Archived: Using the Sensorex S8000 pH Electrode with NI Wireless Sensor Networks (WSN)

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This document describes the use of the Sensorex S8000 pH Electrode Platform with the NI Wireless Sensor Networks (WSN) system for wireless pH measurements. This document is one in a series of documents describing how to use specific sensor products with the NI WSN system to wirelessly enable a variety of applications, such as environmental monitoring, climate studies, resource monitoring, etc. For more information on using other sensors with the NI WSN system, please refer to the WSN Sensor Solutions document.


Sensorex S8000 Series Flat Surface Self-Cleaning pH Electrode

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.

 Wireless pH Measurement

By combining the Sensorex S8000 pH Electrode with the NI Wireless Sensor Network (WSN) platform, you can easily develop a wireless pH measurement system. The NI WSN-3202 voltage input node can remotely collect pH data and transmit it wirelessly back through the NI Wireless Sensor Network for observation and analysis.

The addition of multiple WSN-3202 voltage nodes on a system allows a user to monitor pH and other water quality parameters in any number of locations.  This benefits applications where it is necessary to monitor large bodies of water, or multiple tanks, reservoirs, etc.  Adding WSN nodes to the system allows a user to incorporate a number of other environmental sensors for complete weather or environmental applications.

Connecting the S8000 to the NI WSN-3202 Node

In order to interface the S8000 pH electrode to the WSN-3202 voltage input node, you should use the Sensorex EM-800 battery powered unity gain amplifier.  This amplifier reduces the output impedance of the electrode, and outputs a signal that varies linearly about 59 mV per pH unit.  The cable leading from the Sensorex S8000 pH Electrode Measurement system has BNC cable connectivity. You can use a BNC to bare wire converter to connect your sensor to an analog input channel of the WSN-3202, as seen in Figure 3.

Figure 3.  Connecting the Sensorex S8000 pH Electrode to the NI WSN-3202

Programming NI WSN for use with the Sensorex S8000 pH Electrode

Using LV on a host PC with the NI WSN-3202 with the Sensorex S8000 pH Electrode

The LabVIEW Project Explorer Window can be used on a host computer to read data from the WSN node as it measures the voltage differential from the Sensorex S8000 pH Electrode.  Right click to configure the Properties of the node. Under the Channels tab, configure all channels for Type 0: Analog Input and an input range of  -0.5 to 0.5  Volts.  Under the Node tab, set the node sample interval (seconds) hardware configuration setting to an interval appropriate for your application.  The higher the sample interval value, the less often the WSN-3202 will take a reading from the Sensorex S8000 pH Electrode.  A typical sample rate for pH applications might be one sample every hour, or 3600 seconds.

You can read the Sensorex S8000 pH Electrode’s data into LabVIEW by reading the current value of the shared variables available from the NI WSN-3202.  Simply read the analog input channels that the Sensorex S8000 pH Electrode is wired to, and then scale the acquired analog signals to the appropriate pH. You will need to calibrate the sensor measurements as described in the sensor's User Manual.  The output of the pH sensor will be 0 V at pH 7, and varies linearly with pH by 59.2 mV per pH unit.  The output voltage will increase positively for pH less than 7, and decrease for pH greater than 7.  An example LabVIEW block diagram is shown in Figure 4.

 Figure 4.  LabVIEW block diagram to acquire, scale, and display wireless data from S8000

Using LV on a host PC with the NI WSN-3202 with the Sensorex S8000 pH Electrode

With LabVIEW WSN, you can download and run LabVIEW VIs on a programmable version of the WSN-3202  node for local data processing and control.  For example, you could perform the data scaling to engineering units locally on the node itself, returning pH directly, as well as other application specific functions.