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Opacity Analyzers

Opacity analyzer readings spike during cold weather

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

2. Observe your opacity readings for five to 30 minutes

    - did the spiking go away?

3. 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.

OPM2000, OPM2000A reading 105%

Background: The 105% indication and reading is Rosemount’s way of showing an error message; it was presumed that everyone in the opacity business would recognize that there is no such thing as 105% opacity and that it would instantly mean ‘analyzer failure’ to anyone observing it on the monitor’s display. It was also an easy way to get the milliamp output signal to rail high at about 21 maDC.

Components affected: LCW (liquid crystal window), lamp (bulb), power supply (SLB, Stack LON Board), G-64 LON Board, interconnecting cabling, & temperature.

The fault alarm (105% opacity) can come from any of the following:

  1. failing bulb/lamp or lamp power supply
  2. failing LCWs or LCW power supply
  • – VLTH [volts too high]
  • – LMPF [lamp failure per software algorithm]
  1. loss of Eshelon communications (LON)
  2. failing wire harness (to lamp or LON communications)
  3. failed calibration
  4. corrupted software on the Stack LON Board
  5. failing detector board (±15 vdc power comes from the SLB)

But not:

  1. actual stack opacity conditions (high opacity)
  2. misalignment
  3. dust on barrier window and/or corner cube
  4.  steam that has changed phase to vapor

Call us to help you diagnose this. Please provide the following:

  • model number
  • age of LCWs
  • age of bulb/lamp
  • reference voltages (8) (under Cal, Reference Voltages)
  • current ‘run’ voltages (4) (under Data, Volts)
  • temperature