Hydropower Head Measurements – 3 Methods

Zip Level Pro 2000 Front View Base Unit and Measure Cable

DoradoVista's Zip Level Pro 2000™ made by Technidea - Used to measure Hydropower Head

Hydrostatic head is the pressure rise caused by gravity acting on a column of water or fluid that is not in motion. In the case of a hydropower system it comes from the difference in elevation between the intake of the hydro system penstock and downhill to the input side of the water turbine.

Head can be given as either the difference in elevation or vertical height (Head is a measured height in feet or meters) or it can be expressed as the pressure of the same water column at the turbine inlet (Pressure stated in PSI, KPA, Newtons/m^2, etc.)

As part of each of the 12 Step Small & Micro Hydropower we will provide one or more pages featuring the tools we discuss. For hydropower head measurements we have collected some;  Featured tools related to your hydro site head survey. On this page we feature more on the Zip Level Pro 2000 used in Method 3 below and links for tools mentioned in all 3 methods.

Some Important Notes about Hydro System Head Measurements

The hydro system head measurements will need to be made very accurately since these head measurements play an important role in determination of other hydroelectric system parameters as well.

These head-pressure related system parameters include things such as; hydro turbine selection, system efficiency and fluid dynamic issues. For example; Kaplan, Cross-flow, Francis or Pelton turbines, hydrodynamics of turbine runner blade or bucket design, penstock material and strength, valve types, etc. are all impacted directly by the head measurement. All of these in turn impact the engineering and financial side of the hydro system design.

Because good hydro system design is so dependent on the accuracy of head measurement we will skip the use of mechanical altimeters and GPS units. Typically these have altitude errors on the order of tens of feet or more unless special survey quality GPS instruments are used. As you will see in method 3 below there are instruments that can do the job accurately, repeatedly, with ease and reasonably low cost.

Three accurate hydropower head measurement methods for measuring hydrostatic head are given in the links below, pick one that fits your need or budget and feel free to let us know about any new hydro head measurement methods you have:

Method 1 – Hydro Head Measure using a Surveyor’s Transit or
Levels and a Pole

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This method uses a Surveyor’s transit or contractor’s levels and a marked pole. You can use a 20 ft section of PVC pipe marked with a measuring tape attached for easier reading. The transit can be replaced by a straight board and level for economy, watch out for hydro head error buildup with short segments or warped boards or bad leveling though.

Figure 1 Hydrostatic Head Measure using Surveyors Transit or Straight Board & Levels

Head measurement using Surveyors’ Transit or Levels and a measuring pole – Tools & Resources:

  • Two workers who can measure accurately and can hike/climb your prospect
  • Surveyors’ transit or contractor’s level and straight board or a sight tube or laser level pointer
  • Tripod for holding transit or level
  • Level for vertical pipe
  • PVC Pipe approximately 20 ft section
  • Tape measure for horizontal/diagonal measure, 330 ft or 100 m segments
  • Tape measure for vertical measurement. Attach to PVC pole for ease of use
  • 2 radios help if legs of each segment are far apart
  • Chalk and flags to mark measure points as you move

Figure 2 Hydrostatic Head Procedure using Survey Transit or Board & Levels

Hydrostatic Head Measurement using Surveyors’ Transit or Levels and a measuring pole – Procedure:

  1. Pick a path to work downhill that gives line of sight between the end points of each segment of the measured head. The difference in elevation for each segment must be less than your measured 20 foot pipe length. Steep grades will require some adaptation, so be ready to think like a mountain goat.
  2. Set up the survey transit level for each measurement point on a horizontal reference line. This must be level!
  3. Set up point to measure by holding measuring pipe vertically at the selected point.
  4. Surveyor reads off and records vertical incremental elevation.
  5. Measure and record the horizontal distance between points if you want to determine slope or grade and length of penstock for that segment. Note: Either horizontal or diagonal distance will do you can convert horizontal to diagonal average using a little math later. You will be summing the diagonal segments to get an estimated Penstock length.
  6. Repeat steps 2-3-4-5 until the water level for your power house turbine inlet level is reached (centerline of the hydro turbine inlet pipe)
  7. Now take the last set of measurements from the turbine inlet centerline down to where your tail race or final water outlet level is reached.
  8. The elevation change at a point is simply the sum of incremental elevation changes measured from the start elevation reference up to that point. You need to measure turbine altitude data if the system requires a reaction turbine and associated draft tube. More on turbine selection later.

Note: The incremental elevation errors add up in this process, so you must be careful to keep vertical errors small for each segment.

Method 2 – Measure Head indirectly using a Hose and Pressure gauge

If the distance is short enough, you can use one or more garden hoses to measure Head. This method relies on the constant that each vertical foot of HEAD creates 0.433 psi of water pressure. (100 vertical feet would create 43.3 psi.) By measuring the pressure in the hose, you can calculate the elevation change of your system.

