RainSavor
RainSavor | Updates
Project Leads: Cameron Clonch and Cameron Whitlow
Working with Dr. Catalina Segura and the OSU Department of Forestry, a rainwater collection system is being developed to test the electrical conductivity of rainwater during storms in Oregon.
Using a cup and siphon design, 5mL of rain water will be collected and then released every 1mm of rainfall (about once every 15 minutes), with electrical conductivity measurements taken of the rain prior to the water's release. The data will be used to evaluate how conductive storm water is, indicating a presence of various minerals from the ocean and/or other nearby relevant geological sites.
| Specification | Metric | Value | Met |
|---|---|---|---|
| Battery life | Months | 2 | Yes |
| Reads electrical conductivity of water | µs/cm | 0-100 | Yes |
| Records data at specified rainfall increments/intervals | mm | 0.1 - 1 | In Testing |
| Holds rainwater and then disposes | mL | 5 | In Testing |
| Logs Data to SD | time | date/hour | Yes |
| Serviceable for battery changing and clogging | Yes | ||
| SD and USB are easily accessible | Yes | ||
| Removable for winter time | Yes |
The system of the cup and siphon relies on gravity and potential energy to "push" the water out. Fluid behavior retains pressure throughout the entire volume. Once the maximum fluid height is reached, which is when the height of the fluid in the container and the maximum siphon height match, the fluid is able to flow downward, to a lower pressure (like water flowing down a mountain). The siphon now behaves like a vacuum since there is nothing to break the flow of the fluid until air is able to enter. However, air can't enter the siphon until all of the fluid has been removed and the base of the siphon is exposed.
The current design employs the Form 2 printer with SLA resin to create a see-through mechanism that enables the user to observe water passing through the system. The siphon drains at increments of approximately 5mL of water collection. Water is collected by way of funnel, then stored in a small reservoir, where the electrical conductivity is able to be measured; the water simultaneously fills a small siphon connected to the reservoir. The siphon also houses the interrupt probes, which trigger electrical conductivity readings just before a draining event occurs.
The funnel/reservoir/siphon trio will be contained in a frame similar to the Davis Instruments Rain Gauge and will include a mechanism for keeping debris out of the system. This aspect of the technology is currently being designed. All electronics will be kept inside a Pelican box, which is a weatherproof storage system that will protect everything from getting damaged. The mechanical and electrical design will be connected by way of CAT5 cables between the funnel setup (outside the Pelican) and electronics (inside the Pelican).
The RainSavor project consists of two main components – the mechanical and electrical pieces. The electrical system has a number of goals to meet including temperature readings, interrupt handling, EC sensor connections and signal processing, data sampling hardware, and a microcontroller. These last few items are handled with pre constructed solutions with a Feather M0 microcontroller and OPEnS Hypnos board which hosts the SD card for data logging and an RTC for time recording. The rest of the system will need to be designed purposefully to meet the rest of the system requirements, primarily around the EC measurements. Many projects here at the OPEnS lab have water EC measurement requirements and utilize the same board. This PCB was designed for specific use by the RainSavor and the Smart Rock teams, although it can easily be used by other teams who require these tools. Most of this system was built to kill two birds with one stone.
The design of the electrical system takes advantage of many tools that the OPEnS lab is familiar with a handful of additional components. Many of the features in the system are designed around this existing technology, such as the ADS1115 which is fully implemented in OPEnS Loom library allowing for very simple integration. The PCB hosts all of the components required with a handful of wires leading offboard via connectors. All of the connections traveling off of the PCB are 2mm pitch JST connections, varying between 2 and 4 pins. Only some of these connectors are utilized by the RainSavor system, the others are utilized by Smart Rock. All components are utilized by the RainSavor system except for the LEDs and the Reed Switch which are Smart Rock specific.
The PCB is small enough to be placed inside of a pelican box alongside a battery with enough room for wires to connect and travel outside of the box through a waterproof port. The size of the PCB was kept fairly small for these purposes and for Smart Rock size limitations. There are also mounting holes on the corners of the board. Many of the components are SMD, including all of the resistors, capacitors, and most of the chips. The components that require hand soldering is the 14 pin DIP op amp IC, the 5 diodes, various headers/connectors, and the switch. Using a stencil and oven to solder the SMD components is heavily recommended as the ICs are very difficult to solder correctly by hand. Overall the layout is built with as easy a flow as possible. All connectors to the left, microcontroller attached in the center/bottom, temperature connectors and indicators at the top, and all of the EC sensor circuitry in the center and right side. Supporting ICs, such as voltage regulators, the ADS, and a variable resistor, are towards the left side of the board to better optimize the routing of the I2C lines. This board communicates with all on board chips and off board sensors via I2C with no SPI support as all used components utilize I2C. There is an additional general I2C connector for any sensor.
