Moisture Battle Part 1

From the initial testing release of the Night Sky Pi, it became obvious that there was an issue with moisture within the camera dome. Fortunately, there is no moisture entering the system which could damage the Raspberry Pi.

This moisture is affecting the image quality and resulting in some observations being unusable due to poor visibility. The moisture in the dome isn’t the issue that’s causing the poor image quality, it’s what happens to that moisture throughout the day. The moisture condenses and collects on the inside of the camera dome, causing it to either fog up or water droplets form. Also known as dew1

Dew Point

This collection of water on the inside is down to the amount of water that can be stored within the air inside the dome, and the effects of the moisture content at different temperatures. Air can hold a substantial amount of water vapor, however as the temperature drops then so does the ability of the air to hold the moisture. There is a key temperature point where the air can not hold moisture, which is called the dew point. This point moves with the temperature and moisture/humidity of the air.

You can see the effects of dew point most mornings, the most common image that comes to mind is that of little droplets on blades of grass. Interestingly the reason grass always seems to have dew, is that some species of grass can radiate heat more efficiently causing the air around the grass to drop, creating a micro-climate and causing the water to condense onto it.

Combatting the Moisture

From my initial investigation, there are three ways to battle the moisture within the dome of the Night Sky Pi. The first is to create a completely sealed environment with some silica gel. This is not feasible as creating a completely sealed environment is much harder than it would seem. Also, this system is exposed to the elements 24 hours a day, resulting in a short life for a sealed container.

Heating the air inside the dome seems to the the most common approach. The concept here is to measure the moisture and temperature within the dome, and then calculate the dew point. With a set threshold, if the dew point and current temperature are within range, then turn on a heater to warm up the air within the dome. This is to ensure that the dew point is never reached therefore the moisture stays within the air an not condense onto the dome surface.

Finally, there is active ventilation, the idea is to move the air around the entire system. This air is constantly exchanged with that outside the system, this is with the hope of removing the moisture from the dome and keeping the inside and outside environments as close as possible.

Further Investigation

Before selecting one or multiple solutions I would like to delve a little more into the differences between the outside environment and that inside the dome. This will be done by installing two environment sensors to the current Night Sky Pi running in the garden. One will be installed within the dome compartment alongside the camera, while the other will be outside the entire system but shrouded by a Stevenson screen. They will take measurements every 5 minutes of the temperature, and humidity and calculate the dew point. This will allow me to see if there is any real difference between the two areas and have a baseline for when I try various solutions. I intend to run this experiment for at least 30 days. This is to ensure that I have enough data and that I have encountered dry, wet, and still periods.

While these two sensors are running, I’ll also be doing a similar data reading with the Open Weather API which is currently being used within the Night Sky Pi / Open Weather Dumper. The reason for this is that if I find that the open weather API data is accurate enough not to have to install sensors and increase the complexity of the entire system, then the API data will be used instead. This would be the preferred option as the data is already on hand and proven to be working within the Night Sky Pi system.

Next Steps

So I want to leave the Night Sky Pi for a little longer, as I’d like to measure how long it would take if it ever does sort itself out. Just for my understanding. I’m currently creating another instance of the Night Sky Pi to build and test the circuitry before taking the live system out of action.

This experiment will also have to wait till I’m able to acquire another environment sensor as I’d like to have the same model/serial as the one I already have on hand to ensure reliable data readings. Progress on this series can be followed by using the tag moisture battle. Once I have the data collected, checked over, and graphed, I’ll be posting the results before attempting to address the issues.

  1. What is Dew and Dew Point Met Office ↩︎

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