Apera Instruments SX725 User manual

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SX725 Portable pH/DO Meter
User Manual
APERA Instruments (Europe) GmbH
aperainst.de

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Table of Contents
1Brief Instruction..............................................................................................................- 3 -
2Technical Specifications ..................................................................................................- 3 -
3About the Meter .............................................................................................................- 5 -
4What’s in the kit ............................................................................................................. - 6 -
5Preparation.....................................................................................................................- 7 -
6pH Measurement ............................................................................................................- 7 -
6.1 pH Electrode .............................................................................................................. - 8 -
6.2 pH Calibration ............................................................................................................ - 8 -
6.3 Notes about pH Calibration ......................................................................................... - 9 -
6.4 Sample Test.............................................................................................................. - 10 -
6.5 Recommended pH Electrodes for Different Applications .............................................. - 10 -
6.6 Parameter Setting..................................................................................................... - 11 -
6.7 Cleaning pH Electrode ............................................................................................... - 13 -
6.8 The Self-diagnose Information ................................................................................... - 14 -
6.9 Notes about pH ........................................................................................................ - 14 -
7DO Measurement ......................................................................................................... - 15 -
7.1 Preparation before use ............................................................................................. - 15 -
7.2 DO Calibration (Saturated Oxygen)............................................................................. - 16 -
7.3 DO Measurement ..................................................................................................... - 16 -
7.4 Notes about DO Measurement .................................................................................. - 16 -
7.5 DO Electrode Maintenance ........................................................................................ - 17 -
7.6 Zero Oxygen Calibration ............................................................................................ - 18 -
7.7 Salinity Calibration ................................................................................................... - 19 -
7.8 Barometric pressure manual compensation ................................................................ - 19 -
7.9 Parameter Setting..................................................................................................... - 19 -
8Limited Warranty .......................................................................................................... - 21 -
9Appendix ...................................................................................................................... - 22 -
9.1 Appendix 1 Content of Saturated Oxygen in Water at Different Temperature................. - 22 -
9.2 Appendix 2 Content of Saturated Oxygen at Different Barometric pressure ................... - 23 -
9.3 Appendix 3 Content of Oxygen at Different Altitude .................................................... - 24 -
Scan the QR code below to watch the tutorial video on Youtube.

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1 Brief Instruction
Thanks for purchasing Apera Instruments SX725 Portable pH/DO Meter (will be called “the meter” in short
in the following content). Before using the meter, please read this instruction manual carefully in order to
help you properly use and maintain it.
The meter can measure the parameters of pH, DO, and temperature of water solutions with high accuracy.
It is suitable for various applications in different industries e.g., water treatment, aquaculture, horticulture,
pools and spas, beverage making, mining, power plants, environmental monitoring, etc., especially ideal
for in-field use.
The meter has the following features:
1.1 Built-in microprocessor chip with the intelligent functions of auto. calibration, auto. temperature
compensation, auto. salinity compensation, manual baromatric pressure compensation, data
storage, function settings, auto. power off, and low voltage display etc.
1.2 Adopts digital filter technology to intelligently improve meter’s response speed and accuracy. The
smiley face icon will appear when the reading is stable.
1.3 Pure water modes available for pH, significantly increasing the accuracy in pure water measurement.
1.4 The polarographic DO electrode has a special electrode calibration sleeve. The electrode
polarization only takes 5 minutes. Electrode adopts a combo-style membrane cap, which is very
convenient to use. Each DO electrode comes with three backup membrane caps.
1.5 Meter’s LCD screen has clear and bright backlit display.
1.6 Built-tough. IP57 waterproof and dust-proof.
2 Technical Specifications
2.1 pH
Measurement range
(-2.00 to 19.99) pH
Resolution
0.1/0.01 pH
Accuracy
Meter: ±0.01pH; Overall: ±0.02pH
Input current
≤2×10-12 A
Input impendance
≥1×1012 Ω
Stability
±0.01 pH/3h
Temp. compensation range
(0 to 100) °C (automatic)

