11. Measuring Techniques Reducing Accuracy of Measurement

Regularly maintained equipment, properly installed in an ideal location, will still give inaccurate discharge measurements if the operator uses poor measuring techniques. Head measurement is very important. The techniques in use often are not compatible with the relationships between head and discharge known to exist. Operators should make sure that calibration curves or tables match devices being used both in size and kind.

(a) Faulty Head Measurement

Measurement of the head on a sharp-crested weir, a seemingly simple matter, can be difficult under all but ideal conditions. The head is the height of water above the blade edge or the bottom of a V­notch, measured at a point where the velocity head (or approach velocity) is negligible (figure 5­6). In practice, this point is located four to six times the measuring head upstream from the center of the weir blade. If the head is measured too far upstream, a head not related to the water surface profile at the weir can be measured. If the head is measured closer to the weir blade, some drawdown (caused by increased velocity near the weir) may occur and less than the true head will be measured. If the head is measured at the side of the approach channel, more or less than the true head may be measured depend-ing on the geometry of the approach pool (figures 5-2 and 5­13).

Figure 5-13 -- Cipoletti weir operating with good flow conditions in the approach pool.

Figure 5-13 shows a Cipoletti weir performing properly for the discharge shown. Flow is well distributed across the wide pool and shows no evidence of excessive turbulence. Accurate or "standard" discharges can be expected under these conditions. At larger discharges, the nonsymmetrical approach may produce undesirable conditions.

The principles described above also apply to head measurements on broad-crested weirs and flumes, meter gates, or any other device dependent on a head measurement for discharge determination.

Improper gage location, or an error in head measurement in a flume, can result in large discharge errors because of water surface curvature. Incorrect flume throat width dimensions and weir lengths can also produce errors. The relative ease of making accurate length measurements usually keeps these errors small. However, operators should check lengths in the field rather than rely on values stated or shown on drawings. Readings obtained from stilling wells, whether visual or recorded, should be questioned unless the operator is certain that the well intake pipe is not partially or fully clogged with sediment or air pockets. Data from an overactive stilling well can also be misleading, particularly if long­period surges occur in the head pool. In fact, all head determinations should be checked to ensure that the reading is not part of a long­period surge. A sufficient number of readings, about 10, should be taken at regular time intervals of about 15 seconds, and averaged to obtain the average head. More readings may be required if consecutive readings indicate that the pool is continuing to rise or fall. If this process takes too much time, the cause of the instability should be determined and eliminated.

Readings from gages or staffs which may have slipped or heaved should be avoided. Periodic rough checks can sometimes be made with a carpenter's level or square from a reference point on another structure. A still-ponded water level at the weir crest height is a valuable check on the staff gage zero.

Each operator should understand the desired measurement and then critically examine each operation to ensure that the correct measurement is being taken. The operator should try to find fault with every step in the head measurement process and attempt to improve techniques wherever possible.

(b) Infrequent Measurement

When a head or velocity measurement is taken, the operator must assume that the resulting discharge occurred only at the moment of the measurement. The operator cannot conclude that the same discharge occurred 5 minutes or even 5 seconds earlier. Therefore, accurate water deliveries can be ensured only if enough measurements are made to establish the fact that the discharge did or did not vary over the period that water was delivered.

In many systems, measurements are taken only once a day, or only when some physical change in supply or delivery has been made. Problems introduced by falling head, rising backwater, gate creep, or hunting are often ignored when computing a water delivery. The problem is not simple; many factors must be considered in determining the number of readings to be made per day or other unit of time. If the discharge in the supply system is increasing or decreasing, multiple readings will be required. If the rate of rise is uniform, the average of two readings, morning and night, would be better than one. Erratic rates of change will require frequent readings. A need for many readings may justify the use of a recording device.

When the discharge in the supply system remains constant, the water level or velocity reading may change because of a change in control or because checks have been placed in operation. Temporary changes in the main supply system discharge may occur; for example, because water, in effect, is being placed in storage as a result of the rising water level. Conversely, the discharge may temporarily increase in parts of the system if the operating level is being lowered. The changing water level may require more frequent head readings. If accurate, instantaneous recording of highly variable flows is required, then stilling wells may need larger connecting pipe diameters to reduce time lag. In some situations, weir pooling volumes need accounting.

Here again, the operator should try to visualize the effect of any change in discharge in the supply system, upstream or downstream from a measuring device, and attempt to get more than enough readings to accurately compute the quantity of water delivered.

(c) Use of Improper Measuring Device

Every water­ measuring device has unique limitations; thus, a single type cannot be used in all locations under all possible conditions. Therefore, several devices might be suitable for a given set of conditions, but none could be considered entirely satisfactory. If flow conditions change considerably for any reason because of modified operations, a formerly suitable device may become totally inadequate. A formerly marginally suitable device may become useless for a small change of operation needs. An incorrect device may have been selected in the first place, and no matter how much care the operator takes, accurate measurements cannot be obtained. The operator should call attention to such situations and attempt to have remedial measures taken. Chapter 4 gives guidance for selection.

For example, with sharp-crested weirs, accuracy cannot be expected if the head is appreciably less than 0.2 ft or greater than about one­third of the weir blade length. Large measurement errors can be expected (departure from standard) if these limits are exceeded appreciably.

Large errors are introduced if a sharp-crested weir blade is submerged by backwater. Designing a device that is to be submerged throughout all or part of its flow range requires using a calibration related to a measuring head differential. Having a second or downstream measuring head station doubles the chance for wrong readings. Despite the appearance of handbook submergence discharge determination methods, discharge is related to small differences in measuring heads. Small imprecisions in water elevation measurement cause large errors. As submergence increases, the measuring head differentials decrease and approach values that are about the same magnitude as for minor variations of form and friction loss. Thus, corrections for large submergence are very inaccurate.

Propeller meter devices should not be permanently installed where weeds, moving debris, or sediment are apt to foul the meter or wear the bearings. Submerged devices, such as meter gates and other types of orifices, should not be used where a moving bedload can partly block the openings.

The flow conditions at a particular site must be analyzed. Only then can the measuring device be selected that can best cope with conditions to be encountered. The user of irrigation-type measuring devices should not expect accuracy to exceed about +2 percent, even for standard devices that have been properly selected, set, and maintained.