % P30 ÷ Elapsed Time (minutes)
The 15-minute index will generally be the lowest of the three,
and should be used for filter sizing purposes.