Adjusting your LISST data when you are using a PRM
[Sequoia, April 13, 2008]
When using a LISST-100X or LISST-STX with an Optical Path Reduction Module (PRM) in place, it is necessary to be aware of two facts:1) It is very important that you do a zscat measurement with the PRM in place. The reason is that the PRM introduces 2 extra optical surfaces that the laser beam will have to pass through. The polished glass surfaces on the PRM will scatter the light so that the zscat with a PRM in place will usually be a little bit higher than the factory zscat. Also, once you have performed a zscat with a PRM in place, be sure that the PRM is not removed and re-inserted in another position (e.g. using the opposite stand-off), as this will change the way the laser light scatters from the glass surfaces. If the PRM is re-moved and re-inserted you should always do a new zscat measurement.2) Once you have processed the data collected with the PRM in place it is necessary to manually adjust a) the volume concentration and b) the beam attenuation in the .ASC file that is being produced by the LISST-SOP version 4.65. The reason for this is that version 4.65 of the SOP does not take the existence of a PRM into account when the data are being processed. What happens is that the volume conversion constant that has been computed for your instrument is computed assuming a 5 cm path length. If the path length becomes smaller, then the volume conversion constant is suddenly too large, and the particle volume will become too small when the SOP is computing the volume from the data.a) The volume concentration in columns 1-32 must be multiplied with 2, 5 or 10 for data obtained with 50, 80, and 90% PRM’s, respectively (because the optical path length has been reduced by a factor of 2, 5, and 10).b) The beam attenuation, c, (column 42) in the .ASC file must be recomputed from the optical transmission, tau, (column 41) as follows:
PRM
c
50%
-ln(tau)*(1/0.025m)
80%
-ln(tau)*(1/0.01m)
90%
-ln(tau)*(1/0.005m)
An alternative option is to simply multiply the beam attenuation in column 42 with 2, 5, and 10 for data obtained with 50, 80, and 90% PRM’s, respectively.
