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HydroCycle-PO4 Phosphate Sensor

The HydroCycle-PO4 is a single analyte wet chemistry sensor engineered for environmental monitoring.

The HydroCycle-PO4 is a wet chemical sensor engineered for environmental monitoring to enable valid, scientifically defensible results that lead to better management recommendations. A NIST traceable on-board standard, factory calibration check, a 2.3 µgP/L detection limit, and an accurate calibration mean you can trust the data, line up grab sample records, and swap sensors with minimal offsets.

Autonomous, low-power operations and five month reagent stability enable extended deployments in remote locations. Over 1500 samples between servicing, up to four samples per hour and both SDI-12 and RS232 telemetry options allow for higher frequency phosphate data than before.

The HydroCycle-PO4 has increased reliability over its predecessor, the Cycle-PO4, and many wet-chemical sensors, by maximizing data reliability and sensor uptime. Fluidics have been advanced to provide > 90% uptime free of bubbles for data quality in high oxygen saturation environments and to ensure it stabilizes rapidly after deployment. Filter life has been extended to minimize clogging and enhance data quality during high sediment events.

QC flags have been developed to speed up troubleshooting and provide confidence in data quality and uptime. With the HydroCycle-PO4 you can know that your instrument is operating properly and justify your data quality, so you can spend your time analyzing what the data means.


  • Reagent cartridges
  • On-board standard
  • Standardization scheduling
  • Nanomolar resolution
  • Easy-to-use software
  • Autonomous or externally controlled
  • Scattering insensitive optical cell
  • 1500 samples between servicing
  • Over three month deployments
  • Smart sampling to minimize power consumption
  • Calibrated output (user defined units of μM, mg/L PO4-P)

1.1 Mechanical

Rated Depth 200 m
Weight in Air 6.8 kg (with reagents)
Weight in Water 0.37 kg
Height (with handle) 56 cm
Diameter 18 cm
Temperature Range, Operation 0 - 35 C


1.2 Electrical

  Main Port (6-pin) RS232/SDI-12 Port (8-pin)
Input  10.5 -18 VDC
Current Draw, Operation 115 mA, 3 A max 125 mA avg, 3 A max 
Current Draw, Low Power 0.25 mA 1.0 mA
Power Dissipation 1.15 W avg, 12 W max
Communication Interface RS 232 ASCII RS232/SDI-12
Data memory 1 GB CF Card Type II


1.3 Optical

Wavelength 870 nm
Pathlength 5 cm
Linearity > 95% R2


1.4 Analytical

Detection limit, three standard deviations of 18 MOhm water

less than or equal to 0.075 µm
less than or equal to 0.0023 mg/L PO4-P

Quantification limit, ten standard deviations of 18 MOhm water

less than or equal to 0.025 µm
less than or equal to 0.0077 mg/L PO4-P

Range, nominal

0–10 µm
0–0.3 mg/L PO4-P

Note: A higher range of 0–40 µm, 0–1.2 mg/L PO4-P is possible but outside specifications.

Number of runs per reagent set

greater than or equal to 1000 runs for CSF=2 greater than or equal to 1500 for CSF=6

Sampling rate

Standard operation: 15 min./sample Spiked operation: 35 min./sample

Reagent life

Refer to the "Best by" date on the cartridge (5 months from manufacture)

Filter pore size

5–10 µm (7.5 µm average)

Filter material

Ultra-High-Molecular-Weight Polyethylene

Units: M = moles per liter.
mg/L PO4-P refers to mg of atoms phosphorus per liter measured in the form of reactive phosphate.
ppm ~ mg/L, ppb ~µg/L

Specifications are subject to change without notice.
The list below includes (as applicable) the current product brochure, manual, and quick guide; software manual(s); and application notes.

