Archived: Using the Vaisala HMP 50 Humidity Probe with NI Wireless Sensor Networks (WSN)

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This document describes the use of the Vaisala HMP 50 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 HMP 50 Humidity Sensor

The HMP 50 is part of Vaisala’s humidity transmitter line for volume applications or integration into other manufacturers’ equipment, utilizing Vaisala’s INTERCAP® humidity sensor technology.


Figure 1: Vaisala HMP 50

As the HMP 50 is a humidity transmitter, it relies on an internal sensor to take the measurements.  The HMP 50 utilizes the Vaisala INTERCAP®, a capacitive thin-film polymer 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 voltage output which indicates the humidity.

A number of variations exist for the HMP 50 model.  These variations consist of different output voltages and measurement types.  With regard to output options, it is possible to choose a model with an analog output signal of 0-1 V, 0-2.5 V, or 0-5 V.  Additionally, the HMP 50 can be configured with a temperature sensor to output both humidity and temperature readings.  These operating configurations and specifications are set during order time and cannot be changed by the user.

As is, the HMP 50 can be installed to work with the NI WSN indoors.  It features an IP65 classification housing.  Optionally available within the HMP 50 is a current output converter kit for those applications requiring an analog current output signal.

Wireless Humidity Measurements

By combining the Vaisala HMP 50 with the NI WSN system, complete remote humidity monitoring is capable.  The NI WSN voltage node can source the power required by the HMP 50 to operate, eliminating the need for an external power supply.  The data collected remotely using the HMP 50 can be transmitted wirelessly back through the NI Wireless Sensor Network for observation and analysis.

Expanding the number HMP 50s 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 HMP 50 to the NI WSN-3202 Node

Physically connecting the HMP 50 to the NI WSN-3202 Node is extremely straightforward.  The sensor power output from the NI WSN-3202 should be connected to the positive DC Voltage input on the HMP 50 (brown wire).  The DC Voltage input requires 7-28 VDC, with a current draw of 2 mA.  The sensor power output of the NI WSN-3202 can therefore power one HMP 50.  Furthermore, the voltage output channel of the HMP 50 (white wire) should be connected to an analog input channel of the NI WSN-3202.  The black wire is reserved for the temperature output signal, should the HMP 50 be configured for temperature measurements.  All HMP 50 signals should be referenced to one of the analog input ground channels on the NI WSN-3202.

The 0-1 V voltage output of the HMP 50 is routed directly to the analog input of the WSN-3202.  Additionally, the WSN-3202 is capable of powering the HMP 50.

Figure 2.  Connecting HMP 50 to WSN-3202

Programming NI WSN for use with the HMP 50

Using LV on a host PC with the NI WSN-3202 and the HMP 50

Facilitating communication between the NI WSN and HMP 50 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 HMP 50 returns an analog signal from 0-1 V.  Furthermore, under the node tab, set the sensor power attribute value to 25 ms Before Sampling.  This will ensure that once the HMP 50 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 HMP 50.  A typical sample rate might be one sample every second.

Taking readings from the HMP 50 in LabVIEW is accomplished by reading the shared variables available from the NI WSN-3202.  Simply read the analog input channel that the HMP 50 is physically wired to, and then scale the acquired analog signals.  Since the 0-1 V sensor output maps linearly 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.

HMP 50 Block Diagram.jpg

Figure 3: Reading HMP 50 Data in LabVIEW

Using LabVIEW WSN Embedded Programs on the NI WSN-3202 with the HMP 50

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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