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WSN1120L Range Test

We did a quick range test of our new CC1120 radio based sensor nodes. We did this within Pruksa (community in east Bangalore).  This  was a non line of sight test in which one WSN1120L node was  configured to send packets to a  coordinator node (a WSN1120CL) once every second. The nodes  were around 526 meters apart with lots of houses in  between.  The nodes were at a height of  around 5  feet above ground. The walls in these houses are fully concrete  (no bricks). Even at this distance the signal strength was pretty good (around  -75  dBm). The nodes were  configured  to  transmit at +14 dBm at a data rate of 1.2  kbps (GFSK modulation) with channel bandwidth configured to 25  kHz.


Map showing location of non-LOS range test

We tried to do a line  of sight test along a nearby highway but we could not find a  flat  stretch longer than 1.1 kilometers close by. The two nodes  were able to communicate up to a distance of 1.1 kilometers. The signal strength  was  again very good (around -80 dBm). We will be doing further  tests to find out the max range  under line of  sight and non line  of  sight conditions.

Here  is a  pic of one of the nodes used in the range test. The node was powered by two AA batteries.


WSN1120L Sensor Node 


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Measuring Relative Humidity

Ambient humidity is an important atmospheric parameter. Humidity measurement and control is critical in wide ranging industrial and medical applications. Let’s begin with a review of some basic concepts to learn about humidity and how it changes.

Humidity refers to the amount of water vapor present in air. When water evaporates, it turns to an invisible gaseous state, called water vapor. Absolute humidity is measured in grams per cubic meter. Typically, the amount of water vapor present in air is dependent on the temperature of the air. Higher the temperature, more water can evaporate into it. The pressure exerted by the water vapor is called vapor pressure . When no more water can be evaporated into air, then the air is called saturated.

Relative Humidity (RH) is a more useful metric for describing the vapor content of air. It indicates how close to saturation the air is. RH is given as a percent. So, 100% means that the air is saturated and 0% means that the air is completely dry. When we think of controlling moisture or vapor, it is RH that we should be measuring and controlling. For instance, for building climate control, RH values of 50-60% are considered to be ideal for human comfort.

How is Relative Humidity Measured?

There are many ways to measure the RH of air. Two commonly used techniques in commercial RH sensors are:


1. Capacitive Sensor: This sensor is based on the principle of change in capacitance with change in relative humidity. In these sensors, the capacitance increases when RH increases.  Typically, the capacitance increases linearly with relative humidity, making it a useful principle to deduce the humidity. The change in capacitance is due to the change in the dielectric constant of the material used in the sensor. The main limitation of these type of sensors is that they lose accuracy at very low (less than 5%) or very high (greater than 90%) RH values. Also, the range of capacitance between 0 and 100% RH may be so high that circuit designs could be complicated.

2. Resistive Sensor: This sensor is based on the principle of change in impedance with change in relative humidity. The material used is hygroscopic (one that absorbs water) whose impedance varies with RH. Typically, the relationship is inverse. i.e., impedance drops as RH increases.

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