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Lighting
December 2006
HOW MANY CUPS OF FOG IN A PINT?
By Nathan Kahn
How do you measure fog? With a ruler? A scale? A meter? And why would anyone want to measure fog anyway?
Because you, as a potential consumer of fog and fog machines, need to know how much fog you’re getting for your dollar. And manufacturers need to know how to convey that information to you. And most importantly, because you need to know the accuracy of the information you’re getting from a manufacturer. For answers to questions about fog metrics, you’ve come to the right place.
WHAT IS FOG?
Natural fog (i.e. low-lying clouds) consists of water droplets suspended in the air. The droplets are clear, but they appear to be white because they reflect the color of the light that’s illuminating them. If you were to put a huge green color filter in front of the sun, then natural fog would be green.
Theatrical fog is similar to natural fog in that it consists of droplets suspended in the air, and the droplets are clear and take on the color or the light that’s illuminating them. The difference is that the droplets are made of fog fluid rather than water. Fog fluid is usually a mixture of glycol and water. Theatrical fog machines convert this fluid to fog by forcing it at high pressure through a heated pipe. Some people mistakenly believe that, since fog machines get hot and create a smoke-like effect, they burn the fog fluid to make smoke. This is incorrect. Properly functioning fog machines don’t burn the fluid; they simply convert its physical form from liquid to aerosol. Smoke is made of solid particles while fog is made of liquid droplets. What comes out of a fog machine that is working within its specifications is definitely tiny droplets of fluid vapor.
WHY MEASURE FOG?
One reason to measure fog output is related to health. Although it’s not dangerous to inhale water-based fog fluid droplets in reasonable quantities, you don’t want to go gulping in the stuff. What is a reasonable quantity? Safe limits for glycol inhalation have been established and are published in an ANSI Standard entitled “ANSI E1.5 - 2003 Entertainment Technology - Theatrical Fog Made with Aqueous Solutions of Di- and Trihydric Alcohols.” (This publication is available free at www.esta.org/tsp/.) It’s easy to create cool fog effects that do not exceed the limit, but to do that you must be able to measure your fog, and for that you need a meter. (Did you guess correctly from the three choices given at the start?)
Meters that measure particles in the air are available, and they can be used for water-based glycol fog provided the size and constitution of the particular fog droplets you want to measure are known. Luckily, this information is provided by all the major fog manufacturers. (It’s different for every machine/fluid combination.) The data is published in the form of “calibration factors,” which can be programmed into the particle meter. You can rent one of these meters from the Entertainment Services & Technology Association (www.esta.org), and you can find a current list of calibration factors on the Actors Equity website (www.actorsequity.org) in the Document Library.
WHY ELSE WOULD YOU WANT TO MEASURE FOG?
Maybe you want to know which fog machine has higher output - a Super Fog X150 or a Foggy Fog Whiteout Creator. (The names have been changed to protect the measly.) Some manufacturers will list output in the form of cubic feet per minute. The problem is that there is no established density for this measurement. A spec sheet may say 5,000 cubic feet per minute, but does that mean 5,000 cubic feet of thick fog, of thin fog, or something in between? The answer is - yes.
A more accurate way to compare machines is by fluid consumption. What goes in is what comes out. So if the Super Fog X150 consumes 100 milliliters per minute while the Foggy Fog Whiteout Creator consumes 50, it’s a safe bet that the X150 actually creates a better whiteout than the Foggy machine. You should also take into account any reheat cycle or automatic ramp-down feature that the machine has. (A ramp-down feature automatically reduces the fog output in order to prevent the heater temperature from dropping below a certain, critical point.) If the X150 will only operate for 20 seconds before shutting down to reheat, while the Foggy machine goes forever at full output, then maybe the fog from the X150 isn’t so super after all.
Regardless of what the specs say, you should understand what they mean and how to use them. Reading the ANSI standards is a great place to start.
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