APERA INSTRUMENTS / SX725 / AI465

Two water checks.
One probe connection at a time.

SX725 pairs a DO500 polarographic oxygen probe with a 201T-S pH/temperature electrode for sequential testing of general water solutions. Change electrodes to change measurement tasks. Its backlit screen, 300-set memory and fitted carrying case support a practical field workflow.

Choose AI465 when you need pH and DO in one case, not simultaneous multi-channel monitoring. The 8-pin input recognizes the connected electrode; temperature accompanies the selected measurement. The included probes are for general aqueous samples, not every liquid the meter can display a number for.

Compensate for the water you are testing

DO500 contains temperature and salinity sensors. DO compensation is automatic over 0 to 45°C and 0 to 45 ppt; barometric pressure is entered manually over 66 to 200 kPa. The P3 example of 101.3 means kPa, not mbar. These corrections do not remove the need for calibration, clean sensors or adequate water movement.

Recognize the electrode, then select the task

Connect the 201T-S for pH and temperature or the DO500 for dissolved oxygen and temperature. Automatic electrode recognition is not simultaneous pH/DO acquisition. The manual also specifies an mV mode, but an ORP electrode and any required connection accessory are not included.

Replace the membrane, not the whole workflow

The DO503 cap holds a sensitive membrane over the gold cathode and silver anode. DO502 inner solution completes the serviceable assembly. Keep spare caps clean and avoid touching their membrane faces. The kit includes the maintenance materials and a screwdriver; replacement frequency depends on condition, not a promised fixed service interval.

Know what this configuration supplies.

Aquaculture and surface-water rounds

Compare oxygen conditions in compatible pond, aquarium or surface-water samples. Let the probe approach sample temperature and maintain flow across the membrane. Record location and collection time separately so readings remain meaningful after on-meter recall.

Water-treatment pH and oxygen checks

Use the two electrodes in separate measurement steps. Rinse between solutions and calibrate each method independently. Conductivity, TDS and resistivity are not included measurement channels; the DO salinity sensor provides oxygen compensation.

Training with visible maintenance steps

The replaceable membrane, sleeve and original electrode figures make preparation and care easy to explain. A field reading does not establish potability, regulatory compliance or biological safety. Strong acidic or alkaline water and organic solutions are outside DO500’s listed suitable samples.

Polarize first.
Calibrate, then keep the water moving.

Connect DO500 to the powered meter and allow five minutes of polarization after every power-on. After changing the membrane cap or electrolyte, allow 30 minutes instead. The response specification is no more than 30 seconds to 90% response at 25°C, not a guarantee of a fully stable result in 30 seconds.

For routine 100% calibration, moisten the sleeve sponge with several drops of distilled water without making it drip. Insert the probe, tighten the sleeve cap and enter CAL. Allow three to five minutes, longer if unstable, then confirm only after stabilization. Keep calibration and sample temperatures within 10°C; for a larger difference, precondition the probe in sample water for about ten minutes.

Immerse above the temperature sensor and hold the probe at 45° to 75° to the water surface. In flowing water, wait for stability and swirl slowly if needed. In static water, keep swirling faster than 5 cm/s for one to five minutes and continue while recording. A large temperature difference requires more than three minutes. Stopping motion lets the electrode deplete nearby oxygen.

The specification also lists air-saturated water, but the detailed routine procedure uses water-saturated air in the sleeve. Optional zero calibration is intended for demanding work below 1.0 ppm. Follow the manual’s fresh sodium-sulfite solution, 15-minute polarization and acceptance checks. Arbitrary custom DO calibration points are not documented. Salinity is factory-calibrated; after electrode replacement, the P2 procedure uses separately purchased 12.88 mS/cm standard.

For pH, rinse the 201T-S, briefly stir in buffer, then hold still until stable. Use one, two or three points from the selected USA, NIST or China series; the kit supplies pH 4.00, 7.00 and 10.01. Calibrate a new pH electrode at all three points. Do not apply the DO requirement for continuous swirling to a pH reading.

Keep the membrane clean.
Keep the sleeve sponge moist.

Between uses, protect DO500 in its calibration/storage sleeve with the sponge moist from a few drops of distilled water, not dripping. Do not fill this sleeve with pH KCl storage solution. Rinse the probe after testing and inspect the membrane and electrolyte for damage or bubbles. Keep the electrical connector clean and dry.

Replace a wrinkled, cracked or damaged membrane, or investigate slow response and obvious errors. Unscrew the cap, rinse the exposed electrode with distilled water and shake off excess. Wipe the gold cathode with clean tissue. Slowly add fresh DO502 to a new DO503 cap without bubbles, stand it on a table and screw the electrode vertically into it. Wipe expelled electrolyte, rinse and check again for bubbles; reassemble if necessary.

For a contaminated gold cathode, use DO504 polishing paper lightly along the curved gold surface, then clean, rinse and refill as instructed. Never brush the platinum-black salinity sensor; swirl it in pure water instead. Re-polarize after fresh electrolyte and recalibrate before measurement. Handle electrolyte with appropriate protection and consult its safety instructions.

The 201T-S needs different storage: keep its glass bulb in the supplied KCl storage bottle, never in pure water. Its gel reference electrolyte is not refillable. Rinse before and after pH use; never rub the glass membrane. Do not transfer DO500 electrolyte or cathode-polishing instructions to the pH electrode.

The current 201T-S listing excludes low-ion water, strong-base samples above pH 12 and highly corrosive liquids. Its probe guide also cautions against frequent testing below pH 2 or above pH 12 and prolonged exposure above 50°C; 0 to 60°C is not a continuous-use guarantee.

Read the display. Check the measurement mode.

Use the physical controls for measurement, calibration and the documented record functions. Check units, temperature, calibration indicators and the selected parameter before assigning a result to a sample. Screen values in photographs are illustrative, not sample results. The model-specific record and output workflow is explained below.

Follow the exact parameter-specific calibration procedure, using the required standard, solution or calibration atmosphere. Temperature compensation does not make different sample temperatures, preparation methods or chemistry equivalent. Follow the model-specific meter and electrode instructions before extending the method to a new sample.

Save 300 sets.
Recall locally and keep a separate field log.

When the smile remains on screen, a short press of M+/RM stores the reading. SX725 holds up to 300 sets containing serial number, measurement value, unit and temperature. The manual states 300 sets for the meter, not 300 per mode; it does not specify a per-mode allocation.

A long press recalls the last saved record; use the review keys to browse. The manual’s delete operation in recall mode clears all saved data, so transcribe needed readings first. No date/time fields, real-time clock setting, timed automatic logging, USB, Bluetooth or PC-Link export workflow is documented for this model. “Data logging” here means operator-initiated save and recall.