Two Cheap, Temperature Stable, Battery-Operated Devices Producing a Current Linearly Proportional to Capacitance or Resistance
R. S. Clymo, S. P. Gregory · Journal of Applied Ecology · 1975
It is becoming common for ecologists to measure water level or water content by a capacitative method. In a simple case water rises or falls between two metal plates, one at least of which is insulated. The dielectric constant of water (about 80) is much greater than that of air (about 1), so the capacitance between the plates is nearly proportional to the water level. Design criteria are discussed by Wilner (1960). The measurement of capacitance in field conditions requires a device which is accurate and reliable over a wide temperature range. It should operate from a low voltage supply and use little current. Because ecologists need-or at least they often want-to make measurements at several points and to have these recorded automatically, the device should be cheap and should allow the sensor and recorder to be earthed. To avoid a multiplicity of batteries and voltage stabilizers the power supply should also be earthed. It is desirable to be able to offset stray capacitance in the sensor cable and useful if the output is a linear function of capacitance. Computer processing can nowadays cope easily with non-linear responses but non-linearity must still give variation in accuracy and precision. So far as we know, this combination of properties-cheapness, reliability, linearity, low power consumption, low voltage, temperature stability, zero offset and common earthing of power supply, capacitor and meter-is not possessed by any published design. The recent development of integrated circuit (IC) complementary metal oxide silicon (COS/MOS) devices has now made such designs possible. These have considerable advantages over bridge circuits, including Lion's (1964) non-linear twin-T, and over the usual resistor-capacitor delay circuits. A single IC contains four gates or six inverters and costs about ?O 5. Two such designs are described here.