Potentiometric Sensors
Potentiometric Sensors : Sensing Principle
Potentiometric sensors are designed based on Equation 1—a conductor’s resistance R (in ohms, Ω) is a function of the resistivity of the conductor material ρ (in ohmmeter, Ω ⋅ m), its length l (in meter, m), and its cross-sectional area A (in meter square, m2):
Although any change in l, A, and ρ will cause a change in resistance, potentiometric sensors (also called potentiometers or pots for short) are often designed by varying the length l only for the sake of simplicity and for saving cost. Some chemoresistive sensors are designed based on materials’ resistivity ρ change caused by chemical reactions.
The resistance of a potentiometer can be evaluated using Ohm’s Law by applying an electric current I (in amperes, A) and measuring the voltage V (in volts, V) across the potentiometer.
Potentiometric Sensors : Configuration And Circuitry
Figure 1. Linear (a) and rotary (b) potentiometer configurations
Potentiometric sensors are available in two configurations: linear and rotary, as shown in Figure 1.a and b, respectively. In both configurations, resistance change is the result of position variation (x or θ) of a movable contact (wiper) on a fixed resistor, resulting in an output voltage change. The circuit symbols and typical circuits of potentiometric sensors are shown in Figure 2.a, through c.
The potentiometer R2 in Figure 2.b functions as a voltage divider. The voltage across R2 is the measured output. If a load RL is placed across R2 , as shown in Figure 2.c, the amount of current “diverted” from R2 will depend on the magnitude of RL relative to R2. The output voltage across R2 (which is also the load voltage) is then.
If RL ≫ R2, ideal measurement conditions exist where the power extracted by the load, such as a meter, oscilloscope, or data acquisition unit, is negligible.
Figure 2. Potentiometer circuits: (a) circuit symbols; (b) a voltage divider circuit; (c) a voltage divider circuit with a load
