Archived: Using the Sensorex DO6400 Series Dissolved Oxygen Sensor with NI WSN

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This document describes the use of the Sensorex DO6400 Series Dissolved Oxygen Sensor with the NI Wireless Sensor Networks (WSN) system for wireless dissolved oxygen 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 DO6400 Series Dissolved Oxygen Sensor

Sensorex manufactures a wide variety of sensors including Dissolved Oxygen sensors, which can be used in water, chemical processing, medical, laboratory, and environmental applications. The Sensorex DO6400 Series Dissolved Oxygen Sensor can be seen in Figure 1.

Figure 1: Sensorex DO6400 Series Dissolved Oxygen Sensor

Dissolved oxygen (DO) is the term used for the measurement of the amount of oxygen dissolved in a unit volume of water, usually presented in units of mg/L or ppm.  The DO6400 sensor consists of two electrodes, an anode and cathode, immersed in electrolyte and separated from the water of interest by an oxygen permeable membrane, as seen in Figure 2.  Oxygen diffuses across the membrane and reacts with the cathode to produce a current proportional to the amount of oxygen that has diffused into the sensor. DO sensors therefore actually measure the partial pressure of the oxygen in water; higher pressure allows more oxygen to diffuse across the membrane and more current to be produced.  The current is converted to a millivolt output, which can be directly measured with a WSN wireless node.

Figure 2: Diagram of DO Sensor Electrodes

Wireless Dissolved Oxygen Measurement

In water quality applications, such as fish farming and waste water treatment, the level of DO must be kept high in order to ensure the survival of the fish or the waste-decomposing bacteria. Another application for monitoring dissolved oxygen is industrial applications including boilers. DO levels must be kept low to prevent corrosion and boiler scale build-up which would inhibit heat transfer.

By combining the Sensorex DO6400 Series Dissolved Oxygen Sensor with the NI Wireless Sensor Network (WSN), you can easily develop and deploy wireless DO measurements. The NI WSN-3212 thermocouple node can remotely collect DO data and transmit it wirelessly back through the NI Wireless Sensor Network for observation and analysis.  

The addition of multiple WSN-3212 nodes on a system allows a user to monitor DO and other water quality parameters at multiple 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-3212 Node

The full scale output of the DO6400 sensor will be around 50 mV (in air saturation).  Therefore, we can use the WSN-3212 thermocouple input node, which features a high-resolution 24-bit ADC and can be configured for a mV input signal.   The cable leading from the Sensorex DO6400 Series Dissolved Oxygen Sensor contains a red wire, which is the positive lead, and a black wire, the negative lead. These can be connected directly to the WSN-3212, along with the drain wire, as shown in Figure 3.

Figure 3.  Connecting the Sensorex DO6400 Series Dissolved Oxygen Sensor to the NI WSN-3212

Programming NI WSN for use with the Sensorex DO6400 Dissolved Oxygen Sensor

Using LV on a host PC with the NI WSN-3212 with the Sensorex DO6400 Dissolved Oxygen Sensor

The LabVIEW Project Explorer Window can be used on a host computer to read data from the WSN node.  Right click on the NI WSN-3212 node in the Project Explorer to configure its properties. Under the channels tab, in the range drop down menu, configure thermocouple (TC) channel for voltage input by selecting  -0.73 to 0.73 Volts.  Under the node tab, set the node sample interval (seconds) hardware configuration setting to an interval appropriate for your application.  A typical sample rate for DO applications might be one sample every hour, or 3600 seconds.

You can read the Sensorex DO6400’s data into LabVIEW by reading the current value of the shared variables available from the NI WSN-3212.  Simply read the analog input channels that the Sensorex DO6400 is wired to, and then scale the acquired analog signal to the appropriate dissolved oxygen level.  You will also need to calibrate the sensor as described in the sensor’s User Manual. This involves letting the sensor sit in open air to determine the voltage returned for 100% saturation. Using this reading and the fact that 0 volts will be returned for 0% dissolved oxygen, you can create a scaling factor that can be used to convert from voltage readings to % saturation of dissolved oxygen.

In the block diagram pictured below, calibration of a test DO sensor resulted in a scaling coefficient of 2790.  Therefore, the % dissolved oxygen will equal the measured voltage multiplied by 2790. 

Figure 4.  LabVIEW block diagram to acquire, scale, and display wireless data from DO64000

Using the LabVIEW WSN Module on the NI WSN-3212 with the Sensorex DO6400 Dissolved Oxygen Sensor

With LabVIEW WSN, you can download and run LabVIEW VIs on the programmable version of the WSN node for local data processing and control.  For example, you could perform the data scaling to engineering units locally on the node itself, returning Dissolved Oxygen concentration directly, instead of volts.