measuring the water flow

Flow is a measure of the quantity of water that would be run through your water turbine and is usually measured in gallons per second or gallons per minute.  If you are using metric then the measure would be liters per second or minute. There are several methods you can use to measure flow: the Container, Weir and Float methods. Each will be described in detail below. Once again, accuracy is important to ensure correct system design and optimum power generation.

Method 1: Measuring Time to Fill Container

The Container Fill method works only for very small systems. Build a temporary dam that forces all the water to flow through a single outlet pipe, Using a bucket or larger container of a known volume, use a stopwatch to time how long it takes to fill the container. Then, divide the container size by the number of seconds. Example: Container = 5 gallon paint bucket Time to fill = 8 seconds 5 gallons / 8 seconds = 0.625 gallons per second (gps) To convert into Cubic Feet per Second (cfs): 7.481 gallons per second = 1 cubic foot per second, so 0.625 gps / 7.481 = 0.0835 cubic feet per second (cfs).

Method 2: Measuring with a Weir

A Weir is perhaps the most accurate way to measure small and medium sized streams. All the water is directed through an area that is exactly rectangular, making it very easy to measure the height and width of the water to compute flow. A Weir is a temporary dam with a rectangular slot, or Gate. The bottom of the Gate should be exactly level, and the width of the Gate should allow all the water to pass through without spilling over the top of the dam. A narrower Gate will increase the depth of the water as it passes through, making it easier to measure.

It’s important to note that your depth measurement is not taken at the gate itself because the water depth distorts as it moves through the gate. Instead, insert a stake well upstream of the Weir gate and make the top of the stake exactly level with the bottom of the Wier gate. Measure the depth of the water from the top of the stake.

Once the width and depth of the water are known, a Weir Table is used to compute the flow. The Weir Table shown below is based on a Gate one inch wide; you simply multiply the table amount by the width (in inches) of your Gate. For example, let’s assume your Weir Gate is 6” wide, and the depth of the water passing over it is 7-1/2 inches. On the left side of the table, find “7” and move across the row until you find the column for “+1/2”. The table shows 8.21 cfm flow for a one-inch Gate with 7-1/2” of water flowing through it. Since your gate is 6” wide, simply multiply the 8.21 by 6 to get 49.26 cfm.

Weir Flow Calculation Table
Inches	+0/8	+1/8	+1/4	+3/8	+1/2	+5/8	+3/4
0	0.00	0.01	0.05	0.09	0.14	0.19	0.26
1	0.40	0.47	0.55	0.64	0.73	0.82	0.92
2	1.13	1.23	1.35	1.46	1.58	1.70	1.82
3	2.07	2.21	2.34	2.48	2.61	2.76	2.90
4	3.20	3.35	3.50	3.66	3.81	3.97	4.14
5	4.47	4.64	4.81	4.98	5.15	5.33	5.51
6	5.87	6.06	6.25	6.44	6.62	6.82	7.01
7	7.40	7.60	7.80	8.01	8.21	8.42	8.63
8	9.05	9.26	9.47	9.69	9.91	10.13	10.35
9	10.80	11.02	11.25	11.48	11.71	11.94	12.17
10	12.64	12.88	13.12	13.36	13.6	13.85	14.09
11	14.59	14.84	15.09	15.34	15.59	15.85	16.11
12	16.62	16.88	17.15	17.41	17.67	17.94	18.21
13	18.74	19.01	19.29	19.56	19.84	20.11	20.39
14	20.95	21.23	21.51	21.80	22.08	22.37	22.65
15	23.23	23.52	23.82	24.11	24.40	24.70	25.00
16	25.60	25.90	26.20	26.50	26.80	27.11	27.42
17	28.03	28.34	28.65	28.97	29.28	29.59	29.91
18	30.54	30.86	31.18	31.50	31.82	32.15	32.47
19	33.12	33.45	33.78	34.11	34.44	34.77	35.10
20	35.77	36.11	36.45	36.78	37.12	37.46	37.80

A Weir is especially effective for measuring FLOW during different times of the year. Once the Weir is in place, it is easy to quickly measure the depth of the water and chart FLOW at various points in time. Design Flow Even though your Flow may be very high after exceptionally rainy periods, it probably won’t be cost effective to design your turbine system to handle all that water for just a few days of the year. Instead, it makes sense to build a system that uses Flow you can count on for much of the year. This is called Design Flow, and it is the maximum Flow your hydro system is designed to accommodate. Design Flow, along with Net Head, determines everything about your hydro system, from pipeline size to power output.

 Method 3: Measuring with a Float

The Float method is useful for large streams if you can locate a section about 10 feet long where the stream is fairly consistent in width and depth.

STEP 1: Measure the average depth of the stream. Select a board able to span the width of the stream and mark it at one-foot intervals. Lay the board across the stream, and measure the stream depth at each one-foot interval. To compute the average depth, add all of your measurements together and divide by the number of measurements you made.

STEP 2: Compute the area of the cross section you just measured. Multiply the average depth you just computed by the width of the stream. For example, a 6-foot wide stream with an average depth of 1.5 feet would yield a cross section area of 9 square feet.

STEP 3: Measure the Speed. A good way to measure speed is to mark off about a 10-foot length of the stream that includes the point where you measured the cross section. Remember, you only want to know the speed of the water where you measured the cross section, so the shorter the length of stream you measure, the better. Using a weighted float that can be clearly seen (an orange works well), place it in the stream well upstream of your measurement area, and then use a stopwatch to time how long it takes to cover the length of your measurement section (e.g. 10 feet). The stream speed probably varies across its width, so record the times for various locations and average them.

With these time and distance measurements, you can now compute the water speed. For example, let’s assume it took 5 seconds for your float to travel 10 feet: 10 feet / 5 seconds = 2 feet per second, or 2 feet per second x 60 = 120 feet per minute You can then compute flow by multiplying the feet traveled by the cross section area. Using our cross section area and speed examples: 120 feet per minute x 9 square feet = 1,080 cubic feet per minute (cfm) FLOW STEP 4: Correct for Friction. Because the stream bed creates friction against the moving water, the bottom of the stream tends to move a little slower than the top. This means actual flow is a little less than what we computed. By multiplying our result by 0.83, we get a closer approximation of actual flow: 1,080 CFM x 0.83 = 896.4 cfm (cubic feet per minute), or 896.4 CFM / 60 = 14.94 cfs (cubic feet per second)