Head is the vertical drop between the water surface at the intake and the turbine, not the length of the pipe or the slope you walk. The most direct cheap measurement is a water-filled hose from the intake down to the turbine site with a 0-10 bar pressure gauge on the lower end: head in meters is the reading in bar times 10.2, so 2.5 bar is 25.5 meters.
Part of Micro-Hydro Turbine
Head is the height water falls between where it is diverted and where it does work, and every watt a hydro site can make is proportional to it.
Only The Vertical Height Counts
A stream that runs 200 meters along a gentle slope and drops 5 meters has 5 meters of head. A penstock laid the full 200 meters does not change that; it only adds friction. Water pressure depends on vertical height alone: pressure = density × gravity × height, so a 1-meter column of water at 1,000 kg/m³ presses with 1,000 × 9.81 × 1 = 9,810 pascals, about 0.098 bar. Turned round, one bar is 1 ÷ 0.0981 = 10.2 meters of head.
Two numbers matter. Gross head is the raw height from the intake water surface down to the turbine. Net head is gross head less the losses in the intake screen and pipe, which the existing Head Measurement page gives as a loss fraction of 0.05 to 0.15 for well-designed systems. Use gross head to compare sites, and net head when you size a turbine.
For a Pelton wheel the working height ends at the nozzle, so the few decimeters between the nozzle and the tailwater are lost. A reaction runner such as a propeller sits in the water and can use the height down to the tailwater, as that page also explains.
Method: Hose And Pressure Gauge
Step 1: Fill the hose. Lay a garden or plastic hose from the planned intake pool down to the planned turbine position. Submerge the upper end in the pool and fill the hose completely. Air anywhere in the hose breaks the water column and makes the gauge read low, so tilt and tap the hose until no bubbles remain.
Step 2: Attach the gauge. Fit a 0-10 bar gauge at the lower end and seal it. With no water flowing, the needle settles on a steady value.
Step 3: Convert. Head in meters = reading in bar × 10.2. A reading of 2.5 bar gives 2.5 × 10.2 = 25.5 meters. A 0-10 bar gauge can therefore read up to about 102 meters; for a taller site, survey in stages.
Step 4: Check the gauge itself. Hang the filled hose from a point of known height, such as 5 meters up a tree measured with a plumb line, and read the gauge: 5 ÷ 10.2 = 0.49 bar. A gauge that is off by a tenth of a bar is off by a meter of head.
The hose may snake over rocks and hold any length; only vertical difference matters. The reading is the height from the pool surface to the gauge, so the gauge must sit at the exact level where the nozzle or runner will be.
Method: Level And Pole, As A Cross-Check
The carrier’s second method walks the slope in stages. Hold a straight pole horizontally from the lower point (confirm level with a carpenter’s level or a water level), measure the vertical distance from the pole to the ground uphill, move to where the pole meets the hillside, and repeat. The sum of the vertical measurements is the total head.
If each stage rises about 2 meters, a 25.5-meter site takes 13 stages. A small error repeated at every stage adds up: a pole 2 centimeters low every time gives 13 × 0.02 = 0.26 meters, about 1% of the head, which is acceptable; a systematic 10-centimeter error gives 1.3 meters, about 5%, which is not.
Worked Example: Two Methods, One Answer
- Hose and gauge: 2.4 bar × 10.2 = 24.5 meters.
- Level and pole, 13 stages: 25.1 meters.
- The two differ by 0.6 meters, 0.6 ÷ 24.8 ≈ 2.4% of their average, so accept 24.8 meters gross.
- With a 10% loss fraction for the intake and pipe: net head = 24.8 × (1 − 0.10) = 22.3 meters.
If the two methods differ by more than about 5%, repeat the weaker one before building anything. An altimeter or clinometer, described in the Head Measurement page, is for a quick survey only.
Common Mistakes
- Measuring along the slope — a 40-meter run that drops 8 meters has 8 meters of head, not 40.
- Reading the gauge with water flowing — flow friction lowers the reading; close the lower end and wait for the needle to settle.
- Using gross head in the power calculation — the 5-15% of head eaten by the intake and pipe never reaches the runner.
Safety
Survey slopes are wet
Stream banks and slopes beside a waterfall are slippery. Survey in pairs, keep out of the water above any drop, and do not work during or after heavy rain when the flow can rise within minutes.
What’s Next
- Learn next: Power Calculation (P = Q × H × 7) to turn head and flow into watts
- Combine with: Flow Measurement for the other half of the product