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Hydrolab HL4 Multiparameter Sonde

New Hydrolab Platform: Reliability, Ease-of-use, Metadata

The Hydrolab HL4 is the next generation multiparameter water quality sonde from OTT Hydromet.  Know the instrument is working correctly and troubleshoot quickly. Streamline calibration tasks and produce valid, scientifically defensible conclusions.

Use with the Hydrolab Surveyor HL for attended monitoring, or for unattended continuous monitoring applications, the HL4 has on-board data logging and dedicated communications modules for easy integration with external loggers and telemetry.

 Hydrolab HL4 Intro Video 

  • Attended, Unattended
  • Temperature, Conductivity, Depth, pH, Dissolved Oxygen (LDO), Turbidity, ORP, Clorophyll a, Blue-green algae, Rhodamine, Ammonium, Nitrate, Chloride
  • The next generation multiparameter water quality instrument
  • SDI-12, RS-485, RS-232, TTY, USB
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  • Self-monitoring system reports the status of the instrument, flags the data, and shows the user where the problem is with assistance on how to solve the issue
  • User-scheduled calibration and maintenance intervals indicate when they are due
  • Guided and semi-automated calibration routines lead the user through the calibration process
  • Calibration results are stored with date and time, calibration type, user identification, and user notes
  • Check Calibration process allows the user to verify calibration and store the results
  • Calibration reports contain information about previous calibrations and calibration checks
  • Sensor status is saved with every line of data and is contained in the log file
  • Dedicated communications modules allow easy integration with data loggers and telemetry systems
  • Compatible with the Surveyor HL – a fully IP67 handheld designed for field use with a full-color screen that is visible in direct sunlight

For measurement of popular water quality parameters in:

  • Freshwater rivers and streams, lakes and reservoirs, and groundwater wells
  • Salt or brackish water bays, estuaries, and near-coastal areas
  • Attended monitoring, continuous unattended and real-time monitoring

Click here to visit the download site for Hydrolab sensor firmware

Communication Module Wiring - 6748600, 9039600, 9039700, 9039800, 9312900
HL Series Sensors - Bedienungsanleitung | User Manual | Manuel d'utilisation | Manual de usuario
HL Series Sonde - Bedienungsanleitung | User Manual | Manuel d'utilisation | Manual de usuario
HydroLab HL Series - ISE Sensors - FAQs
Compilation of the most important FAQs about ISE sensors for the HydroLab HL series of water quality sondes
Hydrolab HL4 Overview Video
Hydrolab HL4 Overview Video
Hydrolab Operating Software
Hydrolab Operating Software 64 bit install
Hydrolab Operating Software
Hydrolab Operating Software 64 bit install
Hydrolab Standard Methods Sheet
Leaflet - Multiparameter-Sonde Water Quality Hydrolab HL Series
Operate the Hydrolab SDI-12 / Modbus / RS232 TTY Communications Module (HL Series Sonde)
Video Hydrolab HL4/HL7 - Conductivity I Calibration Tutorial Series - EN
Video Hydrolab HL4/HL7 - Dissolved Oxygen - Calibration Tutorial Series - EN
Video Hydrolab HL4/HL7 - pH - Calibration Tutorial Series - EN
Video Hydrolab HL4/HL7 - Temperature - Calibration Tutorial Series - EN
Video Hydrolab HL4/HL7 - Turbidity - Calibration Tutorial Series - EN
Water Quality Solutions Overview Video
Whitepaper: QA QC (Quality assurance and control) in hydrometry
Whitepaper: Turbidity measurements
Turbidity Whitepaper
Download PDF
Sensors Temperature sensor and up to four additional sensors plus depth and ORP
External 6 ... 30 VDC (12 VDC nominal) applied to the communications module,
12 VDC: 250 mW average, 19 W peak
Internal (Optional) Internal alkaline D-cell battery, non-rechargeable.
Sleep mode at 12 V 20 mA
Communications Hydrolab communications modules: USB, SDI-12, RS232, RS485, or TTY
Memory 4 GB of internal memory; 1 second interval minimum
User Interface
PC Software Hydrolab Operating Software
Handheld (Optional) Hydrolab Surveyor HL
Sonde Depth Rating 200 m (656 ft)
Maximum Deployment Cable 200 m (656 ft)
Diameter w/o bumpers 4.4 cm (1.75 in.)
Diameter with bumpers 5.3 cm (2.1 in.)
Weight with internal battery pack (IBP) and storage/calibration cup 2.2 kg (5 lb)
Length - no IBP, standard sensor guard 51.4 cm (20.25 in.)
Length - no IBP, extended sensor guard 66.4 cm (26.125 in.)
Length - IBP and standard sensor guard 62.2cm (24.5 in.)
Length - IBP and extended sensor guard 77.8 cm (30.625 in.)
Environmental conditions
Operating temperature -5 ... 50 °C (23 ... 122 °F), non-freezing
Storage temperature 1 ... 50 °C (34 ... 122 °F)
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What does ISE actually mean?

