Measuring Head and Flow
Now that you have a basic understanding of
hydro system components, it’s time to begin assessing your own
hydro power potential.
The Four Things You Need to Know
Before you can begin planning your systems
or estimating how much power you'll produce, you'll need to make
four essential measurements:
Head (the vertical distance between
your intake and turbine)
- Flow (how much water comes down the
- Pipeline (Penstock) length
- Transmission Line length
In this section, we'll discuss how to make
these measurements and how they affect the design and efficiency
of your hydro system.
HEAD and FLOW are the two most important
facts you need to know about your site. You simply cannot
move forward without these measurements.
Your Head and Flow will determine everything about your hydro system – pipeline size, turbine
type, rotational speed, generator size – everything. Even
rough cost estimates will be impossible until you’ve measured
Head and Flow.
Also keep in mind that accuracy is
important. Unless your measurements are accurate, you
could end up with a hydro system designed to the wrong specs,
producing less power at a higher cost-per-watt than would
otherwise be possible.
HEAD is pressure, created by the
difference in elevation between the intake of your pipeline, and
your water turbine. Head can be measured as vertical
distance (feet or meters) or as pressure (pounds per square
inch, newtons per square meter, etc.). Regardless of the size of
your stream, higher HEAD will produce greater pressure – and
therefore power – at the turbine.
The following conversions may be helpful:
1 vertical foot = 0.433 pounds per
square inch (psi) pressure
1 psi = 2.31 vertical feet
Accuracy is critical when measuring
HEAD. It not only affects power, but also determines the
type of turbine to use (such as a Francis or Pelton design), as
well as the hydrodynamic design of the turbine buckets or
blades. An altimeter can be useful in estimating Head for
preliminary site evaluation, but should not be used for the
final measurement. It is quite common for low-cost
barometric altimeters to reflect errors of 150 feet or more,
even when calibrated. GPS altimeters are often even less
There are two accurate methods for
measuring HEAD: direct distance measurement, and water pressure.
You can use a surveyor’s transit, a
contractor's level on a tripod, or a level taped to a straight
board to measure head. You will also require a pole with
graduated measurements. (A measuring tape affixed to a 20'
section of PVC pipe works well.) Direct measurement
requires an assistant.
As shown in the diagram, make a series of
vertical measurements using the transit level and the vertical
measuring pole. Make sure each transit setup is exactly
level, and ensure the measuring pole is vertical. Keep
detailed notes at each step, and then add up the series of
measurements (A,B,C,D,etc.) to find total HEAD.
If the distance is short enough, you can
use one or more garden hoses to measure Head. This method
relies on the constant that each vertical foot of HEAD
creates 0.433 psi of water pressure. (10 vertical feet
would create 4.33 psi.) By measuring the pressure in the
hose, you can calculate the elevation change of your system.
Run the hose (or hoses) from your proposed
intake site to your proposed turbine location. If you
attach multiple hoses together, ensure each connection is tight
and leak-free. Attach an accurate pressure meter to the
bottom end of the hose and completely fill the hose with water.
Make sure there are no high spots in the hose that could
If necessary, you can measure total HEAD
over longer distances by moving the hose(s) and taking multiple
readings. Keep in mind, however, that there is less than a
half-psi difference for every vertical foot. Except for
very steep hillsides, even a hundred foot hose may drop only a
few vertical feet. The chance for error
significantly increases with a series of low-Head readings.
Use the longest possible hose, along with a highly accurate
pressure meter, to measure HEAD. The pressure meter must
be graduated so that measurements are taken in the middle of the
pressure gauge's range. Don't use a 0 - 800 PSI gauge to
measure 5 -15 PSI pressure. Select instead a 0 - 30 PSI
By recording these actual measurements,
you have determined Gross Head. As described later
Computing Net Head, however, the effective Head at the
nozzle is actually lower when water begins to flow, due to
pipeline (penstock) friction. A properly designed pipeline
will yield a Net Head of about 85%-90% of the Gross Head
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