HydroCycle-PO4 Phosphate Sensor

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

CUSTOMER ALERT: June 14, 2017 Cycle-PO4 and HydroCycle-PO4 Product Problems and Service-time Update

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 data, line up grab sample records, and swap sensors with minimal offsets.

Autonomous operations, low-power, five month reagent stabilityenable extended deployments in remote locations. Over 1500 samples between services, up to four samples per hour and both SDI-12 and RS232 permit capturing 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
  • Over 1000 samples
  • Over three month deployments
  • Smart sampling minimizes power consumption
  • Calibrated output (user defined units of μM, mg/L PO4-P)

Note to current Cycle Owners: We have a new firmware updater program available. Please click here for more details.

1.1 Mechanical

Rated Depth200 m
Weight in Air6.8 kg (with reagents)
Weight in Water0.37 kg
Height (with handle)56 cm
Diameter18 cm
Temperature Range, Operation0 - 35 C


1.2 Electrical

 Main Port (6-pin)RS232/SDI-12 Port (8-pin)
Input 10.5 -18 VDC
Current Draw, Operation115 mA, 3 A max125 mA avg, 3 A max 
Current Draw, Low Power0.25 mA1.0 mA
Power Dissipation1.15 W avg, 12 W max
Communication InterfaceRS 232 ASCIIRS232/SDI-12
Data memory1 GB CF Card Type II


1.3 Optical

Wavelength870 nm
Pathlength5 cm
LInearity> 95% R2


1.4 Analytical

Detection limit, three standard deviations of 18 MOhm water


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

Quantification limit, ten standard deviations of 18 MOhm water


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

Standard deviation of standard solution of 2.6 µM


equal to or less than 0.0015 mg/L PO4-P

Range, nominal


0-0.3 mg/L PO4-P

* A higher range of 0 - 1.2 mg PO4-P is possible but outside of specifications.

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.

CycleHost software makes it easy to:

  • Quality control flagging of data that is backwards compatible with any past data collected with the Cycle-PO4
  • Configure any Cycle settings, schedule Cycle sampling, or integrate it with a data logger.
  • Configure Cycle using the deployment wizard.
  • Control the PO4 concentration units.
  • Track reagent usage.
  • Control on-board standardization.
  • - Graphically plot and interact with processed and/or raw Cycle data in real time or via file playback.
  • - Generate deployment reports of configuration/settings used.
  • - Manage files and offload data.
  • - Access terminal-like command line control.

Please see the release notes for a complete list of features and improvements and refer to the HydroCycle User Manual for more information.

Cycle Host 1.08

  • Control of output concentration units and autoconversion
  • Faster file offload and better management,


Download Cycle Host 1.0.8

Released May 12, 2016
CycleHost1.08_installer.exe.zip for Windows XP/Vista/7

Cycle Firmware Updater is a program to update the firmware in HydroCycle nutrient sensors.

Released May 12, 2016

Sea-Bird Scientific (WET Labs) has updated the firmware for the Cycle PO4 to include automated real-time quality control (QC) flags and a QC post processing utility in the Host software. QC flags provide confidence in the data quality and uptime so users can know their instrument is working properly. QC flags can help speed up troubleshooting, help justify published data, and permit users to spend more time on advanced data analysis rather than reviewing data to separate good data from bad.

SDI-12 users should not use the link below. SDI-12 users must have their firmware updated at the factory.

LOBOviz users need to contact support prior to upgrade as some STOR-X and LOBOviz setting must be changed to accomodate the firmware change.

Firmware Changes (update from version 1.04 to 1.06)
Each data point receives a general quality flag (good, suspect, or bad) in real time. This flag is a composite of the six individual flags. Those flags include: out-of-range measurement, optical noise, low signal, bubbles and calibration spikes, and mixing spikes. In addition to teh overall quality flag and the five individual quality flags the firmware can output combined specific flags (for condensed telemetry) and flag values (algorithm outputs that trigger flags based on threshold settings). More information about communication output formats can be found in the HydroCycle-PO4 User Manual. Section 9 covers SDI-12 output and Section 10.1 covers RS-232 output.



CycleUpdater_installer1_0_6_1.exe.zip for Windows XP/Vista/7
Customer Alert

Cycle-PO4 and HydroCycle- PO4 Product Problems and Service-time Update

Wednesday, June 14, 2017
Cycle-PO4 and HydroCycle-PO4 Product Problems and Service Time Update.pdf
Product Datasheet

HydroCycle-PO4 Datasheet

Thursday, February 23, 2017
Product Datasheet

Phosphate Standard Safety Data Sheet

Wednesday, February 15, 2017
Safety Data Sheet Phosphate Standard.pdf
Product Datasheet

Mixed Molybdate Reagent Safety Data Sheet

Wednesday, February 15, 2017
Safety Data Sheet Mixed Molybdate SDS.PDF
Product Datasheet

Ascorbic Reagent Safety Data Sheet

Wednesday, February 15, 2017
Safety Data Sheet Ascorbic Reagent.PDF

CYCLE product drawing

Tuesday, February 12, 2013
Product Manual

Cycle Firmware Updater

Monday, February 4, 2013
Product Manual

HydroCycle Phosphate Sensor User Manual

Thursday, May 12, 2016
White Paper

Case Studies of WET Labs' Cycle-PO4: Simple and Reliable Nutrient Monitoring

Thursday, August 15, 2013
Case Studies of WETLabs Cycle-PO4.pdf
Technical Note

CYCLE PO4 Low Power Fault

Wednesday, November 14, 2012
Technical Note

Technical Note: Cycle-PO4 Window Staining

Wednesday, February 15, 2017
Technical Note

Technical Note: Change to Pin Out from Cycle-PO4 to HydroCycle-PO4

Wednesday, February 15, 2017
Technical-Note-Change to Pin Out from Cycle-PO4 to HydroCycle-PO4.pdf
Technical Note

