Your opacity analyzer normally runs fine, but then cold weather sets in and now the opacity readings are a bit flakey, spiky, noisy, weird, unusual, annoying, etc. Well, you should initially rule out the opacity analyzer … it’s not its fault! It’s probably physics! Or earth sciences! But it’s not sunspot activity.

What is most likely happening is:

  • Your stack has significant moisture in its effluent
  • Your purge air system for the opacity monitor draws makeup air from the very cold ambient air
  • When the very cold ambient air meets the very hot, moist stack effluent, the effluent temperature drops significantly


  • forces the relative humidity to approach 100%
  • then forces steam (water) molecules to coalesce (condense) … a phase change
  • then creates visible (water) vapor (some say ‘steam’ but it is really vapor)
  • if you can see it, so can the opacity analyzer’s stack light beam

So, how would you prove this before investing any money in the solution?  How about a very simple test:

  1. Somehow stop the purge gas flow temporarily
    •     block or partially block the blower’s (or blowers’) suction(s)
    •     turn off the blower(s)
    •     remove the feeder hose(s) to the injection ports
  1. Observe your opacity readings for five to 30 minutes
    •     did the spiking go away?
  1. Re-employ the blower system
    •    did the problem return?

If answers to questions 2 and 3 are “yes” then you have “Phase Change”

So, how do you fix it? Here are a few ideas:

  • if it only occurs once in a Blue Moon, just declare it on your quarterly reports and take it as “down time”
  • if it happens too often, your Regional EPA won’t enjoy seeing a high down time number
  • install ducting so that the opacity analyzer draws makeup air from a warmed room
  • install purge air preheaters (1500 watts per side minimum; 3000 watts per side for extreme conditions)

Call us at 877-616-0600 to discuss this in greater detail.

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