Hobo MX2501 User manual

HOBO® MX pH and Temperature Logger (MX2501) Manual
22511-Q
The HOBO MX pH and Temperature logger is designed for long-term monitoring of pH in
aquatic systems. With a replaceable pH electrode and anti-biofouling copper guard, this
durable logger can be deployed in both fresh and salt water environments. This logger is
Bluetooth® Low Energy (BLE)-enabled for wireless communication with a mobile device or
computer. Using the HOBOconnect® app, you can easily calibrate and configure the logger,
download logged data to your phone, tablet, or computer, or automatically upload the data to
HOBOlink® for further analysis. You can also configure the logger to calculate statistics, set up
alarms to trip at specific thresholds, or enable burst logging in which sensor readings are logged
at a faster interval when the pH or temperature rises above or falls below certain limits.
Specifications
pH Sensor
in pH in 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
Logger
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 or Burst
Memory Modes Never Stop (Overwrites Old Data) or Stop When Full
Start Modes On Save (immediate), On Next Interval, On Push Button, On Date/Time
Stop Modes Never Stop (Overwrites Old Data), On Date/Time, After a specified
about of time, Stop When Memory Fills
Time Accuracy ±1 minute per month 0° to 50°C (32° to 122°F)
Battery Type One AA 1.5 Volt, user-replaceable
HOBO MX pH and
Temperature Logger
MX2501
Included Items:
•pH electrode installed and
submerged in storage
solution
•Extra electrode storage
solution
•Anti-biofouling copper guard
•Silicone-based O-ring grease
and foam swab
Required Items:
•HOBOconnect app
•Mobile device with
Bluetooth and iOS, iPadOS®,
or Android™, or a Windows
computer with a native BLE
adapter or supported BLE
dongle
•Deionized or distilled water
and squirt bottle
•Calibration solutions
Accessories:
•Replacement pH electrode
(MX2500-ELECTRODE)
•Calibration kit with three
bottles: pH 4.01, 7.00, and
10.00 calibration solution
(MX2500-CAL-KIT) and
calibration beakers
(MX2500-CAL-BKR)
•Maintenance kit with
calibration kit plus storage
solution (MX2500-MAINT-
KIT)
•pH 4.01 calibration solution
(MX2500-CAL-4)
•pH 7.00 calibration solution
(MX2500-CAL-7)
•pH 10.00 calibration solution
(MX2500-CAL-10)
•Electrode storage solution
(MX2500-STORE-SOLN)
•Replacement sensor end cap
(MX2500-END-CAP)
•Replacement anti-biofouling
copper guard (MX2500-
GUARD)
•Replacement clear storage
cap (MX2500-STORAGE-CAP)

HOBO MX2501 MX pH and Temperature Logger Manual
U.S. and International Sales: 1-508-759-9500 2 www.onsetcomp.com
Specifications (continued)
Battery Life 1 year typical at 25°C (77°F) with logging interval of 1 minute and
Bluetooth Always On selected 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 further 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
The CE Marking identifies this product as complying with all relevant
directives in the European Union (EU).
See last page
Logger Components and Operation
Anti-Biofouling Copper Guard: Use this guard to protect the pH
electrode when deploying the logger in water prone to
biofouling (see Deploying and Mounting the Logger).
Closure Caps: These caps are screwed onto the sensor end and
the mounting end (or top end) of the logger.
Sensor End Cap: This is the bottom end of the logger where the
pH electrode is installed, and the temperature sensor and
water detection screws are located.
Temperature Sensor: The temperature sensor is located at the
base of the sensor end cap.
Water Detection Screws: These two screws on the sensor end
cap can detect the presence of water.
pH Electrode: This is the electrode that is plugged into the
logger. The clear plastic end houses a glass pH sensor bulb that
must always be placed in storage solution when not deployed
in water. See Installing the pH Electrode for details.
Logger Housing: This is the main body of the logger.
Battery Holder: This is where one AA battery is installed under
a battery strap (see Battery Information).
Battery Holder
Sensor
End Cap
Anti-Biofouling
Copper Guard
Closure Cap
Closure Cap Logger Housing Mounting
Hole
Switch
Reed Switch
LEDs (Not
Visible)
pH Electrode
Water Detection
Screw (one of two)
Temperature
Sensor
Disassembled logger
Assembled logger with copper guard installed Assembled logger without copper guard installed
Mounting (or
Top) End Cap

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Reed Switch: This is used in conjunction with the magnetic
switch on the mounting end cap.
LEDs: The blue LED blinks every 4 seconds when the logger is
logging (unless Show LED is disabled as described in Configuring
the Logger). The blue LED blinks every 8 seconds if the logger is
waiting to start logging because it was configured to start On
Button Push or with a delayed start. The blue LED blinks every 4
seconds when an alarm is tripped (unless the Show LED option
is disabled). The blue and red LEDs blink once when you press
the magnetic switch to wake up the logger or blink four times
when you press the switch to start or stop logging. If you select
Illuminate Logger in the app, the red LED is illuminated for 5
seconds.
Mounting Hole: Use the hole on the mounting end of the
logger to attach it to a cable or cord for deployment (see
Deploying and Mounting the Logger).