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2.2 ORP (mV)
Measurement range (mV/ORP/EH)
-1999mV to 0 to 1999mV
Resolution
1mV
Accuracy
Meter: ±0.1% FS, Overall: ±15mV
2.3 Dissolved Oxygen
Measurement range
(0 to 19.99) mg/L (ppm); (0 to 200.0)%
Resolution
0.1/0.01 mg/L (ppm) 1/0.1%
Accuracy
±0.30 mg/L (ppm), including electrode error
Response time
≤30s (25˚C, 90% response)
Temp. compensation range
(0 to 45) °C (automatic)
Salinity compensation range
(0 to 45) ppt (automatic)
Barometric pressure compensation range
(66 to 200) kPa (manual)
Automatic calibration
Air saturated by water; water saturated by air
Electrode type
Polarographic
2.4 Other Technical Parameters
Data storage
300 sets
Data content
Serial number, measurement value, measurement unit, and
temperature
Power Supply
Two AA alkaline batteries (1.5V x2)
Dimensions and weight
Meter: (65×120×31) mm/180g
Complete Kit: (255 x 210x 50) mm/1380g
Quality and safety certification
CE, ISO 9001:2015
2.5 Working Condition
Working temperature
5 to 35°C
Working humidity
≤85%
IP rating
IP57 water-resistant

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3 About the Meter
3.1 Screen Display
① —— Parameter icon
② —— Measurement value
③ —— Serial number and icon as measurement
to be saved and recalled and indication icon for
special states.
M+ — measurement value to be saved;
RM — Saved data to be recalled;
④ —— Measurement unit ⑤ —— Temperature measurement value and unit
⑥ —— Measurement stabilization icon ⑦ —— Electrode calibration indication icon
⑧ —— Indication icon of low battery power; appears when the voltage is less than 2.6V, calling
attention to replace the batteries.
3.2 Keypad
The meter has 5 operation keys in total.
Short press: press time <1 seconds; Long press: press time >2 seconds
3.2.1. — Short press to power on or off.
3.2.2. — Calibration key
(a) When in measurement mode, short press to enter calibration mode.
(b) When in parameter setting mode, short press to make changes.
3.2.3. — Function key
(a) In measurement mode, short press (<1.5s) to switch between and . Long press
to enter parameter setting (P1, P2, P3…)
(b) In other measurement mode, short press this key to enter the parameter setting mode (P1, P2, P3…)
3.2.4. — Back light and confirmation key
(a) In measurement mode, short press to turn on or off the back light.
(b) In calibration mode or the parameter setting, press this key to make confirmation, and return to
measurement mode.
(c) When in mode, hold this key to change pH resolution: 0.01 → 0.1 pH in turn. Release key when
you confirm the selection.

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(d) When in mode, hold this key to change DO in turn: mg/L → ppm → %. Release the key when
you confirm the selection.
3.2.5. — The key for data saving and recalling
(a) When in measurement mode, short press to save the measurement data; Long press to recall the
saved data.
(b) When in parameter setting mode, press the key to make changes.
3.3 Data Log, Recall and Delete
3.3.1. Save the measurement:
In measurement mode, when the reading is stable and stays on screen, short press to save
the measurement data. M+ icon and the data serial number will show up on the upper right corner. The
meter can store up to 300 sets of data.
3.3.2. Recall saved data:
(a) In measurement mode, long press , the meter will recall the last saved data and the serial
number. RM icon will appear in the upper right corner of the screen.
Short press or again, the meter will recall all the data in turn, hold or
to quickly review the data.
(b) In data recalling mode (RM and serial number in the lower right corner), short press to return
to measurement mode.
3.3.3. Delete data:
In data recalling mode, hold for 5 seconds, CLr will show up for 2 seconds, meaning that all the
saved data have been eliminated. Then the meter will return to measurement mode.
4 What’s in the kit
SX725pH/DO meter *1
201T-S plastic pH/ATC three-in-
one combination electrode *1
DO502 DO electrode inner
solution (30mL) *1
DO503 membrane cap *3
DO500 DO electrode *1
Spare AA batteries *2
Cathode polishing paper *2
Screw driver *1
pH standard buffer solution
(pH4.00, pH7.00, pH10.01) *1
50ml bottle/each
Operation manual *1
Carrying case *1

