measuring the water pressure (head)
In order to determine the feasibility of using microhydro, and to
determine the type of turbine you should use, you will need to measure
the head, or water pressure, that your site can generate. At the
most basic level this involves a measurement in the change in altitude
between the water intake and the turbine. There are several
approaches you can take to measure the head for your system.
Remember that head is the pressure created by the force of gravity
and is a function of the difference in elevation between the intake of your pipeline, and your water turbine.
Normally head pressure is measured in pounds per square inch, or for
those of you on the metric system, newtons per square meter.
Fortunately, as Newton taught us, the force of gravity is a constant
and therefore it is possible to exactly calculate the pressure that
gravity will create given a given vertical drop. The formula is
1 vertical foot = 0.433 pounds per square inch (psi)
1 psi = 2.31 vertical feet
Direct Reading using a GPS or Altimeter
By far the simplest method is to take direct measurements using an
altimeter or a portable GPS unit. In our experience most
altimeters do not have sufficient accuracy for most measurement
situations. However, GPS units have been getting progressively
more accurate and are often an excellent alternative if you can get
access to one. In any event, look in the manual and determine
your instruments accuracy level. If it is accurate within a
meter or so you are probably fine. Then just take a reading at
the area you are planning to put the water intake, and another reading
where the turbine will be placed and compare the difference.
Then just multiple the feet times the pressure constant of 0.433
pounds per square inch per foot and you have your pressure
Direct Distance Measurement
It would be nice if you could simply measure the change in altitude
using an altimeter or a GPS unit. Unfortunately, neither of
these types of devices is accurate enough to give you a useful
reading. Therefore, to do a direct measurement you will probably
have to directly measure the height difference using a long pole,
measuring tape and a level. If you have access to a surveyor’s transit on a tripod
that is great. If not, you can make something similar by taping
a measuring tape to a long piece of wood or PVC pipe.
To do the actual measurement you will need someone to assist
you. The process is straightforward. Have someone hold a marker
to use for the initial spot. This should be at the same height
as where you plan to place the turbine to be placed. Then work your
way uphill to where the intake valve will be taking measurements as
you go. To do each measurement walk uphill, select a spot that
is not higher than the top of your measuring stick, and visualize a
horizontal line from where you are standing to your measuring stick.
Use the level both to ensure your pole is vertical and to make sure
the horizontal measurement is exactly 90 degrees from vertical.
For those of you who have access to a laser level, these can be
extremely handy in that they create a straight line that is easy to
spot even over a significant distance.
For most locations you will probably have to do multiple
measurements to get a total measurement of the vertical drop.
Once you have added up all of your measurements take the total and
multiply it by the pressure constant .(433 pounds per square inch per
foot) to get your head measurement.
Water Pressure Measurement
Another approach to determining the pressure is to simply measure
it directly using a long garden hose or piping. Get one or more hoses
and start them at your proposed intake site and run the hose to the
proposed turbine location. If your slope is shallow then you may need
to connect several garden hoses together to do the measurement.
Then attach a pressure gauge to the end of the hose, fill the hose
with water, and take the measurement.
There are a couple of tricks to making this approach work.
First you need to use a pressure meter that is designed for the range
of pressures you are likely to be working with. Don't use a 0 -
500 PSI gauge to measure 20 PSI pressure. Use instead a 0 - 30 PSI gauge.
Another thing to watch for is getting air in the hose, this can throw
your reading off significantly. If you are using multiple hoses,
also make sure there are no leaks where the hoses are connected.
Gross Head vs. Net Head
Once you have determined the head pressure, which we will refer to
as Gross Head Pressure, you then need to account for the loss of
pressure that will actually occur as the water flows through pipes.
In real use conditions the actual pressure at the turbine nozzle is
always a bit less because of this loss of energy due to friction in
the pipes. Typically, the actual pressure your turbine will have
is about 85%-90% of the gross head pressure. This is referred to
as the Net Head Pressure. When you go shopping for a turbine use
the net head pressure as your indicator, not the gross.