Figure 3 Measure Hydro Head with a Pressure Gauge & Hose

Head measurement using a Hose and Pressure Gauge – Tools & Resources:

  • Two workers who can measure accurately and can hike/climb your prospect One can do this.
  • Several segments of garden hose or pressure hose with a burst strength in excess of the maximum head expected.
  • A pressure gauge that has a range approximately 110-125% of maximum head pressure expected (the smallest range that fits gets the best accuracy) 
  • Hose seals – fresh ones you will want gas & water tight seals
  • Fittings to mate hose, valves and pressure gauge.
  • Valves to seal ends and retain water if needed (optional).
  • Tape measure for horizontal/diagonal measure, 330 ft or 100 m segments
  • 2 radios help if legs of each segment are far apart
  • Chalk and flags to mark measure points as you move

Run the hose (or hoses) from your proposed intake site to your proposed turbine location. If you attach multiple hoses together, check that each connection is air tight and leak-free. Also make sure the hoses can handle the maximum pressure you are going to generate. Attach an accurate pressure gauge to the bottom end of the hose and completely fill the hose with water. Set the hose up so there are no high spots in it that could trap air. Note: For accurate readings, you should use a minimum height difference of about 5 feet to be able to read a minimum of a 2 psi change that is the typical resolution of a garden variety 100 psi pressure gauge. More vertical difference is better!


Figure 4 Procedure to use a Pressure Gauge and Hose to Measure Head Pressure

If necessary, you can measure total HEAD over longer distances by moving the hose(s) and taking multiple readings. Keep in mind, however, that there is less than a half-psi difference for every vertical foot (0.433 psi/ft or 2.31 ft/psi.) Except for very steep hillsides, even a hundred foot hose may drop only a few vertical feet. The measurement error significantly increases with a series of low-head readings. Use the longest possible hose or set of hoses, along with a highly accurate quality pressure gauge, to measure head. The pressure gauge must be graduated so that measurements are taken in the middle to upper end of the pressure gauge’s range. You shouldn’t use a 0 – 300 PSI gauge to measure 2 -20 PSI pressure. You should use a 0 – 30 PSI gauge instead.

To purge your hose of air so you get accurate readings, use this water pressure gauge measure procedure —

  1. Fill the hose with water, close off the ends using shut off valves
  2. Submerge one end of the hose in the water at the intake point, and the other at the outlet point.
  3. Open both ends of the hose, start with the intake end. The water should begin to flow by siphon – maybe quite slowly at first depending on slope
  4. Try submerging a few coils of hose to fill them then plug the end with your thumb, then lift the coils to start the siphon releaseing your thumb, quickly submerge the intake end to continue siphon action.
  5. Let the water run back into the stream down hill from the intake. Now take the upper end and hold it below the surface until all air is purged.
  6. Allow the water to flow through the hose for several minutes – to be certain all the air is removed. Sometimes you will need to suspend the hose and move down hill moving the air with you.
  7. Now place a pressure gauge in the outlet and seal tightly with a good washer. This will stop the flow and keep air out. See earlier comments about the pressure range of the gauge and transient pressures.
  8. Allow the gauge pressure transients to settle to a stable reading. Multiply by 2.31 that will give you feet of head, write it down and move on.  Use similar conversion factor for meters see below.
  9. Refill and seal the hose as you measure each segment or keep all hoses tight, making sure to seal it and prevent air from entering.

Notes: Use this formula –>  A psi x 2.31 ft/psi = B feet of head.  So, 75.0 psi x 2.31 ft/psi = 173.3 ft.

Use a similar conversion factor for metric results –>  A psi x 0.704 m/psi = B Meters.  So, 75 psi x 0.704 m/psi = 52.8 m — or… you can just get a metric pressure gauge, we’re working on getting you a supplier!

Method 3 – Measure Hydro Head using a Precision Zip-Level Pro 2000™

This method is unique to DoradoVista’s approach to hydropower head measurement. We will also show how to use it for accurate flow measurement in a later post about flow measurement methods. It is a very accurate differential pressure based geotechnical instrument. It is extremely accurate when used according to the instructions given by Technidea the manufacturer. The tool is also known as the Stanley Tools CompuLevel™ .  This great instrument saves time, labor (one person operation is easy.)  The Zip-Level is the most accurate method we know of for obtaining an easy and accurate measure of hydro head at a decent price.

Figure 5 Use of a Zip Level Pro 2000™

Head measurement using Zip Level Pro 2000™ – Tools & Resources:

  • One worker that can measure accurately and can hike/climb your prospect
  • Zip Level Pro 2000 Precision elevation measuring instrument
  • Tape measure for horizontal/diagonal measure, 330 ft or 100 m segments
  • 2 radios (option if two workers are used)
  • Chalk and flags to mark measure points as you move

We have also found that the engineer and contractor time savings during Zip Level use on DoradoVista’s ranch more than paid for the cost of the Zip level during the first project we completed!

