Archived: Using the Vaisala HMM 30D Humidity Sensor with NI Wireless Sensor Networks (WSN)

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This document describes the use of the Vaisala HMM 30D sensor with the NI Wireless Sensor Networks (WSN) system for wireless humidity 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 HMM 30D Humidity Sensor

The HMM 30D is part of Vaisala’s humidity transmitter line for environmental chamber applications, utilizing Vaisala’s HUMICAP® humidity sensor technology.    

Figure 1: Vaisala HMM 30D

As the HMM 30D is a humidity transmitter, it relies on an internal sensor to take the measurements.  The HMM 30D utilizes the Vaisala HUMICAP®K sensor, a capacitive thin-film capacitive humidity sensor.  The thin-film polymer either absorbs or releases water vapor as the relative humidity of the ambient air rises or drops. The dielectric properties of the polymer film depend on the amount of water contained in it: as the relative humidity changes, the dielectric properties of the film change, and so the capacitance of the sensor changes. The electronics of the instrument convert the capacitance into a current or voltage output which indicates the humidity.    

A number of variations exist for the HMM 30D model.  These variations consist of different input and output power options, as well as output quantities.  With regard to input and output power options, it is possible to configure the model to accept a power input of 14-35 VDC, 19-35 VDC, 20-35 VDC, or 12-24, 16-24, 9-24, 17-24 VAC, and an analog output signal of either either 0-5 VDC, 0-10 VDC, or 0-20 mA.  These operating configurations and specifications can be changed by the user by augmenting the jumper configuration.

As is, the HMM 30D can be integrated into equipment to work with the NI WSN in environmental chamber applications.

Wireless Humidity Measurements

By combining the Vaisala HMM 30D with the NI WSN, complete remote humidity monitoring is capable.  The NI WSN voltage node can source the power required by the HMM 30D to operate, eliminating the need for an external power supply.  The data collected remotely using the HMM 30D can be transmitted wirelessly back through the NI Wireless Sensor Network for observation and analysis.

Expanding the number HMM 30Ds used with WSN-3202s enables expansion of the remote humidity and temperature 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 HMM 30D to the NI WSN-3202 Node

Physically connecting the HMM 30D to the NI WSN-3202 Node is extremely straightforward.  The sensor power output from the NI WSN-3202 should be connected to the positive power input on the HMM 30D.  The power input requires 10-35 VDC with a current draw of 15 mA.  The sensor power output of the NI WSN-3202 can therefore power one HMM 30D sensor.  The negative power input on the HMM 30D should be connected to the analog input ground terminal of the NI WSN-3202.  Finally, the 0-1 V signal output terminal should be connected to an analog input terminal of the NI WSN-3202.

Figure 2.  Connecting HMM 30D to WSN-3202

Programming NI WSN for use with the HMM 30D

Using LV on a host PC with the NI WSN-3202 and the HMM 30D

Facilitating communication between the NI WSN and HMM 30D 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 2Volts, as the output channel from the HMM 30D returns an analog signal from 0-1 V.  Furthermore, set the sensor excitation channel attribute value to 25 ms Before Sampling.   This will ensure that once the HMM 30D receives power from the WSN-3202, it has sufficient time to start up and output accurate data before it is sampled by the WSN.  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 HMM 30D.  A typical sample rate might be one sample every second.

Taking readings from the HMM 30D in LabVIEW is accomplished by reading the shared variables available from the NI WSN-3202.  Simply read the analog input channel that the output signal of the HMM 30D is physically wired to, and then scale the acquired analog signal.  Since the 0-1 V sensor output on Ch1 maps to 0-100% relative humidity, multiply the voltage acquired by 100 to achieve proper scaling.  Then route this scaled value to a chart to view the values as they are acquired.  In this particular example, the charts are updated only when a difference in the timestamps of the sampled date on the analog input is seen.

hmm 30d.jpg

Figure 3: Reading HMM 30D Data in LabVIEW

Using LabVIEW WSN Embedded Programs on the NI WSN-3202 with the HMM 30D

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 humidity directly.

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