Using bentonite is a bit tricky.  You have to use enough to keep the sandy walls of your hole from caving in but if you use too much you can seal the hole so effectively that groundwater can't get in later when it comes time to pump the well.  That is, after all the whole point of drilling a well!  So, how do you know how much bentonite to use?

A Marsh Funnel is just the tool for this.  It measures the viscosity of your drilling fluid.  It is extremely simple to use.

From Wikepedia:  Based on a method published in 1931 by H.N.Marsh, a Marsh cone is a flow cone with an aspect ratio of 2:1 and a working volume of at least a litre. A Marsh funnel is a Marsh cone with a particular orifice and a working volume of 1.5 litres. It consists of a cone 6 inches (152 mm) across and 12 inches in height (305 mm) to the apex of which is fixed a tube 2 inches (50.8 mm) long and 3/16 inch (4.76 mm) internal diameter. A 10-mesh screen is fixed near the top across half the cone.

If that didn't make sense, suffice it to say that a Marsh Funnel is a very specifically shaped and sized funnel.  You fill it up to a pre-designated point with drilling fluid and then you measure the length of time it takes for the drilling fluid to run out.  The thinner the drilling fluid is, the faster it runs out of the funnel and the less time it takes.  The thicker the drilling fluid is, the slower it runs out of the funnel and the more time it takes.

Take a moment and watch the video below posted on YouTube by CETCO:





To accurately measure the drilling fluid viscosity:

1.  Rince the Marsh Funnel with water.  It is important that the opening at the bottom of the funnel be clear.

2.  Hold your finger over the funnel and pour drilling fluid through the screen.  It is important to pour through the screen so that any big solids will be removed before the time measurment.  Continue to fill the funnel until you reach the line marked on the inside of the funnel.

3.  Put your quart measuring cup below the funnel.  Remove your finger from the bottom of the funnel and measure the time in seconds it takes to fill the container up to the quart level.

Adjust the viscosity of the drilling fluid by adding more water or bentonite mix.  Be guided by the chart below.


Natural swelling clays   32 - 37
Fine Sand   40 -45
Medium Sand   45 - 55
Coarse Sand   55 - 65
Gravel   65 - 75