the effect of the particle size distribution on non-newtonian turbulent ...

Chapter 3 Experimental Work Page 3.16
The following procedure was undertaken for calibrating <strong>the</strong> magnetic flow meters.
I. The pump speed was set to <strong>the</strong> desired speed.
2. The weigh tank scale reading was noted (Ml)'
3. The data logger was first started and <strong>the</strong>n <strong>the</strong> diverter valve was opened and <strong>the</strong>
stopwatch started at <strong>the</strong> same time.
4. The diverter valve was closed and <strong>the</strong> stopwatch stopped at <strong>the</strong> same time when
sufficient slurry had been collected. The data logger ran for a pre-set time, enough
to cover <strong>the</strong> whole procedure.
5. The weigh tank reading .was noted (M,).
The flow rate for each reading is given by <strong>the</strong> equation:
(3.9)
The average transducer reading cannot be used for calibration purposes as it is not accurate.
In order to obtain <strong>the</strong> correct transducer reading for each point a graph <strong>of</strong> current signal vs
time was plotted. Figure 3.13 shows a typical output.
As can be seen from <strong>the</strong> graph <strong>the</strong> current signal drops sharply after <strong>the</strong> diverter valve is
opened (point A) before constantly decreasing (point B to point C). After <strong>the</strong> diverter valve
is closed (point C) <strong>the</strong> current signal reading returns to <strong>the</strong> original value (point D). The
slope <strong>of</strong> <strong>the</strong> line BC remains constant at <strong>the</strong> varying pump speeds. The transducer reading
is thus read by extending a line EF from <strong>the</strong> slope <strong>of</strong> line BC halfway during <strong>the</strong> time taken
to divert <strong>the</strong> slurry to <strong>the</strong> weigh tank (point A to point C).
The calibration equation is <strong>the</strong>n derived by performing a linear regression on <strong>the</strong> flow rate
and <strong>the</strong> transducer readings in each case.
No difference was found between <strong>the</strong> laminar and turbulent flow data when calibrated in this
Way, as indicated by Fig 3.14 showing a typical calibration.