In fact, regarding making accurate head or pressure measurements, after investigating the hydropower head measurement use of the Zip Level tool with the manufacturer, we found that each segments reading is well within ±1/8 inch specified for each 200 ft span. That means a 10-segment 2000 foot span of measurements will result in less than 10×1/8 inch = ±1.25 inch total head measurement error. This is the equivalent of about ± (1.25in/12ft x 0.433 psi/ft) = ±0.05 psi error! You will have much bigger errors coming from other sources if you carefully use this tool according to instructions. Wind bounce of the measuring tube and thermal changes can affect the result so some small amount of reasonable care must be exercised regarding how to use it and when to recalibrate the instrument. The good news is the automatic Zip-level calibration is very easy to do.

Zip Level Pro 2000 Base Unit and Measure Module

Zip Level Pro 2000™ Base Unit and Measure Module. Used on DoradoVista's Ranch too!

Another great feature of the Zip Level Pro 2000™ is that it will measure around and over blind corners. That has come in useful more than once when working hydro head measurements for us at DoradoVista. I do not know of any streams that follow straight lines so as with other methods expect to think outside the box a lot… “How can I measure around, over or inside that?” The cool thing about the Zip Level is there is often several ways you can use it to make head and elevation measurements, so study the user guide. Lasers and line of sight tools like a surveyor’s transit cannot touch the Zip Level’s blind measurement feature. There are other features that allow easy one man operations too. That’s enough on the Zip level’s useful features for now. Let’s get back to hydropower head measurement since that is why we are here.


Figure 6 Procedure to use Zip Level Pro 2000 to Measure Head

Hydrostatic Head Measurement using Zip Level Pro 2000 – Procedure:

  1. Pick a path to work downhill that does not have to be in line of sight between the end points of each segment of the measured head. The difference in elevation for each segment must be less than your measured 40 foot elevation change. The Zip Level will issue a warning beep if you exceed 40 ft. I personally try and keep this more like 30 feet since this is trading more segments for a more accurate reading. Steep grades will require some adaptation, so again be ready to think like a mountain goat. Make sure you anchor your Base unit each time if it is likely to shift.
  2. Set up the Zip-Level Pro 2000™ and calibrate it 100 ft downstream. It is best to work in the cool of the morning or late afternoon as large heat shifts will require recalibration due to density changes in the working fluid in the line between the measure module and base unit. Now zero the elevation at the start point where the penstock intake will be.
  3. Set up point to measure by holding measuring module at the selected point.
  4. Surveyor reads off and records vertical incremental elevation.
  5. Measure and record the horizontal distance between points if you want to determine slope or grade and length of penstock for that segment. Note: Either horizontal or diagonal distance will do you can convert horizontal to diagonal average using a little math later. You will be summing the diagonal segments to get an estimated Penstock length.
  6. Repeat steps 2-3-4-5 until the water level for your power house turbine inlet level is reached (centerline of the hydro turbine inlet pipe)
  7. Now take the last set of measurements from the turbine inlet centerline down to where your tail race or final water outlet level is reached.
  8. The elevation change at a point is simply the sum of incremental elevation changes measured from the start elevation reference up to that point. You need to measure turbine altitude data if the system requires a reaction turbine and associated draft tube. More on turbine selection later.

Useful Hydropower Site Head Measurement Tools

As mentioned earlier for SmallHydro.com, part of each of the 12 Step Small & Micro Hydropower articles, we will provide you with one or more pages featuring the tools we discuss. For hydropower head measurements we have collected some of these; Featured tools related to your hydro site head survey. The Zip-Level Pro 2000 is featured as part of a complete set of tools mentioned for the 3 methods of hydro head measurement.

Computing Hydro Head – Gross Head vs. Net Head

By recording these actual hydro penstock head measurements, you have determined your hydro site’s Gross Head. As described later in Computing Net Hydro Head, however, the effective Head at the nozzle is actually lower when water begins to flow, due to pipeline (penstock) friction. Net Head will vary with hydro system flow rates in a dynamic way. A properly designed penstock pipeline will yield a Net Head of about 85%-90% of the Gross Head you measured. Less system Net Head efficiency may be acceptable if the system penstock and turbine costs and energy produced are in balance to meet ROI requirements. More on this Hydroelectric system ROI balance later.

More Notes on Hydro Site Head or Pressure Measurement

  • 1 foot elevation difference gives 0.433 pounds per square inch (psi) rise in water pressure at the bottom.
  • 1 PSI pressure rise means there is 2.31 feet of water column above the water pressure gauge.
  • 1 meter elevation difference gives a 9.807 kilo Pascals (kPa) rise in water pressure at the bottom.
  • 1 kPa pressure rise means there is 10.2 cm of water column above the water pressure gauge.
  • For some reaction turbines, it will be important to log the turbine outlet altitude as well although not as accurately as head. This is used to compute the required net positive head to prevent any damage from turbine runner cavitations.

Hydro Prospector Jess

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