Do PIR sensors work for humans in an environment under high UVC radiation??? - Page 1
Forum section: electronics / projects

author: Dhanushka
author_rank: Regular Contributor
timestamp: October 15, 2020, 04:12:40 am
content: Dear all,I am designing a room with UVC lights (UVC lights can harm to human body) and this room is not allow for humans. But, if someone is trying to enter the room I need to turn off the UVC lights for safety. To detect the a human, I hope to use a industrial PIR sensors due to UV and IR are separated by the visible light in the electromagnetic spectrum. So, I need to know whether my assumption is correct?Thanks

author: coppercone2
author_rank: Super Contributor
timestamp: October 15, 2020, 06:34:45 am
content: when the door is opened have it turn off with a switch that has to be pushed on the outside with the door closed to turn it back on

author: ace1903
author_rank: Regular Contributor
timestamp: October 15, 2020, 07:42:38 am
content: PIR sensor are far from 100% reliable for detecting humans. It needs several seconds to see movements in order to filter false alarms.From personal experience even one second looking into UVC lamp creates temporary blindness. Sight recovers after couple of minutes but this temporary blindness can cause injures when someone tries to find door handle in a hurry. I think intrusion in UVC disinfection space everywhere is protected with mechanical interlock switch.

author: Zero999
author_rank: Super Contributor
timestamp: October 15, 2020, 12:08:06 pm
content: To answer the original question: yes, because PIR sensors work in the IR range, not UVC.As mentioned above, using an ordinary PIR sensor in a safety critical application, such as this, is a bad idea. Door interlocks are a much more reliable solution. The only sane reason for using a PIR sensor is as an additional layer of protection, such as in the event someone is still in the room when the lights are turned on, although operating safety procedure should prevent that.

author: sandalcandal
author_rank: Unknown
timestamp: October 15, 2020, 04:38:48 pm
content: Agreed with Zero999. Motion detection is not suitable as a primary safety barrier. Typical PIR sensors do not function based on the presence of heat alone, they detect the movement of an object with a different temperature to the background environment i.e. they need to "see" movement to activate. There are a variety of other sensors you could use that would perform similarly for detecting human presence through movement such as radar or ultrasonic sensors. I suspect these options might be more durable against long term UVC exposure too compared to the plastic lens/filters used in typical PIR systems. None of these are really the right tool for the job however.What it seems you are looking for is "Presence Detection"/"Presence Sensing" I'm only aware of two solutions, pressure safety mats and laser scanners. Both these might not work depending on the installation environment since they need to be setup/calibrated for a fixed environment so if anything in the room needs to move around regularly then these systems won't work. I'm not sure what levels of UVC are like for your application (and even if I did, I don't have the experience to tell you for sure) but degradation of plastics and other materials used in safety equipment (and other equipment) could be a problem.Probably well beyond the scope of what you're doing but machine vision seems to be a possible development in this area given similar application in automotive safety currently.This might fall on deaf ears (I'm not sure what the safety culture is like in Sri Lanka) but you should only use safety rated devices for safety applications.

author: TimFox
author_rank: Super Contributor
timestamp: October 15, 2020, 05:57:58 pm
content: Be sure to mount the door interlocks near the edge away from the hinge, so that the switch opens immediately when the door starts to open.Refrigerator-light door switches, mounted near the hinge edge of the door, will not activate until the door has been opened most of the way, which is not safe for a radiation interlock.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 15, 2020, 07:58:17 pm
content: Why not ultrasound like a supermarket?

author: amyk
author_rank: Super Contributor
timestamp: October 16, 2020, 03:13:09 am
content: Another alternative may be the Doppler radar sensors.Verysensitive to movement and (depending on the material) can even see through walls.

author: Dhanushka
author_rank: Regular Contributor
timestamp: October 16, 2020, 03:22:12 am
content: Thanks everyone. I gathered lot of ideas for my design make simple and I found that there are microwave motion sensors. As I know, they works well.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 16, 2020, 04:22:33 am
content: Quote from: Zero999 on October 15, 2020, 12:08:06 pmDoor interlocks which are in series with the on/off control circuits will not only drop the UVC lights off, but while the door is open, will prevent them from being turned on again.The best way is two use two independent switches separated by some distance on the door, both in series with the control circuit wire.That way, if one fails in the "on" direction, you are still protected.Don't "cheap out." on the switches & operating setup----fridge switches are inadequate.

