Archived: Using the Vaisala GMM 221 Carbon Dioxide Module with NI Wireless Sensor Networks (WSN)

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This document describes the use of the Vaisala GMM 221 sensor with the NI Wireless Sensor Networks (WSN) system for wireless humidity and temperature monitoring. This document is one in a series of documents describing how to use specific sensor products with the NI WSN system to enable a variety of applications, such as environmental monitoring, climate studies, and resource monitoring. For more information on using other sensors with the NI WSN system, please refer to the WSN Sensor Solutions document.


Vaisala GMM 221 CO2 Module

The GMM 221 is part of Vaisala’s carbon dioxide line for Original Equipment Manufacturers (OEMs), utilizing Vaisala’s CARBOCAP® carbon dioxide sensor technology.


Figure 1: Vaisala GMM 221

As the GMM 221 is a carbon dioxide module, it relies on an internal sensor to take the measurements.    The GMM 221 utilizes the Vaisala CARBOCAP® sensor, a silicon based non-dispersive infrared (NDIR) sensor.  An infrared source at the end of the measurement chamber emits light into the gas chamber, where any CO2 present absorbs a part of the light at its characteristic wavelength.  A measurement is also taken at a wavelength where no absorption occurs.  The electronics of the instrument convert the ratio of the absorption measurements into a voltage output which indicates the CO2 reading.

The GMM 221 is a member of the GMM 220 Series of modules.  The variations within the series consist of different measurement ranges for CO2 concentrations.  The GMM 221 is designed for high CO2 concentration measurements, and can be calibrated to operate within one of five concentration ranges: 0-2%, 0-3%, 0-5%, 0-10%, and 0-20%.  The GMM 222 is designed for low CO2 concentration measurements, and can be calibrated to operate within one of five concentration ranges: 0-2000 ppm, 0-3000 ppm, 0-5000 ppm, 0-7000 ppm, and 0-10000 ppm.

The GMM 221 can be integrated into equipment to work with the NI WSN in incubator applications.  The probe features an IP65 classification housing.


Wireless CO2 Measurement

By combining the Vaisala GMM 221 with the NI WSN, complete remote CO2 monitoring is capable.  The NI WSN voltage node can source the power required by the GMM 221 to operate, eliminating the need for an external power supply.  The data collected remotely using the GMM 221 can be transmitted wirelessly back through the NI Wireless Sensor Network for observation and analysis.

Expanding the number GMM 221s used with WSN-3202s enables expansion of the remote CO2 monitoring system.  Now an entire network of sensors and nodes can communicate between multiple locations, over larger areas if needed, acquiring more data for concurrent analysis.


Connecting the GMM 221 to the NI WSN-3202 Node

Physically connecting the GMM 221 to the NI WSN-3202 Node is extremely straightforward.  The positive reference terminal of an external power supply should be connected to the low power input on the GMM 221.  The low power input requires 11-20 VDC with an average current draw of 100 mA.  Furthermore, the voltage output channel of the GMM 221 should be connected to the analog input channels of the NI WSN-3202, referenced to one of the analog input ground channels on the NI WSN-3202.  If the current output channel of the GMM 221 is preferred, it should be connected over a precision resistor to the analog input channels of the NI WSN-3202, also referenced to one of the analog input ground channels.


Figure 2.  Connecting GMM 221 to WSN-3202


Programming NI WSN for use with the GMM 221

Using LV on a host PC with the NI WSN-3202 and the GMM 221

Facilitating communication between the NI WSN and GMM 221 can be accomplished through the node properties, accessible from the LabVIEW Project Explorer Window.  Under the channels tab, configure all channels for Type 0: Analog Input and the range for -2 to 2 Volts, as the GMM 221 returns an analog signal from 0-1 V.  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 GMM 221.  A typical sample rate might be one sample every second.

Taking readings from the GMM 221 in LabVIEW is accomplished by reading the shared variables available from the NI WSN-3202.  Simply read the analog input channels that the voltage output of the GMM 221 is physically wired to, and then scale the acquired analog signal.  Since 0-1 V sensor output maps linearly to 0-20% CO2 concentration, multiply the voltage acquired by 20 to achieve proper scaling.  Then route those scaled values to a chart to view the values as they are acquired.  In this particular example, the chart is updated only when a difference in the timestamps of the sampled data on the analog input is seen.


Block Diagram.jpg 

Figure 3: Reading GMM 221 Data in LabVIEW


Using LabVIEW WSN Embedded Programs on the NI WSN-3202 with the GMM 221

With LabVIEW WSN, you can download and run LabVIEW VIs on a programmable 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 CO2 concentration directly.


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