Mix of coursework and my internship, so I’d like the why as well as the what.

A district heating substation: a PI loop drives a 3-way valve to hold the secondary supply temperature. Outdoor temperature is the main disturbance and we measure it, so I want to add feed-forward. Two questions:

1. How do I pick the loop’s execution interval (Δt) when I know the disturbance? My notes say “fast enough compared with the disturbance” which isn’t a number.

2. My mentor half-jokingly suggested doing the control in Hermetry with a Rules card, since the readings are there anyway. Is that a terrible idea or just a bad one?