pH/ORP/Temperature swimming pool controller with analog outputs BL121-20

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Control with Confidence

  The BL121 Swimming Pool Controller is a complete system designed for maintaining swimming pool, hot tub, and spa disinfection water quality. The BL121 is available in two configurations. The basic version is the in-line model which allows for direct installation of probe and chemical injection fittings into existing piping. The other version is a panel mounted version with a bypass flow cell. The bypass flow cell allows for calibration and maintenance of the probe without having to shut down the recirculation pump. Additional features of the BL121 include LED indicators for dosing, meter status and service, real-time graph display, programmable alarms, and password protection.

Controller Overview




Multiple Configurations

The controller is available in one of two configurations: an in-line model which allows for direct installation of the probe and chemical injection fittings into existing piping (BL121-10) or a panel mounted system with a bypass flow cell (BL121-20). The bypass flow cell allows for calibration and maintenance of the probe without having to stop the recirculation pump. 



Digital Combination Probe

The HI1036-1802 is a digital probe that measures pH, ORP, and temperature. This probe also incorporates a potential matching pin. The pin is considered the “earth ground” connection and is used to prevent ground loop effects from causing erratic readings and damage to the system. 


Dosing & Control



Peristaltic Dosing System

The BL121 is equipped with two peristaltic dosing pumps with replaceable chemical resistant tubing. When using a diaphragm pump, chlorine gas formed from outgassing can collect in the pump head resulting in the pump losing its prime; the buildup of chlorine gas is not a problem with peristaltic pumps that use rollers and tubing.

Proportional Pump Control

The BL121 features proportionally controlled dosing pumps. Based on the sensitivity of the process to chemical addition, these controllers allow the user to adjust a proportional band. This setting determines the amount of time that the pumps are dosing as a percentage of the deviation from the set point allowing for very fine control in maintaining the desired set point.

Adjustable Flow Rate

The flow rate from the dosing pumps is adjustable from 0.5 to 3.5L/h. Larger bodies of water require more chemical to be dosed than small bodies since it takes more chemical to realize a change in the reading. The adjustable flow rate, like the proportional band, allows for better control in maintaining a desired set point.

ORP Dosing Consent

With chlorine disinfection there is an inverse relationship between pH and ORP. As the pH level increases, the ORP level decreases. The BL121 utilizes 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 prevents wastage of chemicals and avoids a higher than necessary chlorine concentration.


Connections & Display




Circulation Pump Monitoring

An inline flow switch or a mechanical relay connected to the recirculation pump power source may be connected to the hold input of the BL121. With no flow or when no power is applied to the recirculation pump, the hold circuit disables the dosing pumps. This will prevent any dosing of chemical when there is no movement of water in the system.


Programmable Alarm System

Hanna controllers allow users to enable or disable the low and high level of alarms for all parameters. When an alarm is activated, all dosing will stop. The alarm system also offers overdosing protection if the value is not corrected within a specified time interval then the meter will go into alarm status



Multifaceted Display

The versatile display of the BL121 allows for three screen modes. The LCD can display all three parameters at one time, a 3-second cycle of single parameters, or a real-time plot screen with options for parameter selection, zooming, and log recall.


Multicolored LED Indicators

The BL121 offers multiple LED indicators for status, servicing, and pump operation. The STATUS LED changes color based on the operational state; green when setting are in range, yellow when user intervention is required, and red for problems. The Service LED indicates any alarms and process errors experienced by the controller.


Communication & Security ​




Automatic Logging

The readings for each parameter are automatically logged every 10 seconds. A new log is started each time the instrument is calibrated or at the start of a new day. Logged data include pH, ORP, and temperature values, last calibration data, setup configuration, and any event data. Each log is saved as a .csv file for easy transfer.


Analog Output

The BL121 offers three 4-20 mA outputs for pH, ORP, and temperature. Each output may be disabled or connected to an external recording device and is adjustable to be proportional to the measured value. For greater flexibility and better resolution, the analog output can be scaled; users can define any two points within a parameter range to correspond to the analog output span. This adjustment allows better resolution in the range of interest. 


USB Connectivity

For review and storage the users can easily transfer data to a PC using a flash drive and the USB port.


Password Protection

Hanna’s BL121 controller features a password protection solution that offers restricted access to calibration, setup, and review of logged data. The password can be set and enabled/disabled during general setup of the instrument.




pH Range

0.00 to 14.00

pH Resolution

0.01 pH


±0.05 pH


pH buffer calibration: automatic, two-point (4.01, 7.01, 10.01 pH) pH process calibration: one-point, manual input

Temperature Compensation

automatic, -5.0 to 105.0ºC (23.0 to 221.0ºF) for pH

ORP Range

±2000 mV


1 mV


±5 mV


ORP (mV) calibration: one-point, manual input

Temperature Specifications Range

-5.0 to 105.0ºC (23.0 to 221.0°F)


0.1°C (0.1ºF)


±1ºC (±1.8ºF)

pH Dosing

proportional with adjustable set point and proportional band delay to start at power-on and overdosing protection

ORP Dosing

proportional with adjustable set point and proportional band delay to start at power-on and overdosing protection pH dosing interlocked


intuitive alert system based on LEDs alarm filtering options alarm relay control based on user setup

Alarm relays

output SPDT 5A/230 VAC activated by pH/ORP/temperature selectable alarm conditions Analog outputs 4 to 20 mA, sourcing, configurable output impedance ≤ 500 Ohm accuracy < 0.5 % FS galvanically isolated up to 50 V relative to earth ground

Auxiliary output

low level in acid/base tank (contact open) low level in chlorine tank (contact open) hold input (contact open)

Pump Control

automatic and manual modes adjustable flow rate from 0.5 to 3.5 L/h

Log Feature

automatic logging of pH, ORP, and temperature measurements, GLP and events including alarms, errors and power failure capacity for 60 days with 10 second sampling interval all data .csv files are transferred by USB flash drive

Password Protection

setup, calibration and log recall options features are password protected

Storage Interface

USB GLP calibration information including date and time for pH and ORP sensors Digital Probe input galvanic isolated digital input HI1036-1802 pH/ORP/temperature/matching pin combined probe with DIN waterproof connector

Power Supply

100 - 240 VAC

Power Consumption

10 VA Environment 0 to 50 ºC (32-122 ºF); max 95% RH non-condensing


245 x 188 x 55 mm (73 mm with pumps); 9.6 x 7.4 x 2.2” (2.9” with pumps)


1700 g (60 oz)

Ordering information

BL121-10: Swimming Pool & Spa Controller with HI1036-1802 combined pH/ORP/temperature digital electrode, with probe fittings and chemical injectors