An ISE is an Ion-Specific Electrode. It converts the activity of a specific ion dissolved in water to an electrical potential.

What are ISE sensors best suited for?

The ISE sensors are best suited for spot monitoring activities. These sensors drift more than other sensors and are not well-suited for long-term deployment. Do frequent QA/QC and calibration on ISE sensors, especially ammonium and nitrate, if they are to be used for extended deployment.

What is the life expectancy of ISE sensors?

Ammonium and Nitrate sensor tips will last from three to six months whether they are stored on a shelf or installed in a sonde. Chloride sensor tips may last a year or more in the same conditions.

What is the maximum submersion depth of ISE sensors?

Maximum of 15 meters for all ISE sensors. If deployment exceeds 15 meters, the ISE sensor must be removed and a dummy plug (included with sensor) must be installed in the sensor adaptor or the sensor will be damaged.

What type of water should ISE sensors be used in?

Use Ammonium and Nitrate sensors in fresh water whose specific conductivity is less than 1000uS/cm. Sodium is a major interference for Ammonium and Chlorate ions are major interferences for Nitrate activity. Chloride sensors suffer interferences when the concentrations of bromide, iodide, cyanide, silver, and sulfide ions are much higher than the chloride ion concentration.

What are the best practices for calibrating the HL series ISE sensors?

  • Rebuild the reference if needed, and calibrate conductivity and pH before calibrating any ISE sensor.
  • Calibrate ISE sensors with standards that either bracket your expected measurements or that are close to what you expect to measure.
  • The ISE sensors need frequent maintenance and calibration compared to other water sensors. ISE calibrations do not last as long and will drift faster. Bio-fouling and water conditions will affect maintenance and calibration frequency. A difficult to calibrate ISE sensor indicates that it needs to be replaced.
  • Before calibrating a new sonde with ISE sensors, soak the sensors in the calibration cup with a high conductivity standard, like 47.6 mS/cm, for 2 to 4 hours to condition the reference electrode. When the reference mV is stable, it is ready.
  • Soak a new ISE sensor in any of its standards overnight to condition it. It will calibrate more easily.
  • To calibrate an ISE sensor reliably will take a minimum of 20 minutes per sensor. ISE sensors need more time to stabilize than other sensors during calibration as well as during measurements in the field.
  • For best results, calibrate the ISE sensor at or near the water temperature it will be sampling. In any case, be consistent in your calibration temperatures.
  • When calibrating sensors, it is important to use deionized (DI) water to rinse between calibration steps.
  • If at all possible, use a stir plate to calibrate ISE sensors. Keeping the standard well-mixed improves calibration stability.

What are the best practices for installing the HL series ISE sensors?

  • When installing, be sure to calibrate pH and conductivity first and then ISE sensors as a best practice.
  • Sensors work best in flowing water. We recommend choosing a site in a location with good water flow to ensure movement of water over the sensor.
  • After placing the sonde in the water, allow several minutes for the sensors to stabilize for consistent measurements.

How important is the maintenance of ISE sensors?

Maintenance of the reference electrode is critical because ISE potentials are measured with respect to the reference electrode.

