BS&W is nothing but water cut meter used in oil & gas industries. They are basically use for calcualting/detecting the water content in crude oil etc. We have used such meters from ROXAR.
Roxar meters cost between USD 30,000 to 50,000 each depending on the size. You can also purchase from another company called Agar Corporation. Other companies are Delta-C, EESIFLO, Phase Dynamics and FMC. Haimo make an incredibly good full cut meter (the only one we know that works) but they have a radioactive source and this is inconvenient for us.
Prices range from about USD 4000 - 50,000 depending on your application. After much research and site testing ,I have no idea why there is such a price difference. We have tried all the brands and they work about the same. We do not trust watercut meters for fiscal metering and rely on automatic sampling systems from Welker or Jiskoot. With the price of oil more than 75 USD/barrel, every drop counts. We have just purchased 120 crude oil sampling systems. Now is the time to monitor every drop!
Shell did some work and patented a method but basically the answer can be found using density. Suggest you contact Emerson and ask about their 7835 density meters and ask to see a manual. That will show you how they find BS&W from density.