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5 Preparation
What you need in addition to what’s in the kit:
Pure water (RO, distilled or deionized water) for rinsing off the electrode.
6 pH Measurement
Short press to turn on the meter. Connect 201T-S pH electrode to the meter (See Section 5 for
connection tutorial). The meter will automatically enter pH measurement mode.

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6.1 pH Electrode
The meter comes with the 201T-S 3-in-1 combination electrode with a built-in temperature sensor, which
enables automatic temperature compensation. This electrode is only suitable for general water
solutions’ pH testing. Please refer to Section 6.5 for ideal pH electrodes to use for other applications. A
3M KCL storage bottle comes with the electrode, which is for storing the electrode when not in use to
keep the sensitivity of the probe.
6.1.1 Technical Specifications of the 201T-F pH Electrode
Measurement Range: 0 – 14 pH, 0 – 80°C (32 – 176°F); Junction: Single Ceramic
Reference Electrode: Ag/AgCl Connector: 8-pin
Size: ø12*160 mm; Cable length: 1 meter
Temperature unit: 30K Thermistor
6.1.2 Use the Electrode
Screw off the KCL storage bottle, and put it aside (do not dump or spill the KCL solution). Rinse the
electrode with distilled or deionized water. Shake off excess water or blot-dry with clean tissue or kimwipe.
Stir the electrode for a few seconds after it’s dipped into the test solution and then hold it still. Wait for the
reading to get fully stabilized ( appears and stays) and then take the measurement. When the test
is finished, place the electrode in the storage bottle and tighten the cap to keep the sensitivity of the pH
sensor.
6.2 pH Calibration
6.2.1 After powering on the meter, short press to enter calibration mode, c1 flickers on LCD,
indicating the meter enters the first point calibration.
6.2.2 Rinse off the pH electrode in pure water and remove excess water, then insert it into the pH 7.00
buffer solution, make a quick stir and then hold it still. When the reading is stabilized and stays on
screen, short press again to finish the 1st point calibration. “7.00 pH” will start flickering and a
flickering c2 will show up, indicating the 1st point calibration has been finished and the meter is entering
the 2nd point calibration.
6.2.3. Rinse off the pH electrode in pure water and remove excess water, then insert it into the pH 4.00
buffer solution, make a quick stir and then hold it still. When the reading is stabilized and stays on
screen, short press again to finish the 2nd point calibration. “4.00 pH” will start flickering and a

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flickering c3 will show up, indicating the 2nd point calibration has been finished and the meter is entering
the 3rd point calibration.
6.2.4. Rinse off the pH electrode in pure water and remove excess water,
then insert it into the pH 10.01 buffer solution, make a quick stir and then
hold it still. When the reading is stabilized and stays on screen, short
press again to finish the 3rd point calibration. “10.01 pH” will start
flickering and then the meter will return to measurement mode.
will show up at the bottom left (see picture 4-1), indicating all 3 calibrations are finished.
6.3 Notes about pH Calibration
1) Keeping the freshness and cleanliness of calibration buffers is essential for accurate pH
measurement. The small bottles of pH buffers come with the meter should be replaced within 3
months after opening. The new buffer solutions should be made by a legitimate lab supply
manufacturer. Avoid sunlight and air contact when the buffers are not being used, and store at room
temperature.
2) This meter can adopt random 1-point, 2-point or 3-point automatic calibration. During calibration,
short press to return to measurement mode. When the measurement accuracy is ≤ ±0.1pH,
choose a buffer that’s close to the estimated measurement range and perform 1-point calibration
would be good. Choose pH 4.00 and pH 7.00 to calibrate if the measurement range is within the
acidity range (<pH 7) and choose pH 7.00 and pH 10.01 to calibrate if just within the alkalinity range
(>pH 7).
3) Choose 3-point calibration to achieve a more accurate measurement if the measurement range is
wide, or if the electrode has not been used for long. When connecting a new pH electrode, it must
be calibrated at 3 points.
4) The frequency that you need to calibrate your meter depends on the tested samples, condition of
electrodes, and the requirement of the accuracy. For high-accuracy measurements (≤ ±0.02pH), the
meter should be calibrated before test every time; For ordinary-accuracy measurements (≥ ±0.1pH),
once calibrated, the meter can be used for about a week or longer. In the following cases, the meter
must be re-calibrated:
•The electrode hasn’t been used for a long time or a new electrode is connected.
•After measuring strong acid (pH<2) or strong base (pH>12) solutions.
•After measuring fluoride-containing solution and strong organic solution.
•There is a significant temperature difference between the test sample and the buffer solution.

