For all you Pulp & Paper historians out there, here's an "oldie but goodie" from the Electron Machine archives.
In 1972 The Institute of Paper Chemistry wrote a paper titled "Instrumentation Study - Black Liquor Solids Content" detailing a comparison between an Electron Machine Corporation refractometer and an NUS Corporation sonic velocimeter and their respective capability of measuring black liquor solids. Below is the document to view online, or you can download your own copy here.
