We have been a little remiss in reporting progress on the MMA Weather Station over the last few months, but we’ve had some early prototypes out in the elements for testing and we learned some important lessons on durability and reliability. Here’s an update on what we’ve been up to:
Outer Shell and Structure
Aside from electronic components, parts for the stations are made on a 3D printer. This includes some of the circuit ‘boards’ and wire connectors. In late spring / early summer, we placed a station at the Marshall Field Site in Boulder, CO. The site is used by NCAR/UCAR to test stations and sensors. While it’s only been out at Marshall for few months, we were nonetheless pleased to see that the components show no sign of aging in the sun, wind, and rain. Connections between modules hadn’t moved, the plastic did not discolor or crack, and the movement of the anemometer remains unchanged. Since placing the station at Marshall, we’ve made a number of changes to further strengthen and improve the ruggedness of the design.
We struggled for some time with how ‘smart’ each station should be, as well as how best to perform basic data logging, data processing, time stamping and synchronization, and communications functions you would expect or need. We initially threw out any notion of developing our own circuits, as this would undermine the spirit of the project in several ways. First, we did not want to mass produce boards, as this then created one more highly specialized external dependency for the meteorological services that would be making the stations. Our goal is to enable the manufacture and assembly of weather stations, by using readily available off the shelf base components. This allows for long term maintenance, as well as customization. Secondly, if we opted for the boards and circuits to be built in-house by each NMHS, then this would require special training, skills, and equipment. If too much skill is required, then the ability for the system to work in areas of limited resources is threatened.
We therefore had the option of using some of the common hobby micro-controllers out there, such as the Arduino. While inexpensive and easy to setup, we eventually rejected such devices, due to the cost of add on components and features. For instance to add data logging would cost $15, wireless communications $30; so on and so forth.
Eventually we looked at single board computers, and chose the Raspberry Pi. It quite literally is a small computer. It comes with ports for Ethernet and USB connections, wireless functionality is added easily and cheaply, data can be logged to the same card that holds the operating system, and it still hosts all the GPIO functionality we need. As it is a full on operating system, we expect to be able to provide some more advance features, which make sense and are nice for remote use and access locations. Data processing can be done at the weather station, rather than waiting for collection and centralization of information. A GUI management interface is easy to create and provide. Updates of scripts (Python and shell) used to read the sensors, manage data, and create communication links are easily updated on the fly, as opposed to a micro-controller.
Currently, we have all the basic scripts and functionality for the station working on the Raspberry Pi. There is a lot of polishing and further testing to do, but at this point, we’re very pleased with the selection.
Junction Box and Circuits
The Junction Box gathers together all the cables from the various sensors and also houses the rain gauge amplifier, a real time clock (RTC) IC, and an analog to digital converter (ADC) for the analog sensors (temperature, humidity, rain and wind direction, necessary because the Raspberry Pi will only accept digital input). The introduction of the ADC, like the amplifier — a 16 pin IC chip, prompted a rethink of the “Steinson Circuit” that we were using with the amplifier. While the circuit seemed to be pretty reliable, it was somewhat fiddly to make and certainly had the potential to cause problems, so the prospect of having a “Steinson Circuit 2” for the ADC did not seem like a good idea. We still wanted to avoid soldered connections and preferred the use of off-the-shelf, push fit jumper cables to standardize the design of all the other multi-cable connectors that will link the various components of the weather station (see below). We also wanted to enclose and seal all the connections to avoid corrosion and temperature differential issues, which can be a problem with these types of sensor in this kind of application. We believe the new approach achieves all these goals while at the same time being easier to assemble and very robust (all connections are tightly clamped and enclosed under a sealing layer of hot glue).
Connectors and Plugs
We started off using CAT 5e punch-down jacks to connect different components of the weather station together but during initial testing it was clear that this approach would not be durable enough. We’ve developed a series of connectors which clamp together multiple push-fit jumpers in a way that allows easy disconnect of individual components while also providing very stable connections, thanks to the precision of the 3D printer and liberal application of hot glue to secure and seal. These connectors have evolved into plugs that are integrated into the cases of the junction box, rain gauge and radiation shield, providing a plug-and-play approach that should make assembly in the field foolproof and minimize disruption during maintenance (a replacement rain gauge could be swapped out in a matter of minutes without the need to open up either the new gauge or the junction box). The plugs are designed so that they can rotate in their socket, which allows a convenient way of leveling the component pieces before they are clamped in place (particularly important for the rain gauge).
Little of consequence has changed on the rain gauge since we originally described it (see Hybrid Tipping Bucket Rain Gauge Design), other than the introduction of the connector plugs described above. The plug in approach provides much better protection to the very delicate wires on the load cell, which tended to break easily in previous iterations.
Low Cost Anemometer
No self-respecting weather station can do without a means of measuring wind speed and direction. We have some ideas for a combined gauge that will do both at once but it will need some extensive development and testing, so in the meantime, we plan to include a conventional, three cup anemometer and wind vane in our station. We’re using a digital (latch) Hall Effect sensor triggered by four small magnets to count rotor revolutions for the anemometer (these will then be converted to wind speed by a script in the Raspberry Pi), and an analog Hall Effects rotational sensor, again triggered by magnet, to determine angular displacement of the vane. Once again, the majority of components are made on the 3D printer, and a small, sealed ball bearing allows low friction rotation.
We did some initial testing of the anemometer in our own “high tech testing center” (A.K.A. the warehouse with various fans and blowers.) and the results from three prototype units demonstrated consistent enough performance for us to move to formal testing. We later gave one unit a brief run in a wind tunnel, where it survived a wind of around 115mph (low category 3 hurricane), although there were some indications that it was not going to last much longer. With the lessons learned from the brief wind tunnel experiment we did some redesign so that the various components are locked in place and we are now ready for a full calibration test which we hope will be conducted by the National Weather Service Field Support Center in Sterling, VA.
The radiation shield protects various sensors (temperature, humidity, pressure) from direct exposure to the sun, while allowing sufficient circulation so that the air being sampled is representative of outside conditions. We have loosely based our design on the Maximum Minimum Temperature Shield (MMTS) that is used by the US National Weather Service, although the diameter of the leaves was dictated by the size of the build plate on the 3D printer. We’re trying something that we haven’t seen in commonly used shields – we’ve added a mesh screen to all air inlet/outlet passages to dissuade wasps etc. from making their home inside the shield. We figure that the remote locations that we are targeting with the MMA project will likely mean sporadic maintenance for many of the stations, so colonization by creepy crawlies is a distinct possibility that we should try our best to prevent. In addition to field testing, we also hope to eventually test all the components in the wind tunnel to see how they withstand extreme winds. All the components of the Radiation Shield are made on the 3D printer.
Sensor Connections and Holders
The various electronic sensors that we’re using all tend to have very delicate connectors, so to ensure reliable and consistent connections we’ve conceded that this is the one place where we cannot avoid the soldering iron. Fortunately, these are all simple, easy access soldering jobs that even the dexterity-challenged (such as your humble scribe!) can handle.