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You can reference attributes throughout your AF hierarchy via paths when building an expression. You can of course hand type the paths into your expression or you can use the "attribute" pane to select the attribute that you want to use, which I have come to learn is a bit hidden from users. See here:
If you're building expression at the template level, make sure you first choose an "example" element, then when using attributes outside of the templated element, make sure you choose to use either relative path or absolute path for your needs.
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Since you mentioned that your wet bulb temperature attribute isn't on any of the system templates or the building template, where is it? There are two different ways I can think to do what you're asking, however they'll each be better in different situations.
I imagine you have something like this, albeit probably way more intricate:
1) You can use substitution parameters to reference attribute on other elements, however you do so by navigating through the layers of your hierarchy to the attribute (essentially you say where the element is relative to where you're referencing it from). So if I know that web bulb temperature is on my attribute two levels up from Steam, I could use '..\..|Web bulb temperature' since each set of two periods goes up one level (details here).
2) You use the full path to the web bulb temperature attribute, starting from the top of y our hierarchy
Bingo - this is exactly what I was looking for, using the path structure to get out of the template and up a couple levels to where the wet blub would be located. That way I only have to reference the wet blub once for the campus, or at least for the building, and all the sub elements can have access to it.
Thanks! Problem solved.
So if I've got this right, the syntax above would go up one level from my chill water template to my building template, grab the attribute wet bulb temperature, search over the last year for a value that matches the current wet bulb temperature.
Correct? And if I wanted to go up two levels, it would be '..\..|Wet Bulb Temperature' - and three levels would be ..\..\..|Wet Bulb and so on and so on
By the time this example is done, I should be calculating the average tonnage from three different years, based on the current wet bulb temperature matching a point from the past. I can't evaluate this right now because I need to do a PI to PI move to get the temp in from our plant system, but I'll get on that next week.