From Fig. 12.8a it is possible to calculate the values of q, R1, R2, and Vout for
various liquid levels. These are shown in Table 12.1.
Figure 12.9a shows a plot of the output voltage versus the volume of liquid in
the tank from Table 12.1 for the uncompensated curve. The best-fit straight line
(dashed) gives an error of about ±15 percent of full scale reading (FSD). To correct
the mid-point of the scale for 5 V output when the tank is half full (Fig. 12.8b),
the circuit would require a 6.66-kΩ resistor from the wiper to ground as shown
in Fig. 12.9b (the resistor should be made with the same material as the potentiometer).
Table 12.2 gives the liquid levels, resistance values with the potentiometer
in parallel with the 6.66 kΩ resistor, and the output voltage for the
circuit in Fig. 12.9b. The new values are also plotted in Fig. 12.9a as the compensated
curve. The best-fit straight line (dashed) gives an error of less than
±5 percent showing the improved linearity with simple conditioning. The output
voltage from the float sensor can also be compensated by the control processor
before being fed to other elements in the system.
An alternative to the float attached to an arm is a float with a counter balance
as shown in Fig. 12.10a. This arrangement will give a linear scale with liquid level
or if a rotary potentiometer is attached to the pointer pivot, the output voltage
from the potentiometer will be linear with liquid level as shown in Fig. 12.10b.