Switch: This magnetic switch on the mounting end of the logger
works in conjunction with the reed switch. Press this switch into
the gap beneath the mounting lanyard for 3 seconds to start or
stop the logger when it is configured to start or stop On Button
Push (see Configuring the Logger). Press this switch for 1 second
to wake up the logger (if configured with Bluetooth Always Off
as described in Configuring the Logger).
Downloading the App and
Connecting to a Logger
Install the app to connect to and work with the logger.
1. Download HOBOconnect to a phone or tablet from the App
Store® or Google Play™.
Download the app to a Windows computer from
www.onsetcomp.com/products/software/hoboconnect.
2. Open the app and enable Bluetooth in the device settings if
prompted.
3. Wake up the logger by pushing the switch on the mounting
end of the logger into the gap beneath the mounting
lanyard.
4. Tap Devices and then tap the logger tile in the app to
connect to the logger.
If the logger does not appear in the list or if it is having trouble
connecting, follow these tips:
•If the logger was configured with Bluetooth Always Off
(see Configuring the Logger), make sure the logger is
awake by pressing the switch for 1 second. This brings
the logger to the top of the list if you are working with
multiple loggers.
•Make sure the logger is within range of your mobile
device or computer. The range for successful wireless
communication is approximately 30.5 m (100 ft) in air
with full line-of-sight.
•Change the orientation of your device to ensure the
antenna is pointed toward the logger. Obstacles between
the antenna in the device and the logger may result in
intermittent connections.
•If your device can connect to the logger intermittently or
loses its connection, move closer to the logger, within
sight if possible. If the logger is in water, the connection
can be unreliable. Remove it from water for a consistent
connection.
•If the logger appears in the app, but you cannot connect
to it, close the app and then power down your device to
force the previous Bluetooth connection to close.
Once the logger is connected, you can:
Tap this: To do this:
Specify logger settings and save them onto the
logger to start logging. See Configuring the
Logger.
Tap to view live data.
Mark the logger as a favorite. You can then filter
the list of devices to show only loggers marked as
favorites.
Calibrate the logger. See Calibrating the Logger.
Start logging if the logger was configured to start
with a button push. See Configuring the Logger.
Stop logging data (this overrides any Stop
Logging settings described in Configuring the
Logger).
Download logger data. See Downloading Data
From the Logger.
Lock the logger. Set a password for the logger so
that it is not mistakenly stopped or reconfigured.
Tap the icon, enter a password, and then tap Set.
Once a password is entered on a phone or tablet,
that mobile device is the only one to
“remember” it. This means you can connect to
the logger without entering a password on that
mobile device, but you must enter a password on
all other mobile devices. For example, if you set
the password for the logger with your tablet and
then try to connect to the device later with your
phone, you must enter the password on the
phone but not with your tablet. Similarly, if
others attempt to connect to the logger with
different devices, they are also required to enter
the password.
To reset a password:
•Press the Start/Stop button for 10 seconds,
or
•Tap Manage Password and then tap Reset.
Manage Password. Reset your existing password
or set a new password for the logger.
Illuminate the logger LEDs for 5 seconds.
Update the firmware on the logger. A logger
readout will be completed automatically at the
beginning of the firmware update process
Important: Before updating the firmware on
the logger, check the remaining battery level
and make sure it is no less than 30%. Make

HOBO MX pH and Temperature Logger (MX2501) Manual
U.S. and International Sales: 1-508-759-9500 4 www.onsetcomp.com
sure you have the time to complete the
entire update process, which requires that
the logger remains connected to the device
during the upgrade.
Calibrating the Logger
If the logger is new or if you have installed a new pH electrode,
you must calibrate the logger with the app before configuring
and deploying it. The app prompts you to calibrate the logger if
the pH electrode has never been calibrated, the last calibration
is more than seven days old, or the logger firmware was
updated.
Important: The pH sensor must be kept in storage solution
when not being calibrated or deployed in water. The hydration
layer surrounding the sensor starts to break down after 10
minutes if the pH sensor is not submerged in storage solution
or calibration solution, or deployed in water.
You will need:
•Deionized or distilled water and a squirt bottle to rinse
the pH sensor.
•The logger with the pH electrode installed and the copper
guard removed.
•Calibration solution (also called buffer solution) poured
into beakers. All three buffers are available in a kit
(MX2500-CAL-KIT) or individually (MX2500-CAL-4,
MX2500-CAL-7, or MX2500-CAL-10).
Important: The logger should be calibrated based on the
conditions at the location where it will be deployed.
Specifically, if the water temperature is expected to be
different than 25°C (77°F), it is important for the calibration
buffers to be close to the expected temperature of the analyte
(see the chart at the end of this section for the effect of
temperature on buffers). Similarly, if the logger will be
deployed at an altitude other than sea level, perform the
calibration at or near the deployment altitude.
To calibrate the logger:
1. If the sensor is in the storage solution, unscrew the clear
storage cap and remove it from the logger.
2. If you removed the clear storage cap, screw the closure cap
onto the logger.
3. In the HOBOconnect app, tap Devices.
4. Remove the logger from water. If the logger was previously
configured with Bluetooth Always Off, press the switch on
the logger to wake it up.
5. Tap the logger in the app to connect to it.
6. Tap Calibrate if you are not automatically prompted to
calibrate the logger.
