Measuring Head And Flow With A Hose And A Bucket

Two Numbers, One Verdict

Measuring head and flow tells you whether your stream can power your home. Head is how far the stream falls. Flow is how much passes each second. Those two numbers determine roughly how many watts are waiting for you.

Professionals use a gadget like the one above, a little propeller on a pole that spins faster when the current is faster. You don't need one. A garden hose, a bucket and a stopwatch get you surprisingly close, and you can check everything before a single part is installed.

What Is Head?

Head is the vertical drop from your intake (where the stream enters the pipe) down to your turbine, and it is the first number to determine.

Not the length of the stream. The drop. If your stream runs 100 meters but the ground only falls 1 meter, your head is 1 meter.

Head gives the stream its punch. The further it falls, the harder it hits the turbine, so a big drop with a small flow can make more power than a huge flow with no drop at all.

For a home hydro system, you generally want at least 2 meters of head. Three is much better, ten is great, and anything over 30 is mountain stream territory and a Pelton wheel paradise.

Gross Head And Net Head

The drop you measure on the hillside is called the gross head. But the stream rubs against the inside of the pipe on the way down and loses some push, a bit like a bike slowing down when you stop pedaling.

What reaches the turbine is the net head. US farm guides say friction in the pipe often takes about 20 percent of the gross head, and a wider, shorter penstock loses less.

So if you measure 10 meters, plan around 8.

Measuring Head With A Hose

The most common do-it-yourself method needs a long garden hose and a friend.

Fill the hose with no air bubbles. You hold one end at ground level at the high point, while your friend walks downhill with the other end.

Your friend lifts their end until the water inside just stops running out. The hose works like a giant U-tube, and water always finds its own level, so the water at your friend's end now sits exactly as high as your end.

Measure from the ground at your friend's feet up to that water level. That is the drop for this stretch.

Then follow the stream downhill and repeat, adding up every stretch. Slow, but accurate to a few centimeters.

Measuring Head With A Pressure Gauge

A faster trick: run a long hose from your intake spot down to your turbine spot, fill it, and screw a cheap pressure gauge onto the bottom end. When nothing is moving inside, the gauge reads the pressure of everything stacked above it.

To calculate head, multiply: each psi equals 2.31 feet of head, so 40 psi means 92.4 feet. Turbine manufacturers such as Canyon Hydro publish the same conversion. If your gauge reads in bar, 1 bar is about 10.2 meters.

The best part? If you already have an existing pipe running downhill, like an old irrigation line, you can read its pressure and get your head directly.

Measuring Head With A Phone

Many phones have a barometer, a tiny pressure sensor that can tell your height above sea level. Start at your intake, note the altitude, walk to your turbine spot, note it again, and subtract.

It is fast but rough, because weather conditions change air pressure and the reading can drift by a meter or two. A GPS or a topographic map is just as rough.

What Is Flow Rate?

Flow rate is how much passes by each second, and it is the number that changes most with the seasons. Scientists call it discharge, which is just a fancy word for the volume moving down a stream per second.

Not the speed of the current. The volume. A wide slow river can have a bigger flow rate than a narrow fast one.

Flow is expressed in liters per second, or in the US in gallons per minute (GPM) or cubic feet per second. One liter per second is about 15.85 GPM.

A small home hydro system wants at least 5 liters per second, and a serious one wants 30 or more.

The Bucket Method

The easiest and most common way to measure flow is the bucket method. You need a bucket of known volume (10 liters works great) and a stopwatch.

Find a place where you can catch the whole stream at once, like where it pours over a rock or log. Hold the bucket under it and time how long it takes to fill.

Divide the bucket's volume by the time in seconds. For example, a 10 liter bucket that fills in 4 seconds means 10 / 4 = 2.5 liters per second.

Want gallons per minute instead? Divide the gallons by the seconds, then multiply by 60. For example, a 5 gallon bucket that fills in 10 seconds means 30 GPM.

Do it three or four times and average the results.

The Float Method For Bigger Streams

When the stream is too big for a bucket, use a float.

Mark out a straight stretch long enough that a stick needs at least 20 seconds to drift through it, then drop one in and time it.

Measure the width and average depth at three or four different spots along that stretch. Width times depth gives the cross-sectional area, the size of the stream's slice.

Then calculate:

Flow = area × (length / time) × 0.85

The 0.85 is a correction factor. The surface moves faster than the bottom, because the stream bed drags against it like sandpaper. The US Geological Survey uses 0.85 for most surface floats.

Example: a stream 1 meter wide and 20 centimeters deep, and a stick drifts 10 meters in 20 seconds.

Area = 1 × 0.2 = 0.2 m²

Speed = 10 / 20 = 0.5 m per second

Flow = 0.2 × 0.5 × 0.85 = 0.085 m³ per second = 85 liters per second

Try a few different sticks, because one float only gives an estimate.

For very small streams there is also the weir method. You build a temporary dam with a notch of known size, often a V shape, and the height of the flow through the notch tells you the discharge.

How To Calculate Power From Head And Flow

Once you have both numbers, you can calculate the power your stream produces.

Power in watts = net head (meters) × flow (liters per second) × 7

Where does the 7 come from? Gravity and the density of water (a liter weighs a kilogram) give about 9.8 watts per meter of head per liter per second. No turbine catches all that energy, and the 7 assumes your turbine and generator keep about 70 percent.

US extension guides play it safer and assume about 50 percent. In feet and gallons their rule is net head times GPM divided by 10. In meters and liters, that is closer to times 5.

Try it with the float example and 5 meters of net head:

5 × 85 × 7 = 2,975 watts, or about 2,100 watts with the careful rule.

Even the careful number runs a fridge, lights, a laptop and a TV, day and night. Check whether that covers your home, then choose the turbine type that fits your numbers.

Measured in feet and gallons, or want the pipe loss and kWh a day worked out for you? Type your numbers into the hydro power calculator.

Measure In The Dry Season

Streams roar in spring with melting snow and trickle in August.

So measure your flow in the driest month. That worst case is what you size your turbine around, because a turbine sized for spring flow sits dry all summer.

If you cannot wait, ask neighbours how the stream behaves in late summer, or check for a government stream gauge nearby that records its discharge. The USGS alone runs more than 8,200 of them in the US.

→ Back to the full hydro overview

Sources

• Missouri Extension, Micro-hydropower site assessment
• Wyoming Extension, Micro-hydropower B-1285
• USGS, How Streamflow is Measured
• USGS training, Miscellaneous discharge measurements
• Canyon Hydro, Formulas and conversions
• Wikipedia, Hydroelectricity