Understanding trigger expressions

Triggers are quite simple to create and configure—choose a name and a severity, define a simple expression using the expression form, and you are done. The expression form, accessible through the Add button, lets you choose an item, a function to perform on the item's data, and some additional parameters and gives an output as shown in the following screenshot:

You can see how there's a complete item key specification, not just the name, to which a function is applied. The result is then compared to a constant using a greater than operator. The syntax for referencing item keys is very similar to that for a calculated item. In addition to this basic way of referring to item values, triggers also add a comparison operator that wraps all the calculations up to a Boolean expression. This is the one great unifier of all triggers; no matter how complex the expression, it must always return either a True value or a False value. This value is,...