GROUP 1, EXERCISE 5: FLOW MEASUREMENT USING THOMPSON WEIR AND VOLUMETRIC METHOD

Today, October 10th, it was time for flow measurement. Our entire group was present. The weather was partly cloudy and quite windy, but fortunately, it didn’t rain much. The temperature was around 7 degrees Celsius above zero.

The task consists of meassuring the flow of the inlet and outlet stream (locations in figure 1) of the Pieni-Valkoinen using the Thompson weir and the volumetric method. It´s important to have a correct installation (right angle to the watersurface and no side water flow) and messure as precise as we can. The idea is that the water is guided to a v-shaped opening, flows through it and there the water level can be read of it. Afterwards the flow of the stream in volume per time will be meassured at the same spot.

Figure 1: Inlet location: 62,92864° N, 27,66486° E; Outlet location: 62,93057° N, 27,65662° E

For both locations we searched for a good place to set-up our equipment, which has turned out to be more difficult than we thought. The outlet should be deep enough and needs to be the right width for our Thompson-messurement device.

To do the setup we used the messurement device, which is a V-shaped metal plate with a scale on it (as shown in figure 2) and has a plastic tarp attached. It is placed in the water and used as a dam, also the edges of the tarp are beeing sealed with stones and sand on the ground with the help of a shovel.

Figure 2: Example of the scale (Output stream)

The surface height at the outlet-stream is h_o=0,025m and h_i=0,030m for the inlet-stream. We can calculate the discharge Q in m³/s with the following formular:

 

Constant c = 0,58

Gravity of earth g = 9,81 s/m²

Water surface height h

 

Since there were difficulties with getting the whole water flow through the meassurement devices (espacially at the inlet channel, like shown in figure 3), we add an estimated 5% to the outlet flow and 15% to the inlet flow.

Figure 3: Set up at the input stream with side flow

For the next part, the determination of the discharge with volumetric meassurements, we need a stop watch (Handy), measuring containers and a bucket. The measuring cointainer is hold against the discharge opening of the measuring dam and fills up with water. During the meassuring cup fills up, the time needs to be meassured with a timer. To get a average in l/s, we repeat this 5 times.  

Figure 4: Meassurements and calculations

Conclusion:

As mentioned in the inlet channel experiment, the channel was too wide for accurate measurement. It is uncertain whether we were able to estimate the amount of side flow correctly, which may cause distortions in the results. The outlet channel was narrower, and we also received help from the teacher, so we can assume that those results are accurate.