Overview
This article explains how the DIS sensor is used in the analyzer: the measured oxidant depends on the parameter selected in the instrument (chlorine / free chlorine, chlorine dioxide, or ozone). It also lists which influences apply to all of these measurements and which apply only to chlorine.
Applies to: ZIRKON® DIS, Neon® DIS, Neon® Multi, Krypton® DIS, and Krypton® Multi systems that use the DIS amperometric measurement.
What does the DIS sensor measure?
The sensor is referred to as DIS. Depending on the measurement parameter configured in the analyzer, the DIS channel reports chlorine (free chlorine), chlorine dioxide, or ozone. The instrument setup must match the oxidant and units in use (see channel configuration, factory defaults, and application notes for your system).
For chlorine (free chlorine), the DIS sensor responds to hypochlorous acid (HOCl), the species commonly meant in practice when discussing free chlorine. For chlorine dioxide or ozone, the DIS measurement follows the parameter selected for that oxidant.
What factors influence the measurement?
The following apply to all DIS parameters (chlorine, chlorine dioxide, and ozone): the signal must be interpreted in the context of stable hydraulics, temperature, and a clean measuring surface.
Variables of influence (all parameters)
Flow — The DIS measurement is amperometric: a differential current is produced at a measuring electrode with an applied potential. That requires a continuous and stable flow of sample water at the sensor.
Kuntze reduces flow variability using the Argon Stabiflow® sensor assembly, which regulates flow to a stable 30 L/h (8 GPH) and limits process-side changes that would otherwise alter the flow rate through the sample line.
Fouling — All on-line analyzers can foul depending on process conditions. Particulates and organic or inorganic deposits may coat the sensor and obstruct the measurement path.
Kuntze’s patented ASR® (Automatic Sensor Cleaning) technology reduces sensor fouling and can be triggered automatically up to seven times per week.
Temperature — Temperature and temperature changes affect signal strength.
Kuntze measures temperature with a built-in Pt 100 probe and applies temperature compensation. The same Pt 100 element also serves as the flow indicator.
pH (chlorine / free chlorine only)
pH dependence applies only to chlorine (free chlorine), not to chlorine dioxide or ozone. For free chlorine, the DIS sensor primarily tracks the HOCl fraction; as pH rises, a larger share of free chlorine exists as hypochlorite (OCl⁻), which changes the signal even when total free chlorine is unchanged.
Kuntze uses a continuous pH measurement to compensate for signal gain or loss on the DIS free-chlorine measurement when pH fluctuates. This pH compensation is not required for the same underlying physical reasons when the parameter is set to chlorine dioxide or ozone.
HOCl and OCl⁻ across pH (chlorine / free chlorine only)
The diagram below applies only when the analyzer is set to measure chlorine (free chlorine). It shows how the distribution between HOCl and OCl⁻ shifts with pH. The two curves cross near about pH 7.5 (roughly 50% HOCl and 50% OCl⁻). Example points on the HOCl curve:
- pH 7 — about 77% HOCl
- pH 8 — about 25% HOCl
- pH 9 — about 3% HOCl
That is why pH-aware compensation matters for free chlorine: from pH 7 to 9, the HOCl fraction collapses even when total free chlorine is similar—so the raw amperometric response to HOCl changes strongly with pH unless compensated.
Related Articles
- ZIRKON® ONE DIS / Zirkon® DIS Sensors: No raw signal - what to do?
- Does ZIRKON® DIS / ZIRKON® ONE DIS Need a Zero-Point Calibration?
- Chlorine Dioxide Measurement - Choosing the Right Kuntze Parameter (ClO2 vs Cl2)
- Why are Argon Stabiflow® / STABIFLOW® ONE important?
- Argon StabiFlow® Cleaning and Maintenance (cell diagram)
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