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HEAD MEASUREMENT
Head may be measured using various
techniques. A garden hose or length of pipe can be
submerged with one end upstream and the other end
downstream. Anchor the upstream end with rocks or have
an assistant hold it; water should flow out the low
end, especially if the pipeline is pre-filled. Once
water is flowing, raise the downstream end until it
stops. Do this slowly since the water tends to
oscillate. When the flow has stabilized, measure the
distance down to the level of water in the stream with
a tape measure. This will give a very accurate
measurement of that stream section. Mark the spot and
then repeat the procedure until the entire distance is
covered.
Another technique is to use a surveyor's
transit. This method can also be approximated using a
carpenter's level using a measuring stick or a
"story pole." This technique is also done in
a series of steps to arrive at the overall head. Note
that with this reaction type machine, the entire head
is used. No head is lost as with an impulse machine.
FLOW MEASUREMENT
The weir method can be used for
the higher flows used with this machine. This
technique uses a rectangular opening cut in a board or
piece of sheet metal set into the brook like a dam.
The water is channeled into the weir and the depth is
measured from the top of a stake that is level with
the edge of the weir and several feet upstream.
WEIR MEASUREMENT TABLE
Table shows water flow in gallons/minute
(GPM) that
will flow over a weir one inch wide and from 1/8 to
10-7/8 inches deep.
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Inches
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1/8
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1/4
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3/8
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1/2
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5/8
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¾
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7/8
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0
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0.0
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0.1
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0.4
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0.7
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1.0
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1.4
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1.9
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2.4
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|
1
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3.0
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3.5
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4.1
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4.8
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5.5
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6.1
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6.
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7.6
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|
2
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8.5
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9.2
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10.1
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10.9
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11.8
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12.7
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13.6
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14.6
|
|
3
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15.5
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16.5
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17.5
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18.6
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19.5
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20.6
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21.7
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22.8
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|
4
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23.9
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25.1
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26.2
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27.4
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28.5
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29.7
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31.0
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32.2
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5
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33.4
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34.7
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36.0
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37.3
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38.5
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39.9
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41.2
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42.6
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6
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43.9
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45.3
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46.8
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48.2
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49.5
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51.0
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52.4
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53.9
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7
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55.4
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56.8
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58.3
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59.9
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61.4
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63.0
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64.6
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66.0
|
|
8
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67.7
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69.3
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70.8
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72.5
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74.1
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75.8
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77.4
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79.1
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9
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80.8
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82.4
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84.2
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85.9
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87.6
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89.3
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91.0
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92.8
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10
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94.5
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96.3
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98.1
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99.9
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101.7
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103.6
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105.4
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107.3
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Example of how to use weir
table:
Suppose depth of water above stake is 9 3/8 inches.
Find 9 in the
left-hand column and 3/8 in the top column. The value where they intersect is
85.9 gpm. That's only for a 1-inch weir, however. You multiply this value by
the width of your weir in inches to obtain water flow.
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Click
picture for larger view
Measuring the flow at different times of the year
helps you estimate maximum and minimum usable flows.
If the water source is seasonally limited, you may
have to depend on some other source of power during
dry times (solar, wind). Keep in mind that a
reasonable amount of water must be left in the stream
(Don't take it all, that water supports life forms).
When head and flow are determined, the expected power
output can be determined from the following chart.
Keep in mind that chart values represent generated
output and that actual power delivered to the
batteries will be reduced by transmission lines, power
converters, and other equipment required by the
system. All systems should be carefully planned to
maximize power output.
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