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6.4 Sample Test
Power on the meter. Connect the pH electrode and the meter goes to pH measurement mode
automatically. Screw off the KCL storage bottle. Rinse the electrode with pure water. Shake off excess
water or blot-dry with clean tissue or kimwipe. Stir the electrode for a few seconds after it’s dipped into
the sample solution and then hold it still. Wait for the reading to get fully stabilized ( appears and
stays). Then save the measurement by pressing .
According to the pH isothermal measurement principle, the closer the test sample’s temperature is to the
calibration solution’s, the higher the accuracy of the measurement. This principle is recommended to
follow for the best result. For example: If you have to test at 150°F, we recommend warming up the
calibration solutions to the same temperature before performing calibration in order to get the most
accurate readings.
6.5 Recommended pH Electrodes for Different Applications
Application
Ideal Apera pH Electrodes
General water solutions
201T-S, LabSen 211, LabSen 213
Beverage, beer, wine
LabSen 211, LabSen 213
Low ionic strength solutions (RO water, distilled water,
deionized water…)
LabSen 801, LabSen 803
Liquid food (milk, cream, yogurt, jam, sauce, etc.)
LabSen 821, LabSen 823
High-Temperature solutions
LabSen 841, LabSen 861
Low-temperature liquid
LabSen881
High salinity solutions
LabSen 841
Complex and caustic solutions (e.g. electroplating)
LabSen 861
Raw meat
LabSen 761, LabSen 763
Micro sample testing
LabSen 241-6, LabSen 241-3
Cosmetics, skincare products
LabSen 851-1, LabSen 851-S
Soil
LabSen 551, LabSen 553
Solid or semi-solid samples (cheese, dough, fruits, meat
products, etc.)
LabSen 251, LabSen 751, LabSen
753

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Strong acid solutions, HF containing solutions (HF
concentration<1%)
LabSen 831
Strong alkaline solutions
LabSen 841
Surface test (skin, paper, carpet, etc.)
LabSen 371
TRIS buffer solutions
LabSen 213, LabSen 223
Viscous solutions
LabSen851-S, LabSen 851-H
Wastewater or emulsion
LabSen 333
* An 8-pin to BNC convertor (AI7103) is required to connect LabSen pH electrode to this meter.
* Visit aperainst.de or contact us at [phone] for more details.
6.6 Parameter Setting
Table (4-1)
Prompt Mark
Parameter Setting Items
Code
Parameters
P1
pH buffer solution series selection
USA (U.S.A series)
NIS (NIST series)
CH (China series)
P2
Pure water pH temperature
compensation setting
OFF-On (shut-set)
P3
Ammonia added pure water pH
temperature compensation setting
OFF-On (shut-set)
P4
Temperature unit setting
°C - °F
P5
Back light display time setting
0-1-3-6min
P6
Auto power off setting
0-10-20min
P7
Restore to producer setting
OFF-On
6.6.1 pH buffer solution series selection (P1)
(a) Long press , meter enters P1 mode: see picture (4-2).
(b) Press or to choose buffer solution series:
(U.S.A series) — 1.68, 4.00, 7.00, 10.01and 12.45 pH
(NIST series) — 1.68, 4.01, 6.86, 9.18 and 12.45 pH
(China series) — 1.68, 4.00, 6.86, 9.18 and 12.46 pH
(c) Press to enter next parameter setting or press to confirm and return to measurement
mode.

