Grit Testing Methodology Comparison
The method used to collect grit during a test drastically affects the amount collected, and therefore, the results of the test.
Cross-Channel Sampling (CCS)
- Developed by respected engineers and S&L in 1977 to validate equipment
- S&L approved technique; reduces the turbulence in the flume
- Ensures that truest mix of grit is obtained
- Samples collected across entire width of flume
- Sampling probe is patented 1” (25 mm) sampling probe
- Siphoning is preferred (diaphragm pumps ok as alternative)
- No submersible pumps (inaccurate grit samples with turbulence from the pump)
- Steady velocities between 2.0 and 3.5 FPS (0.6 and 1.10 m/s), with constant & steady flows
- Minimum of 500 grams (1.5 cups), combined, post-burn influent sample size
- Minimum required provides the most representative sample of how the system is performing.
- Small sample sizes do not accurately portray how effective the system is functioning.
- Use an approved, established lab experienced in processing grit samples
Example of data collected via the multi-positioned CCS method of sampling across the channel.
A scene from a CCS grit test.
(0.5 – 1.1 m/s).
Vertical Integrated Sampling (VIS)
- Based on the use of a VIS sampling probe (also called VSS in places)
- Utilized by Black Dog Analytical
- Typically placed in center, in one collection location (grit is not homogenously spread in the stream)
- VIS sampler more prone to clogging.
- Brings unnecessary turbulence to the flume, disturbing grit’s natural path away from flume.
- The technique uses a “WYE” after the pump and before the sample collectors. The “WYE” has a leg straight ahead going back to the sampled flume, same size as incoming sample, with the wye split at a smaller diameter at an angle directed to the sample collector basins. Directing the small leg of the wye splitter to the sample collectors creates a problem. PROBLEM – The grit will naturally go straight.
Wye Splitting practices should be discontinued because grit will tend to go straight then suddenly flowing left and flow consistently, which can taint the sample and consequently the grit characterization results — often in the favor of lighter grit.