7. Use a squirt bottle to rinse the pH sensor with deionized or
distilled water.
8. Place the sensor end of the logger in the pH 7 solution and
tap Start. Make sure the sensor end cap, temperature
sensor, and closure cap are submerged in the solution as
shown in the illustration. Once the pH reading is stable, tap
Confirm.
9. Rinse the pH sensor with deionized or distilled water.
10. If you are using the pH 4 buffer, place the sensor end in the
pH 4 solution and tap Start. If you are not using the pH 4
buffer, tap Skip and go to step 12.
11. Once the pH reading is stable, tap Confirm. Rinse the pH
sensor with deionized or distilled water.
12. If you are using the 10 buffer, place the sensor end in the
pH 10 solution and then tap Start. Once the pH reading is
stable, tap Confirm. If you are not using the pH 10 buffer,
tap Skip.
13. Verify that the slope and offset are acceptable and tap
Save. If the slope and offset are not acceptable, tap the
back arrow and repeat the calibration steps in this section
as needed.
14. Rinse the pH sensor with deionized or distilled water again.
If you are not deploying the logger immediately, place the pH
electrode in storage solution. Unscrew the sensor end cap
installed on the sensor end of the logger. If the clear storage
cap is empty, pour in storage solution to the marked fill line.
Carefully screw the clear cap with storage solution onto the
logger. Only use storage solution in the clear cap; do not use
water. Do not use the copper guard when the pH electrode is in
storage solution.
Unscrew the clear
cap filled with
storage solution
from the logger
Screw the closure
cap onto the logger
before calibrating
Closure cap
Sensor end cap Temperature sensor

HOBO MX pH and Temperature Logger (MX2501) Manual
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Notes:
•The date of the last calibration, the slope and offset
values, and the buffers used for the calibration are listed
under the Deployment Information in the Configuration
screen for reference.
•Once a logger is connected to the app, a notice appears if
the last calibration is more than seven days old prompting
you to recalibrate the logger. Although you can postpone
the calibration, we recommended you recalibrate the
logger before configuring it.
•If the status message during calibration indicates the
reading is invalid or it never stabilizes, try the following:
Make sure the sensor is in the correct buffer solution or
try rinsing the sensor and restarting the calibration. If
the problem persists, clean the sensor (see Maintaining
the Logger) and inspect it to make sure it is not
damaged. Note: There could be microcracks on the
glass bulb of the electrode that may not be visible to
the human eye.
Remove the sensor from the calibration solution and
clean it (see Maintaining the Logger). Restart the entire
calibration process.
Wait longer to see if it stabilizes, keeping in mind that a
pH electrode has a 6-month typical maximum life and
may produce unstable readings if it is older than 6
months. The slope and offset values that are provided
are the best indication for the condition of the
electrode. A healthy electrode has a slope between
85% and 100% and an offset between ±30 mv.
If the reading reported in app is acceptable for your
deployment even though it is reported as unstable, you
can tap Stop and then Confirm and proceed with the
calibration.
•You can calibrate the logger while it is logging. After the
calibration is saved, any new data logged uses the new
calibration. Data logged before the new calibration was
saved uses the old calibration data. If you do not want
data in a single file to be based on more than one
calibration, then stop the logger and read it out. You can
then calibrate it and reconfigure it.
•pH values of calibration buffers and samples vary
depending on their temperature. It is important that the
logger is calibrated in buffers that have the same
temperature as that of the sample. To do this, you must
use buffers that have known values at different
temperatures. The app automatically compensates for
temperature variances from 25°C (77°F) based on the
actual temperature-dependent pH value (from the chart
below) of buffers and samples.
Temperature
°C °F
pH 4.01
Buffer
pH 7.00
Buffer
pH 10.00
Buffer
0 32 4.01 7.12 10.31
5 41 4.00 7.09 10.24
10 50 4.00 7.06 10.17
15 59 4.00 7.04 10.11
20 68 4.00 7.02 10.05
25 77 4.01 7.00 10.00
Temperature
°C °F
pH 4.01
Buffer
pH 7.00
Buffer
pH 10.00
Buffer
30 86 4.01 6.99 9.95
35 95 4.02 6.98 9.92
40 104 4.03 6.97 9.88
45 113 4.04 6.97 9.85
Configuring the Logger
Use the app to set up the logger, including selecting the logging
options, configuring alarms, and other settings. These steps
provide an overview of setting up the logger. For complete
details, see the HOBOconnect User’s Guide.
1. Remove the logger from water. If the logger was previously
configured with Bluetooth Always Off, press the switch on
the logger to wake it up. If you are working with multiple
loggers, pressing the button also brings the logger to the
top of the list in the app.
2. Tap Devices and then tap the logger tile in the app to
connect to the logger.
3. Tap Configure & Start to configure the logger.
4. Tap Name and type a name for the logger (optional). If you
do not enter a name, the app uses logger serial number as
the name.
5. Tap Group to add the logger to a group (optional).
6. Tap Logging Interval and choose how frequently the logger
will record data unless operating in burst logging mode (see
Burst Logging). Note: If you configure an alarm, the logger
uses the logging interval you selected as the rate to check
for alarm conditions (alarms are not available if burst
logging is configured). See Setting up Alarms for more
details.
