Sequoia’s LISST-200X has been designed to cover a wide range of particle concentrations. However, at high concentrations the light scattering from the particles can include multiple scattering. This will affect the measurement of particle size and concentration. The concentration where multiple scattering can take effect is dependent on particle size. The finer the particles, the lower the concentration where multiple scattering will occur. The rule-of-thumb we recommend is that if the optical transmission is less than 30%, then multiple scattering may be significant enough to cause errors.
This is not a hard number: Multiple scattering gradually increases as the concentration increases. Therefore, 30% optical transmission is a warning level where multiple scattering may be present. It may still be possible to get reasonable data down to 10% transmission. Below is a table with estimates of the recommended maximum concentration for different sized particles and a standard 25 mm path length.
To measure higher concentration with the LISST-200X, the optical path length must be reduced. The maximum concentration scales linearly with the optical path: If we reduce the optical path by ½ we double the maximum concentration. The LISST-200X PRM reduces the optical path from 25 mm to 5 mm; this increases the maximum concentration by a factor of 5 compared to the 25 mm standard path length. See table:
The LISST-200X PRM consists of a highly polished quartz cylinder and mounting hardware. The quartz cylinder presses against the smaller transmit window on the LISST-200X with a small
drop of highly filtered water in between. The plastic holder and spacers hold the quartz cylinder in place during deployments. The LISST-200X software must be configured for use with the PRM. This will ensure the proper path length is used when computing the particle concentration.
The standard LISST-200X zscat chamber does not fit when the LISST-200X. Sequoia recommends purchase of a large volume test chamber for the LISST-200X if you are purchasing a PRM. This will make it a lot easier to make a zscat measurement.