Measuring Water Pressure or Head

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In this section you can find information on small scale microhydro systems such as small water turbines. Check out the Resources section for information on books,magazines and articles on water energy applications.

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MEASURING THE WATER PRESSURE (HEAD)

In order to determine the feasibility of using microhydro, and todetermine the type of turbine you should use, you will need tomeasure the head, or water pressure, that your site can generate. Atthe most basic level this involves a measurement in the change inaltitude between the water intake and the turbine. There are severalapproaches you can take to measure the head for your system.

Remember that head is the pressure created by the force of gravityand is a function of the difference in elevation between the intake ofyour 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 persquare meter. Fortunately, as Newton taught us, the force of gravity is a constant andtherefore it is possible to exactly calculate the pressure that gravity will create given a givenvertical 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 portableGPS unit. In our experience most altimeters do not have sufficient accuracy for mostmeasurement situations. However, GPS units have been getting progressively more accurateand are often an excellent alternative if you can get access to one. In any event, look in themanual and determine your instruments accuracy level. If it is accurate within a meter or soyou are probably fine. Then just take a reading at the area you are planning to put the waterintake, 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

FINDING THE BEST

One of the services wewant to provide our EBmembers is a listing ofRecommendedMicrohydro Contractors.If you have used amicrohydro contractor foryour home or businessand were happy withtheir work please provideus their contact info byclicking here and we willdo the rest. As soon as wehave a sufficient listtogether we will publishit on the site. Thanks!--Editor

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The multiple nozzlearrangement allows muchmore water to impact the

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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 aGPS unit. Unfortunately, neither of these types of devices is accurate enough to give you auseful reading. Therefore, to do a direct measurement you will probably have to directlymeasure the height difference using a long pole, measuring tape and a level. If you haveaccess to a surveyors transit on a tripod that is great. If not, you can make something similarby 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 isstraightforward. Have someone hold a marker to use for the initial spot. This should be atthe same height as where you plan to place the turbine to be placed. Then work your wayuphill to where the intake valve will be taking measurements as you go. To do eachmeasurement 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 thelevel both to ensure your pole is vertical and to make sure the horizontal measurement isexactly 90 degrees from vertical. For those of you who have access to a laser level, these canbe extremely handy in that they create a straight line that is easy to spot even over asignificant distance.

For most locations you will probably have to do multiple measurements to get a totalmeasurement of the vertical drop. Once you have added up all of your measurements takethe total and multiply it by the pressure constant .(433 pounds per square inch per foot) toget your head measurement.

Water Pressure Measurement

Another approach to determining the pressure is to simply measure it directly using a longgarden hose or piping. Get one or more hoses and start them at your proposed intake site andrun the hose to the proposed turbine location. If your slope is shallow then you may need toconnect several garden hoses together to do the measurement. Then attach a pressure gaugeto 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 pressuremeter that is designed for the range of pressures you are likely to be working with. Don't usea 0 - 500 PSI gauge to measure 20 PSI pressure. Use instead a 0 - 30 PSI gauge. Another thingto watch for is getting air in the hose, this can throw your reading off significantly. If you areusing 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 flowsthrough pipes. In real use conditions the actual pressure at the turbine nozzle is always a bitless because of this loss of energy due to friction in the pipes. Typically, the actual pressureyour turbine will have is about 85%-90% of the gross head pressure. This is referred to as theNet Head Pressure. When you go shopping for a turbine use the net head pressure as yourindicator, not the gross.

runner resulting in greateroutput at any head, andmore usable power atlower heads. Multi-nozzlesystems include a PVCpenstock and individualball valves on each nozzle.The pelton type runner islost wax cast of siliconbronze. The wheel is 70 -90% efficient, dependingon nozzle size and headpressure. The bucketshape allows highefficiency for nozzles andprovides a flow range ofover 100/1. $1640 fromHarris Hydroelectric.

WATER FACTBOOK

The use ofhydroelectricity isgrowing. Today 6.6% ofthe total electricalgeneration of the U.S.comes from hydrosystems. In Canada 20%of the energy comes fromhyrdro.


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Measuring Water Pressure or Head