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6.6.2 Pure water pH temperature compensation setting (P2)
(a) Short press in P1, the meter enters mode P2, see picture (4-3).
(b) Press or to choose
(pure water pH temperature compensation setting) or
(c) Press to enter next parameter setting or press to confirm
and return to measurement mode.
(d) The factory default setting is OFF
Note: will appear in the upper right corner of the LCD if pure water temperature compensation
function is set.
6.6.3 Ammonia added pure water pH temperature compensation
setting (P3)
(a) Short press in mode P2 to enter mode P3, see picture (4-4).
(b) Press or to choose (ammonia added pure water pH
temperature compensation setting) or
(c) Press to enter next parameter setting or press to confirm and return
to measurement mode.
(d) The factory default setting is OFF
Note: If the ammonia pure water pH temperature compensation function is set,
will appear in the right upper corner of the LCD when in measurement mode.
6.6.4 Temperature unit °C/°F setting (P4)
(a) Short press in P3 to enter P4, see picture (4-5).
(b) Press or to choose temperature unit: °C or °F.
(c) Press to enter next parameter setting or press to confirm
and return to measurement mode.
6.6.5 Back light display time setting (P5)
(a) Short press in P4 to enter P5, see picture (4-6).
(b) Press or to choose the time of back light auto off: 0min,
1min, 3min or 6min. The back light will be closed if choosing 0 min.
(c) Press to enter next parameter setting or press to confirm
and return to measurement mode.
(d) The factory default setting for P5 is 1 min.

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6.6.6 Auto power off time setting (P6)
(a) Short press in P5 to enter P6, see picture (4-7).
(b) Press or to choose the time: 0min, 10min or 20min. The
auto power off function will be closed if choosing 0 min.
(c) Press to enter next parameter setting or press to confirm
and return to measurement mode.
(d) The factory default setting for P6 is 10 min.
6.6.7 Restore to factory default setting (P7)
(a) Short press in P6 to enter P7, see picture (4-8).
(b) Press or to choose , meaning that the parameters
have been restored to the producer setting mode, and return to
measurement mode after 2 seconds. Returning to factory default setting
is to restore the meter to theoretical value (zero potential pH is 7.00, slope
is 100%), and set all the parameters to default settings. When the meter’s calibration or measurement is
performing abnormally, users can use this function, and then calibrate and test again. Please note that
this function is irreversible once used.
6.7 Cleaning pH Electrode
1) The measurement is only as accurate as the electrode is clean. Always thoroughly rinse off the
electrode before and after each test with pure water.
2) For tough contaminants, soak the electrode in Apera electrode cleaning solution (AI1166) for at
least minutes. Then use a soft brush to remove the contaminants. Afterwards, soak the electrode
in 3M KCL soaking solution for at least 2 hours. Rinse it off, then re-calibrate the tester before
using again.
3) For other special contaminants, refer to the table below:
Contamination
Cleaning Solution
Inorganic metal oxide
Dilute acid less than 1mol/L
Organic lipid, protein containing samples
Apera cleaning solution (AI1166)
Resin macromolecule
Dilute alcohol, acetone, ether
Paints
Dilute bleacher, peroxide

