How can a pool be monitored and controlled automatically?

For private pools and public swimming pools or large facilities in spas and hotels there are many ways to ensure automated control and monitoring of the water.

For this purpose, there are automatic chlorine dosing systems which permanently check the pH and redox potential value of the pool system with fine sensors. An alarm system warns the owner as soon as preset limit values are exceeded. Peristaltic pumps already integrated in the system create an optimal rearrangement of the water and thus no harmful chlorine gas can form in the device, which could disturb the fine sensor technology.

In addition, there are systems that adjust the optimum dosage by means of photometric measurement of chlorine and pH. Especially in public swimming pools with large volumes, large pumping systems are required to continuously mix the water masses. The automatic dosing systems with photometric control allow the connection of pumping systems of different sizes.

automation control

Chlorine, pH, ORP and Temperature Analyzers

  • Backlit LCD display
  • Nema 4X protection
  • DPD chlorine measurement method
  • Colorimeter diagnostics
  • Reagent reminder
  • Amplified pH/temperature probe
  • Data logging of up to 3500 measurements
  • GLP data for review of calibration information
  • Digital RS485 output
  • Two analog outputs for recording or dosing devices (PCA340)
  • Two dosing relays
  • SPDT alarm relay
  • SPDT system error relay
  • Warning messages
PCA Chlorine ORP Analyzers

The PCA family are process analyzers for the continuous measurement of chlorine, pH (PCA320, PCA330, PCA340) and temperature. These analyzers feature built in data logging, RS485 digital output, dosing relays, and alarm relays packaged in a wall mount Nema 4x enclosure. The PCA340 also features two analog outputs.

This family uses DPD Colorimetric method in which N, N-Diethyl-p-phenylenediamine indicator and a buffer are mixed together with the sample. The resulting chemical reaction causes a magenta color to form in the presence of chlorine. The color intensity is proportional to the concentration. The color intensity is measured photometrically  (light source at a specific wavelength and a photodetector) and converted to chlorine concentration, in mg/L, which is displayed on the front panel. The sampling interval for chlorine measurement is adjustable from 3 to 90 minutes. These analyzers have a dosing relay for the addition of chlorine by a dosing pump or chlorine generator when a reading is below the programmable set point. The technology used by this family for chlorine measurement is the same as that found in portable and benchtop colorimeters providing for consistent results when performing process verification with one of
those types of meters.

The PCA320, PCA330 and PCA340 also utilize the HI1005 amplified pH electrode with a built in pt100 temperature sensor and matching pin to measure both pH and temperature. The built in amplifier and matching pin provide for exceptional performance against any electrical noise generated by pumps and motors. These analyzers have a programmable dosing relay for the adjustment of pH. The dosing relay can be activated by either on/off or proportional control. The PCA340 features two selectable 0-20 or 4-20 mA signal output that are scalable for the transmission of readings to external recording devices. The analog outputs can also be set for dosing and used with dosing pumps that accept a 4-20 mA analog input. The analog outputs can be used for any of the three measured parameters. Through the system setup menu, users have the ability to enable or disable the low and high level of alarms for all parameters. ThePCA family also offers overdosing protection that generates an alarm if something within the system is not working properly. The system will stop processes until the user corrects the error.

Swimming Pools and Chlorine for Disinfection

In regards to swimming pool treatment, disinfection or sanitizing basically means to rid the pool of bather contamination, destroy bacteria, and control nuisance organisms like algae, which may occur in the pool, filtration equipment, and piping. Of the many techniques used (chlorine, bromine and iodine dosing systems), chlorine is the most common.


