Aurora Waterlevel is a capacitive level sensing. The sensor contains an array of sensor elements, able to detect the capacitive parameters of adjacent mediums. This allows you to programmatically detect and distinguish between different types of fluids and mediums, for example under the manholes. This information can be transferred wireless, for real-time monitoring.
Four noncontact capacitive sensors can be found in the device;
- The first sensor is 53 cm long. It starts 1 cm from the top of the 60 cm enclosure tube. (Hereby: CHARGE_LEVEL)
- The second sensor, 1 cm long, is at the bottom of the first sensor. (Hereby: CHARGE_BOTTOM)
- The third sensor, 1 cm long, is in the middle of the first sensor. (Hereby: CHARGE_MIDDLE)
- And the fourth sensor, 1 cm long, is at the top of the first sensor. (Hereby: CHARGE_TOP)
These sensors measuring changes in an electrical property called capacitance. Capacitance describes how two conductive objects with a space between them respond to a voltage difference applied to them. Aurora Waterlevel uses two conductive electrodes to form a capacitor in the sensors. When sensing, an electric field is stored between the electrodes, the ability for the capacitor to store energy increases when placing non-conductive material (dielectric material) between electrodes. When dielectric material such as water rises or falls between electrodes and capacitor, the capacitance changes progressively and the water level can be determined.
The change in capacitance will not tell us how many inches or centimeters the water has risen or dropped, because the capacitance varies from medium to medium, such as salt water and fresh water, they have different dielectric constants. You can make a reference measurement for "empty" and "full", then you can calculate how many liters or centimeters the water has raised or dropped.
Measuring Different Mediums
Aurora Waterlevel can be used in water, sand, corn etc. The sensor measuring changes in the capacitive level of the medium you are measuring. Use all four sensors to detect changes in the mediums. Let's say you have your sensor in a manhole, as the water rises, CHARGE_LEVEL rises as well. The changes in CHARGE_BOTTOM, CHARGE_MIDDLE and CHARGE_TOP are also important, it can help you determine that the water has reached the top, middle or still at lower levels.
When sand, gravel or sludge is formed at the bottom, the CHARGE BOTTOM provide different reading than if it's only surrounded by water. Note that sand gives a different charge value than sludge. Overtime, you can programmatically detect different types of medium around your sensor.
To maximizing effectiveness and minimizing error when capacitive sensors are used, it requires that the measurements be made under the same conditions in which the sensor was calibrated. Whether it’s a sensor calibrated at the factory, or one that is calibrated during use, repeatable results come from repeatable conditions, this mean during calibration process all variables must be constant.
To calibrate Aurora Waterlevel is simple, all you need to do is to send a calibrating downlink message to the sensor. If you want all your sensors to have the same reading, make sure you have a repeatable calibrating environment so that all sensors you calibrate is under the same condition. Such calibration assuming all your installation sites are identical.
However, we recommend you to calibrate the sensors at installation sites. For example: If you calibrate the sensor in an ideal environment without any medium or objects near the sensor and then out at the installation site the sensor is mounted next to a wall. The wall will give different sensor reading than the one you tested in your lab.
Calibrating Downlink Message
To calibrate the sensor, send following downlink message:
FA 01 00 00 02 91 00 00 09 02 95 00 00 00 00 CE
For LoraWAN, the payload needs to be sent with FPort 2.
It is always good to calibrate the sensor because it will reset the zero point.
Example: you put the sensor 20 cm down into a water pool and you want that 20 cm water level to be zero point. On site, sending a calibration message down to the sensor will reset the zero point to the level the sensor is in. In practice, when the water is lower than 20 cm, you will get negative charge value.
Protecting the sensor
Aurora Waterlevel sensors are protected with plastic tubing. If you install the sensors in harsh environments and need additional protection, you can use non-conductive materials such as plastic to further protect the sensors. The capacitance will change in relationship to the thickness or density of the material, and therefor the reading may be less sensitive.