Onset HOBO MX2500 pH logger underwater

Part Number - MX2501

HOBO pH and Temperature Data Logger

Bluetooth-enabled logger

$795.00 USD
A pH and temperature data logger for long-term monitoring of pH in water, leveraging Bluetooth for easy setup and data dowload.

Important Information

Requires the HOBOconnect app and a compatible mobile device or Windows computer. System requirements for the app can be found at the bottom of our HOBOconnect software page. NOTE: pH electrodes should always be stored in storage solution when not deployed.

Compatible with


Designed for long-term monitoring of pH in estuaries, lakes, streams, rivers, and oceans. This logger leverages Bluetooth technology to communicate wirelessly with Onset's free HOBOconnect app running on your mobile device or Windows computer, making logger setup, calibration, and data offload quick and easy. Plus, a guided pH calibration process in the HOBOconnect app makes an otherwise complicated process easier to follow. This affordable, compact logger delivers high resolution data and dramatically cuts the time and effort needed to collect it.



  • Guided pH calibration following on-screen prompts in HOBOconnect app
  • Rugged PVC housing for deployment in both freshwater and saltwater environments
  • Quick and easy data offload via Bluetooth to mobile devices or Windows computers
  • Water detection system for longer battery life and less maintenance
  • Potentiometric pH electrode with plastic body, gel electrolyte, and double cloth junction
  • User-replaceable battery, pH electrode, and anti-biofouling copper guard
  • Accuracy of ±0.10 pH units within ±10°C of temperature at calibration


pH Sensor
pH mV
Range 2.00 to 12.00 pH -512 to 512 mV
Accuracy ±0.10 pH units within ±10°C of temperature at calibration ±0.20 mV
Resolution 0.01 pH 0.02 mV
Response Time 1 minute typical to 90% at constant temperature in stirred water
Sample Ionic Strength ≥ 100 µS/cm
Temperature Sensor
Range -2° to 50°C (28.4° to 122°F)
Accuracy ±0.2°C (±0.36°F)
Resolution 0.024°C at 25°C (0.04°F at 77°F)
Response Time 7 minutes typical to 90% in stirred water
Operating Range -2° to 50°C (28.4° to 122°F) — non-freezing
Buoyancy Fresh water: 13.6 g (0.48 oz) negative
Salt water: 19.6 g (0.69 oz) negative
Waterproof To 40 m (131.2 ft)
Water Detection Water conductivity level of 100 µS/cm or greater is necessary for reliable detection of water. Deionized water or water below 100 µS/cm may not be detected.
The water conductivity circuit may not reliably detect water that has frozen around the electrodes, i.e. below 0°C (32°F).
Radio Power 1 mW (0 dBm)
Transmission Range Approximately 30.5 m (100 ft) line-of-sight in air
Wireless Data Standard Bluetooth Low Energy (Bluetooth Smart)
Logging Rate 1 second to 18 hours
Logging Modes Fixed interval (normal, statistics) or burst
Memory Modes Wrap when full or stop when full
Start Modes Immediate, push button, date & time, or next interval
Stop Modes When memory is full, push button, date & time, or after a set logging period
Time Accuracy ±1 minute per month 0° to 50°C (32° to 122°F)
Battery Type One AA 1.5 Volt, user-replaceable
Battery Life 1 year typical at 25°C (77°F) with logging interval of 1 minute and Bluetooth Always On selected in software.
2 years typical at 25°C (77°F) with logging interval of 1 minute and Bluetooth Off Water Detect enabled in software.
3 years typical at 25°C (77°F) with logging interval of 1 minute and Bluetooth Always Off selected in software.
Faster logging intervals and statistics sampling intervals, burst logging, remaining connected with the app, excessive downloads, and paging may impact battery life.
pH Electrode Typical Minimum Life 6 months in sample with ionic strength ≥ 100 µS/cm
Memory 152 KB (43,300 measurements, maximum)
Full Memory Download Time Approximately 60 seconds; may take longer the farther the mobile device is from the logger
Dimensions 22.86 x 4.27 cm (9.0 x 1.68 inches); mounting hole 0.64 cm (0.25 inches)
Weight 268.2 g (9.46 oz)
Wetted Materials Logger: PVC housing and sensor end cap, polycarbonate closure caps and mounting end cap with a TPE switch
pH electrode: plastic-bodied with Pellon® junctions and gel electrolyte, glass pH sensor bulb
Environmental Rating IP68

Description: ce

The CE Marking identifies this product as complying with all relevant directives in the European Union (EU).

Compatible items

Replacement Parts

$129.00 USD
$25.00 USD
$119.00 USD
$15.00 USD


$6.00 USD
$16.00 USD
$16.00 USD
$16.00 USD
$25.00 USD

Compliance Certificates

$29.00 USD


$375.00 USD

What's in the box

  • MX2501 pH and Temperature Data Logger
  • pH electrode
  • Bottle of storage solution that can also be used as a storage container for the electrode
  • Anti-biofouling copper guard
  • Tube of silicone grease
  • AA battery


Quick Start for the HOBO® MX pH and Temperature Logger (MX2501)

View View

HOBO® MX pH and Temperature Logger (MX2501) Manual

View View

Getting Started with MX Data Loggers

This Getting Started guide will walk you through the basic steps of data logging, setting up your new data logger, downloading and sharing data, and the option to upload your data to the cloud.
View View


Customers Also Bought

Data Logger

HOBO Pendant MX Water Temperature Data Logger

A waterproof logger enabled with Bluetooth wireless access to deliver accurate temperature measurements straight to your mobile device or Windows computer using our HOBOconnect app.
View product
Data Logger

HOBO TidbiT MX Temperature 5000' Data Logger

A waterproof temperature logger that uses the power of Bluetooth to deliver high-accuracy data straight to your mobile device or Windows computer running Onset's free HOBOconnect app
View product
Data Logger

HOBO Salt Water Conductivity/Salinity Data Logger

A cost-effective data logger for measuring salinity, conductivity, and temperature in saltwater environments with relatively small changes in salinity (±5,000 μS/cm).
View product
Data Logger

HOBO Dissolved Oxygen Data Logger

An affordable, precise data logger that measures oxygen concentrations in lakes, streams, rivers, estuaries, and coastal waters.
View product