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6.8 The Self-diagnose Information
During usage, the following icons may show up on LCD. This is the meter’s self-diagnose information,
which can help you understand what may go wrong with the meter or electrode.
4.6.1. The stable icon -2.00 pH or 19.99 pH — this icon appears when the pH value exceeds the
measurement range. This icon will also show up when the electrode is not well connected with the meter
or when the electrode is not insertd into solution. This is a normal phenomenon.
4.6.2. — Electrode offset out of range (<-60mV or >60mV)
4.6.3. — Electrode slope out of range (<85% or >105%)
When or shows up:
1) Make sure there is no damage on the electrode’s glass bulb (if so, an electrode replacement is
necessary). Check if there is air bubble inside the glass bulb. If so, shake the electrode with force
for several seconds to remove the bubble.
2) Check the quality of buffer solution. Make sure it’s fresh and clean and conforms to the meter’s
buffer series setting.
3) Set the meter to factory default setting mode (refer to section 6.6.7), then recalibrate it.
If the meter is still not working after the above checkings, it’s time to replace a new pH electrode or contact
us at [phone] or [e-mail] for help.
6.9 Notes about pH
1) Never store pH electrode in pure water as it will damage the electrode. Always store the electrode
in 3M KCL soaking solution (AI1107).
2) If you find any air bubble in the glass bulb of the pH sensor, simply shake the probe with force for
a few times to remove it. The existence of an air bubble in the glass bulb will significantly decrease
the stableness of measurement.
3) pH electrode is technically a chemical battery. Every pH electrode will eventually die even if you
don’t use it at all. The typical service life of a pH electrode is 1-2 years depending on how you
use and maintain it. We recommend replacing your pH electrode after one year of use to ensure
the best result.
4) Always keep the meter’s connector clean and dry, otherwise it may lead to an inaccurate
measurement. If contaminated, clean the connector with alcohol prep pads and blow-dry.

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7 DO Measurement
7.1 Preparation before use
1) Connect the DO electrode (See Section 5 for connection tutorial). Then Press to power on.
2) Choose DO unit: Hold to switch among mg/L, ppm and %. When the desired unit shows up,
release the button to confirm the unit selection.
7.1.1 Check the sponge
Check on the DO500 DO electrode: see picture (4-1). Screw off the bottom lid of electrode calibration
sleeve, inspect whether the sponge inside is moist. If it is dried out, add in several drops of distilled water
to the sponge to maintain the satured air inside the calibration sleeve (Do NOT add too much water so it
won’t be dripping).
7.1.2 Electrode polarization
After each power on, the DO electrode must be polarized for 5 minutes before measurement or calibration.
So do not to turn off the meter in the middle of your DO measurement. Otherwise, you need to polarize
the DO electrode again after next power on (The auto. power off function in DO mode has been turned
off by default).
7.1.3 How to polarize the DO electrode
Just connect the DO electrode to the powered-on meter without measurement or calibrating.
Electrode polarization is to consume the remaining oxygen inside the inner solution of the membrane cap
so as to ensure fast response and high accuracy. If a new membrane cap is replaced, or new inner
solution is added, the polarization time should be 30 minutes instead of 5 minutes.
1 — DO500 DO electrode
2 — Calibration sleeve cap
3 — Calibration sleeve
4 — Water stored sponge
5 — Calibration sleeve bottom lid
6 — Temperature electrode
7 — Membrane cap
8 — Cathode (gold)
9 — Anode (silver)
10 — Salinity electrode

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7.2 DO Calibration (Saturated Oxygen)
Power on the meter, press , the meter enters calibration mode, CAL will be flashing at the upper-
right corner. Insert the DO electrode to the calibration sleeve, and tighten the sleeve cap. Let the electrode
sit for 3 to 5 minutes.
When the reading is fully stabilized, press again to finish the calibration. A flashing 100% will show
up, and the calibration will be finished in several seconds and the meter will return to measurement mode.
If the reading is unstable, please wait for several more minutes. Do NOT press until the reading is
fully stabilized.
7.3 DO Measurement
7.3.1 Measurement DO in flowing water
Power on the meter. Place the DO electrode into the flowing water. The water surface level should be
higher than the location of the temperature electrode. The angle for the electrode and water surface
should be 45° to 75°.
Hold the electrode in the flowing water and wait for a stable reading, then press to record the
measurement. If the reading cannot be stabilized, keep swirling the electrode slowly in water to help you
get a stable reading (typically takes 1-5 minutes).
7.3.2 Measurement DO in static water
Power on the meter. Place the DO electrode into water. The water surface level should be higher than
the location of the temperature electrode. The angle for the electrode and water should be 45° to 75°.
Keep swirling the electrode in water slowly (>5cm/s) for 1 to 5 minutes. When the reading is fully stabilized,
record the measurement (do NOT stop the swirling while recording the measurement).
Never stop the DO electrode swirling in static water while measurement. Otherwise, the dissvolved
oxygen will be consumed by the DO electrode and the reading will keep dropping.
7.4 Notes about DO Measurement
a) When calibrating, the environment temperature and water temperature should be close to each
other (≤10°C). If the difference is large, please dip the electrode into the sample water for about
10 minutes, then calibrate the electrode according to Section 7.2.

