The drilling world has a few practices reinforced countless times in literature, as well as in mud-schools and seminars. The importance of regular drilling mud testing ranks high among these. While I certainly agree, I believe it is time to throw a proverbial wrench in the system and explain one of the limits of all that collected testing data.
You could fairly call the Marsh funnel the number one, most common tool utilized by drillers in the field for testing drilling fluids. I would go so far as to argue that, in our industry, no other piece of testing equipment even comes close to the ubiquity of the humble Marsh funnel. In some cases, however, the Marsh funnel may not be the best tool.