7. Tap Start Logging and select when logging begins:
•On Save. Logging begins immediately after
configuration settings are saved.
•On Next Interval. Logging begins at the next even
interval as determined by the selected logging interval.
•On Button Push. Logging begins when you press either
the Start/Stop logging button on the logger for 3 seconds
or start logging with the app.
•On Date/Time. Logging begins at a date and time you
specify. Select the Date and Time.
8. Tap Stop Logging and select the options for when logging
ends:
•Never Stop (Overwrites Old Data). The logger does not
stop at any predetermined time. The logger continues
recording data indefinitely, with newest data overwriting
the oldest.
•On Date/Time. The logger stops logging on a specific
date and time that you specify.
•After. Select this if you want to control how long the
logger should continue logging once it starts. Choose the
amount of time you want the logger to log data. For
example, select 30 days if you want the logger to log data
for 30 days after logging begins.
•Stop When Memory Fills. The logger continues recording
data until the memory is full.

HOBO MX pH and Temperature Logger (MX2501) Manual
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9. Tap Pause Options, then select Pause On Button Push to
specify that you can pause the logger by pressing its switch
for 3 seconds.
10. Tap Logging Mode. Select either Fixed Logging Mode or
Burst Logging. With fixed logging, the logger records data
for all sensors and/or selected statistics at the logging
interval selected (see Statistics Logging for details on
choosing statistics options). In burst mode, logging occurs at
a different interval when a specified condition is met. See
Burst Logging for more information.
11. Enable or disable Show LED. If Show LED is disabled, the
alarm and status LEDs on the logger are not illuminated
while logging (the alarm LED does not blink if an alarm
trips). You can temporarily turn on LEDs when Show LED is
disabled by pressing the button on the logger for 1 second.
12. Select the power saving mode, which determines when the
logger advertises or regularly sends out a Bluetooth signal
for the mobile device or computer to find via the app.
•Bluetooth Always Off. The logger advertises only during
logging when you push the switch on the logger. This
wakes up the logger when you need to connect to it. This
option uses the least battery power.
•Bluetooth Always On. The logger always advertises. You
never need to push a switch to wake up the logger. This
option uses the most battery power.
13. Set up an optional alarm to trip when the pH and/or
temperature sensor reading rises above or falls below a
specified value. See Setting up Alarms for details on
enabling sensor alarms.
14. Tap Start to save the configuration settings.
Logging begins based on the settings you selected. See
Deploying and Mounting the Logger for details on mounting
and see Reading Out the Logger for details on downloading.
Setting up Alarms
You can set up alarms for the logger so that if a sensor reading
rises above or falls below a specified value, the red logger LED
blinks and an alarm icon appears in the app. Alarms alert you to
problems so you can take corrective action.
To set an alarm:
1. Tap Devices. If the logger was configured with Bluetooth
Always Off enabled, push the switch on the logger to wake
it up.
2. Tap the logger tile to connect to the logger and tap
Configure & Start.
3. Tap a Sensor/Channel.
4. Tap Enable Logging, if necessary.
5. Tap Alarms to open that area of the screen.
6. Select Low if you want an alarm to trip when the sensor
reading falls below the low alarm value. Enter a value to set
the low alarm value.
7. Select High if you want an alarm to trip when the sensor
reading rises above the high alarm value. Enter a value to
set the high alarm value.
8. For the Duration, select how much time should elapse
before the alarm trips and select one of the following:
•Cumulative. The alarm trips once the sensor reading is
out of the acceptable range for the selected duration any
time during logging. For example, if the high alarm is set
to 85°F and the duration is set to 30 minutes, the alarm
trips once the sensor readings have been above 85°F for
a total of 30 minutes since the logger was configured.
•Consecutive. The alarm trips once the sensor reading is
out of the acceptable range continuously for the selected
duration. For example, the high alarm is set to 85°F and
the duration is set to 30 minutes, the alarm trips only if
all sensor readings are 85°F or above for a continuous 30-
minute period.
When an alarm trips, the red logger LED blinks every 4 seconds
(unless Show LED is disabled), an alarm icon appears in the app,
and an Alarm Tripped event is logged. The alarm state clears
when the readings return to normal if you selected Sensor in
Limits in step 9. Otherwise, the alarm state remains in place
until the logger is reconfigured.
Notes:
•Alarm limits are checked at every logging interval unless
burst logging is configured. For example, if the logging
interval is set to 5 minutes, the logger checks the sensor
readings against your configured high and low alarm
setting every 5 minutes.
•The actual values for the high and low alarm limits are set
to the closest value supported by the logger. For
example, the closest value to 85°F that the logger can
record is 84.990°F. In addition, alarms can trip or clear
when the sensor reading is within the specified
resolution specifications.
•When you read out the logger, alarm events can be
displayed on the plot or in the data file. See Logger
Events.
