Head Measurement

Part of Hydro Generator

Accurately measuring the available vertical drop (head) of a stream or river to calculate hydro power potential and select the right turbine type.

Why This Matters

Head — the vertical distance water falls from intake to turbine — is the single most important parameter in a hydro power assessment. Along with flow rate, it determines how much power is theoretically available. Underestimate the head and you’ll size your turbine too small; overestimate it and you’ll build an oversized penstock and turbine for power that doesn’t exist.

More practically, the head value determines which turbine type you can use. Low head (under 3m) requires water wheels or specialized low-head turbines. Medium head (3-30m) suits breastshot/overshot wheels, crossflow turbines, and Francis turbines. High head (30m+) enables Pelton wheels, which are more efficient and generate more power per unit of flow than any other type.

Measuring head sounds simple — it’s just a height difference — but getting an accurate number without surveying equipment requires some ingenuity. The methods described here are accurate to 1-5% using simple tools, which is sufficient for turbine design purposes.

Understanding the Three Types of Head

Gross head: The raw vertical distance from water surface at the intake to the turbine location. This is what most people measure first.

Net head: Gross head minus friction losses in the penstock pipe, intake screen, and other hydraulic components. This is the effective head that actually drives the turbine. Net head = Gross head × (1 - loss fraction), where loss fraction is typically 0.05-0.15 for well-designed systems.

Turbine-specific head: For water wheels, only the head from intake to turbine inlet matters. For reaction turbines (Francis, Kaplan), the head from the turbine inlet to the tailrace surface also contributes (suction head). For impulse turbines (Pelton, crossflow), only the head up to the nozzle matters.

For initial site assessment, gross head is sufficient. For turbine design, calculate net head. For final detailed design, measure both static head (with no flow) and dynamic head (with design flow through the penstock) to determine actual head loss.

Method 1: Water Level and Measuring Pole

The simplest method for small head differences (under 5m):

Walk the stream from potential intake to turbine site. At each change in gradient, hold a measuring pole vertically in the water and sight horizontally across the water surface to a mark on the bank above the water at the next station. The vertical difference between the water surface and the sighting point is one incremental head value. Sum all increments.

More precisely: two people with a marked pole and a hand level (or even a sighting tube of water that serves as a level). Person A holds the pole; Person B sights horizontally across their hand level from the current water surface to the pole. The pole mark at the horizontal line of sight is read and the next upstream station is established at that mark. Repeat until the intake is reached. Sum all marked values.

Accuracy: 2-5% with careful technique. The main error is in establishing horizontal — a small tilt in the level instrument creates systematic error. Check your hand level on a calm water surface before starting.

Method 2: Altimeter or Barometric Pressure

A pocket altimeter (aneroid barometer calibrated in elevation) can measure head directly: read elevation at intake and turbine site. Difference = gross head.

Accuracy: ±1-3 meters for most altimeters, depending on atmospheric pressure changes during the survey. Take both readings within 30 minutes to minimize pressure change error. Re-check by returning to start point — your reading should match the original.

GPS elevation is less accurate (typically ±5-15 meters vertical) and not suitable for head measurements of less than 50m.

Method 3: Builder’s Level or Dumpy Level

A builder’s level (optical instrument with a leveling telescope on a tripod) is the most accurate simple method and gives results within ±5mm over distances of 50m.

Process: Set up level midway between two stations. Read rod (leveling staff) held vertically at each station. The difference in rod readings gives the elevation difference between stations. Repeat along the stream profile from intake to turbine site.

This is standard surveying practice (differential leveling). A hand level, carefully used, approximates this with less accuracy. If you can obtain or build a builder’s level (a simple telescope with a spirit level mounted on a rotating head), the accuracy is dramatically better than sighting with a pole.

Making a simple level: A spirit level with a sight tube. Mark the tube at the center of the spirit bubble. Sight through the tube when the bubble is centered. Less accurate than a builder’s level but much better than free-hand sighting.

Method 4: Clinometer and Slope Measurement

For steep streams where water falls rapidly:

A clinometer measures the angle of a slope. Walk along the stream, measuring distances along the channel and angles at each change of slope. Convert to vertical using: Vertical rise = Horizontal distance × tan(angle), or for angles under 30°: Vertical rise ≈ Distance × sin(angle).

A simple improvised clinometer: a protractor with a plumb bob hanging from the center. Sight along the straight edge; read angle from the plumb string.

Accuracy: 5-10% due to difficulty reading angles precisely and measuring along irregular stream banks. Adequate for initial feasibility, not for final design.

Calculating Power from Measured Head

Once head (H) and flow rate (Q) are measured:

Power = 9.81 × H × Q × η (in kilowatts, with H in meters, Q in cubic meters per second, η = efficiency)

Examples:

• H = 5m, Q = 0.1 m³/s (100 liters/second), η = 0.65: Power = 9.81 × 5 × 0.1 × 0.65 = 3.19 kW
• H = 15m, Q = 0.05 m³/s, η = 0.75: Power = 9.81 × 15 × 0.05 × 0.75 = 5.52 kW
• H = 2m, Q = 0.5 m³/s, η = 0.55: Power = 9.81 × 2 × 0.5 × 0.55 = 5.39 kW

Flow measurement is the other critical variable. Simplest method for small streams: bucket and stopwatch (divert a small sluice into a known-volume container and time the fill). For larger flows: float method (measure channel cross-section area × float velocity × correction factor ~0.85), or velocity-area method with improvised current meter.

Always measure flow at the driest time of year you expect to rely on the system — this gives the minimum dependable flow. Design for minimum flow; the system will automatically generate more during high-flow periods.