Low-flow and zero-flow chlorine monitoring
Actual low-flow / zero-flow setup uses the HiRes impeller so the MP5 / MP5-A can read in a wet-tap, tank, or remote site without a conditioned sample panel or waste stream.
Quick Answer
How does the MP5-A work at low-flow or zero-flow?
Best for
- Wet-tap mains, tanks, clearwells, and remote sites with little or no pipeline flow
- Replacing a sample panel that cannot hold a stable conditioned stream
- Sites that cannot discharge a waste stream
Not best for
- A dry or frequently air-locked line — the sensor must stay flooded
- Chloramine / nitrification residual — use MP-TOTAL
- Sites that must keep online DPD colorimetric as the method of record
Why flow independence matters
Analyzers that need a conditioned sample stream (including DPD panels) depend on pressure, a flow cell, and a drain. At low-flow and zero-flow those conditions disappear: the sample line siphons, the cell runs dry, or the waste stream is unacceptable. The MP5-A measures in place. The HiRes impeller is the internal circulation that makes low-flow and zero-flow readings possible.
| Published MP5 spec | Value |
|---|---|
| Free Chlorine Range | 0–10 mg/L (ppm) |
| Accuracy | ±5% of reading or 0.04 mg/L (0.04 ppm), whichever is greater |
| Response Time (T90) | 90 seconds |
| Pressure Range | −0.7–10 bar (vacuum to 145 psi) |
| Operating Flow | 0–4 m/s |
Common failure modes
| Mode | What operators see | First check |
|---|---|---|
| Air | Erratic residual; 4–20 mA may drop on error | Bleed, tap body, confirm ejection stream |
| Dry running | Error 31; pH may later fail to calibrate | Keep the cell flooded; do not run in air |
| Error 31 | Air-in-sensor alarm | Re-prime; bucket-test if it persists after full submersion |
| Impeller binding | No internal circulation; stale residual | Inspect HiRes impeller and annual wear parts |
| pH dry-out | Chlorine compensation wrong after air exposure | Rewet ~3 min; stabilize ~5 min; wait 20 min before cal |
Install / check workflow
1. Confirm the sensor stays flooded
Install in a wet-tap, tank immersion, or sample panel so the measuring cell stays full of water. Flow-independent measurement still requires a wetted sensor — not a dry line.
2. Set orientation and bleed air
Keep the outlet / ejection path where trapped air can leave. Tap the body to release bubbles. Error 31 (air in sensor) is the usual first alarm when the cell is dry or air-locked.
3. Check HiRes impeller circulation
The HiRes impeller draws sample across the electrodes so the sensor can read at low-flow and zero-flow. Binding, debris, or a dry run stops that circulation. Inspect wear parts if the impeller does not turn.
4. Allow pH rewet and stabilize
After reimmersion, the pH cartridge typically rewets in about 3 minutes and the sensor stabilizes in about 5 minutes. Do not treat the first minutes as a calibration window.
5. Wait 20 minutes before calibration
Halogen’s BTSB / product guidance: allow the sensor to run in process water at least 20 minutes before any calibration. Then compare to a fresh DPD grab.
6. Log residual and inspect annually
SensiCLENE beads keep the electrodes clean in service. Plan an annual wear-parts swap. Recheck after any dry-out, air event, or impeller service.
Field evidence
“The MP5 is installed on a remote site with low and variable sample pressures, which had chronic alarm issues. The Halogen MP5 has continued to operate successfully.”
Maui’s remote low-pressure site started without flow adjustments and has run without calibration since June 2024. Lakewood’s wet-tap MP5 eliminated the waste stream that blocked monitoring at some wells — Lakewood case study.
Videos
There is no dedicated low-flow troubleshooting YouTube video. These are the closest published clips.
System Startup & Function Testing
Power-on, pairing, and first circulation check after a low-flow install.
Reference Calibration & Calibration Defaults
DPD grab comparison after the 20-minute wait.
Non-Hazardous Sensor Wear-Parts Replacement
Annual beads, impeller inspection, and reassembly.
Troubleshooting Sensor Communication
Closest published comms / controller troubleshooting clip.
Low-flow troubleshooting demonstration — video coming soon
A dedicated low-flow / zero-flow troubleshooting demonstration has not been filmed. No YouTube ID is invented here. It is also listed on the videos upcoming list.
Frequently asked questions
Can the MP5-A measure chlorine at zero flow?
Yes. The HiRes impeller circulates water across the electrodes, so the MP5 / MP5-A is flow-independent and can read at low-flow and zero-flow when the sensor remains flooded. That is why wet-taps, tanks, and remote sites do not need a conditioned sample stream.
What is Error 31 on a low-flow install?
Error 31 means air in the sensor (AUX voltage out of range). It is the common alarm for dry running, trapped bubbles, or a siphoning sample line. Re-prime with the sensor flooded, tap to release air, and confirm a visible ejection stream. The error typically clears once water flow inside the sensor is restored.
How long after reimmersion before I can trust the reading or calibrate?
Site FAQ: the pH cartridge rewets in about 3 minutes and the sensor stabilizes in about 5 minutes after reimmersion. Halogen’s BTSB guide: wait 20 minutes in process water before calibration.
Why does flow independence matter versus a DPD analyzer?
DPD analyzers such as the Hach CL17sc need a conditioned sample stream and a drain. At a tank, dead-end, or remote wet-tap there is often no reliable sample flow to waste. A flow-independent in-pipe sensor measures in place and does not consume a waste stream.
What wears out on a low-flow MP5-A?
SensiCLENE beads and the impeller / wear-parts kit are the annual service items. Impeller binding and pH dry-out are the usual low-flow failure modes besides air. There is no monthly reagent cycle.
Is there a dedicated low-flow troubleshooting video?
Not yet. A “Low-flow / zero-flow troubleshooting demonstration” is listed as coming soon. Until that is filmed, use the existing startup, calibration, wear-parts, and comms troubleshooting clips.
References & sources
- 1.MP5 / MP5-A specifications — accuracy, operating flow 0–4 m/s, NSF/ANSI 61 — Halogen Systems Inc.. https://halogensys.com/products/sensors/mp5-free-chlorine
- 2.Site FAQ — pH cartridge rewet ~3 minutes; stabilize ~5 minutes after reimmersion — Halogen Systems Inc.. https://halogensys.com/resources/faq
- 3.BTSB / product guide — allow at least 20 minutes in process water before calibration; Error 31 air-in-sensor — Halogen Systems Inc.. https://halogensys.com/resources/videos
- 4.County of Maui, HI — remote low-pressure site; MP5 started without flow adjustments — Halogen Systems Inc.. https://halogensys.com/resources/case-studies/maui-county-hi
- 5.City of Lakewood, CA — wet-tap / zero-waste MP5 pilot alongside Hach CL17 — Halogen Systems Inc.. https://halogensys.com/resources/case-studies/lakewood-ca
Author: Michael Silveri, Halogen Systems Inc.
Reviewed 2026-08-31. datePublished / dateModified 2026-08-31.