Burst Logging
Burst logging is a logging mode that allows you to set up more
frequent logging when a specified condition is met. For
example, a logger is recording data at a 5-minute logging
interval and burst logging is configured to log every 30 seconds
when the temperature rises above 85°F (the high limit) or falls
below 32°F (the low limit). This means the logger records data
every 5 minutes as long as the temperature remains between
85°F and 32°F. Once the temperature rises above 85°F, the
logger switches to the faster logging rate and records data
every 30 seconds until the temperature falls back to 85°F. At
that time, logging then resumes every 5 minutes at the fixed
logging interval. Similarly, if the temperature falls below 32°F,
then the logger switches to burst logging mode again and
record data every 30 seconds. Once the temperature rises back
to 32°F, the logger then returns to fixed mode, logging every 5
minutes. Note: Sensor alarms, statistics, and the Stop Logging
option Never Stop (Overwrites Old Data) are not available in
burst logging mode.
To set up burst logging:
1. Tap Devices. Remove the logger from water. If the logger is
configured with Bluetooth Always Off enabled, press the
button on the logger to wake it up.
2. Tap the logger tile to connect to the logger and tap
Configure & Start.
3. Tap Logging Mode and then tap Burst Logging.

HOBO MX pH and Temperature Logger (MX2501) Manual
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4. Set the burst logging interval, which must be faster than the
logging interval. Keep in mind that the faster the burst
logging rate, the greater the impact on battery life and the
shorter the logging duration. Because measurements are
being taken at the burst logging interval throughout the
deployment, the battery usage is similar to what it would be
if you had selected this rate for the normal logging interval.
5. Select Low and/or High and type a value to set the low
and/or high values. Select Low and/or High and either type
or drag the slider to set the low and/or high values.
Notes:
•The high and low burst limits are checked at the burst
logging interval rate whether the logger is in normal or
burst condition. For example, if the logging interval is set
to 1 hour and the burst logging interval is set to 10
minutes, the logger always checks for burst limits every
10 minutes.
•If high and/or low limits have been configured for more
than one sensor, then burst logging begins when any high
or low condition goes out of range. Burst logging does
not end until all conditions on all sensors are back within
normal range.
•The actual values for the burst logging limits are set to
the closest value supported by the logger. In addition,
burst logging can begin or end when the sensor reading is
within the specified resolution. This means the value that
triggers burst logging may differ slightly than the value
entered.
•Once the high or low condition clears, the logging
interval time is calculated using the last recorded data
point in burst logging mode, not the last data point
recorded at the fixed logging rate. For example, the
logger has a 10-minute logging interval and logged a data
point at 9:05. Then, the high limit is surpassed and burst
logging begins at 9:06. Burst logging then continues until
9:12 when the sensor reading falls back below the high
limit. Now back in fixed mode, the next logging interval is
10 minutes from the last burst logging point, or 9:22 in
this case. If burst logging had not occurred, the next data
point would have been at 9:15.
•A New Interval event is created each time the logger
enters or exits burst logging mode. See Logger Events for
details on plotting and viewing the event. In addition, if
the logger is stopped with a button push while in burst
logging mode, a New Interval event is automatically
logged and the burst condition is cleared, even if the
actual high or low condition has not cleared.
Statistics Logging
During fixed interval logging, the logger records data for
enabled sensors and/or selected statistics at the logging
interval selected. Statistics are calculated at a sampling rate you
specify with the results for the sampling period recorded at
each logging interval. The following statistics can be logged for
each sensor:
•The maximum, or highest, sampled value
•The minimum, or lowest, sampled value
•An average of all sampled values
•The standard deviation from the average for all sampled
values
For example, a logger is configured with the logging interval set
to 5 minutes and Fixed Logging Mode. In addition, all four
statistics are enabled with a statistics sampling interval of 30
seconds. Once logging begins, the logger measures and records
the actual temperature and pH sensor values every 5 minutes.
In addition, the logger takes a temperature and pH sample
every 30 seconds and temporarily store them in memory. The
logger then calculates the maximum, minimum, average, and
standard deviation using the samples gathered over the
previous 5-minute period and log the resulting values. When
downloading data from the logger, this results in 10 data series
(not including the mV series): two sensor series (with
temperature and pH data logged every 5 minutes) plus eight
maximum, minimum, average, and standard deviation series
(four for temperature and four for pH with values calculated
and logged every 5 minutes based on the 30-second sampling).
To log statistics:
1. Tap Devices. Remove the logger from water. If the logger
was configured with Bluetooth Always Off enabled, press
the button on the logger to wake it up.
2. Tap the logger tile in the app to connect to the logger and
tap Configure & Start.
3. Tap Logging Mode and then select Fixed Logging Mode.
4. Select the statistics you want the logger to record at each
logging interval: Maximum, Minimum, Average, and
Standard Deviation (average is automatically enabled when
selecting Standard Deviation). Statistics are logged for all
enabled sensors. In addition, the more statistics you record,
the shorter the logger duration and the more memory is
required.
5. Tap Statistics Sampling Interval and select the rate to use
for calculating statistics. The rate must be less than, and a
factor of, the logging interval. For example, if the logging
interval is 1 minute and you select 5 seconds for the
sampling rate, the logger takes 12 sample readings between
each logging interval (one sample every 5 seconds for a
minute) and use the 12 samples to record the resulting
statistics at each 1-minute logging interval. Note that the
faster the sampling rate, the greater the effect on battery
life. Because measurements are being taken at the statistics
sampling interval throughout the deployment, the battery
usage is similar to what it would be if you had selected this
rate for the normal logging interval.
