METHODS: Hydro Measurements

Beginner
An accurate pressure gauge can be used to determine head in an already-existing pipe.
Measuring head with a handheld sight level (see below) and a friend. Adding all of the legs determines the entire head.
A site level can be used for determining head.
A topographical map can give good estimates of elevation at the intake and turbine sites, giving you information to compute total static head.
A GPS can give good estimates of elevation at the intake and turbine sites, giving you information to compute total static head.
If you can get a container under the majority of the flow, the bucket method can be an easy way to measure it.

A comprehensive microhydro resource survey will include four measurements:

•           Head (vertical drop)
•           Flow
•           Penstock length
•           Transmission length

The last two measurements are fairly straightforward. Once you have established the location of the intake and the turbine, you can determine the best penstock route and then measure the distance, so you’ll know how much pipe to buy. The transmission distance is similarly easy to determine once you know the locations of the turbine and the point of use for the energy.

The first two measurements are not as easy, but they are critical to understanding the potential of your hydro site and how to develop it. Head and flow will determine what pipeline and turbine you choose, and will affect other hydro installation factors (see “Designing a Microhydro System” in this issue).

Measuring Head

There are a number of ways to measure head. Which you choose will depend on your site and equipment.

If there is an existing pipeline on the site, such as a water supply pipe, it may be possible to use it to measure some or all of your head. Head, or vertical drop, is what determines pressure. Every 2.31 feet of vertical drop gives 1 pound per square inch (psi) of static water pressure. So a pressure gauge on an existing pipeline—with the water in the full pipe, but not flowing—can give you a very accurate head measurement for that portion of your site’s vertical drop. Use a pressure gauge with good accuracy in the measurement range needed and make sure the pipe is full to the intake and there is no air in the line—an air vent just downstream of the intake can be useful in many systems.

Without a pipeline, using a level and a tape measure is common and accurate, if done carefully. A sight level is a simple and inexpensive tool that can be used with two people to measure sections, moving up the hill from the turbine site in stages. If the area is clear enough, it is helpful to have one person with brightly colored shoes going up the hill while a person below with a site level follows, shooting full-height levels (from the site-level person’s eye to the bottoms of the brightly colored shoes), tallying each measurement.

A GPS or altimeter can give a reasonable idea of different elevations if used properly. It’s important to have an accurate device—there’s a lot of variety in accuracy, and an error of 20% will give you an equivalent error in your power and energy predictions. If your altimeter is based on barometric pressure, do both measurements (proposed intake and powerhouse locations) on a stable weather day in a narrow period of time. Checking your device and method against a known elevation, and rechecking measurements will verify your data’s accuracy.

Accurate topographic maps can give you solid information, especially for higher-head systems. Most of these maps don’t show better than 20-foot elevation lines and, in lower-to-medium-head systems, this leaves a wide margin of error. But if you have a few hundred feet of head, that sort of inaccuracy may not be as important.

Using more than one method to confirm your head measurement often makes sense. Your whole system design will be affected by this measurement, so getting it right is worth the time and investment.

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