Physics of theatrical fog/haze?

[sparkyb]

Does anyone know the formula for the rate that theatrical haze dissipates? Of course, the constants will change depending on the space and the specific fog/haze used, but what's the general form? Basically, the question I want to answer is this. If I run a hazer for 1 minute to fill a room and it takes 10 minutes to clear out completely, if I need to re-fill the room after only 5 minutes, how long do I run the hazer so I don't over-fill? Do I run it for half the time, or is it something other than linear?

If you're wondering why I need to know this, here's my situation. My "show" is an interactive walk-through adventure. One of the rooms is a laser maze. We're using a Look Solutions Unique 2.1 hazer to fill the room so you can see the lasers. The hazer's manual suggests running it on some high setting for a fixed amount of time before the show to fill the space, and then run it during the show on a low setting to keep the space filled. However, unlike a normal stage production that will only do one show a night and starts out with no haze each night, our shows run whenever there is a group ready, potentially back-to-back. I don't want to run it and keep it filled all day, because there can be gaps between shows, but it isn't consistent. The whole show is automated so there isn't a person to manually decide when there's enough haze. It hazes automatically before each show, but right now it always does the same amount. The problem with that is if it does it too much then when there are back to back shows it gets much too dense. However if I do less, then the first show of the day or a show after a long gap doesn't have enough. So I'd like to program my show control system to estimate how long it needs to run the hazer based on how long it has been since the last show. Is this possible?

 

[carproelsofly]

Can you employ an optical sensor that feeds info to the show control system? Teach it to regulate itself?

 

[MNicolai]

The complication of a formula to do this is that HVAC systems will greatly affect how long the haze will hang in the air. If it's spring or fall and it's a comfortable 76° outside, the HVAC systems may run less and haze will persist in the air for a long time. Whereas if it's summer and the air-conditioning is running often, or winter and the heating is running often, more ventilation will reduce the time the haze hangs in the air.

To automate this in any sane implementation, you may need to integrate IR sensors or the like to test the haze concentration of the air. Even then, depending on the size of the room, that may or may not be practical. If it's a really large room and the fans of the ventilation system aren't running, some parts of the room may have heavy haze while others have no haze. If the fans are running, you may have evenly distributed haze throughout the space but the fans may pull the haze out of the room faster via the HVAC systems.

 

[sparkyb]

Ah, you raise a good point about HVAC. I think maybe that's the reason we're having so much more trouble getting haze to stick around in this show compared to last similar one we did when we didn't have HVAC connected to the interior of the show. I like the idea of using sensors. The spaces aren't that big and I don't need to be that exact, just good enough. Do you know of some kind of sensor I can buy that will measure this kind of thing?

 

[carproelsofly]

Try searching for "particulate sensor". If you're running from an arduino, there are a number of tested options, as well as some shared code.

 

[StradivariusBone]

Speaking of particulate sensors, your HVAC may have them and they may be tied to your fire alarm control panel. In our building when we haze to a certain point the alarms go off and trucks are coming. Depending on your AHJ, you can sometimes request it disabled if you run a fire watch.

 

[Dalamar]

A few things here:

A) Yes, haze is entirely dependent on air movement. Your particulate is a minuscule droplet of a liquid substance (almost) suspended and supported by air. if the air goes one way, odds are the particulate will, too.

B) Particulate detectors are efficient - painfully dumb - but efficient. I don't understand why we still use them in performance halls / venues.... longer, far more complicated discussion meant for another thread / time.

C) I need to preach for my own Parrish, and I apologize if that sounds opportunistic, but what you require here is an MDG ATMOSPHERE . It's longer lasting effect would allow you to simply leave it at a constant low level output once you've figured out your sweet spot output vs HVAC ratio.

spec sheet

 

[FatherMurphy]

Air temperature and humidity can also have a bearing on haze hang time. The more variables you can control, the more predictable your results are going to be.

Strategically placed fans (or ceiling fans) can help distribute the haze evenly. From what you've said above, it sounds like your best bet might be to find a remote corner for some sort of particulate sensor, and have it shut down the hazer when a certain concentration away from the hazer is reached. Essentially, starting the show would send an 'on' signal to the hazer, and the sensor would send the 'off' signal, or prevent the hazer from starting if the density is already high.

 

[wolfman005]

Why not do some testing and figure out some sort of time limit where the start of the pre haze is cancelled if a previous instance occurred within X amount of time as the new cue? That's not really worded well but hopefully you get the idea.