Setting a Password
You can create an encrypted password for the logger that is
required if another device attempts to connect to it. This is
recommended to ensure that a deployed logger is not
mistakenly stopped or purposely altered by others. This
password uses a proprietary encryption algorithm that changes
with every connection.
To set a password:
1. Tap Devices. Press the switch on the logger to wake it up, if
necessary. Tap the logger tile in the app to connect to it.
2. Tap Lock Logger.
3. Type a password and then tap Set.

HOBO MX pH and Temperature Logger (MX2501) Manual
U.S. and International Sales: 1-508-759-9500 8 www.onsetcomp.com
Only the device used to set the password can then connect to
the logger without entering a password; all other devices are
required to enter the password. For example, if you set the
password for the logger with your tablet and then try to
connect to the logger later with your phone, you must enter
the password on the phone but not with your tablet. Similarly,
if others attempt to connect to the logger with different
devices, they are also required to enter the password. To reset
a password, press the switch on the logger for 10 seconds or
connect to the logger and tap Manage Password and tap Reset.
Downloading Data From the Logger
To download data from the logger:
1. Tap Devices.
2. Tap Download Data. The logger downloads data to the
phone, tablet, or computer.
3. Once the download is complete, tap Done to return to the
previous page or tap Export and Share to save the file in the
specified format.
4. When the export file has successfully been created, tap
Done to return to the previous page or tap Share to use
your device’s usual modes of sharing.
You can also upload data automatically to HOBOlink, Onset’s
web-based software, using the app or the MX gateway. For
details, see the HOBOconnect User Guide and see the HOBOlink
help for details on working with data in HOBOlink.
Logger Events
The logger records the following events to track logger
operation and status. You can view events in exported files or
plot events in the app.
To plot events, tap Data and select a file to open.
Tap (if applicable) and then tap . Select the events you
want to plot and tap OK.
Internal Event Name Definition
Host Connect The logger was connected to the mobile
device.
Started The logger started logging.
Stopped The logger stopped logging.
Calibration End pH calibration ended while the logger was
logging.
Calibration Cancel pH calibration was canceled while the
logger was logging.
Calibration Values The slope and offset values that were set
during calibration while the logger was
logging.
Calibration Buffers The buffers used during calibration while
the logger was logging.
Alarm
Tripped/Cleared
An alarm has occurred because the reading
was outside the alarm limits or back within
range. Note: Although the reading may
have returned to a normal range during
logging, an alarm cleared event will not be
logged if the logger was set up to maintain
alarms until reconfigured.
Internal Event Name Definition
New Interval The logger has switched to logging at the
burst logging rate or back to the normal
rate.
Water Detect The logger has been placed in water or
removed from water.
Low Power The battery level dropped below 0.8 V.
Safe Shutdown The battery level dropped below a safe
operating voltage and the logger
performed a safe shutdown with all data
stored in memory.
Deploying and Mounting the Logger
Important: The pH sensor must be kept in storage solution
when not being calibrated or deployed in water.
Follow these guidelines when deploying the logger:
•If the logger is in storage solution, remove the clear
storage cap from the logger and replace it with a closure
cap as shown in the next diagram before deploying it.
•If your water sample is prone to biofouling, insert the
optional copper anti-biofouling guard over the pH sensor
as shown. Gently twist the copper guard so that the holes
line up with the holes in the sensor end cap and pH
electrode. Do not use the copper guard when the pH
sensor is being calibrated or in storage solution, or if the
sensor is placed in a very small container with still water
or in water where copper can affect the environment.
Line up holes in copper guard with holes
in sensor end cap and pH electrode
Optional copper guard
installed to protect
against biofouling
Make sure the closure
cap is installed before
deploying the logger

HOBO MX pH and Temperature Logger (MX2501) Manual
U.S. and International Sales: 1-508-759-9500 9 www.onsetcomp.com
•There are two mounting options:
Attach a nylon cord or other strong cable to the
mounting hole to deploy the logger.
Alternatively, secure the logger to a pipe or similar
surface using cable ties wrapped around the two large
grooves in the logger housing.
•The logger should be appropriately secured and
protected depending on water conditions and desired
measurement location.
Maintaining the Logger
•The pH sensor must be kept in storage solution when
not being calibrated or deployed in water. Additional
storage solution (MX2500-STORE-SOLN), clear storage
caps (MX2500-STORAGE-CAP), and a maintenance kit
containing pH 4.01, 7.00, and 10.00 calibration solution
and storage solution (MX2500-MAINT-KIT) are available.
See www.onsetcomp.com.
•Periodically inspect the logger for biofouling. To clean the
logger housing, rinse the logger in warm water. Use mild
dishwashing soap if necessary on the logger housing only,
taking care not to touch the glass bulb in the pH
electrode. Do not use harsh chemicals, solvents, or
abrasives. Replacement sensor end caps (MX2500-END-
CAP) and copper guards (MX2500-GUARD) are available if
they cannot be cleaned effectively. See
www.onsetcomp.com for replacement parts.
•Periodically uninstall the pH electrode from the logger to
inspect it for deposits. Clean and recondition it as follows
depending on the type of deposits found. To remove a
build-up of scale or other dissolve salts, briefly soak the
pH sensor in 5% hydrochloric acid. To remove oil
deposits, soak the pH sensor in a solution with mild
dishwashing soap. Never touch the glass bulb inside the
clear plastic end of the probe. Always calibrate the
logger after cleaning the pH electrode with anything
other than distilled or deionized water.