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b) Temperature has a large impact on DO measurement. When the electrode temperature and water
temperature have larger difference, the reading time must be greater than 3 minutes. Otherwise,
there could be some large errors.
c) Salinity and barometric pressure also affect DO measurement. The meter has automatic salinity
compensation and manual barometric pressure compensation. For details, refer to Section 7.7
and 7.8.
d) When the meter is performing abnormally, please set the mode P7 to be “On”to restore meter
to factory default setting, and then do the calibration again before measurement.
7.5 DO Electrode Maintenance
7.5.1 Always maintain a moist sponge
Screw off the bottom lid of electrode calibration sleeve (see picture 4-1), inspect whether the sponge
inside is moist. If it is dried out, please add in several drops of distilled water to the sponge to maintain
the satured air inside the calibration sleeve (Do NOT add too much water so it won’t be dripping).
7.5.2 Remove air bubbles
Inspect the inner solution within the electrode membrane cap. Make sure it doesn’t have air bubbles. If
there are air bubbles, please screw off the membrane cap and add in new inner solution to remove the
bubbles. Then screw the cap back on.
7.5.3 Clean the gold cathode
When the gold cathode is contaminated, use the polishing paper (DO504) to lightly wipe the gold cathode
(along the arc of the gold surface); clean the gold surface with a clean tissue or kimwipe afterwards; rinse
off the electrode with distilled water, then shake off excess water; Add some new inner solution into the
membrane cap; screw on the cap. Then perform the saturated oxygen calibation according to 7.2 before
measurement again.
7.5.4 Replace the membrane cap
When the DO electrode’s response time becomes slower, obvious errors occur in measurement, or when
the sensitive membrane of DO electrode is wrinkled, cracked, or damaged at any extent (the well-
functioning membrane surface should look perfectly smooth), it’s time to replace a new membrane cap
(DO503) according to the following steps.
a) Screw off the membrane cap;
b) Rinse off the electrode without membrane cap with distilled water and shake off excess water.
c) Clean on the cathode surface (gold slice) with a clean tissue or kimwipe;

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d) Add new inner solution (DO502) into a new membrane cap (DO503) slowly and do not let any air
bubbles appear. If you found any air bubbles in the cap, carefully flick on the membrane cap to
eliminate them.
e) Place the membrane cap on the table and put in the electrode vertically, slowly twist in, and then
screw on the cap tightly. The excess inner solution will be squeezed out. Please wipe it off with
tissue and rinse off the electrode in distilled water.
f) Check if there are any air bubbles in electrolyte (except for the smaller air bubbles), If so, re-
assembly is needed.
g) Do not touch the sensitive membrane with fingers directly because the sweat and grease will
affect the quality of the membrane.
7.6 Zero Oxygen Calibration
Usually there is no need to do zero oxygen calibration unless you have a high requirement for accuracy
in low oxygen level (<1.0ppm). To do zero oxygen calibration, follow the steps below:
Prepare 100mL of zero-oxygen water: In a 100mL beaker, add in 5.0g anhydrous sodium sulfite (Na2SO3);
add distilled water to 100mL, mix well to dissolve. Note that the zero-oxygen water is only effective within
24 hours.
1) Polarize the meter for 15 minutes, and then perform the saturated oxygen calibration according
to Section 7.2.
2) Place the electrode into zero-oxygen water, press to enter calibration mode. When the
stabilized reading is 0.02 ppm to 0.15 ppm after 5 minutes, press again to finish the
calibration, then flashing 0.0% will show up, the calibration will be finished in several seconds and
0.00 ppm will show up. Rinse off the electrode with distilled water afterwards.
3) If the reading is less than 0.02 ppm after 5 minutes, it means the electrode is in good condition
and there is no need to perform zero oxygen calibration. Just press
to return to measurement mode.
4) If the reading is greater than 0.15 ppm after 5 minutes, it’s time to replace the membrane cap
according to section 7.5.4 or remove the membrane cap and use the polishing paper (DO504) to
lightly polish the gold cathode according to section 7.5.3. Then perform the saturated air
calibration and zero oxygen calibration before measurement.

