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CHEMTROL® is introducing a new line of PPM sensors for direct measurement of FREE CHLORINE in water in parts per million (ppm) or milligrams per liter (mg/l).

Unlike other chlorine sensors on the market, these sensors are very precise and so reliable that they are covered by a 2-year warranty. They are easy to install and practically maintenance free.

The PPM sensor can be used alone or in combination with an ORP sensor to give the operator complete control of water quality.

New membrane-less sensors are also available for Chlorine Salt Generators.


The new PPM Sensors are now available with the following controllers:
  • CHEMTROL® CH265 Digital Controller for Free Chlorine and pH control with 4-10 mA outputs.
  • CHEMTROL® CH255 Digital Controller for Free Chlorine and pH control.

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 a non-chlorine oxidizer.

Free Chlorine Sensor


  • FREE CHLORINE measurement (same as DPD #1) in ppm or mg/l including:

    - free chlorine (HOCl and OCL-) and

    - chlorinated isocyanurates if present,

  • pH RANGE of 5.5 to 9.5,

  • TEMPERATURE RANGE of 5 to 50C (40 to 120 F)

  • CYANURIC ACID compatible,

  • PRESSURE RANGE up to 3 bars (45 psi),

  • PVC BODY with 1-in MPT fitting,


  • SENSOR CELL with specially designed probe housing,

  • EASY CALIBRATION on startup.


The following standard ranges are available:

- P/N PPM002 from 0.02 to 2 ppm,

- P/N PPM010 from 0.1 to 10 ppm.

Other sensors are available for measurements up to 200 ppm.


Like all CHEMTROL® products, free technical support for the PPM Sensors is based on our experience with over 50,000 installations worldwide. It is always available by toll-free phone, e-mail or fax.

PPM Sensor Cell


In November 1985, Dr. J. Steininger, President of Santa Barbara Control Systems, wrote an article in Swimming Pool/Spa Age called: "PPM or ORP: Which Should Be Used?". This article contributed greatly to the recognition and use of ORP (Oxidation-Reduction Potential) for chemical automation in swimming pools and spas.

ORP has the great advantage of showing the activity of an oxidizing sanitizer - such as chlorine or bromine. Because it shows the effects of pH and cyanuric acid on the effectiveness of the sanitizer and correlates very well with its bactericidal activity, it has become an invaluable tool for proper water sanitation.

However ORP sensors cannot show chlorine concentration. Approximate values of PPM (parts per million) or mg/l (milligrams per liter) values can be derived indirectly through algorithms or more precisely with amperometric sensors. The problem is that these sensors are expensive and difficult to use because they require special pressure cells and expendable reagents.

Unlike conventional amperometric sensors, the new solid state PPM sensor is simple to install and operate because it does not require expensive reagents or special flow cells.

A significant advantage of the PPM sensor is that it's not affected by cyanuric acid (CYA) and/or oxidizers, thereby assuring constant readings of Total Chlorine et all times. Extensive testing has shown that this sensor is very accurate and reliable.


PPM or ORP Paper


Absolutely not!

With the new PPM sensor, the operator can now control either free chlorine or ORP, or both.

The DP High School graph below shows the data log for a CHEMTROL® PC6000 controller using the new solid state sensor for PPM control.

The ORP variations show the diurnal effects of sunlight in a cyanuric-stabilized pool. During the day, the chlorine decomposes more rapidly under UV radiation and the ORP decreases accordingly. At night, the ORP increases again. At all times though, the Free Chlorine level remains stable.

Another advantage of the new PPM sensor is that the two control modes can be used simultaneously with PPM control for chlorine and ORP control for an oxidizer, such as Potassium Monopersulfate, to eliminate chloramines.

For the first time in automated water treatment, the combination of PPM and ORP (Oxidation-Reduction Potential) sensors gives the operator the capability to control either chlorine residual or oxidizing activity. This also makes it ideal for control of a separate oxidizer in the Automated Chloramine Treatment (ACT) program.

The sensor readings can be displayed in parts per millions (PPM) or milligrams perliter (mg/l). They can also be used to control Total Chlorine levels in water.

Daily Variations of ORP
with Cyanuric acid stabilizer


The sensor is designed to be installed in a specially designed flow cell for controlled flow of water.

Install the sensor cell on a bypass line with 3/8” tubing as shown on the right. Make sure that the bypass line is located after the filter and that there is a sufficient but not excessive flow of water. The recommended flow rate is about 30l/h (0.2 gal/min).

PPM Sensor Cell

The chlorine sensor is shipped in a molded foam package containing:

- One (1) chlorine sensor with serial number (needed for warranty),
- One (1) compression fitting with O-ring and compression disk,
- One (1) electrolyte cap with membrane,
- One (1) electrolyte bottle and screw-on filling tip,
- One (1) screwdriver (not needed if pre-wired).

In order to prevent serious damage to the sensor, be careful not to touch the membrane or the electrodes.

To facilitate shipment and storage of the sensor, it is not fitted with the electrolyte cap until ready for installation. The sensor cap must be filled with the electrolyte solution before usage and emptied during storage.

Fill the sensor cap with the electrolyte taking care to prevent air bubbles. Fill the cap to the bottom of the threaded section. Thread the sensor into the cap as far as it will go. Excess air and electrolyte may escape through the hole below the rubber seal. Wipe any excess electrolyte with a soft tissue. Contact your CHEMTROL® dealer If you need additional electrolyte.

Slide the O-ring and the compression disk over the sensor tip up to the compression ring. Finally, insert the sensor with the compression fitting in the flow cell and make it finger tight.


To calibrate the sensor, use a DPD #1 test kit for free chlorine. It is recommended to do the first calibration 2 hours after startup and to repeat it after 24 h. After that, check the calibration as required and generally every 3 to 4 weeks.


There is no special maintenance requirements except to make sure that the membrane remains clean and to remove the sensor to prevent freezing in winter.

The sensor is covered with a standard manufacturer’s warranty of 2 years. Under proper water conditions, the membrance should last 1 to 2 years. Do not reuse the membrane if removed. Replace with new membrane.


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