author: coppercone2
author_rank: Super Contributor
timestamp: October 16, 2020, 06:02:22 am
content: you dont want the switch to turn it back on because if someone goes into the room and closes the door behind them it turns on when their in there closin the door behind them, thats practically a trap.It needs to force someone to check the room before engaging the system!!it needs to check if the door was recently closed and give you a operating window, i.e.door is opened, turn off systemdoor is closed, enable on switch for 5 seconds (which is outside of the room)on switch - light on until door openedif 5 seconds pass by after the door was closed and the switch is pressed, so nothing.that way if someone is walking by they cant just hit the switch with the door closed, they would at least need to fully open the door before they turn on the device, so they HAVE to check who is in there and hopefully the person in there senses the door was opened, preferably with a buzzer, but I don't know how this relates to deaf people with the law, probobly special training is requiredthen you put the human detector on top of that system,i.e. within 1 minute window : door has to be opened, the inside sensor has to be triggered (perhaps another button on the far side of the room), the door has to be closed, within 5 seconds the switch has to be pushed. that way it forces the person turning it on to walk into the room and check it,because you can always have a employee that passed out, fell asleep or is simply hiding in that room for whatever reason,. (some people would set them selves on fire to avoid a performance review) I heard of employees hiding in weird places in a building to avoid bosses having bad days and all sorts of stories. (say... tell Joe to see me if you see him around, I am too busy to look right now, but I am obviously going to scream at him because the project is late)All these motion sensor ideas dont work because people fall asleep all the time, so someone can be sleeping in there. Most PIR sensors don't work unless you move, I think if you are still infront of them for long enough, they turn off and only turn on if you move. I have seen this while sitting in a room reading, the light turns off and I need to stand up.Anyone remember this episode ?

author: wizard69
author_rank: Super Contributor
timestamp: October 16, 2020, 06:49:16 am
content: Quote from: Dhanushka on October 15, 2020, 04:12:40 amFirst off anything you implement will have to be reviewed based on compliance with OSHA, UL, and other regulations. This includes a formal LOTO (Lock Out Tag Out) procedure for room entry and the associated hardware to effectively lock out the UV light sources. Effectively it should be impossible to turn on the UV system while any lock is still applied.The second problem is infrared detection that is qualified for use in such an application. A secondary detection method might be required but high UV flux raises hell with common control hardware. You will at the very least need to find a vendor willing to certify its detectors for such use. It is actually pretty hard to find detectors for UV that have reasonable lifetimes even for modest exposures as I've looked a bit for a much more modest application. A good optical filter might be of help to prevent flooding the infrared sensor. On the other hand if the UV light is already on then your safety systems have failed. Detection needs to happen before any high energy UV is turned on.You might be better off with video detection but even here you have the issue of finding a supplier willing to take on the risk. You also run the same risk of damage to the detector which might be addressed with filtering. Such a system is largely software based and could potentially be faster than an infrared detector. The problem here is that the state of the art is moving pretty fast when it comes to vision systems!!!!!!! You still have the same issues as with PIR sensors which is that they will not detect hidden figures well. However I get the sense that a good infrared video system might do better than a PIR.Door safety switches will be required but this really does nothing to prevent the condition of somebody left in the room. The door switch may require a lock and then you will need for that lock to be defeatable from inside the room. By the way this stuff gets expensive pretty fast depending upon the standards it must meet. An E-stop inside and outside the room will be required too. All of these will have to be monitored by some sort of safety controller(s).The next issue you will have is loading of the room. I'm assuming that this is a sterilization process but it could be some sort of cure process. In an ideal world the operator would never enter the room. The ideal situation is to push in work from one end and pull out work from the other. Entering the room can become problematic as it is all to easy to be in a location not covered by monitoring.There is more to this seemingly simple question than one might first assume. I wouldn't be surprised if your insurance carrier will want to review what you implement.