At Brass Monkey, we understand that safety is a top priority when it comes to chemical dosing in pool and spa environments. We've received a few customer queries regarding the Hanna Dosing Unit—particularly around the use of chemicals like sodium hypochlorite (chlorine) and acid. We're happy to share the following information, provided by Hanna Instruments, which outlines the extensive safety features designed to minimise risk and ensure peace of mind.

Why Are Two Chemicals Used?

Sodium hypochlorite (chlorine) is a widely used and highly effective pool sanitiser. However, it raises the pH of the water, which reduces chlorine’s effectiveness. To correct this, a liquid acid is introduced to bring the pH back to an ideal level—around 7.0 pH. This dual-chemical approach is standard in pool systems around the world.

Built-in Safety Features of the Hanna Pool Controller

The Hanna Pool Controller has been engineered with a range of features specifically designed to prevent the accidental mixing of chemicals and minimise any associated risks:

Consent Dosing:
The controller ensures that chlorine and acid are never dosed at the same time. Only once the pH has been corrected does the controller allow chlorine to be added, and vice versa.
Low Volume, Proportional Dosing:
Chemicals are dosed in extremely small amounts—often drop by drop. The controller adjusts the dosing rate based on how close the water is to the target value, significantly reducing the volume added and the potential risk in the event of a leak.
Automatic Safety Shut-Off:
The system can be programmed to shut off after just 30 minutes of continuous operation. This prevents over-dosing and mitigates risks if a chemical reservoir runs dry or if there’s a system issue.
Tubing Safety & Design:
In rare instances where tubing might split, Hanna’s system design—with separate dosing times, low volumes, and automatic shut-down—prevents both chemicals from being dispensed simultaneously. In over 1,000 units installed across Europe, reports of split tubing have been extremely rare, and no cases have involved simultaneous failure of both chemical lines.
Sensor Innovation:
Hanna’s sensors are purpose-built for pool environments. They feature built-in signal amplification to avoid electrical interference and a unique pH configuration to prevent incorrect dosing even if a sensor fails.

Why We Recommend Sulphuric Acid Over Hydrochloric Acid

Although hydrochloric acid is commonly used in commercial pools due to its low cost, combining it with chlorine can produce hazardous chlorine gas (Cl₂). Hanna strongly recommends the use of sulphuric acid instead. When used correctly, sulphuric acid reacts with sodium hypochlorite to produce hypochlorous acid (HClO)—a much safer by-product.

Reaction Comparisons:

Sulphuric Acid:
2NaOCl + H₂SO₄ → Na₂SO₄ + 2HClO (safe)
Hydrochloric Acid:
NaOCl + 2HCl → Cl₂ + H₂O + NaCl (produces chlorine gas)

Due to the low volumes, dilute concentrations, and carefully controlled dosing environment, the likelihood of chlorine gas formation in standard pool systems is extremely low—even in rare fault scenarios.

The Hanna Pool Controller is a highly refined system trusted by operators across Europe. Its intelligent dosing logic, precision control, and emphasis on safety set it apart in the industry. We continue to recommend it with confidence for any application where chemical safety is paramount.

If you have any questions or would like to know more about how the system works, please don’t hesitate to get in touch with our team or Hanna Instruments.