There are a couple options for temperature sensing for a system like this and this can still be updated for future implementations. The main argument lies between using a thermistor or a thermocouple for temperature measurements. This system uses a thermocouple for a couple of reasons. The first is the dimensions of the sensor itself. The thermocouple has a small metal rod at the end of the wire with the tip sensing rather than the whole rod. In comparison the thermistor is a silicon rod that senses with the entire rod. Since the amount of fluid being probed is very small it wouldn’t be feasible to submerge the entire thermistor, the thermocouple makes the physical aspect of sensing much simpler. Each type also has different supporting circuitry. The thermocouple is more complicated with an entire breakout board being connected while the thermistor only requires a resistor in a voltage divider layout. The advantage on size and ease of implementation belongs to the thermistor for the minimal space. Unfortunately, there is an additional downside to the thermistor where the resistor changes value dependent on the temperature as well, which is difficult to fix in software. The thermocouple doesn’t have this challenge as the breakout board handles everything already and uses a cold solder as a temperature control point and varies much less than a thermistor. We decided it was worth the added complexity and space to have the thermocouple and greater temperature accuracy. Below are some temperature tests of the two methods that helped us come to this decision.
This graph shows how the thermistor and thermocouple react to extreme temperature changes and how noisy they can be. Note that the Y axis is different for the two sensors. The thermocouple measurements are in degrees Celsius while the thermistor is a value of voltage read in by the microcontroller ADC. Generally the thermocouple has less noise in the temperature signal and has a faster response to a drastic change in temperature.
To get more information out of the temperature graph comparisons the thermistor y axis was altered to match the scale of the thermocouple graph as a relationship to the input voltage. This allows for a more direct comparison that shows the limits of the thermistor in the upper temperatures and the greater variances across the graph. We aren't expecting rain to be the temperature of boiling water, so this drawback of a thermistor isn't important. Regardless these two graphs helped us to decide that a thermocouple is worth using. The effects of temperature on the resistor in the voltage divider network of the thermistor is slightly seen here, where the thermistor has a bigger or smaller error to the thermocouple at different temperatures.
The only reason temperature is needed is to give us more accurate descriptions of the water EC. The EC is fairly dependent on the temperature of the water and can have a large swing when the temperature is higher or lower than 25 degrees Celsius. Anyone using this system has the option to ignore the temperature sensing component but risks having data that isn’t as accurate.
This section will go more into how the electronics get data from the water to the board for processing. As explained above JST connectors lead all signals into the board. There are only two pairs of wires, four wires total, that connect to the funnel for the RainSavor system. One pair is for the EC sensor and the other pair is for an interrupt system. The interrupt system is straightforward. The first wire is a common ground and the other is attached to the interrupt pin with a large pullup resistor (large in this case is around 250k ohms). This large resistor is needed to pull the pin up and still allow it to fall when the large resistance of the water is introduced. When water in the siphon shorts the two wires the pullup is then shorted to ground, pulling down the interrupt pin and triggering the interrupt in the software. This allows the system to pull itself out of sleep mode when water is present in the system to take a measurement, saving battery and making sure the system gets a measurement every time there is enough water to measure. These probes are epoxied inside small holes in the siphon.
The main job of the electrical system is to measure the EC value of the water and log it on the SD card. The EC sensing circuitry takes up the most space of the board. The value measured by this circuitry is directly related to the conductivity of the water in the siphon. Some of the circuitry on the PCB is there to better tune the range and measurements of these readings and can be utilized on a project by project basis. The tuning resistor can be set as a ratio of 10k or 100k ohms depending on which circuit is being used. Calibration will be required for each set PCB as inherent small variations of component values may have an effect on the EC measurement. The calibration will be used to set this resistor value and find the correct equation for translating the results once the data is collected.
All of the tuning for the EC sensor is done using stock solutions. Each PCB will have slight variations but the same behavior should be observable. The first major test and comparison of the system is verification against the initial calibration data against these stock solutions. The next two graphs show a curve with generic probes and a curve with the siphon used for RainSavor. These curves are expected and similar data should be seen for this sensor.
Paying attention to the siphon measurements the resolution of the data is a poor fit for the system expectations. Our specifications detail that the range of measured EC is between 0 and 100 uS/cm whish has a very small range of readings from the initial measurements. The highest value shown is an EC value that is very outside the expected range. Better utilizing the tuning variable resistor will help to give a much larger resolution. There are two variable resistors that can be used between the 10k ohm and 100kohm base. The next graph will show the 10k ohm range on 100 uS solution, showing a linear relationship between the increased resistor tuning and the EC measurement.
This resistor sets the upper value of our circuit and since our high end if 100 uS it is useful to increase this value as high as possible. Since the maximum reading of the sensor is around 13000 there is still lots of room to use, allowing for good use of the larger tuning resistor value.
To better show the resolution changes when updating the resistor value, the next graph will display how the data reading is effected.
The full measurements of the EC reading as the EC sensor is swept is valuable information for determining what value of resistor is best for the application. The next graph shows some data that can be useful in this goal.
If an even greater resolution is required the larger 100k ohm version of the resistor can be used on the board instead. When running the same tests as above similar results are seen but with larger values.
