A pH meter is a specific type of voltmeter or ion meters with high impedance of input channels. The high impedance is a necessary part of the equipment because of high resistance of pH glass electrode typically used with pH meters. In the present days almost all ion meters satisfy this requirement. Common pH meter has several inputs: for indicator( ion-selective electrode or redox electrode), reference electrode and temperature sensors like thermoresistor or thermocouple.
pH scale of the device should be calibrated by several pH buffer solutions . It allows to correlate measured potential of indicator electrode and pH scale.