Sensorex manufactures a wide variety of water quality sensors, including pH Electrodes, which can be used in water, chemical processing, medical, laboratory, and environmental applications. The Sensorex S8000 Series Next Generation Flat Surface Self-Cleaning pH Electrode, seen in Figure 1, is a modular, easy maintenance, cartridge-type, combination pH/Reference electrode for measurement of the acidity or alkalinity of water and other liquid medias.
Figure 1: Sensorex S8000 Series Next Generation Flat Surface Self-Cleaning pH Electrode
Sensors for pH use electrochemical means to measure and digitize pH values, enabling automated data acquisition and control. This is done by using a pH electrode whose output voltage changes as the pH (hydrogen ion concentration) of the medium of interest changes. The output voltage is then compared to a reference electrode whose voltage output stays constant. The Sensorex S8000 Series pH electrode is a combination electrode which contains both the pH electrode and the reference electrode in the same body housing, or cartridge.
A pH electrode is constructed from a doped glass membrane, labeled in Figure 2, that is sensitive to hydrogen ions. Filling solution on the other side of the glass membrane responds to the presence of the hydrogen ions and a voltage is generated in the silver wire contained in the electrode. The silver wire passes the voltage signal from the solution being measured to the electrode's cable or connector, where it is differentially compared to the voltage generated by the reference electrodes silver wire. At 25 degrees C, voltage differential output due to pH is 59.16 millivolts/pH unit. The electrode will give zero output at a pH of 7.0 at all temperatures. For more information on this process, refer to Related Links section below.
Figure 2: Diagram of pH electrode cross section
The Sensorex S8000 Series pH platform is a modular system that consists of a group of components, configurable by the user at time of purchase or at a later time, should the requirements change. The system will typically include a pH electrode cartridge, the mounting interface, electronic signal conditioning if required, and a Cap-cable connector. The flat sensing surface used on the Sensorex S8000 is rugged, abrasion resistant, and self-cleaning. Turbulent flow over the electrode's flat measuring surface scrubs off particles for a self-cleaning effect.
The bare Sensorex pH electrode provides a very high output impedance millivolt signal that can be difficult to acquire depending upon your measurement device. It is recommended that you use the Sensorex EM800 or EM801 battery powered unity gain amplifier to ensure quality signal transmission and acquisition. The amplifier will also allow you to transmit the signal over long cable distances. Alternatively, you can use a Sensorex EM802/pH 4-20mA transmitter for use with DAQ products that support 4-20mA inputs.
Additional important considerations when using pH sensors include the possible requirement for temperature compensation, and proper calibration of the pH probes using buffer solutions.