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7.7 Salinity Calibration
Dissolved oxygen is also affected by salinity in water. The meter has automatic salinity
compensation, and the salinity electrode is pre-calibrated before leaving the factory. In general,
there is no need to perform salinity calibration. Salinity calibration is only needed when replacing a
new electrode. For the procedure of salinity calibration, please refer to section 7.9.3 in P2.
For the salinity electrode installed in DO electrode (see picture 4-1), its surface is plated with a layer
of platinum black to reduce electrode polarization. When cleaning salinity electrode, never brush it
otherwise the coating will be peeled off. Just rinse it off by swirling in pure water to remove
contaminants. For organic contaminants, rinse off with warm water containing detergent or alcohol.
7.8 Barometric pressure manual compensation
Dissolved oxygen is also affected by barometric pressure. The meter has the function of manual
barometric pressure compensation. We suggest resetting the value according to the standard
barometric pressure table when there is a big change in barometric pressure to ensure the accuracy
of barometric pressure compensation (Please see Appendix 2 and Appendix 3). For the procedure
of barometric pressure calibration, refer to 7.9.4.
7.9 Parameter Setting
7.9.1 DO parameter settings (Chart 4-1)
Prompt Mark
Parameter Setting Items
Code
Parameter
P1
Resolution selection
0.01/0.1(mg/L and ppm)
0.1/1(%)
P2
Salinity calibration
P3
Barometric pressure setting
P4
Temperature unit setting
°F/°C
P5
Back light display time setting
0-1-3-6 min
P6
Auto power off time setting
0-10-20 min
P7
Restore to producer setting
OFF-On (shut-set)

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7.9.2 Resolution selection (P1)
(a) Press to enter P1 mode, see picture (4-2);
(b) Press to choose resolution: 0.01 → 0.1;
(c) Press to enter the next parameter setting or press to
return to the measurement mode.
7.9.3 Salinity calibration setting (P2)
(a) In P1, short press to enter P2, see picture (4-3).
(b) Immerse the DO electrode into 12.88mS/cm calibration solution
(sold separately), solution level should exceed salinity electrode; let
it stand still after stirring; then press when the value is stable
and smiley face icon appears. 12.9 will show up on the screen. The
calibration will be finished after several seconds, and the meter will display a stable salinity value.
(c) Short press to enter the next parameter setting or short press to return to
measurement mode.
7.9.4 Barometric pressure setting (P3)
(a) In P2, short press to enter P3, the screen displays a
barometric pressure measurement value set earlier (unit is kPa),
such as 101.3 kPa, shown as picture (4-4).
(b) Press or to adjust according to standard barometric
pressure value, and long press them to quickly change the value.
(c) Press to enter next parameter setting or short press to confirm and return to
measurement mode.
7.9.5 Temperature unit °C/°F setting (P4)
(a) Short press in P3 to enter P4, see picture (4-5).
(b) Short press or to choose temperature unit: °C or °F.
(c) Short press to enter next parameter setting or short press
to return to measurement mode.