Opacity is determined by observing how much a background is obscured by a plume. Not all backgrounds are created equal. Here’s how to select a background that will optimize your opacity reading accuracy.
You should always choose a background that has the highest contrast to the plume. If you are viewing a white plume, you would want a dark background such as green foliage. A black plume would best be viewed against a light background such as blue skies or a light building.
Example of high contrast between plume and background.
Negative bias means your readings are lower than the actual opacity. As the contrast between the background and the plume decreases, negative bias increases. Negative bias can also occur on overcast or cloudy days when luminescence is low. If your opacity readings are lower than the actual opacity, the source may be exceeding its opacity limit.
Negative bias usually occurs in low-contrasting conditions.
Positive bias means your readings are higher than the actual opacity. This tends to occur when you have a high contrast background with respect to the plume. It also tends to occur on sunny days, when contrast is increased with luminescence. Positive bias is always better to have because you will usually read higher than the actual opacity; ensuring that you are within your opacity limit.
Sunny days and high-contrasting backgrounds can cause positive bias.
There are some situations where you can’t choose your background. Tall stacks usually have to rely on the sky for background because nothing around is tall enough to read against. In large industrial facilities, you may be forced to read against a building that is not contrasting to the plume. Additionally, the wind may dictate your background due to plume direction. In these situations, you have to make do with what you have. Just keep in mind that your readings may be biased due to the poor contrast.
Video demonstrating the effect of a low-contrasting background.