Measurement of acidity or alkalinity is very important in many industries and processes. Acidity or alkalinity is a measure of the relative ratio of Hydrogen (H+) ions to Hydroxyl (OH+) ions. If they exist in equal amounts, the solution is neutral, 7.0 pH. An increase of H+ will increase acidity, a lower pH, and an increase of OH+ will increase alkalinity, a higher pH.
Common electrodes used to measure pH will, based upon the Nernst equation at 25 °C, output 0 mV at 7.0 pH, +174 mV at 4.0 pH, and –174 mV at 10.0 pH. Although many measuring instruments today use complex microprocessor based circuits to measure pH, a low cost basic circuit can take advantage of the wide DPV-3
offset and span capability to simplify the external interface circuit.
Probes for measuring pH usually require a very high input impedance buffer amplifier, (100 megohms or greater). This can be followed by an inverting high gain amplifier to output a useable voltage signal. For example with a total gain of -2000, pH 4.0 would output approximately –3.48 volts, pH 7.0 approximately 0.00 volts, and 10.0 pH approximately +3.48 volts.
These can be input directly into a DPV-3 set for the 5 Volt range, which can be adjusted to read the desired pH directly.
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