Understanding Upstream and Downstream

The terms “upstream” and “downstream” are frequently used in the stack testing industry to describe direction within a stack.  However, these terms are often confused and used opposite of their intended meaning.

The key to remembering upstream and downstream when talking about flow is to think about another flowing object—a river!

Rivers generally flow from a source, like a mountain lake, to an outlet, like the ocean where the freshwater disperses into the saltwater.  In terms of stack air flow, the air emissions originate from an emissions source and travel through the stack to the stack exit where they disperse into the atmosphere.

The direction of travel from a source to an outlet is with the current, or downstream.  Conversely, the direction of travel from an outlet to a source is against the current, or upstream.

There are two simple mnemonic devices which can help you to remember the difference between upstream and downstream.

“Up the Creek Without a Paddle”

Imagine you are in a canoe enjoying a peaceful trip down the river when you hear the sound of a waterfall ahead.  You look around for your oars, but there are none in sight.  Frantically, you use your arms to try to paddle further upstream to safety, but it is very difficult because you are fighting the current.  This is a situation of being up the creek without a paddle.

Upstream is the direction toward the source and also against the current.

Upstream Downstream Diagram

Diagram Showing Upstream and Downstream in Relation to Current


“Gently Down the Stream”

Now imagine that you made it safely to the river bank and carried your canoe to bypass the waterfall on foot.  You put the canoe back into the river where you can lay back and relax because the current is carrying you gently down the stream to your destination.

Downstream is the direction away from the source and also with the current.

The diagram above is a visual aid demonstrating the relationship between current and upstream/downstream direction.

If you have questions about this topic, or any other emissions testing question, please call Environmental Source Samplers at 1-888-363-0039 and we will be happy to help you.

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