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Free Chlorine Control

A simple and inexpensive controller for:

  • Residential Pools and Spas
  • Commercial Pools and Spas
    • Hotels, Motels
    • Health Clubs
    • Apartments Buildings
    • Condo Associations.
  • Industrial Water Treatment.

The CHEMTROL® 255 PPM/pH Digital Controller introduces the new sensor for Free Chlorine control in automated water treatment.

A microprocessor-based digital controller, it is designed to maintain both free chlorine PPM and water pH in high-end residential pools, small commercial pools and spas and light industrial applications.

Unlike other chlorine sensors on the market, these sensors are easy to installl and maintain, very precise and so reliable that they are covered by a full 2-year warranty.

For better water management results, it is recommended to use ORP control of chlorine oxidizing activity and PPM monitoring of free chlorine. Alternatively, PPM control can be used for control of the sanitizer and ORP control for an oxidizing agent, such as ozone.


The new Free Chlorine Sensor is a solid state sensor with a selective membrane that provides direct readings of Free Chlorine in water at concentrations ranging from 0.1 to 10 ppm (parts per million) or mg/l (milligrams per liter).

These are true readings of Free Chlorine corresponding to the DPD #1 test.

A significant advantage of this sensor is that it's not affected by cyanuric acid (CYA) stabilizer and/or oxidizers, thereby assuring constant Free Chlorine levels at all times. Extensive laboratory and field tests have shown it to be very accurate and reliable.



As shown on the graphic data log on the right, the new sensor gives the operator precise and reliable control of Free Chlorine under the most difficult conditions.


The CHEMTROL® 255 is the only easy-to-use controller offering Free Chlorine and pH control for pools, spas, cooling towers, industrial water treatment, etc.



Designed for affordability, simplicity of installation, ease of use and long-term reliability, it includes features normally not found on this type of controller, such as:

• adjustable setpoints,

• adjustable high and low alarms,

• adjustable overfeed safety timers and

• adjustable proportional feed control.



Unlike conventional amperometric or colorimetric sensors, the PPM sensor is easy to install and operate. It does not require a special pressure flow cell and/or expensive and expendable chemical reagents. And best of all, it’s maintenance free.

Installation of the CHEMTROL® 255 is as easy as 1-2-3.

  1. CONTROLLER - Connect the controller to the 110 or 230V output of the Timer in parallel to the recirculation pump.  This will prevent accidental feeding of chemicals when the pump is not running. Make sure to follow the electrical code.
  2. SENSORS - Install the PPM and pH sensors on the recirculation line or on the optional bypass line.  The two sensors can be mounted directly on the main recirculation line with 2" PVC tees, as shown on the schematic.  For recirculation lines over 2" in diameter lines, the sensors should be mounted on a 1/2" bypass line.
  3. CHEMICAL FEEDERS - Connect the chemical feeders to the 110 or 230V controller outputs, according to the manufacturer's instructions, making sure to inject the chemicals downstream of the sensors.  For a bromine or chlorine erosion feeder, install a corrosion-resistant solenoid valve on the intake line of the feeder.  The controller is compatible with most ozonators and electrolytic generators.

Flow Cell Assembly

For best operation, it is recommended to install the sensors on a bypass line with a Flow Cell Assembly (FCA) as shown below.



The FCA includes a see-through cell with a plastic cover that regulates the flow of water and allows the operator to see the tip of the sensors.

The cell includes the following:

  1. two 1/2" FPT fittings for the connections to the bypass line,
  2. two 1/2" FPT fittings for mounting the sensors,
  3. a spigot for water testing,
  4. a bleed line to prevent air lock in the cell,

In addition it is also recommended to install a safety switch on the bypass line to prevent accidental feeding of chemicals.


The CHEMTROL® 255 PPM/pH Digital Controller automatically maintains both sanitizer PPM and water pH in pools and spas.

All operating functions are clearly labeled and accessible from the front panel, with many safety and easy-reset features not found on other controllers:

  • pH calibration from 7 to 8,
  • PPM setpoint from 0.1 to 10.0 ppm or mg/l (or 0.01 to 2.0 optional),
  • pH setpoint from 7 to 8,
  • Chlorine and pH feed modes: off, manual, automatic,
  • PPM and pH proportional control bandwidth from 0 to 100% of the setpoint,
  • Chlorine high and low alarms from 400 to 900 mV,
  • pH high and low alarms from 4 to 10,
  • Chlorine overfeed safety timer from 0 to 999 minutes,
  • pH overfeed safety timer from 0 to 99 minutes.

To select a function, simply press PPM or pH and then the desired function.  The display shows the current function value.  Use the UP or DOWN arrows to adjust to the desired value.


The standard factory-set Chlorine control level is 1.0 ppm or mg/l, based on WORLD HEALTH ORGANIZATION (WHO) and NATIONAL SPA AND POOL INSTITUTE (NSPI) values for prevention of germs and bacteria. A range of 0.01 to 2.0 is also available for low chlorine level applications.

The standard factory-set pH control level is 7.5. Below 7.4, the water causes eye irritation and corrosion of equipment. Above 7.6, it also causes eye irritation plus water cloudiness, scaling, stains and reduced sanitizer efficiency.

To change the control levels, the operator presses PPM or pH and then SETPOINT. Use the UP or DOWN arrows to adjust the PPM and pH setpoints. To save the new setpoint value and exit, press SETPOINT again.

The controller then automatically activates the chemical feeders for sanitizer and acid (or soda) feed, as required to maintain the set PPM and pH levels - regardless of fluctuations in bather load, contamination and weather.  Like a thermostat maintaining a constant temperature, it prevents overfeeding or underfeeding, both of which are detrimental to good water quality.


When there is an alarm condition, the corresponding display flashes the alarm value:

  • PPM out-of-range,
  • pH out-of-range,
  • PPM overfeed,
  • pH overfeed.

The display stops flashing when the condition is corrected. To reset the overfeed timers, turn the feed mode to off and then back to Manual or Auto.


The ideal values are set as default values.  They can be adjusted by the operator at any time.  To reset all factory default values, press PPM and pH for 5 seconds.



Adjustable from 0.1 to 10.0 ppm (mg/l) or 0.01 to 2.0 (specify).


Adjustable from 4.0 to 10.0


Solid state electrode with 10-ft (3 m) cable


Glass electrode with 10-ft (3 m) cable


PPM (3 digits) and pH (2 digits) LEDs


Manual, automatic, proportional


Setpoint, calibration, limits, feed rate


LEDs for feed mode, feed status, alarms


Audio, visual and remote alarms
Out-of-range limits and overfeed safety timers


Polystyrene 9x7x4 in. (22x18x10 cm)


110V, 230V or 24 VAC (specify)


PVC tees for 1-1/2" or 2" line


5 years on electronic module
1 year on all other parts


12x12x12-in (30x30x30 cm) carton
8 lbs (3 kg) without pumps
16 lbs (6 kg) with 2 chemical feed pumps


The CHEMTROL® 255 controller is based on 20+ years of experience in pool and spa automation with over 25,000 installations worldwide. Featuring a 5-year electronics warranty, it actually pays for itself in 1 to 2 years through cost savings in operating and maintenance expenses.

Savings continue to accrue from the more efficient use of chemicals, avoiding over- and underfeeding, and from lower labor costs for water testing and repairs of damaged equipment. In particular, commercial operators save on specialty chemicals, customer complaints and health department shutdowns.


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