Hi Erin,
There are a couple of pieces here.
In your images, you have your Reduce expression selected as the alert condition. I'm unsure of how this is actually evaluated to true or false as an alert condition, but it is not what you want selected. Only a single expression can be selected as the alert condition, so the Threshold expression won't work for your case. You'll want to use the Math expression and write a comparison such as (-800 < $B < -600 || 600 < $B < 800). See Flux Query Alert Example for a nice walk through.
You don't have this included in your screenshots, but make sure to set the evaluation duration to 0 if you'd like to fire the alert on the first instance of your triggered alert condition. Otherwise, the alert will be in a pending state until the condition has been satisfied for the specified duration.
Also, it's important to understand how the alerts are evaulated. When you query a dataset, you are pulling some range of time series data. For example, (now-10m to now). You then execute a Reduce function to convert the time series data into a single value. This is then passed into the alert condition function for evaluation. Alert conditions are only evaluated every 10s at a minimum, so this means its possible to have at least a 10s delay between when data enters your threshold and when the alert condition is checked and fired.
TLDR, these alerts are not really set up to check each data point as they arrive. It is not a real time alarming system. Keep in mind that decreasing alert evaluation frequency increases RAM and CPU usage.
