Akiba's NukeBox Sterilizes N95 Masks for Reuse, While the NukeMeter Confirms That It's Doing Its Job

With SARS-CoV-2, and its ability to survive on a variety of common surfaces for hours or even days, at the forefront of everyone's mind, there's considerable interest in using ultraviolet light to decontaminate everything from face masks to smartphones — but how do you know if your UV lamps are doing their job? If you're Chris "Akiba" Wang, you build a UV-C meter dubbed the NukeMeter.

Akiba published a guide to building an ultraviolet sterilization box, the NukeBox, for still-in-short-supply N95 face masks late last week. The NukeBox uses lamps which produce UV-C at a wavelength between 100–280nm to perform ultraviolet germicidal irradiation (UVGI) — a process which kills the virus responsible for COVID-19 without damaging the underlying material on which it rests.

"The dosage is measured in μWs/cm2 (micro Watt seconds/squared centimetre) or sometimes μJ/cm^2 (micro Joules/squared centimetre)," explains Akiba. "Although the units are intimidating, the formula to obtain it is quite simple: Dosage = (Intensity at that particular point) x (time exposed).

"For example, in this particular HOWTO, I’m using a 10W germicidal fluorescent bulb with a rated output of 2.7W of UV-C light (UV light at 254 nm). This light theoretically puts out 1,364 μW/cm² of intensity at a point 7 cm away. The COVID-19 virus requires 5,000 μWs/cm² (5 mJ/cm²) for sterilization. Based on the above formula, it would take 3.67 seconds of UV irradiation to inactivate the virus."

But how can you know that the lamps you're using are performing to spec? Enter the NukeMeter. "NukeMeter is a device that allows you to measure the intensity of UV-C light put out by standard germicidal bulbs," Akiba writes. "It relies on a low-cost sensor and some open source hardware and software. For the prototype in this HOWTO, total cost in materials is ~$2.50 for the sensor module and around $3 for the Arduino-compatible board if you get them off eBay. Theoretical models are nice, but since the stakes are so high, I wanted to see how much theory deviated from reality."

As well as verifying the output of the NukeBox lamps, the NukeMeter allowed Akiba to check on some other metrics — including how the output varies over time, confirming that you need to leave it running for at least a minute to each 80 percent of its peak output, and how the intensity varies along the axis of the tubular bulbs.

"I was surprised to find that there was very significant deviation in the light intensity depending on the location along the bulb axis," Akiba concludes. "The two ends of the bulb were the weakest radiators whereas the centre of the bulb was unsurprisingly the strongest. This means that the intensity variation would need to be taken into account when using the lamp as a sterilizer."

Akiba has published how-to guides for both the NukeBox and the NukeMeter on the HackerFarm website.