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UVC Sterlization Cabinet

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redgear:
Hi all,

I am planning to build a disinfection cabinet using UVC radiation. Many of my friends and family work in the health sector and I plan to gift it to them. I am a beginner, i'm sorry if my questions are too easy or stupid. I have been collecting information online about the same and I am pretty confused. The cabinet will have a surface area of 7300 cm2. I am planning to use 6 X Philips TUV T5 16W tubes with UVC wattage of 4W and irradiance of 45 uW/cm2 at 1m. The maximum distance between the object and the tube will be 30cm in my case, as irradiance is inversely proportional to distance, I am simply multiplying the irradiance value by 3, so I get 135 uW/cm2. The total UVC wattage will be 24W(6*4) and irradiance will be 810 uW/cm2(6*135)(correct me if I'm wrong). I will be covering the interiors of the box with polished aluminium sheets to increase reflectivity.

Different resources suggest different dosages and different formulas to calculate the dosage. Studies based on SARS-CoV-1 suggest a dosage of 7-2410 J/m2. The IUVA recommends 1-3 J/cm2 for real world conditions but only 10-20 mJ/cm2 under controlled lab conditions. Since the cabinet will be closed and effectively almost all UVC radiation will reach the object can I achieve 99.9% deactivation using UVC dosage in order of few hundred mJ?

The next problem is properly calculating the exposure time. I find different formulas that give completely different results.

* According to this study, the exposure time formula is given by : Exposure Time(s) = UV Dose(J/cm2)/Irradiance(W/cm2), substituting values I get around 62 minutes
* According to this Research gate answer, the exposure time is given by : Exposure Time(s) =
( Desired_UV_dose(J/m2 * 4 * pi * (UV_bulb_distance(m)^2 )/ UV_bulb_power(W), substituting values, I get 23.55 minutes. 
* A Youtube video uses the formula, Exposure Time(s) = Surface Area(cm2) * Dosage(J/cm2)/Power(W), substituting values I get around 15 minutes.
As you can see, the differences are huge, idk which is the correct one.

Do I still need a Ballast for the tube if I step down the voltage to the tube's operating voltage?

I'm also planning to include a timer, will any 555 timer or timer relay be enough?

Since, UVC is harmful to humans, I will have to add some mechanism to stop the equipment from being operator if the doors of the cabinet are opened, I planning to use a contact switch.

Thanks

orolo:

--- Quote from: redgear on June 23, 2020, 12:28:02 pm ---The maximum distance between the object and the tube will be 30cm in my case, as irradiance is inversely proportional to distance, I am simply multiplying the irradiance value by 3, so I get 135 uW/cm2.
--- End quote ---
I would expect irradiance being inversely proportional to the square of the distance, so you should multiply by 9, to get 405 uW/cm2.

When you later multiply by 6, that could be big underestimate, since if your face of the object is 30cm away from one of the lights, it probably is quite closer to another, and if the closer one is at say 15cm, then it counts as 4 sources at 30cm, due to the inverse square law. Multiplying by 6 could also be a overestimate, because some lightsources (tipically half?) should be occluded (or not facing directly), so only half reach your face of the object, and probably not at a right angle. Then again, the walls are reflective.

I think a good idea is to consider first how will you place the light sources, which are not point-like, and then to get the lower bound of irradiance, try to imagine the face of a typical object that will be farthest (in the inverse square sense) from all light sources, and the more ocluded / facing away. That will be your lower bound. And that doesn't consider the reflectivity of the walls.

Maybe the best advice for your project I can give: try using 4-pin CFLs, because there are goood, compact and efficient electronic ballasts for them, which also give a longer life.

I've put together something like your project recently, but much more crude, with just one 4W Philips CFL. Those two pin CFLs have an internal ballast, so you just need to limit the mains current going into the lightbulb with some kind of inductance. Since I didn't own a limiter, I improvised one with two 12V transformers back to back. They get hot, but get the work done. It would have been much better if I had bought four pin CFLs with the correct electronic ballasts. I was under very strict lockdown then, so I had to stick to this scheme.