Technical Note: Cycle PO4 Updates to Host Software (1.08) and Firmware (1.06)

Wednesday, February 15, 2017
Technical-Note-Cycle PO4 Updates to Host Software 1.08 and Firmware 1.06.pdf
Technical Note

CYCLE PO4 Firmware Release Notes

Tuesday, February 12, 2013
Technical Note

Technical Note: Cycle PO4 Updates on Reagent Best By Dates and Lot Numbers

Wednesday, February 15, 2017
Technical Note

Technical Note: Hardware Upgrades for Cycle-PO4

Wednesday, February 15, 2017
Technical-Note-Hardware Upgrades for Cycle-PO4.pdf
Technical Note

Technical Note: HydroCycle-PO4 and Cycle PO4 Reagent/Filter Compatibility

Wednesday, February 15, 2017
Technical-Note-HydroCycle-PO4 and Cycle PO4 Reagent:Filter Compatibility.pdf
Technical Note

Nutrient Units Primer

Monday, January 30, 2017
Technical Note

HydroCycle-PO4 Quick Start

Wednesday, February 15, 2017
Technical Note

Cable and Bulkhead Connector Maintenance

Thursday, March 9, 2017
Technical Note

HydroCycle-PO4 and Cycle-PO4 Service Offerings

Thursday, March 9, 2017
HydroCycle-PO4 Service Offerings.pdf
Technical Note

Instrument Voltage Ranges (in VDC)

Thursday, June 21, 2012

Why are there bubbles in my sample line?

This can be due to several things, being only half-immersed could contribute. Cycle is designed to be fully submerged for fluidic pressure balancing. The pumps can cause cavitation (create air bubbles) if, for example, the waste tube or sample intake is constricted (pinched or covered). Most frequently this issue is caused by sample outgassing or the Cycle just not being stabilized (i.e. not all air has been pumped out of the system yet). It can take some time to stabilize Cycle (overnight, sometimes extra measures are needed) and extra attention and care must be used when measuring lab water. Please use our Hardware User's Guide (http://www.wetlabs.com/cycle-po4, or in the software help), section 3.4 ("Evaluating Cycle in the lab", esp. section 3.4.2 "Enhanced priming methods") for more on stabilization techniques. Please also note section 10 on data analysis (esp. 10.5 "Poor quality data" and 10.6 "Good quality data"); figure 4 shows sample line bubbles.

What should I use to lubricate bulkhead connectors?

WET Labs recommends 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.

Do you want WET Labs to review your data?

In order for data questions to be answered in an efficient manner, WET Labs requests:

-  The serial number of the instrument data in question. The most current calibration information
can be verified and the instrument settings upon delivery can be checked.

-  Description of the deployment configuration. Questions to consider:

  •   What are the current instrument settings?
  •   What is the instrument orientation?
  •   Are there possible obstructions?
  •   Is it being used in a mooring, a profiler, or being towed?
  •   What were the deployment date and geographical location?
  •   How far into the deployment was the problem seen?
  •   What power supply and cabling are being used?
  •   What was the instrument condition during the last field service or retrieval?
  •   Do you have any pictures?

-  Raw data files of the physical data in question

  •   examples of good raw data.
  •   examples of questionable raw data .

-  Information about how the raw data was processed.

Why should I update my firmware from version 1.03 to 1.04?

A bug exists in version 1.03 that can result in a failure of the Cycle to continue recording on-board data. Use the automatic updater to update your sensor's firmware from version 1.03 to 1.04.

Should I update earlier versions of the cycle firmware?

Firmware prior to version 1.03 does not contain any known bugs, so it is not critical that they be updated. All Cycle sensors are updated to the most current firmware during regular servicing at WET Labs unless the customer requests otherwise.

How should I store my Cycle in the lab after deployment?

No special procedure needs to be taken when storing Cycle, 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.

How can I extend the life of my Cycle cartridges?

It is best to deploy Cycle 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 Cycle 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.


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

  • Matthew J. Cohen, Marie J. Kurz, James B. Heffernan, Jonathan B. Martin, Rachel L. Douglass, Chad R. Foster, and Ray G. Thomas (2013) Diel phosphorus variation and the stoichiometry of ecosystem metabolism in a large spring-fed river Ecological Monographs 83 155-176 Read Now
  • Gilbert M., Needoba J.A., Koch C., Barnard A., Baptista A. (2013) Nutrient Loading and Transformations in the Columbia River Estuary Determined by High Resolution In Situ Sensors Estuaries and Coasts 36, 4 708-727 Read Now
  • WET Labs Science Team (2013) Case Studies of WET Labs' Cycle-PO4: Simple and Reliable Nutrient Monitoring WET Labs Read Now
  • Haggard B.E., L.B. Massey, C.R. Koch, J.T. Scott, M.A. Evans-White, B.J. Austin, E.E. Scott, and D. Procyk (2012) Phosphorus Slug Dynamics in an Urban Stream University of Arkansas 22 Read Now
  • Lauren R. Sherson (2012) Nutrient dynamics in a headwater stream: use of continuous water quality sensors to examine seasonal, event, and diurnal processes in the East Fork Jemez River, NM. University of New Mexico. Dept. of Earth and Planetary Sciences. Read Now
  • Leslie B. Massey, Brian Haggard, Corey Koch, Thad Scott, Michelle Evans-White, Brad Austin and Erin Scott (2012) Real-Time Phosphorus Concentrations in an Urban Stream Arkansas Water Resources Center 27 Read Now
  • Melissa L. Gilbert (2012) Nutrient loading and transformations in the Columbia River estuary determined by high resolution in situ sensors Oregon Health and Science University Read Now