A pH meter is an electronic instrument used to measure the pH (acidity or alkalinity) of a liquid (though special probes are sometimes used to measure the pH of semi-solid substances). A typical pH meter consists of a special measuring probe (a glass electrode) connected to an electronic meter that measures and displays the pH reading.
The meter circuit is no more than a voltmeter that displays measurements in pH units instead of volts. The input impedance of the meter must be very high because of the high resistance — approximately 20 to 1000 MΩ — of the glass electrode probes typically used with pH meters. The circuit of a simple pH meter usually consists of operational amplifiers in an inverting configuration, with a total voltage gain of about -17. The inverting amplifier converts the small voltage produced by the probe (+0.059 volt/pH) into pH units, which are then offset by seven volts to give a reading on the pH scale. For example:
* At neutral pH (pH 7) the voltage at the probe's output is 0 volts. 0 * 17 + 7 = 7.
* At basic pH, the voltage at the probe's output ranges from +0 to +0.41 volts (7 * 0.059 = 0.41). So for a sample of pH 10 (3 pH units above neutral), 3 * 0.059 = 0.18 volts), the output of the meter's amplifier is 0.18 * 17 + 7 = 10.
* At acid pH, the voltage at the probe's output ranges from -0.41 volts to -0. So for a sample of pH 4 (3 pH units below neutral), -3 * 0.059 = -0.18 volts, the output of the meter's amplifier is -0.18 * 17 + 7 = 4.
The two basic adjustments performed at calibration (see below) set the gain and offset of the inverting amplifier
The meter circuit is no more than a voltmeter that displays measurements in pH units instead of volts. The input impedance of the meter must be very high because of the high resistance — approximately 20 to 1000 MΩ — of the glass electrode probes typically used with pH meters. The circuit of a simple pH meter usually consists of operational amplifiers in an inverting configuration, with a total voltage gain of about -17. The inverting amplifier converts the small voltage produced by the probe (+0.059 volt/pH) into pH units, which are then offset by seven volts to give a reading on the pH scale. For example:
* At neutral pH (pH 7) the voltage at the probe's output is 0 volts. 0 * 17 + 7 = 7.
* At basic pH, the voltage at the probe's output ranges from +0 to +0.41 volts (7 * 0.059 = 0.41). So for a sample of pH 10 (3 pH units above neutral), 3 * 0.059 = 0.18 volts), the output of the meter's amplifier is 0.18 * 17 + 7 = 10.
* At acid pH, the voltage at the probe's output ranges from -0.41 volts to -0. So for a sample of pH 4 (3 pH units below neutral), -3 * 0.059 = -0.18 volts, the output of the meter's amplifier is -0.18 * 17 + 7 = 4.
The two basic adjustments performed at calibration (see below) set the gain and offset of the inverting amplifier
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