Edit: another thing about my crude project. I wanted it to sterilize masks. Just leaving the mask lying in the box doesn't work, because there are always parts of it in complete shadow. So I considered (but didn't have time to) using a small synchronous motor from a microwave oven to make a rotating stand for the mask. That way I wouldn't need to manually turn it several times to make sure it is disinfected.

redgear:
Hi, Thanks for the reply.

--- Quote from: orolo on June 23, 2020, 01:42:58 pm ---When you later multiply by 6, that could be big underestimate, since if your face of the object is 30cm away from one of the lights, it probably is quite closer to another, and if the closer one is at say 15cm, then it counts as 4 sources at 30cm, due to the inverse square law. Multiplying by 6 could also be a overestimate, because some lightsources (tipically half?) should be occluded (or not facing directly), so only half reach your face of the object, and probably not at a right angle. Then again, the walls are reflective.

--- End quote ---

Thanks for the correction. I plan to anyway double the exposure time I get. So, I choose not to worry about occluded surfaces. I will also be placing stoppers on all side to ensure the surfaces are not fully hidden. Since the aluminium walls are highly reflective I hope I can effectively achieve 360deg coverage. I checked out few products in the market and they all seem to use the same.


--- Quote from: orolo on June 23, 2020, 01:42:58 pm ---I think a good idea is to consider first how will you place the light sources, which are not point-like, and then to get the lower bound of irradiance, try to imagine the face of a typical object that will be farthest (in the inverse square sense) from all light sources, and the more ocluded / facing away. That will be your lower bound. And that doesn't consider the reflectivity of the walls.

Maybe the best advice for your project I can give: try using 4-pin CFLs, because there are goood, compact and efficient electronic ballasts for them, which also give a longer life.

--- End quote ---
That's a nice idea, I will try it.


--- Quote from: orolo on June 23, 2020, 01:42:58 pm ---I've put together something like your project recently, but much more crude, with just one 4W Philips CFL.

--- End quote ---
Where did you get UVC CFL lamps? Would be great if you can leave a link here.
The main problem is with the dosages, if I considered 0.1 J/cm2 of the effective dosage, even 2 tubes seem to be a overkill.  What dosage did you consider for your project?


--- Quote from: orolo on June 23, 2020, 01:42:58 pm ---Edit: another thing about my crude project. I wanted it to sterilize masks. Just leaving the mask lying in the box doesn't work, because there are always parts of it in complete shadow. So I considered (but didn't have time to) using a small synchronous motor from a microwave oven to make a rotating stand for the mask. That way I wouldn't need to manually turn it several times to make sure it is disinfected.

--- End quote ---

That's cool. I initially thought of using just 2 lamps, and moving them with a stepper motor(just like how the nozzle moves in a 3d printer). But that would increase the total time taken for disinfection as it has to wait few seconds after every step.

orolo:

--- Quote from: redgear on June 23, 2020, 02:07:33 pm ---Where did you get UVC CFL lamps? Would be great if you can leave a link here.
The main problem is with the dosages, if I considered 0.1 J/cm2 of the effective dosage, even 2 tubes seem to be a overkill.  What dosage did you consider for your project?
--- End quote ---
The lamps I got from Amazon, they seem to be popular for disinfecting the water in fish tanks.

Amazon link to the CFLs I bought.

Amazon link to the ones I'd bought had I known better. Maybe lower power, but with the four pins.

Regarding dosage, I read a lot about this. First, it seems that to kill the virus the best is hot air, because UVC attacks the masks and degrades their efficiency. So maybe a simple closed oven at about 80ÂșC is best just for masks and coronavirus.

I attach two of the best articles I found (I read quite a lot, but can't find them / deleted them). One is the implementation of UVC in a hospital. The other is about LED UVC, but gives a nice table of different bugs and kill ratio vs irradiance. I figured covid should be similar to its relatives in the table, and determined irradiance from there. Then doubled the time, just in case.

redgear:
Thanks a lot man! Appreciate it!

I am building it primarily for disinfecting daily use items like food packets, watches, phones,etc.

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