Chlorine is a strong oxidizing agent that destroys mostly organic pollutants and bacteria and can combine with nitrogen containing compounds, forming chloramines. When dosing chlorine for disinfection, only a portion of the dosed chlorine remains active to actually continue the disinfection process. When free chlorine combines with a nitrogen containing compound it becomes a less efficient disinfectant called chloramines. The addition of these two parts gives total chlorine. The target is to keep freeb and total chlorine equal, and thus to maintain the combined chlorine concentration chloramines) near zero. The presence of chloramines is not desired because of the distinctive ‘swimming pool’ smell caused by combined chlorines like di-chloramines. Beside this unpleasant odor,
chloramines can irritate the eyes and the mucous membranes. Commercial chlorine for disinfection may be available as a gas (Cl₂), a liquid like sodium hypochlorite or bleach (NaOCl) or in a solid state like calcium hypochlorite, chloro-hydantoins or chloro-cyanuric acid compounds. These compounds, once dissolved in water do establish equilibrium between the hypochlorous acid (HOCl) and the hypochlorite ions (OCl¯). Although both forms are considered
free chlorine, it is the hypochlorous acid that provides the strongest disinfecting and oxidising characteristic of chlorine solutions; the amount of hypochlorous acid in chlorinated water dependends upon the pH value of the solution. Changes in pH value will affect the HOCl equilibrium in relation to the hydrogen and hypochlorite ion; HOCl decreases and OCl¯ increases as pH increases. At a low pH, almost all the free chlorine is in the molecular form HOCl and at a pH of around 7.5, the ratio between HOCl and OCl¯ is 50:50. Since the ionic form OCl¯ is a slow acting sanitizer while the molecular HOCl is a fast acting, it is
important to regularly measure the pH. As a general rule a pH of about 7.2 is recommended to maintain fast acting disinfection conditions.

BL120 and BL121

pH/ORP Swimming Pool and Spa Controllers with Built-In Chemical Feed Pumps

The BL120 and BL121 are an all-in-one solution for automatic control of pH and chlorine levels in swimming pool, hot tub, and spa water.

The BL120 and BL121 Swimming Pool Controller is a complete system designed for maintaining swimming pool, hot tub, and spa disinfection water quality. These controllers are available in two configurations. The basic version is the inline model which allows for direct installation of probe and chemical injection fittings into existing piping. A panel mounted version with a bypass flow cell is also available. The bypass flow cell allows for calibration and maintenance
of the probe without having to shut down the recirculation pump.

The BL120 and BL121 use a multiparameter digital HI1036-1802 probe that incorporates pH, ORP, and temperature sensors along with a matching pin. All readings are measured within the probe and the data transferred to the controller by a digital connection. Both a digital connection and matching pin provide
for stable, reliable measurements. Without these two components, electrical noise from recirculation pumps and ground loops can interfere, causing erratic readings and premature probe failure.

These controllers have two built-in peristaltic chemical feed pumps that are proportionally controlled with adjustable flow rates. One of the pumps is used to dose acid while the otheris used to dose chlorine. The effectiveness of the available chlorine, as determined by ORP, is inversely related to the water’s pH value.
A pool with a fixed concentration of chlorine will show a decrease in ORP as the pH of the water increases. The BL120 and BL121 utilize a dosing consent feature that will not dose chlorine until the pH value is first corrected, since it is possible to have a low ORP value even though there is sufficient chlorine. The dosing consent feature will prevent chemical wastage and having a higher chlorine concentration level than desired.

For compliance monitoring, BL120 and BL121 have a built-in datalogger. Measurement readings are logged every 10 seconds with a new log starting for each new day or when the instrument is calibrated. Logged data include pH, ORP, and temperature values, last calibration data, setup configuration, and any
event data.

For BL121 models, three 4-20 mA analog outputs are available for users that wish to connect to an external chart recorder or datalogger to monitor any of the three measured parameters. The outputs are scalable, offering increased flexibility and better resolution as needed.

Additional features of the BL121 include LED indicators for dosing, meter status and service, real-time graph display, programmable alarms, and password protection.

These controllers are an all-in-one solution for automatic control of pH and chlorine levels in swimming pool, hot tub, and spa water.