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.