Title Type Publication Date PDF File
HydroCycle-PO4 Datasheet Product Datasheet Thursday, September 14, 2017 datasheet_hydrocycle.pdf
HydroCycle Manual Product Manual Wednesday, November 8, 2017 manual-HydroCycle-RevB.pdf
In Situ Nutrient Sensors Brochure Product Brochure Wednesday, November 22, 2017 webres-seabird_nutrients-October-2017 x 100.pdf
AN124: Early Rinse Command and more Aggressive Sensor cleaning with Bleach Application Notes Friday, December 8, 2017 TN171108-1RinseBleach.pdf
AN118 Nutrient Units Primer Application Notes Friday, June 23, 2017 Nutrient-Units-Primer.pdf
AN107 HydroCycle-PO4 and Cycle-PO4 Service Offerings Application Notes Tuesday, July 4, 2017 AN107-HydroCycle-PO4ServiceOfferings.pdf
AN111 Change to Pin Out from Cycle-PO4 to HydroCycle-PO4 Application Notes Tuesday, July 4, 2017 AN111-Change to Pin Out from Cycle-PO4 to HydroCycle-PO4.pdf
AN112 HydroCycle-PO4 and Cycle PO4 Reagent/Filter Compatibility Application Notes Tuesday, July 4, 2017 AN112-HydroCycle-PO4 and Cycle PO4 Reagent_Filter Compatibility.pdf
In Situ Nutrient Sensors Product Brochure Wednesday, October 11, 2017 web-seabird_nutrients-October-2017-4pager.pdf
Mixed Molybdate Reagent Safety Datasheet Product Datasheet Tuesday, July 4, 2017 datasheet-Hydrocycle-SaftyMixedMolybdateSDS.PDF
Ascorbic Reagent Safety Datasheet Product Datasheet Tuesday, July 4, 2017 datasheet-Hydrocycle-SafetyAscorbicReagent.PDF
Phosphate Standard Safety Datasheet Product Datasheet Tuesday, July 4, 2017 datasheet-Hydrocycle-SafetyPhosphateStandard.pdf
Cycle Mounting Dimensions Product Drawing Tuesday, July 4, 2017 drawing-CycleMountingDimensions.pdf
Case Studies of WET Labs' Cycle-PO4: Simple and Reliable Nutrient Monitoring White Paper Wednesday, July 5, 2017 Case Studies of WETLabs Cycle-PO4.pdf
HydroCycle-PO4 Quick Start Product Quick Guide Tuesday, July 4, 2017 Quick-Start-HydroCycle-PO4.pdf

CycleHost is a graphical user interface for configuring and controlling the HydroCycle P04 sensor and processing and analyzing phosphate measurements.

Features include:

Version 1.08 released May 12, 2016

How can I extend the life of my HydroCycle-PO4 cartridges?

It is best to deploy HydroCycle-PO4 with new reagents right after receiving them from WET Labs.
If this cannot be done, it is recommended to store reagent cartridges in the refrigerator for enhanced stability (DO NOT FREEZE). Keep in mind, the reagent use after the "Best by" date is no recommended. The "Best by" date is approximately 3 months from fill date and is provided on the cartridge label.

How do I calculate power consumption over a deployment period?

  • The Cycle software does provide a calculator for power when a HydroCycle-PO4 Phosphate Sensor instrument is hooked up.
  • In Amp-Hours the values are: 0.034 Ahr for a normal run and 0.0563 Ahr for a spiked run.
  • In Joules the values are: 1.6kJ for a normal run and 2.5kJ for a spiked run.
  • Values can vary due to temperature, battery specific discharge and variation in sensor current draw profile.


How should I store my HydroCycle-PO4 Phosphate Sensor in the lab after deployment?

No special procedure needs to be taken when storing HydroCycle-PO4, expect for these basic steps that can be found in the manual.

1) Wrap the outlet tubing around the eye bolts.
2) Clean the optics directly after recovery (section 11.1.1).
3) Flush Cycle with DI or tap water (section 11.1.1).

Short term storage (1-20 days):

4a) If planning to redeploy: Keep cartridges clicked in to avoid introducing air bubbles.
4b) If returning cartridges to WET Labs: follow manual procedures (11.4) for draining cartridges. Remember to wear proper Personal Protection Equipment (PPE) when working with the red cartridge! See MSDS shipped with the cartridges.

Long term storage (21 + days):

4a) Remove cartridges and store in refrigerator (DO NOT FREEZE). Pay close attention to the "Best by" date. Reagent use after this date is no recommended. The "Best by" date is approximately 3 months from fill date and is displayed on the cartridge label.
4b) If returning cartridges to WET Labs: follow manual procedures (11.4) for draining cartridges. Remember to wear proper Personal Protection Equipment (PPE) when working with the red cartridge! See MSDS shipped with the cartridges.
5) Flush the instrument with DI water (leave passages filled with DI water). DO NOT pump air through the system to dry it out. It is not critical for all passages to remain wet during the duration of a long-term storage, but an initial flush is required.
6) Replace any worn parts (sleeve screws, fitting, filters, copper screens, etc.) with new parts from the spare parts kit.
7) Lubricate connectors with silicone spray.
8) Replace the sleeve
9) Store Cycle in its Pelican case in keep at room temperature. Reagent cartridges should be stored separately in the refrigerator.


Is the deployment wizard the only option to change settings on cartridge volumes?

The command line (on the monitor tab or any terminal emulator) can also be used to reset the cartridge volumes. The command to reset the cartridge volumes is:

$DCA 250.00 250.00 250.00 /S

What should I use to lubricate bulkhead connectors?

We recommend keeping the connectors on your meter lubricated using a good quality silicone spray lubricant, such as 3M Silicone Spray or Loctite 8021 spray. These can be purchased at your local hardware store. Avoid silicone grease. It is messy and attracts contaminants.

Do not use WD-40. It will destroy connectors.

What is the input voltage range for my instrument?