Using LV on a host PC with the NI WSN-3202 with the SDX
The LabVIEW Project Explorer Window can be used on a host computer to configure the WSN-3202 node to measure the output of the SDX transmitter. Right click on the 3202 node in the Project Explorer to configure the node’s properties. Under the channels tab, configure all channels for a range appropriate to the shunt resistor you selected. For example, if you have a 249 Ω resistor, across which you are shunting 4-20 mA, you can expect to see 0.996 – 4.98 V, and would therefore want to configure all your AI channels for Range: -5 to 5 Volts.
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 SDX. A typical sample rate for measurement of pond water level might be one sample every hour, or 3600 seconds.
You can read the SDX’s data into LabVIEW by reading the current value of the shared variables available from the NI WSN-3202. Simply read the analog input channels that the voltage drop across the resistor is physically wired to, AI 0 in Figure 4, and then scale the acquired analog signals.
The user will need to be aware of the depth that their SDX sensor was rated for at the time of purchase, Dmax in the equation below. If R is the resistance value in Ohms of the shunt resistor, you now know two points (Depth d, Measured Voltage V):
( 0 , 0.004 * R) and ( Dmax , 0.020 * R) where 0 is atmospheric pressure and Dmax corresponds to the maximum rated depth.
y = m*x + b
Voltage = m * Depth + b
Voltage = (0.016 *R) / Dmax * Depth + 0.004 * R
Depth = (Dmax * Voltage) / (0.016 * R) – Dmax / 4
Implementation of this measurement and scaling can be seen in the block diagram below. You can also right click on the AI0 shared variable in the Project Explorer and enable Timestamps. This will let you right click on the variable itself on the block diagram, and select Show Timestamp. Now, you can perform a comparison
Figure 5. LabVIEW Diagram to Acquire, Scale, and Display Water Level Reading
Using LabVIEW WSN Embedded Programs on the NI WSN-3202 with the SDX
With LabVIEW WSN, you can download and run LabVIEW VIs on the WSN node for local data processing and control. For example, you could perform the data scaling to engineering units, add custom filtering, or perform alarming locally on the node itself. Another possible use is to utilize a digital output of the WSN-3202 to connect and disconnect the external power supply to the SDX transmitter, under LabVIEW WSN control, in order to minimize power draw by the SDX transmitter.