CEE 3620: Water Resources Engineering Spring 2018

Lab 1: Flow Measurement BACKGROUND

Hydraulic Bench

The Hydraulic Bench provides facilities for performing a number of hydraulic experiments at laboratory bench scale. A small centrifugal pump, drawing water from a sump which lies below the bench, delivers water to an experimental apparatus placed on the bench top (see Figure 1). The flow rate is controlled by a valve in the supply line, and is measured before return to the sump for recirculation. Using the volume-time method, the discharge is measured by timing the filling of a 5-gallon bucket. Around the edge of the bench there is a raised lip so that any water leaking from the equipment does not spill over the edge, but drains back to the sump through a waste hole provided for the purpose. There is no permanent connection between the bench top and the supporting structure, so the top may be removed easily at any time for inspection of the working parts below.

Figure 1. Schematic of Hydraulic Bench

Electromagnetic Flowmeter

Due to the inconvenience of the volume-time method, electromagnetic flowmeters have been added to benches to measure the flow rate. In an electromagnetic or magnetic flowmeter, voltage is induced in a moving electroconductive liquid (e.g., water) crossing lines of the magnetic field. The voltage is directly proportional to the flow rate. The flowmeter consists of a nonconductive pipe (or a conductive pipe with an insulating inner surface lining), two electrodes which are usually flush with the inside surface of the pipe, and an electromagnet (or sometimes a permanent magnet). The electrodes are in contact with the liquid and are oriented perpendicular to both the direction of flow and the lines of the magnetic field. The pipe is mounted in the gap of the magnet. The liquid must be electroconductive; however, the conductivity can be very low. There are a number of modifications that can be made to the flowmeter to improve its accuracy or measurement range. For instance, the local velocities of the liquid in a duct can be measured by immersing two electrodes into a stream and mounting a magnet outside the duct.

OBJECTIVE

To measure flow rates using a magnetic flowmeter and the volume-time method, and to compare these flows as a means of estimating the accuracy and precision of the flowmeter.

EXPERIMENTAL PROCEDURE

1. Turn on the pump by flipping the switch (or pressing the red button).

2. Open the valve slowly to start the flow. Do not turn it on full blast!

3. Adjust the valve to obtain the desired flow rate, as indicated by the flowmeter display. Observe the display for at least 20 seconds to make sure the flow rate is steady (or is changing slightly enough to estimate a value).

4. Once the digital readout is constant (or nearly constant), without changing the flow rate, quickly pull the hose out of the tub and fill the bucket to the specified line. Record the time that it takes for the bucket to fill to the specified line. Be ready to shut off the flow as soon as the bucket is filled to the specified line. Try not to get wet! (Hint: Try to hold the bucket so that the hose discharge is at the same elevation as when discharging to the tub.)

5. Repeat steps 3 and 4 two more times (three trials total) for each of four different flow rates ranging from 6-18 L/min.

RESULTS

Record observations in the tables provided on the attached data sheet. Compile these results in a spreadsheet and plot percent error versus flow rate as indicated below. Be sure to include this plot and your tables of values in your report.

CALCULATIONS

Show sample calculations for one set of trials (i.e., for the three trials at one flow rate) as outlined below. Label variables and use units in your calculations. The flow rate is defined as the volume of water passing through a given area (cross section) per unit time. In other words:

Flow rate = Q = Volume

Time

Flow rates were measured using the volume-time method in this lab by filling a bucket to a given volume and timing the procedure. You have three trials at each of four flow rates. For each set of trials, compute the average time to fill the bucket to the specified line. Use this average time to compute the average flow rate obtained using the volume-time method (see Table 1).

Assuming the volume-time method gives the best estimate, compare the volume-time method and the magnetic flowmeter. Keep in mind the potential human error associated with the volume-time method. The percent error can be calculated as follows:

% Error = 100Qvol Qmag

Qvol

where Qvol = flow rate calculated using the volume-time method (m3/s), and

Qmag = flow rate measured by the magnetic flowmeter (m3/s).

Useful Conversion Factors 1 m3 = 1000 L 1 ft3 = 7.48 gallons 1 m3 = 35.3 ft3

GRAPHS

Plot % Error versus the average Qvol.

DISCUSSION

1. Why do you think there is a difference in the values?

2. What could be some sources of error in each method used?

3. Do you think one method is more accurate than the other? Why or why not?

4. Does the accuracy of each method depend on the flow rate?

DATA SHEET

Table 1. Volume-Time Method Results

Volume Time (s) Avg. Qvol Qmag (L) Trial 1 Trial 2 Trial 3 Average (m3/s) (m3/s)

Table 2. Error Calculation

Avg. Qvol (m3/s)

Qmag (m3/s)

% Error