If there is no pressure gauge or pipe available, survey the site the old fashioned way. A rod is a stick that is eight feet long with each foot marked. Hold it straight up in the air at the starting point. Every place that is level with the top of the ruler is also eight feet higher than the base. Using a level, sight along to a point that is level with the top of the rod. This is also eight feet above the starting point.
Next move the rod so that you can place the bottom on the piece of ground that you marked as even with the top before. Now, every place that is level with the top of the second setting of the staff is 16 feet higher than the starting point. Repeat as necessary. Heavy brush means setting up more often. Add up your totals to get the elevation in feet.
For high head sites, over 200 feet, a sensitive altimeter can be used. Record the elevation at the bottom. Move to the top, and record the altitude again. The difference in feet is your gross head. Repeat the process and average the results for better accuracy. Because altimeters measure the difference in atmospheric pressure, choose a day when the weather is not changing rapidly. Many modern GPS receivers also offer an altimeter. The reliability of this will depend on your GPS unit and the signal strength. Again, take several readings and average them for best accuracy.
Flow is the volume of water per unit of time available to the turbine. It varies seasonally and may vary along a creeks length if tributaries flow into it. Measuring the flow at different times of the year helps estimate the maximum and minimum usable flow. Most micro hydro systems use less than a hundred gallons per minute. These flows can be measured by timing how long it takes to fill a bucket. A hundred gallons per minute will take three seconds to fill a five-gallon bucket.
A micro hydro system typically only uses a small portion of the streams flow and has a very minor impact to the overall stream. Diverting too much of the water from a stream will cause a negative environmental impact and should be avoided.
Usually the pipe is the limiting factor in determining what flow is available. You'd be surprised how small a stream of water even 50 gallons per minute is.
Some micro hydro systems use pipe that is already installed for other purposes such as irrigation or domestic water supply. 40 PSI is a common household pressure and can easily generate quite a lot of power. But in order to predict how much power a pipe can produce, we need to know how long it is, what kind it is, and what its diameter is. Since power can be moved quite a ways, the distance from where the power is to be generated to where it is to be used needs to be known as well.
Micro hydro uses a variety of generators to suit the wide range of sites available. A specially adapted automotive alternator provides low cost, DC output. These units are less expensive, but do require more frequent maintenance. Replacing brushes is a relatively inexpensive and simple procedure that may be required every six months to two years of continuous use. A newer, high efficiency permanent magnet brush-less generator is also available, which makes this maintenance unnecessary. These units have only one moving part and sealed bearings. Generator maintenance is only required at intervals of many years. These units are adjustable to produce power efficiently over a wide range of pressures.
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