Determination of hydraulic parameters  

 

Water flow meter OTT MF Pro

Manufacturer: OTT Hydromet



  
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The velocity measurements are performed with the electromagnetic sensor OTT MF Pro. It consists of a battery-driven control unit which is connected to the measuring probe by a sensor cable. The probe traces the point velocity in natural streams, rivers, creeks and open channels. Additionally, the sensor can automatically analyse the water depth in the water course. When the probe is fully placed in and under flowing water it records the point velocity in a certain position and depth of a selected vertical [2].

 

  Sensor specifications OTT MF Pro [2]

Velocity range

0 to 6 m/s
 

Water depth measurement range

0 to 3.05 m
 

Accuracy of velocity measurements

for 0 to 3 m/s
±2 % of measured value ± 0.015 m/s
for 3 to 5 m/s ±4 % of measured value ± 0.015 m/s


 

 

 

 

 

 

  Preparing measurement device [2]

(1)
Attach sensor with wing screw to hollow measuring rod adapter

ott_mf_pro_measurement_device

Fig. 1: OTT MF Pro measurement device [5]

(2)
Link cable with portable meter

(3)
Screw slim inner poles together (quantity varies by needed height, it should protrude a little bit above your head though)

(4)
Put lowest hollow rod with sensor over inner poles

(5)
Put additional hollow over inner poles and screw hollow rods together (quantity of hollow rods is one less than inner poles)

(6)
Tighten sensor height lock at topmost hollow rod which functions as adjustment and lock

(7)
The control unit can either be attached to the topmost hollow rod or it can be carried around the neck and be controlled with one hand

(8)
Set entire measuring pole including probe into the water course

(9)
Turn on control device and choose method of measurement:

  • real time: select this option to get real time point velocity and depth information
  • profiler: select this option to set up a velocity distribution of a profile for a stream

(10)
Set up profiler and continue with option “stream” for stream and flowing channel measurements, then enter a name for chosen profile

(11)
Usually the reference water level is not that important and can be set to 0 m

 

  Performing velocity measurement [2]

(1)
Enter distance of first vertical to a reference point (bank), it’s important to gauge the distance orthogonally to flow direction

(2)
Confirm input with “OK” and continue to define depth

(3)
For defining depth loosen locking screw and lift probe over water surface, then slowly descent the sensor to streambed, after value of water depth stabilizes, complete metering with “OK”

(4)
Continue with measuring the flow velocity in different heights of selected vertical, quantity of measured points varies and depends on water depth and can be set between one and six measured points

(5)
Follow instructions on portable meter and adjust sensor to correct depth, secure locking screw when depth indicator lights up green and start measurement

(6)
Repeat step 4-5 for other depths in vertical

(7)
Repeat for following verticals

(8)
Always end a complete measurement by saving data before exiting, otherwise recorded data might be lost

(9)
Label saved data but avoid binomials

 

  What to look out for performing velocity measurements with OTT MF Pro [1], [3]

a.
For metering the velocity in high gradient lined channels it’s suggested to use a bridge as a standpoint perpendicular to direction of flow

b.
Selected cross sections shouldn’t have vorticities, back currents or dead water

c.
Direction of flow of each vertical line of measurement is parallel to embankment respectively perpendicular to cross section

d.
To get an optimal result, one vertical line of measurement should not cover more than 10 % of discharge, however cross sections with uniform channel bed and flow velocity can have less verticals for a good result

e.
Reduce distance between verticals at positions with change of water depth and velocity

f.
Flow obstacles affect flow direction; therefore, the sensor should be at least 4 to 8 cm away from these obstacles

g.
The angle of flow should be less than 10° because only the vertical velocity component is measured by the probe

h.
Stand lateral to the device in the water course when performing a measurement, otherwise results might be influenced (Fig. 2)

ott_mf_pro_positioning

Fig. 2: Positioning in the channel bed [3]  


  

  Analysis procedure

The evaluation of the mean flow velocities and further hydraulic parameters depends on the number of point measurements in the respective measuring perpendiculars. To evaluate the measured data the device must be connected to the computer via a USB-cable and then switched on. Via "Setup", "USB", "Mass storage", the computer accesses the memory of the measuring device. The files (#.tsv) from the two folders RT (Realtime) and P (Profiler) are opened with a text editor.  To determine the velocity distribution over the cross section the profiler textfiles are used. In order to evaluate the results of the velocity measurements two possibilities are introduced: The evaluation can be either performed with the application Q-Review provided by OTT (WEBLINK) or with an individual analysis sheet in Excel which is described in more detail below.

(1)
Transfer the textfile (#.tsv) to the raw data chart in the analysis sheet

(2)
The light blue cells are optional, grey cells are necessary to be filled

(3)
The calculation is accomplished for every cross section segment related to a measured vertical according to the mid-section method:

mid_section_methodFig. 3: Mid-section method [3]

(4)
For every segment assigned to a measured vertical the hydraulic parameters are calculated:

         

(5)
To finalize the calculations, individual segmets are summed up and evaluated in the "hydraulic parameters" column: 

    

 (6)
Additionally a stage-discharge-relation for trapezoidal profiles can be displayed using the Manning-Strickler-equation

(7)
For this purpose, the averaged Strickler-coefficient [6] can be used accounting different roughness conditions along the wetted perimeter in the cross section:

     hydraulic_parameters_equations3

 

   References

[1]
OTT Hydromet, (2015): Flow and Discharge Measurement with the OTT MF Pro, WEBLINK
[2]
OTT Hydromet, (2018): OTT MF Pro, Basic User Manual, 09/2018, Edition 7, DOC026.98.80211, WEBLINK
[3]
OTT Hydromet (2018): OTT MF Pro, Operating Instructions, 09/2018, Edition 7, DOC026.53.80211, WEBLINK
[4]
OTT Hydromet (2018): Technical Data OTT MF Pro - Water Flow Meter, WEBLINK
[5]
OTT Hydromet (2019): OTT MF Pro - Less expenditure for precise data, WEBLINK
[6]
Naudascher, E. (1992): Hydraulik der Gerinne und Gerinnebauwerke, Springer Vienna/New York, 2nd edition (German)
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