There are many instances where Phidgets might get stuck outdoors: weather stations, RC vehicles (quadcopters, underwater vehicles, robots, etc), outdoor installations, and applications we haven’t imagined yet. Some specific examples of Phidgets in the outdoors are a giant drum machine, a sky temperature scanner and precision agricultural monitors.
In this series, we explore how to protect Phidgets from outdoor conditions, like rain and dust. In this post, we look specifically at Phidgets that need to be exposed to air, like the humidity sensor and gas pressure sensor.
With electronic devices, like the humidity sensor, dust is a big culprit of throwing off readings by increasing insulation, holding static charge, and holding water (thus increasing humidity). Over time, the effects of collected dust has a serious impact on the accuracy of readings. Because these sensors need to interact with the air, any sealed enclosure will give completely false readings, but there are some enclosures that will work.
Encasing Phidgets that need to be exposed to air
Phidgets, like the humidity/temperature sensor, sound sensor and the gas pressure sensors (used for barometric pressure) can be placed outdoors, but still need to be protected from elements. The temperature sensor must be guarded against heat radiation (preferably even put in a white housing so that the sun rays are reflected off the enclosure), and all sensors need protection from precipitation and excessive dust.
There are many clever solutions to keeping Phidgets safe from weather while still allowing them to be in the open-air. As shown last week, Patrick’s humidity/temperature sensor has proven its worth in the outdoors by simply being mounted to the underside of a piece of wood next to a house. However, over time the humidity values have become offset. What can be done?
- A Stevenson Screen is for serious weather monitoring with louvres to protect against direct radiation and precipitation, and will work with the sound sensor as well. These screens start as low as $110, but most quality-made ones are going to cost in excess of $1000. They can be bought from meteorological weather stores like Hoskin Scientific, Russell Scientific, the Weather Shop (very expensive, well researched designs), and MetCheck (the best value). You can also make your own Stevenson-type screen using white plastic vents, wood, guttering or pot saucers (weatherforschools.me.uk offers some great instructions for making these). Because of the louvre design, it is harder for dust to collect on the electronics, thus extending the time that you’ll get accurate readings.
- A KN Filter and housing is your best bet for the humidity sensor if you’re really worried about dust (especially in Sub-Saharan countries). While testing this filter, we noticed inconsistencies in the readings that don’t follow a steady offset, so they can’t be calibrated. In the graph below, you can see the extent of these inconsistencies. The blue line is the 5-year old humidity sensor we’ve talked about before at Patrick’s house, the orange line is the humidity in the KN filter and the yellow line is a brand new humidity sensor installed outdoors with no filter. We’ve observed that humidity will increase about 10% during and shortly after a rain as the humidity peaks in the KN filter.
KN Filter Assembly
To make your KN filter assembly you’ll need:
- a KN filter
- 3″ PVC pipe cap
- USB2 cable of the correct length for you application
- Phidgets sensor cables
- 3mm cable grip/gland (we used a tight cable grip with internal diameter 2.5-6.5mm by American Electrical; the diameter of the grip will depend on the thickness of your cable)
- Scrap sheets of wood or plywood, about 4 square feet
- Mounting materials
To put it together:
- Cut off the ends of the USB2 cable and place the cable grip onto it, without the lock nut. Run two sensor cables through the USB cable, by following these directions. Make sure that the end that will be put into the KN filter is very well waterproofed with heat shrink and does not have a lot of loose sensor cable.
- Drill a hole the same as the outer diameter of the cable grip in the middle of the PVC pipe cap. Pull the prepared cable through the hole and secure with the lock nut on the other side. Attach the sensor to the sensor cables coming out of the pipe cap.
- Place the pipe cap on the open end of the KN filter.
- Use the spare wood to craft a simple protective housing to shield of rain from hitting the filter directly.
- You can then run the other end of the sensor cables to an interface kit that is located indoors.
Now you have a couple of options to explore for installing Phidgets that need to be exposed to air. If you don’t live in a dusty environment then a Stevenson screen is a very reasonable solution, but some people may require a slightly stronger filter, like the KN filter.