APERA INSTRUMENTS / SX751 / AI469

Four measurement methods.
One portable kit.

SX751 combines pH, ORP, conductivity and dissolved oxygen testing in a battery-powered carrying-case kit. Connect the appropriate supplied electrode, let the meter recognize its mode, and take one measurement at a time. Conductivity also supports TDS, salinity and resistivity displays; these are related outputs, not additional independent probes. Temperature accompanies supported electrode measurements.

Choose AI469 for sequential spot checks with replaceable 201T-S, 301Pt-S, 2301T-S and DO500 electrodes and on-meter recall. The kit includes polarographic DO500, not an optical oxygen sensor. A dedicated 301Pt-S ORP electrode is supplied as well. It is not a simultaneous multichannel logger or a wireless app-based tester.

Recognize the electrode, then select the display

Automatic recognition selects the measurement family for the connected probe. Use the keypad to choose conductivity, TDS, salinity or resistivity, and the desired resolution or units. This changes the displayed result, not the number of sensors connected. The backlit LCD includes temperature, stability, calibration and memory indicators.

Keep instrument range separate from probe suitability

The pH meter spans -2.00 to 19.99 pH, but the current US 201T-S probe page specifies 0 to 13 pH and 0 to 60°C. It is intended for general aqueous samples, not every solution within the meter display range. Older probe instructions show wider limits; use the conservative current probe specification when selecting an application. 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.

Conductivity with an identifiable cell

The supplied 2301T-S uses a K=1 platinum-black cell and a temperature sensor. Current US documentation rates the probe to 200 mS/cm; the meter display ends at 199.9 mS/cm. K=0.1 and K=10 settings accommodate other suitable cells, not extra electrodes in this kit. Verify high-range work against an appropriate standard.

Know what this configuration supplies.

Routine water and treatment checks

Compare compatible water samples using consistent containers, immersion and temperature equilibration. Record the selected parameter and units with every result. Conductivity measures aggregate ionic response; calculated TDS is not a gravimetric solids test, and an ORP or pH reading alone cannot establish drinking-water safety.

Field rounds with deliberate sample records

The IP57-rated handheld format and carrying case suit spot-check routes where the operator can rinse probes between sites. Use the 400-set memory alongside a sample log. Protect the connector from contamination, keep spare solutions closed, and respect the stated 5 to 35°C instrument working environment.

Aquatic systems

For ponds, aquaculture or surface-water checks, allow time for DO polarization and sample flow. Do not treat a stationary oxygen probe as an unattended monitor. Use representative sampling and separately document pH and conductivity rather than implying all readings occurred at exactly the same instant.

Prepare each probe.
Calibrate each method separately.

Align the 8-pin plug notch with the single meter socket, push straight in and tighten only the metal locking ring. Do not twist the black rubber cable section. Rinse the selected electrode with distilled or deionized water before testing, remove excess water gently and avoid contaminating fresh standards.

For pH, use the selected USA, NIST or China buffer series and one to three recognized calibration points. The supplied bottles are pH 4.00, 7.00 and 10.01. The manual calls for three-point calibration with a new pH electrode. Match calibration and sample temperatures where practical; automatic temperature compensation corrects electrode response, not every chemical temperature effect.

For conductivity, calibrate in conductivity mode with one recognized standard close to the sample range. The kit supplies 1413 µS/cm, not every supported standard. Reference temperature choices are 18, 20 or 25°C; the linear coefficient adjusts from 0.00 to 9.99%/°C, default 2.00. Record settings before comparing batches.

For ORP, connect the 301Pt-S and wait for a stable mV value. The manual requires no ORP calibration; use a suitable 222 mV standard to check performance when needed, observing its temperature-dependent reference value. Do not apply the pH automatic-temperature-compensation claim to ORP or assume a clean pH electrode validates this separate probe.

For DO500, polarize for five minutes after power-on before calibration or testing. Use 30 minutes after replacing a membrane cap or inner solution. Calibrate at saturation in the sleeve with a moist, non-dripping sponge and wait for stability. Temperature and salinity compensation are automatic; barometric pressure is entered manually.

Routine DO calibration is saturation calibration. Optional zero-oxygen calibration follows the exact model manual and requires separately obtained supplies. Sodium sulfite and the 12.88 mS/cm standard for salinity-sensor recalibration are not listed kit supplies. DO502 is electrode electrolyte, not a calibration standard.

Different probes need
different storage and maintenance.

Keep the 201T-S glass bulb hydrated in its 3M KCl storage solution. Do not store a pH electrode in distilled or deionized water. By contrast, the 2301T-S conductivity cap uses clean distilled or deionized water. Rinse between samples and keep the electrical plug dry; never interchange these storage routines.

The low-conductivity nonlinear compensation function is described for readings below 10 µS/cm. It does not make the supplied pH electrode suitable for ultrapure water. For very low ionic strength pH samples, select an appropriate specialist electrode. Low-range conductivity may require a compatible K=0.1 cell and a controlled flow-cell method.

DO500 consumes oxygen during measurement. In still water, the manual requires continuous slow swirling above 5 cm/s, including while recording. Keep the temperature sensor submerged and the probe at 45 to 75 degrees to the surface. Inspect the membrane and electrolyte for damage or bubbles; use DO503 caps and DO502 refill rather than treating it as a no-refill optical probe.

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.

DO500 is intended for general water solutions, not strong acidic, strong alkaline or organic solutions. Its probe operating temperature is 0 to 50°C, distinct from DO compensation at 0 to 45°C and meter operating conditions.

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 the readings you choose.
Keep sample identity in your own log.

The exact SX751 manual documents 400 stored sets. A short memory-key press saves a stabilized result; a long press enters recall. Records contain a serial number, measurement value, unit and temperature. The stated capacity is not a promise of 400 records for every mode or simultaneous complete-parameter datasets.

M+ and RM identify saving and recall. The manual does not document a date/time clock, timed-interval storage, USB export, Bluetooth, browser export or an electronic GLP audit trail. Record sample identity and collection time separately. Deleting memory clears all saved records, so transcribe the required values before using that command.