Important Range Selection Rule for JUMO Ozone and Chlorine Sensors

(Types 202634/45, /47, /50, /52 – 4–20 mA Output)

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**Important Range Selection Rule for JUMO Ozone and Chlorine Sensors

(Types 202634/45, /47, /50, /52 – 4–20 mA Output)**

Why This Needs Attention

JUMO ozone and chlorine sensors (types 202634/45, /47, /50, /52) use an electrochemical / amperometric measuring principle. For this technology, correct selection of the measuring range is critical for obtaining stable and reliable measurement results.

In practice, it is not sufficient to select a sensor range that merely “covers” the expected process concentration.

Important Technical Background

Due to:

• Manufacturing tolerances

• Membrane and electrolyte characteristics

• Application conditions (temperature, humidity, water quality, gas composition)

• Sensor ageing

…the actual sensitivity (slope) of ozone and chlorine sensors can vary significantly from sensor to sensor.

As specified, the slope can vary between 65% and 150% of the nominal value.

What This Means in Practice

The sensors are scaled so that:

• 4 mA corresponds to zero concentration

• 20 mA corresponds to the nominal full-scale measuring range

If a sensor has a higher-than-nominal slope, it will reach 20 mA at a lower actual concentration than expected. This leads to early saturation of the output signal, even though the real process concentration is still within the specified operating range.

Once the signal is saturated at 20 mA, the measurement no longer represents the true concentration.

Risk of Selecting Too Small a Range

If the measuring range is selected too close to the actual maximum ozone or chlorine concentration:

• The output signal may reach 20 mA prematurely

• The measurement will saturate and lose linearity

• Control, monitoring, and alarm systems may receive incorrect values

• This can lead to incorrect process decisions and unnecessary troubleshooting

This situation is caused by range selection, not by a defect in the sensor.

Mandatory Engineering Rule for These Sensors

To compensate for the possible slope variation, the following rule must be applied:

For JUMO ozone and chlorine sensors, the selected measuring range must be at least 1.5 times the maximum expected process concentration.

Calculation Rule

Required measuring range ≥ Maximum expected ozone/chlorine concentration × 1.5

Example

Maximum concentration to be measured:

1.6 ppm

Apply safety factor:

1.6 × 1.5 = 2.4 ppm

Available ranges:

• 0…2 ppm → Not sufficient

• 0…3 ppm → Not recommended (too close to limit)

• 0…5 ppm → Recommended

Correct selection: 0…5 ppm

Engineering Justification

Applying this safety factor ensures that:

• The sensor does not saturate due to high slope variation

• The full 4–20 mA signal remains within the linear working range

• Manufacturing tolerances and application influences are covered

• Long-term stable and reliable measurement is achieved

Note on Resolution

Selecting a higher measuring range slightly reduces theoretical resolution. However, this effect is negligible compared to the risk of signal saturation and incorrect measurement. Measurement reliability always has priority.

Summary

• Ozone and chlorine sensors can have slope variation from 65% to 150%

• This can cause early 20 mA saturation if the range is selected too narrowly

• Always apply a 1.5 safety factor to the maximum expected concentration

• Select the next higher standard measuring range

• This ensures stable, linear, and reliable measurement