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 straightforward:

1 vertical foot = 0.433 pounds per square inch (psi)

    or conversely

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 measurement.

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.