We're all aware of the current worldwide shortage of N95 and surgical masks .

Many are trying to build their own, with questionable efficiency, and risky potential outcomes.

And then, I stumble upon the Nukebox project describing how to build a DIY UV-C sterilizer for masks. It seemed too good to be true.

After reading Nebraska Medicine Center's Publication, I got convinced of the plausible technique's efficiency.

UV-C

What is UV-C ? And how it kill virus ?

Wikipedia explain it great

Also, have a look to this great document from Clordisys for germicidal usage of UV-C.

What surprised me the most is how little UV-C it takes to kill 99.99% of germs, bacteria, viruses and many others.

By the way, 4-Log reduction = 99.99%.

Based on Nebraska's project, a dose of 60 and 300 mJ/cm2 did a 6-log reduction of bacterial and viral organisms.

This is 99.9999% killing !

Even if only 2-5 mJ/cm2 seems to be generally accepted to kill SARS-CoV-2 virus that causes Covid-19, a 300 mJ/cm2 dosage target looks to me like having a safe margin. Also considering precision of a DIY sensor, and many out of control effects like the unknown "opacity" of different parts of the mask, like were the seams are, we really need a overkill target of many magnitude bigger.

UV-C light is not present in our every day life. UV-C from the sun is mostly all filtered-out by the atmosphere. This may explain why it so easily kill viruses , bacteria, moulds, ext. and why it is harmful to your eyes and skin . These Lamps are DANGEROUS, never look at them with your naked eye, and avoid skin exposure.

LAMP

First you need a UV-C Germicidal lamp emitting 253.7nm light (sometimes rounded to 254nm in the datasheets). Any other UV flavor is NOT equivalent . Tanning UV lamp or night club UV lamp emit other UV Frequency and are not useful for this. There're some LED available in the UV-C ranges, but the $ per watt is too high to consider them. After some shopping, I've decide to use USHIO tubes. These are high quality, made in japan, that look way more serious that all the no-name Chinese lamps. I mean, the specific glass tube quality composition (that must be quartz), gas filling quality, over-all mechanical quality and longevity of the tube are all details I don't what to mess with. Panasonic seems to build decent tube too. USHIO offer a great variety off tube types and have great documentation.

By the way , the blueish light the bulb emit IS NOT the UV-C we're looking for. What I mean is that if you filter the blue light, the UV-C radiation might still be present, and on the opposite, you may have a transparent surface (like plain glass) in front of the lamp that allow the blue light to pass, but block the UV-C radiation and not have the desired sanitizing effect anymore (more on that later).

These lamps are DANGEROUS, never look at them naked eye and avoid skin exposure.

Fixture and ballast

A bigger challenge is actually finding a fixture and ballast that fit the tube you chose.

These are the four characteristics for the fixture you have or might buy:

• Connectors on the tube. Something like E17, G5, G13 or G23. Look at the tube's specs for the supported fixture if the fixture doesn't supply the info;
• Tube power. This will be ruled by the ballast of the fixture you have. You can use a ballast a bit more powerful than the tube, but not the opposite;
• Tube length. This one is tricky. I found many tube that have the right Power and connectors, but are too short for the fixture/balast available;
• Tube diameter. This one can sometime forgive a little bit , and will usually follow nicely if the other 3 characteristics match. These are the T5, T8 and T10 numbers and means 5/8, 8/8 and 10/8 inches respectively.

Fixture ballast and bulb match I found

First match I found is an old black light 9W ballast I had with a G23 removable bulb socket and standard north-american 120v E26 lamp input. I've try hard to find another ballast like this one with no success, it is probably more than 20 yrs old, and has a purely magnetic ballast. It still work, and easily power a USHIO 3000304 GPX9 - 9 Watt Germicidal UV-C Lamp.

• Connectors : G23
• Tube power:9W Balast power: 5-7 or 9W
• Tube length : na
• Tube diameter: na

Current consumption on the 120v side was similar for Standard White 9w Lamp, my old Black-Lite and the UV USHIO lamp. ( 0.150A , 0.152A and 0.146A ). This look very inefficient because 120v x 0.15A = 18W. Amp reading are questionable because there may be many harmonics or inductive power factor, not giving not a true RMS value. But at least the 3 bulb give the similar reading, so it doesn't disapprove the compatibility of the ballast.

The second match I found was a "under the kitchen cabinet fixtures" type . This is my preferred one since I can purchase more. I've use a 8W Miyako LS-08-08 with a USHIO 3000016 - G8T5 - 8 Watt - T5 Germicidal Lamp.

• Connectors : Miniature Bi-Pin (G5)
• Tube Power:7.2W BalastPower:8W
• Tube length : 11.3 in
• Tube diameter: T5

This time I got a 0.112A reading on the 120v Supply with the normal 8w white tube , and a 0.106A reading with the 7.2W USHIO tube

This mean:

13.44w for a 8W white tube --> 0.59 efficiency

12.72w for the 7.2w USHIO tube --> 0.56 efficiency

This electronic ballast is more efficient than my old magnetic one and seems to drive the UV-C tube nicely, but the efficiency difference make me wonder if it is actual driving the 7.2w tube at 7.2w . Would the efficiency be equivalent for both tubes , it may be actually powering the 7.2w tube at 12.72*0.59 = 7.5W . This may explain the small reading error we will see later.

Also , get rid of any translucent shield the fixture may have, they may block the UV-C

Monitoring

Now the fun part begins.

I've search a bit on major suppliers (digikey, mouser, Newark Av-net, ext) for a UV-C sensor, either photo-diode or photo-transistor, without any luck. I didn't want to pay 100$+ neither was I willing to wait months for not in stock items. When digging deeper and wider I found this guy:

Adafruit 1918 UVA UVB Sensor

I know, I know, it is not a UV-C Sensor per say, but the GUVA-S12SD photo diode it use has just a wide enough detection range that goes a bit into UV-C land. The Low Pressure Mercury Arc tube emit a pretty sharp 253.7nm wavelenght light and this sensor detect from 240 to 370nm . The photo diode is not that sensitive at this wavelength, and we will have to account for it later to get accurate measurement.

Fun fact, I've add the colour spectrum of light on the right graph, so you can see the little spikes in the blue-green and yellow , that give the tube the nice visible blueish colour. Notice how bigger is the invisible UV-C spikes, did I mention it was dangerous?

Now, can we get quantifiable reading from this little guy? Adafruit did a pretty good job of packaging a nice op-amp close to the diode that allow you to simply do a voltage reading to get a measurement.

So , 19.5 mW/cm2 ?!?!?

What does it mean ?

The tube has a diameter of 1.6cm , and emit light on only approx 24cm of its length .

So on the distance I took this measurement (as close as I could) the tube as a surface of Diam x Pi x Length = 1.6cm x 3.14 x 24cm = 120.6cm2

So with this experimental measurement , 19.5 mW/cm2 x 120.6cm2 = 2.32 W of UV light exit the tube

Wait ... why only 2.32 W ?

It is a actually pretty close to specs measurement

Now that we have a UV-C sensor with some precision we can also verify many items for UV-C transmissivity.

Can we reflect UV and thus improve sterilizer box efficiency ?

Aluminium Foil Self Adhesive seems to have a beneficial effect , like a mirror would have

Since plain glass absorb UV-C , i didn't even try a that.