•The hydration layer surrounding the glass pH sensor bulb
in the electrode naturally breaks down over time.
Cleaning and reconditioning the electrode as described in
in the previous bullet can help to extend its life. It is
recommended that you replace the pH electrode after 6
months of continuous use or when the slope and offset
readings reported during calibration are consistently not
within an acceptable range, even after cleaning. The ideal
range for slope is 85% to 100% and for offset is ±30 mv.
See Installing the pH Electrode for steps on replacing the
electrode (MX2500-ELECTRODE).
•Periodically add silicone-based grease to the outer O-
rings on the sensor end cap shown below. Using your
finger, spread a small dot of grease along both outer O-
rings.
Installing the pH Electrode
Important: New pH electrodes are uncalibrated. Once a pH
electrode is installed in the logger for the first time, it must be
calibrated before configuring and deploying the logger (see
Calibrating the Logger).
To install the pH electrode in the logger:
1. Unscrew the closure cap over the sensor end cap of the
logger. Pull the sensor end cap straight out from the logger
without twisting it.
2. Gently and slowly remove the pH electrode from the
sensor end cap. (Note: Removing the electrode too quickly
could result in unseating the internal O-rings.)
Use this hole for
mounting the logger
Outer O-rings on
sensor end cap
Use cable ties around these
grooves for mounting the logger
First, unscrew this closure cap
Then, pull this
sensor end cap
straight out
Gently and slowly
remove the pH electrode
from the end cap

HOBO MX pH and Temperature Logger (MX2501) Manual
U.S. and International Sales: 1-508-759-9500 10 www.onsetcomp.com
3. Use a foam swab dipped in water to clean the internal O-
rings. Use the foam swab to spread a small dot of silicone-
based grease along both the O-rings inside the sensor end
cap. Do not use a cotton swab as the fibers on the swab can
interfere with the operation of the electrode.
4. Remove the replacement pH electrode from the shipping
bottle and wipe the outer plastic body dry with a soft paper
or cloth towel. Never touch the glass bulb inside the clear
plastic end of the probe.
5. Remembering not to touch the glass bulb, push the pH
electrode gently and slowly into the sensor end cap. You
will feel the pH electrode pause as it passes through two
internal O-rings inside the sensor end cap (shown with step
3). Make sure the internal O-rings are still in place as the
probe is pushed into the sensor end cap. Note: If done too
quickly, this process can unseat the internal O-rings.
Gently twist the electrode so that the holes in the clear
electrode body line up with the holes in the end cap.
6. Continue to gently push the electrode into place until the
clear plastic is fully seated in the end cap as shown.
7. Push the sensor end cap back into the logger, making sure
the small tab in the end cap fits in the small groove in the
logger housing. Use the large tab on the end cap and the
arrow on the logger housing to help with proper alignment.
Do not twist or turn the end cap as you push it straight into
position.
8. If you will be calibrating the logger immediately: screw the
closure cap back on the logger and follow the instructions in
Calibrating the Logger.
If you will not be calibrating or deploying the logger
immediately:
a. Pour storage solution into the clear storage cap to the
marked fill line.
b. Carefully screw the cap with storage solution onto the
logger.
Important: Keep the sensor in storage solution unless you are
calibrating the logger or deploying it in water. Only use storage
solution in the cap; do not use water. Do not use the copper
guard while the sensor is in the cap.
Battery Information
The logger requires one user-replaceable AA 1.5 V alkaline or
lithium battery. With a logging interval of 1 minute and at 25°C
(77°F)
, battery life is 1 year with Bluetooth Always On selected
and 3 years with Bluetooth Always Off selected. Expected battery
life varies based on the ambient temperature where the logger is
deployed, the logging interval, the frequency of connections,
downloads, and paging, and the use of burst mode or statistics
logging. Deployments in extremely cold or hot temperatures or a
logging interval faster than 1 minute can impact battery life.
Estimates are not guaranteed due to uncertainties in initial
battery conditions and operating environment.
pH electrode installed
Use a foam swab dipped in
water to clean the internal
O-rings; use a foam swab
to spread a dot of silicone-
based grease on the
internal O-rings
Push the end cap
back into logger,
aligning tabs with
arrow
Sensor end cap
installed with
correct alignment
Large tab
Small tab
Small groove
Gently and slowly push pH
electrode into end cap and
twist the electrode so the
holes in the electrode align
with the holes in the end cap
Pour storage
solution into the
cap to the fill line
Screw on the
filled cap

HOBO MX pH and Temperature Logger (MX2501) Manual
U.S. and International Sales: 1-508-759-9500 11 www.onsetcomp.com
To install or replace the battery:
1. Wipe the logger clean and dry it completely.
2. Unscrew the closure cap on the mounting end of the logger.
3. Pull the mounting end cap and attached battery holder
straight out from the logger body without twisting. Insert a
screwdriver through the mounting hole if you need
additional leverage for removing the mounting end cap
from the logger.
4. Remove the battery strap. Insert a flat-head screwdriver
into the hole on the strap to lift it off the battery.
5. Remove the battery and install a new one, observing
polarity.
