4.1.2 Motor Speed during a measurement

In confirmation of the method described in section 3.3.2, figures 8-10 show the variation of motor speed with time during a measurement. The graphs were obtained with three fluids of different viscosity, and using a chart recorder to measure the voltage across the motor. Strictly speaking, due to mechanical losses in the motor, the voltage across it only approximates the speed; however this approximation is sufficient for these largely qualitative graphs. (Note that in all viscosity measurements, the microprocessor control system adjusts the motor speed using the voltage applied, but obtains the exact speed for the calculations from the optical readings.) A measurement takes around 90 seconds, depending on the test fluid's viscosity.

Figure 8 displays the rotation rate for the calibration reading of a 1000 cst fluid. The speed starts off at the maximum (300 rpm) and decreases slowly in steps, until the displacement angle becomes less than 170 degrees.Next the speed is increased from zero until a small angle is detected. Following this ten revolutions at maximum speed are counted, then the measurement run of ten readings equally spaced throughout the range.
 

Figure 9 is a similar plot, obtained with a fluid whose viscosity was found to be about 670 cst. The lower viscosity results in the measurement range occurring at a generally higher speed.
 

Figure 10 shows the same results for an even lower viscosity liquid, measuring about 280 cst. In this case even at the maximum speed the torque exerted on the glass cylinder was not enough to fully extend the spring to 180 degrees. Neventheless as long as there is at least some displacement a measurement may still be made, as in this example. Here the fastest motor speed is used as the top end of the range, and a lower limit found in the usual way. (Such matters are transparent to the user.)
 
 

4.1.1 Photo-diode Output

Contents

4.1.3 Angular displacement and motor speed