Archived: Using the Vaisala HMT 100 Humidity and Temperature Monitor with NI WSN

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This document describes the use of the Vaisala HMT 100 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 HMT 100 Humidity and Temperature Sensor

The HMT 100 is part of Vaisala’s humidity transmitter line for fixed installations, utilizing Vaisala’s HUMICAP® humidity sensor technology.


Figure 1: Vaisala HMT 100

As the HMT 100 is a humidity and temperature transmitter, it relies on internal sensors to take the measurements.    For humidity, the HMT 100 utilizes the Vaisala HUMICAP® 180, 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 current or voltage output which indicates the humidity.

The HMT 100 utilizes a Platinum Resistance Temperature Detector (RTD) for temperature measurements.  The resistance of an RTD increases as the temperature of the surrounding environment increases.   The electronics of the instrument measure the resistance of the sensor, converting it into a second current or voltage output proportional to the temperature.      

A number of variations exist for the HMT 100 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 choose a model powered with 10-35 VDC or 24 VAC input voltage and an analog output signal of either 4-20 mA, 0-1 V, 0-5 V, or 0-10 V.  Additionally, the HMT 100 can be configured to output either one or two quantities, in the following combinations: relative humidity, dewpoint temperature, relative humidity and temperature, dewpoint temperature and temperature.  These operating configurations and specifications are set during order time and cannot be changed by the user.

As is, the HMT 100 can be installed to work with the NI WSN indoors or outdoors.  It features an IP65 classification housing.  Optionally available within the HMT 100 are an aluminum wall installation plate, a duct installation kit, a weather shield for the transmitter, and a rain / solar radiation kit for the probe. 

Wireless Humidity and Temperature Measurements

By combining the Vaisala HMT 100 with the NI WSN, complete remote humidity and temperature monitoring is capable.  The NI WSN voltage node can source the power required by the HMT 100 to operate, eliminating the need for an external power supply.  The data collected remotely using the HMT 100 can be transmitted wirelessly back through the NI Wireless Sensor Network for observation and analysis.

Expanding the number HMT 100s 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 HMT 100 to the NI WSN-3202 Node

Physically connecting the HMT 100 to the NI WSN-3202 Node is extremely straightforward.  The sensor power output from the NI WSN-3202 should be connected to the power input on the HMT 100.  The power input requires 10-35 VDC, and as such, the NI WSN-3202 can power one HMT 100.  Furthermore, the voltage output channels of the HMT 100 should be connected to the analog input channels of the NI WSN-3202.  All HMT 100 signals should be referenced to one of the analog input ground channels on the NI WSN-3202.

Figure 2.  Connecting HMT 100 to WSN-3202

Programming NI WSN for use with the HMT 100

Using LV on a host PC with the NI WSN-3202 and the HMT 100

Facilitating communication between the NI WSN and HMT 100 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 -10 to 10 Volts, as both Ch1 and Ch2 from the HMT 100 return analog signals from 0-5 V.  Furthermore, set the sensor excitation channel attribute value to 25 ms before sampling.  This will ensure that once the HMT 100 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 HMT 100.  A typical sample rate might be one sample every second.

Taking readings from the HMT 100 in LabVIEW is accomplished by reading the shared variables available from the NI WSN-3202.  Simply read the analog input channels that the Ch1 and Ch2 of the HMT 100 are physically wired to, and then scale the acquired analog signals.  Since the 0-5 V sensor output on Ch1 maps linearly to 0-100% relative humidity, multiply the voltage acquired by 20 to achieve proper scaling.  Furthermore, since 0-5 V on Ch2 maps linearly to 32-122 °F, multiply the voltage acquired by 18 and add 32 °F to achieve proper scaling.  Then route those scaled values to a chart to view the values as they are acquired.

Figure 3: Reading HMT 100 Data in LabVIEW

Using LabVIEW WSN Embedded Programs on the NI WSN-3202 with the HMT100

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 and temperature directly.