6. Using your finger, spread a small dot of silicone-based
grease along both O-rings (shown in step 4) on the
mounting end cap.
7. Place the battery strap back in place, using the notches on
both sides of the holder (shown in step 4) to align it and
guide it into place until you hear a click.
8. Reinstall the battery holder and attached mounting end
cap. Use the alignment tab on the battery holder (shown in
step 4) and the arrow on the logger housing as a guide as
you fit the small tab in the mounting end cap into the small
groove on the logger housing. Do not twist or turn the
mounting end cap as you push it straight into position.
9. Screw the closure cap back on the logger.
WARNING: Do not cut open, incinerate, heat above 85°C
(185°F), or recharge the lithium battery. The battery may
explode if the logger is exposed to extreme heat or conditions
that could damage or destroy the battery case. Do not dispose
of the logger or battery in fire. Do not expose the contents of
the battery to water. Dispose of the battery according to local
regulations for lithium batteries.
Unscrew the
closure cap
Pull the mounting
end cap from the
logger; insert a
screwdriver for
leverage
Insert a flat-head screwdriver in
this hole to remove the battery
strap
Notch to align battery strap
(
on both sides
)
Alignment tab
O-rings
Mounting end cap
installed with
correct alignment
Push the end cap back into the
logger, aligning small tab with
groove on logger housing

HOBO MX pH and Temperature Logger (MX2501) Manual
U.S. and International Sales: 1-508-759-9500
www.onsetcomp.com/support/contact
© 2018–2022 Onset Computer Corporation. All rights reserved. Onset, HOBO, HOBOconnect, and HOBOlink are
registered trademarks of Onset Computer Corporation. App Store, iPhone, iPad, and iPadOS are service marks or
registered trademarks of Apple Inc. Android and Google Play are trademarks of Google LLC. Windows is a registered
trademark of Microsoft Corporation. Bluetooth and Bluetooth Smart are registered trademarks of Bluetooth SIG,
Inc. All other trademarks are the property of their respective companies.
Patent #: 8,860,569 22511-Q
Federal Communication Commission Interference Statement
This equipment has been tested and found to comply with the limits for a Class B digital device, pursuant to Part 15 of the FCC Rules. These limits are designed to provide
reasonable protection against harmful interference in a residential installation. This equipment generates uses and can radiate radio frequency energy and, if not installed and
used in accordance with the instructions, may cause harmful interference to radio communications. However, there is no guarantee that interference will not occur in a
particular installation. If this equipment does cause harmful interference to radio or television reception, which can be determined by turning the equipment off and on, the user
is encouraged to try to correct the interference by one of the following measures:
•Reorient or relocate the receiving antenna
•Increase the separation between the equipment and receiver
•Connect the equipment into an outlet on a circuit different from that to which the receiver is connected
•Consult the dealer or an experienced radio/TV technician for help
This device complies with Part 15 of the FCC Rules. Operation is subject to the following two conditions: (1) This device may not cause harmful interference, and (2) this device
must accept any interference received, including interference that may cause undesired operation.
FCC Caution: Any changes or modifications not expressly approved by the party responsible for compliance could void the user's authority to operate this equipment.
Industry Canada Statements
This device complies with Industry Canada license-exempt RSS standard(s). Operation is subject to the following two conditions: (1) this device may not cause interference, and
(2) this device must accept any interference, including interference that may cause undesired operation of the device.
Avis de conformité pour l’Industrie Canada
Le présent appareil est conforme aux CNR d'Industrie Canada applicables aux appareils radio exempts de licence. L'exploitation est autorisée aux deux conditions suivantes : (1)
l'appareil ne doit pas produire de brouillage, et (2) l'appareil doit accepter tout brouillage radioélectrique subi, même si le brouillage est susceptible d'en compromettre le
fonctionnement.
To comply with FCC and Industry Canada RF radiation exposure limits for general population, the logger must be installed to provide a separation distance of at least 20cm from
all persons and must not be co-located or operating in conjunction with any other antenna or transmitter.
NCC Statement
經型式認證合格之低功率射頻電機，非經許可，公司、商號或使用者均不得擅自變更頻率、加大功率或變更原設計之特性及功能。
低功率射頻電機之使用不得影響飛航安全及干擾合法通信；經發現有干擾現象時，應立即停用，並改善至無干擾時方得繼續使用。前項合法通信，指依電信法規定作
業之無線電通信。低功率射頻電機須忍受合法通信或工業、科學及醫療用電波輻射性電機設備之干擾。
Translation:
Article 12
Without permission granted by the NCC, any company, enterprise, or user is not allowed to change frequency, enhance transmitting power or alter original characteristic as well
as performance to an approved low power radio-frequency device.
Article 14
The low power radio-frequency devices shall not influence aircraft security and interfere with legal communications. If found, the user shall cease operating immediately until no
interference is achieved. The said legal communications means radio communications is operated in compliance with the Telecommunications Act. The low power radio-
frequency devices must be susceptible with the interference from legal communications or ISM radio wave radiated devices.
KC Statement
해당 무선설비는 전파혼신 가능성이 있으므로 인명안전과 관련된 서비스는 할 수 없음
Translation:
The service related to human safety is not allowed because this device may have the possibility of the radio interference.