Most flow switches are a simple magnet and reed switch. But what do you mean by this "good/fair accuracy in regards to proper chlorine production."

Most SWGs are open loop production where you just set the runtime percentage. There are a few that use ORP meters and can regulate production based upon ORP levels but ORP readings can change with other factors such as PH and direct sunlight. Both Autopilot and Hayward ProLogic are two manufactures that have ORP SWGs.