All of this data is useful to understand how each individual system will need to be calibrated. Different combinations and utilizations of the above data can form an equation for calibration in any way the user sees fit. At the moment we are using a linear fit curve using the DI water measurements, a midpoint EC value measurement (such as 23 uS/cm), and the top end measurement (the 100uS/cm measurement), giving a 3 point curve to create a linear fit. Other methods of data estimation can be used, such as other fits, or various theories in information theory. It all comes down to how accurate the final data needs to be for the user. In our case we used this 3 point linear fit to attempt to measure gathered rain water electrical conductivity. In this data we used an equation derived from these tests and an equation derived from a calibration taken right before measurements on slightly different electrical systems. The error between these two is also shown.
This data shows, across three trials from the same source of rain water, that the error between a calibration from the data taken at system verification and the data taken from a new pre test calibration is within one point of EC value. This is promising as the method of calibration won't greatly change the results, although the variation between these errors is higher than we would like to see and we would like to make smaller. The good news that we pull from this data is the system does indeed react to rain water and can pick up EC values. Additionally the higher resolution was easily able to pick up the EC value when compared to DI water. If we had used the baseline resistance of the 10k variable resistor this would not be the case as shown in above graphs, the EC data would be much closer to DI water. The reported EC value of the rain water is correctly within the range of the specifications and expected range. The downsides include the variance in the data. If exact measurements are needed it will be difficult to determine that value with a small amount of measurements per siphon event. This may not be a notable downside in RainSavors application as the trend of the EC from the rain will be picked up in the data over multiple siphon events in a rainfall event.
Using the board defined for the RainSavor and Smart Rock projects data collected from trials, currently in lab and including field tests in the future, will be shown. There is currently an issue with the sensor hanging at the highest input after some impedance changes at the probe end. I am assuming this is an unforeseen impedance issue, possibly impedance matching problems in the pipeline, and am attempting some solutions. One quick fix for the meantime is to set the maximum reported voltages during these hang ups to a mode value, or a value that is close to an open probe measurement. A run time test was done to show this solution first with the raw reported values.
There are multiple plateaus of the same value that seem to cap the voltage. This cap is an interesting effect, a question for the physics going on behind the scenes is how is this voltage actually reacting? It may be the case that the voltage wants to drive much higher but is being physically capped by the reading circuitry, which could be a result of impedance mismatching causing many reflections of the signal driving the voltage to a much higher voltage than vcc. These events also occur around water events, where the probe is submerged, so it may be that the change of impedance at the probes is somehow causing this.
An impedance mismatch occurs when the impedance of the source, load, or transmission line aren't matching each others values properly. This mismatch, as shown in the figure, causes a reflection of the signal on the transmission line and can cause voltages to become high or low depending on the mismatch. Standard gauge wires and PCB traces are used for this board which are a type of transmission line. Some testing will be required to know the reasoning. Fortunately the data can be saved by suppressing these outliers.
The data above is the same as the previously shown data with some post processing. All of the maximum reported values were set to the mode of the data over all samples. This is the value of the flat line in the center and works to make the data usable. A similar value can be used when a mode like this isn't available where the user can select the appropriate suppressed value.
There are other interesting artifacts in this data as well. An important distinction between other EC systems and the RainSavor model is a continuous log of data and a discrete one. Samples are taken periodically and many of the samples have the probe completely dry. This causes some interesting effects when the sampling period lines up with a ramp up or ramp down of water being introduced into the system. This explains some of the 'hitches' in the data. For field data this will be greatly minimized as an interrupt signal is used to tell the system when water is present and to then trigger a sample rather than hoping water is present when a sample is taken.
There are still a few barriers to project completion that will need to be met. Electrically the final system needs to be constructed for rigorous field testing. Currently non permanent connections are made between the PCB and siphon to allow for faster swapping and testing. Custom cables will be constructed with heat shrink to create solid connections. A final battery also needs to be decided alongside a power budget test to make sure we have a long enough field lifetime. Currently the battery does work for some amount of time but a higher maH may be needed for the specified two month operational period. There are potentially minor modifications for the PCB to allow for slightly easier construction as the tolerances for the non SMD components may be too tight but this will not change how the circuit operates. Mechanically a finalized mounting system will be formed to allow for the siphon to be posted well above the ground out of animal reach while being able to keep the siphon level and within range of the electronics without extensive lengths of wire.
- Capture and release equal volumes of rain water repeatedly
- Measure electrical conductivity of storm water
Siphon Size Tests: After testing various siphon sizes, aiming to achieve the smallest amount of water collection while maintaining accurate results, a water collection volume minimum of approximately 2mL has been determined.
In the coming weeks, RainSavor aims to integrate mechanical and electrical design to conduct prototype tests during rain events in Corvallis, Oregon. The project is also looking to improve methods for removing debris and keeping the siphon mechanism clear of any potential blockage sources.
Siphon, Rainwater, EC Measurements