Chlorine makes drinking water safe — and it also makes chlorine-handling rooms some of the most toxic confined spaces in a water-treatment plant. When a cylinder or ton container leaks, the gas attacks the eyes, lungs and airways within seconds. The same plants often use sulfur dioxide (SO2) to dechlorinate the water, and SO2 is corrosive and irritating in its own right. In seawater desalination plants across the Middle East — a region where water treatment is strategically critical — reliable chlorine and SO2 gas detection is not optional.

This guide explains why chlorine and SO2 are so hazardous, the exposure limits that set your alarms, where leaks occur, how electrochemical sensors perform, and how fixed and portable detection should be combined. Regulatory framing follows EAC (TR CU 012/2011), ATEX and IECEx.

Table of Contents

Why Chlorine and SO2 Are So Dangerous

Chlorine (Cl2) is a yellow-green, pungent gas that is heavier than air and settles at low points. It reacts with moisture in the respiratory tract to form acids, causing immediate irritation and, at high concentrations, pulmonary oedema. Because it is heavier than air, a leak in a chlorine room fills from the floor up — exactly where a worker’s breathing zone is.

Sulfur dioxide (SO2) is a colourless, sharp, choking gas, also heavier than air. It irritates the upper airways and is especially dangerous to asthmatics. Both gases are detected by electrochemical sensors in the ppm range.

Gas	OSHA PEL	ACGIH TLV (TWA/STEL)	NIOSH IDLH
Chlorine (Cl2)	1 ppm (ceiling)	0.5 / 1 ppm	10 ppm
Sulfur dioxide (SO2)	5 ppm	0.25 / 0.5 ppm	100 ppm

Because the permissible limits are so low — sub-ppm for chlorine — even a small cylinder leak demands an immediate alarm and ventilation response. Low alarms are typically set at or just above the TLV, with high alarms well below the IDLH.

Where Leaks Happen in Water Treatment

Chlorine storage and ton-container rooms: cylinder connections, fusible plugs, regulators and manifolds.
Chlorine evaporators and feeder panels.
Sulfur dioxide storage and dechlorination skids.
Hypochlorite and chlorine-dioxide generation rooms.
Pumping stations and filter galleries where residual chlorine off-gases.
Seawater desalination intake and pre-treatment buildings in the Gulf, where chlorine is dosed continuously to control biofouling.

Both gases are heavier than air, so detectors are mounted low — near the floor and at worker breathing height — and close to the most likely leak points (valves, couplings, change-out areas).

How Electrochemical Cl2 and SO2 Sensors Work

Chlorine and SO2 are measured with highly specific electrochemical sensors. The sensor produces a current proportional to the target gas, with cross-sensitivity filters designed to reject other plant gases. These sensors are sensitive (sub-ppm resolution is required), but they are also fragile: they are affected by high humidity, by cross-sensitive compounds, and by age.

Service discipline matters. Bump test before each shift, calibrate at least every six months with certified Cl2 or SO2 span gas, and replace the cell when calibration drifts. In a corrosive desalination or chemical-dosing room, protect the inlet with a dust and moisture filter and service it on a short interval.

Fixed vs Portable Detection

A chlorine room or SO2 storage area is a permanently hazardous location and needs fixed detection. The HD9300 fixed transmitter mounts low near the leak source, sends 4–20 mA / RS485 / MBUS to the DCS or plant controller, and switches dual alarms and forced ventilation. With an Ex d IIC T6 Gb enclosure and –40 °C to +70 °C operation it suits cold northern plants and hot Gulf desalination buildings alike.

See the fixed detector: HD9300 Fixed Gas Detector with LCD Display

See the controller: HD6000 Fixed Gas Alarm Controller (2–128 channels)

When technicians change cylinders, enter a chlorine room or respond to an alarm, they need portable detection. The HD600 single-gas portable provides a dedicated Cl2 or SO2 channel on a clear LCD for the worker who goes into the leak area.

See the portable detector: HD600 Single-Gas Portable Gas Detector

For a small chlorine feeder room, the HD6100 single-point wall-mounted all-in-one controller combines detector, display and local alarm in one enclosure.

Hangwei Chlorine & SO2 Detection Solutions

Henan Hangwei Sensing Technology Co., Ltd. has more than 20 years of manufacturing experience, 20 patents and 10 production lines. We supply fixed and portable chlorine and SO2 detectors, controllers and custom sensor packages for EAC, ATEX and IECEx-oriented water and wastewater projects, with OEM/ODM and global distributor recruitment.

Fixed Cl2/SO2 detection: HD9300, Ex d IIC T6 Gb, –40 to +70 °C, 4–20 mA/RS485/MBUS, dual relays, IR remote calibration.
Portable Cl2/SO2 detection: HD600 single-gas instrument with LCD display.
Controllers: HD6000 from 2 to 128 channels; HD6100 single-point all-in-one.
Custom packages: sub-ppm Cl2/SO2 sensors, humidity-protected inlets and detector-plus-controller systems.

FAQ

What is the IDLH for chlorine?

10 ppm, per NIOSH. OSHA’s permissible ceiling is 1 ppm. Because the limits are so low, low alarms are set near the 0.5–1 ppm range and any high alarm must trigger ventilation and evacuation.

Why are chlorine detectors mounted low?

Chlorine is about 2.5 times heavier than air, so it pools near the floor. Mounting low catches the gas before it reaches breathing height.

Can a four-gas detector measure chlorine?

Standard four-gas instruments measure H2S, CO, O2 and LEL — not Cl2 or SO2. For chlorine or sulfur dioxide, use a dedicated single-gas portable such as the HD600, or a fixed transmitter fitted with the specific electrochemical cell.

What does SO2 do in a water-treatment plant?

Sulfur dioxide (or bisulfite) is used to dechlorinate treated water before discharge. It is itself an irritating, corrosive gas, so the SO2 storage and dosing area needs the same detection discipline as the chlorine room.

How often should chlorine sensors be serviced?

Bump test before each shift; calibrate at least every six months with certified span gas. In humid or corrosive desalination service, shorten the interval and inspect the inlet filter.

Are these detectors certified for explosive atmospheres?

Yes. The HD9300 fixed detector is Ex d IIC T6 Gb and can be supplied to support EAC (TR CU 012/2011), ATEX and IECEx documentation.

Get Expert Guidance

Chlorine and SO2 monitoring in water treatment and desalination is a life-safety system with sub-ppm alarm set-points. Hangwei Sensors designs and manufactures fixed and portable Cl2/SO2 detectors and controllers for water, wastewater and desalination clients across Russia, the Middle East and worldwide. Contact our engineers for detector placement, alarm set-points, EAC/ATEX documentation and OEM or distributor terms.

Email: hw@hangweisensors.com | Phone: +86 371 6667 0886

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