Monday, April 6, 2020

How to sterilize corona virus masks

First of all, I want to reiterate that I am not responsible for what I will expose in this article, it is just an idea that came to me reading that ultraviolet rays alter the DNA of viruses and make them harmless. To have a scientific validity it is necessary to know other information such as the intensity of UV radiation and the exposure time necessary to eliminate them. In these times of forced isolation, I thought of reusing the lamp of an old eprom (Erasable Programmable Read-Only Memory) eraser that I had at home. These non-volatile memory chips had a fused quartz window to erase them by exposing them to a special ultraviolet lamp for a few tens of minutes.

The lamp
The recommended exposure time for eproms was 20-30 minutes with a UV lamp with a wavelength of 253.7 nm with an intensity of at least 15W / cm2 and at a distance of approximately 2.5 cm.
The lamp of my eraser, shown in figure 1, may have similar characteristics.
Figure 1
It is a transparent fluorescent tube, having no internal phosphor coating, with a power of 6W. The lamp housing is made of fused quartz or Vycor as the common glass filters UVC rays. The efficiency is around 30-40%, which means that the effective radiated power is at most 2.4W so it is necessary to use an aluminum reflector to put under the lamp, as seen in the prototype photos. Even a thin sheet of aluminum, like the one used in the kitchen, acts very well as a reflector.
As you can see from the photo, the lamp has the writing "GERMICIDAL" G6T5 GL-6. It is not difficult to find them on the market because they are widely used for disinfection of air and water. Philips also produces several lamps with a wavelength of 253.7 nm (UVC) from the TUV TL series. The cost is less than 10 €.
Warning, UVC lamps are very harmful to the eyes and skin, therefore all precautions must be taken, as I did in my project.
The lamp, like all fluorescent tubes, requires a ballast suitable for its power and a starter.
A tanning lamp (UVA) is not good or, in any case, does not have the same effectiveness.
Even the black fluorescent tubes, coated with special phosphors that filter the visible light (> 400 nm), emit UVA radiation.

Germicidal lamps made with UVC led arrays are also on the market, but these are of higher wavelength (265-290 nm). From what I have seen in the datasheets, these LEDs have not high powers and low efficiency (10%), therefore different ones are needed and they are also quite expensive.

My realization
I made my system with what I had at home. I disassembled the lamp, ballast, starter and timer from the old eraser and mounted them in a suitably sized aluminum cabinet, as seen in the images in figure 2.

Figure 2

On the bottom right side I mounted a micro-switch to turn off the lamp when the cover is open, as seen in figure 3. The timer used is mechanical.
Figure 3

Of course you could make this system in better ways, perhaps by inserting a support for the mask, but I made it with what I had at home.
The green light that can be seen at the top acts as a simple slide, lights up with the light of the lamp and indicates that it is working.
The scheme is shown in figure 4.
Figure 4

Parts list
  • 6W UVC lamp (254 nm) Philips TUV TL or Osram HNS 6 W G5 or compatible.
        2 lamp holders with G5 socket.
        Starter complete with 6-9W ballast or electronic ballast.
        0.5 A fuse and fuse holder.
        Double switch, power cord with earth.
        1-5 minutes electronic or mechanical timer.
        25x15x10 cm aluminum case.
        21x12 cm aluminum reflector, 4 + 4 + 4 cm folded (trapezoidal section).

1.       “Ultraviolet”,
2.       “Philips TUV TL Mini”,

3.       “HNS 6 W G5, PURITEC HNS UV-C”, OSRAM GmbH, April 17, 2020

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