Group 7 - Discharge measurement using Thompson weir and current meter

Today was rainy and cloudy, but it didn't slow us down, our task was discharge measurement using Thompson weir and volymetric method.

5. discharge measurement

So we started by building this V-shaped barrage on the ditch, which allowed us to messure the ammount of water that was streaming.

We built 2 barrages, 1 at the beginning of the pond and 1 at the end.
Most important part about building the barrage was to make sure that all of the water flows thruu the barrage (from the V-shaped thing) and minimize the leaks.

We used sand and rocks to seal the barrage best we could and we we're pretty happy with the barrages we were able to build using the tools we had, which were basicly just a spade.

Our best estimate for the leaks was 2% at the beginning of the pond and same 2% at the end.

So the water height was 5cm at the beginning of the pond and 5.5cm at the end.

inflow:

Q=  0.000765 m³/s =   0.765 l/s                               h= 0.05 (m)

outflow:

Q=  0.00097 m³/s  = 0.97 l/s       h= 0.055 (m)

volymetric method.

Here we were supposed to find 2 spots (1 at the beginning and 1 at the end) where we could have messured the flow by filling a bucket with water. We we're unable to find spot where we could have used bucket to messure the waterflow at the beginning of the pond since there wasn't any "waterfalls" but we found a spot at the end.

We messured the waterflow at the waterfall 4 times and our results were:

                L                              S

1.            3.9                          4.11

2.            4.3                          4.33

3.            4.2                          4.4

4.            4.2                          4.3

so the average waterflow was 0.97 l/s.

and here again we estimated that there might be ­around 5% inaccuracy, since we weren't able to catch 100% of the water that came from the pipe.

So if we compare these 2 outflows from discharge measurement and volymetric method

we can see that they both have perfectly same result of 0.97 l/s !

4. Discharge measurement using current meter

Discharge measurement using current meter was our fourth task. In the task we measured the discharge of the small ditch by using current meter. For the measuring we used SEBA Mini Current Meter M1 indicator, stopwatch, roller gauge and tools for the notes.

First we measured the inflow. The width of the ditch was 36 cm and depth in the middle of the ditch was 7cm. We got as a result that the wing speed was 25,43 s/ 10 rounds.

Secondly we measured the outflow of the pond. The width of the ditch was 47 cm and the depth 10,5 cm. As a result we got that the speed was 34,85 s/10 rounds.

Here we measure width of the ditch

Measuring the inflow with current meter

Measuring the outflow with current meter 

We get the speed of the flow by using V=k*n+D formula, where

V = speed of the flow

k = hydraulic lift of the wing

n = wing speed

D = the standard oh the current meter

The formula of the flow is instrument-specific and its multipliers (k=0,2591) and (D=0,005) are defined by the calibration curve.

è Inflow is

k = 0,2581,

n = 10r/25,43s = 0,39r/s  

D = 0,005

V = 0.2581m*0,39 r/s+0,005 = 0,105659 ≈ 0,11m/s

è Outflow

k = 0,2581

n = 10r/34,85s = 0,29 r/s

D = 0,005

V = 0,2581m*0,29 r/s+0,005 = 0,0791 m/s ≈ 0,08 m/s

Then the current (m³/s) is counted by formula Q= V*A

Q  = Current 

V = speed of the flow

A = area

è Inflow

V =0,11 m/s

Width = 36 cm , depth = 7,0 cm 4,0 cm 4,0 cm

A =  198cm² = 0,0198m²

Q = 0,11* 0,0198 m² =  0,0021 m3/s = 2,1 l/s

è Outflow

V = 0,08 m/s

Width = 47 cm, depth = 10.5 cm, 5 cm, 5 cm

A = 364,25 cm² = 0,036425m²

Q = 0,08 m/s* 0,036425m² = 0,0029 m3/s = 2,9 l/s