The Problem With UV Sensors

One of the sensors I would like to add to the weather station is an UV sensor. It would provide local information about the current UV intensity. One sensors that sparked my interest was the SI1145 Digital UV Index / IR / Visible Light Sensor from adafruit.com (product id: 1777). 10 USD excluding shipping make this an affordable sensor and it has a calibrated UV sensing algorithm that can calculate the UV index. This is very convenient because the UV index is the same format that Weather Underground accepts when you want to upload sensor data. So why haven't I installed one yet?

Waterproofing UV Sensors

Almost all hobbyist UV sensors are made available as a circuit board without a waterproof enclosure. So you are probably thinking, why not put it in a clear plastic container and add it to the weather station? Lets have a look at the properties of UV light. For the UV index we are interested in wavelengths between 295 and 325 nm. The main property of interest is the optical transmission of UV light through different materials. 

We are looking for a materials that lets the most amount of UV light through for the most accurate results by the UV sensor. Most materials block UV light to a certain degree and become an UV filter. Thats why 3$ sunglasses almost work as well as 100$ sunglasses. To identify suitable materials we will use the following graph.

The graph plots various materials and their transmission properties. The transmission properties vary by wavelength. The graph uses microns as the unit for wavelength. 300nm equals 0.3 microns. So that is the vertical axis we will look at. Straight away we can identify that the materials acrylic and germanium are unsuitable since they don't let through any UV at a wavelength of 0.3 microns.

I will list possible candidates and their optical transmission in %:

• UV grade fused silica, 5mm                            -   94%
• Semiconductor grade fused quartz, 5mm   -   94%
• Borosilicate, 1mm                                             -   82%
• Soda lime (BK7), 1mm                                     -   82%
• Saphire 1mm                                                    -   82%

So know we know which materials we have to look for to use as a waterproof barrier for the UV sensor. But are there useable pieces available and are they affordable? For this I use eBay. I will be looking for round pieces with a minimum thickness of 3mm and a minimum diameter of 18mm. These are the results (prices are material + shipping).

• Fused silica 50mm dia, 5mm thick    -   ~35 NZD   (not sure if UV grade)
• fused quartz gives same result as fused silica so - ~35 NZD
• Borosilicate 170mm dia, 3mm thick  - ~15 NZD
• Soda lime 25.4mm dia, 4mm thick    - ~50 NZD
• Saphire 21.3mm dia, 3mm thick         - ~110 NZD

Even though Borosilicate is more affordable I would try out the fused silica and do some tests to find out if the optical transmission corresponds to that of the graph. So it will be a 35 NZD investment for the sensor cover. The sensor is 10USD. Converting it to NZD and shipping to New Zealand will bring up the price probably to that of around 30 NZD. So now the cost of adding an UV sensor is already at 75 NZD. That leaves us with designing and making of the actual waterproof case that will hold the sensor with the glass shielding still to be added to the price.

So I won't be adding a UV sensor at this moment because of the cost. When there is a strong demand or a sponsor I might change my mind. But until then the current implemented range of sensors will have to make do.