Which accessories or add-ons are needed, and for which reasons?

We recommend using a stir plate, if you have one, during calibrations as a best practice to ensure an equalized standard.
Constantly stirring ensures a set consistency for the liquid.

Why should I consider HYDROLAB measurements?

HYDROLAB measurement solutions provide these benefits:

  • Ruggedized ISE sensors operate to depths up to 15 meters
  • Automatic calculation of Ammonia and Total Ammonia concentrations
  • User-serviceable reference electrode
  • As well as HYDROLAB’s application advice from the customer support team

Range and accuracy of each, recommended measurements for each?


  • Range: 0 to 250 mg/L-N
  • Accuracy: +/- 10% or +/- 2mg/L-N, whichever is larger
  • Temperature range: 0 to 40 °C (non-freezing)
  • Not recommended for measurements less than 2mg/L-N


  • Range: 0 to 250 mg/L-N
  • Accuracy: +/- 10% or 2mg/L-N, whichever is larger
  • Temperature range: 0 to 40 °C (non-freezing)
  • Not recommended for measurements less than 2 mg/L-N


  • Range: 0 to 18,000 mg/L
  • Accuracy: +/- 10% or 5mg/L, whichever is larger
  • Temperature range: 0 to 50 °C (non-freezing)
  • Not recommended for measurements less than 2 mg/L

What are the best environments for measuring each of: Ammonium, Nitrate, and Chloride?


  • Can suffer interferences from other ions, especially sodium, potassium, and magnesium. These can reduce your accuracy.
  • Specific conductivity of less than 1000 uS/cm.


  • Can suffer interferences from other ions, especially chloride, bromide, bicarbonate, perchlorate, nitrate, and chlorate. These can reduce your accuracy.
  • Specific conductivity of less than 1000 uS/cm.


  • Can suffer interferences from other ions, especially bromide, iodide, cyanide, silver, and sulfide. These can reduce your accuracy.

What are ammonium and nitrate?

Ammonium (NH4+) and nitrate (NO3-) are ionized forms of nitrogen. Nitrate is related to ammonia in that bacteria colonies convert ammonia and ammonium to nitrite and then to nitrate. This final nitrate stage is the least toxic to water life. Ammonia has two forms - the ammonium ion, and the unionized, dissolved ammonia gas (NH3). The form depends on pH, with ammonium predominating when the pH is below 8.75, and ammonia  redominating above pH 9.75. Total ammonia is the sum of ammonium and ammonia concentrations. Ammonia is very toxic to water life, ammonium is less toxic.

How is chloride measured?

Chloride ion concentration is also measured with a chloride ion-selective electrode (ISE). The chloride ISE is a pellet of silver chloride in direct contact with the sample water. Because silver chloride has extremely low solubility in water, the silver chloride pellet never reaches chemical equilibrium with the sample water. Instead, a small amount of chloride ion dissolves into the sample. The resulting relative surplus of silver ions at the surface of the pellet creates a measurable electrical potential that varies with the concentration of chloride ions in the sample. This activity is compared to an electrode filled with a Potassium Chloride and Silver Chloride electrolyte (KCl and AgCl) which has a known ionic activity constant. The difference between these two “half cells” gives electrical potential in millivolts (mV).

Chloride sensors suffer interferences from other ions, working best when the concentrations of bromide, iodide, cyanide, silver, and sulfide ions are much lower than the chloride ion concentration.

Notice that ISEs are sensitive only to the ionized form of the chemical in question. Un-ionized forms of the chemical (for instance, insoluble salts or organic compounds), will not be detected by the ISE.

How is chloride measurement useful in water quality applications?

The chloride ion does not react with, or adsorb to, most components of rocks and soils, and so is easily transported through water columns. Thus, chloride is an effective tracer for pollution from chemicals moving from man-made sources into natural water bodies, or for salt water intrusion.
Applications for Chloride ion measurement include monitoring landfills for leaks, tracing the movement of point or non-point source pollutants within a natural water body (for instance, storm water runoff), monitoring estuary waters for changes in salinity, and detection of salt water intrusion into drinking water supplies.