Using a CWS655, CWS655A, or CWS655E, why would the volumetric water content value be NAN?
There are three reasons that NAN values are reported:
- A communication issue exists between the CWS655x sensor and the CWB100x base station.
- Soil conditions are in an out-of-bounds range beyond the sensor’s calibration.
- A hardware issue exists within the sensor electronics.
The CWS655-series sensors have several logical tests built into their firmware to ensure that the sensors do not report a number that is known to be erroneous. Erroneous readings are either outside the sensor’s operational limits or outside of published accuracy specifications.
A reported value of NAN does not necessarily mean that there is a problem with the sensor hardware. The conditions outlined below can lead to a value of NAN for volumetric water content.
Radio communication issue
When the CWB100 base station is about to poll a wireless sensor, it first populates the variable array specified in the data logger program with a NAN value for each field. Then, when a successful transmission of sensor data is received, those NAN values are overwritten with valid data. If the transmission is unsuccessful, all of the values for that sensor remain as NAN. This makes it easier to tell when there is an issue. Possible causes of a radio communication issue include:
- The sensor is not powered. Press the Setup button on the back of the sensor, and count the number of blue LED flashes on the front of the sensor. Four flashes indicate a fully charged battery, whereas one flash indicates a low battery that needs replacement or recharging. If there are no blue flashes, this indicates that the battery pack is not working at all and likely needs to be checked for damage.
- The wireless sensor has not been detected by the base station. Press the Setup button on the back of the sensor, and count the number of red LED flashes on the front of the sensor. The number of red flashes indicates how many hops the sensor must take to reach the CWB100 base station. One to four red flashes indicate that the sensor has been discovered by the base station. If there are no red flashes, this indicates that the wireless sensor has not been discovered by the base station. Hold the Setup button for eight seconds to force a search for the base station. After eight seconds, the blue LED flashes once per second until the sensor and base station have synchronized or until an unsuccessful search has timed out (approximately 8 minutes).
- If the CWB100 base station cannot discover the CWS655, use DevConfig with the A205 interface to connect to the CWS655 and check its settings as described in the Wireless Sensor Network Instruction Manual. Select Connect in DevConfig, and press the Setup button on the back of the sensor.
If the DevConfig settings are all correct but the CWB100 base station does not discover the CWS655 sensor, then the radio signal strength (RSSI) should be checked. Common causes of failed transmission between the CWS655 and the CWB100 base station include the following: poor line of sight between the sensor and base station, too much distance, radio interference from other sources, and the absorption of RF energy by water or leaves. Sometimes these issues may be resolved by adding one or more wireless sensors to the system to serve as repeaters.
- Check that the radio address of the CWS655 is present in the Radio Address field. If that field is blank, the sensor has a damaged radio module and is unable to detect its own radio. The sensor will need to be replaced.
- Check that the radio address for the CWB100 base station is correctly entered in the Base Station Address field. The base station address is printed on a sticker on the CWB100. The CWS655 needs to have this address entered so that it will respond when the base station tries to discover it.
- Select the Settings Editor tab and scroll down to Measurements. This will query the sensor every 5 seconds and display readings for volumetric water content (VWC), bulk electrical conductivity (EC), temperature (Ts), apparent bulk permittivity (Pe), voltage ratio (AR), period, internal sensor temperature (Ti), and battery voltage (BV). Certain soil conditions may cause NAN values in the VWC, EC, and/or Pe fields, but a working sensor always reports Ts, AR, period, Ti, and BV. If any of those fields report NAN, the sensor is damaged.
Permittivity is greater than 42
The CWS655 calculates real apparent bulk permittivity of soil and then uses the Topp et al. (1980) equation to convert permittivity to volumetric water content. A permittivity value of 42 is the upper limit for making that conversion. This is equivalent to a volumetric water content of 0.52.
If the reported value for VWC is NAN but there is a numerical value for Pe, then volumetric water content numbers higher than 0.52 may be calculated by applying the Topp et al. (1980) equation or another chosen equation to the permittivity reading. This may be done in the data logger program or in post-processing software, such as a spreadsheet. Note that the Topp et al. (1980) equation is typically held to be valid for water content values between 0 and 0.55 m3 m-3.
Permittivity value is NAN
Because volumetric water content is calculated from the permittivity reading, conditions that cause the sensor to report NAN for permittivity will give the same value for volumetric water content.