Four hot-water floor returns contain chlorine. Floor 3’s hot return reads 0.00 mg/L. Once they mix, the basement sensor still reads chlorine — but cannot tell you which floor lost its residual.
No chlorine residual at this monitored point.
Chlorine is present. The affected floor is hidden.
Follow each return down to its basement sensor. Hot mixes with hot; cold mixes with cold. The sensor swarm measures each floor before mixing, so Floor 3’s zero stays visible.
80% × 0.40 + 20% × 0.00 = 0.32 mg/L
Start: equal return flow from five floors (20% each). Move the slider: less flow from the affected floor makes its zero even harder to see in the blend.
Compare the floor readings with the two basement sensors. Modbus polling shows how each separately addressed measurement reaches the controller.
| Cold | Hot |
|---|
Illustrative readings in mg/L free chlorine, not operating limits or field data. Mixed concentration = sum of (return flow × concentration) divided by total return flow. The model assumes complete mixing with no additional chlorine demand or decay between floor sensors and basement sensors. Other floor hot returns remain at 0.40 mg/L; all cold returns remain at 0.50 mg/L. The circuits never mix with each other. Cold circulation represents an engineered recirculating system. A positive mixed residual does not demonstrate adequate residual on every floor. Modbus RTU is a polled bus: the controller queries one sensor address at a time and each sensor answers in turn. Halogen sensors connect directly to Modbus-capable controllers, PLCs and SCADA systems, or through a D20. How many sensors a bus can carry depends on the controller model, cable length and baud rate, so larger buildings may use more than one cable segment. Controller brands are shown as examples. Low residual warrants investigation; it does not by itself establish biofouling.