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The combo pen with the ORP head supplied, not merely compatible. GroStar GS4-P Gen II combines its model-specific sensor setup with a practical calibration routine and TruRead session summaries.
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APERA GROSTAR / GS4-P / GEN II / AI204G
GS4-P supplies the GS4 pH/EC combo pen and an additional GS5-E platinum ORP probe. That makes the extra sensor a visible kit item, not an assumed mode of the installed combo head.
Choose the P kit when pH, EC/ppm and separate ORP checks belong in your liquid-testing workflow. Exchange the head for ORP; it is not simultaneous pH/EC/ORP measurement from one sensor.
2 probes
One pen body
Combo setup or ORP setup after a swap
pH + EC/ppm
Standard combo head
500 and 700 are calculated scales
GS5-E
ORP head included
A separate platinum sensing probe
Choose by the measurement and the probes supplied. Each card opens that model’s product page; optional compatibility does not mean an extra probe comes in the box.
AI101G
Liquid pH
pH probe supplied; ORP head optional.
AI201G
Liquid pH + ORP kit
Two probes supplied; swap to read ORP.
AI102G
Moist soil/media pH
Spear, dibber and cleaning brush.
AI103G
Liquid EC / ppm
EC sensor supplied; no pH probe.
AI104G
Liquid pH + EC / ppm
Combo head supplied; ORP head optional.
AI204G
Combo + ORP kit
Combo and separate ORP heads supplied.
ONE SOLUTION. DIFFERENT CONVERSION SCALES.
Start with the conductivity. Apera’s worked example uses 2.6 mS/cm. The 500 and 700 displays apply different multipliers to that same EC; changing the scale does not change the nutrient solution.
2.6 mS/cm
The measured EC
1300 ppm
500 scale: 2.6 × 500
1820 ppm
700 scale: 2.6 × 700
Match the feed chart’s scale, or compare EC directly. This is a conversion example, not a recommended feed strength or dosing target.
The P kit supplies both the standard probe and GS5-E. Physically change the installed head to move between the standard setup and ORP. The extra probe is not hidden inside the pH or combo head.
pH, EC, calculated ppm and temperature with GS4-E.
Fit the supplied GS5-E and confirm mV mode. ORP is a separate measurement step, not simultaneous capture with the other probe.
GS4 pen with GS4-E combo probe and cap, separate GS5-E ORP probe, pH 4.00 and 7.00 buffers (50 mL each), 2.77 EC standard (50 mL), 3M KCl soaking solution (10 mL), four installed AAA batteries, lanyard and instructions.
The supplied-fluid list is separate from optional compatibility: ORP checking solution and pH 10.01 are separate.
WHAT THIS MODEL IS BUILT TO ANSWER
Use the GS4-E for pH and conductivity-based feed measurements. Keep the pH calibration and EC calibration distinct.
Fit GS5-E to take redox readings as part of an established procedure. An ORP result is not a direct disinfectant concentration or safety verdict.
Compare base GS4 with GS4-P by what is physically supplied. Optional compatibility is not the same as an included accessory.
Supplied probe: GS4-E / AI804G plus GS5-E / AI805G. The standard head combines double-junction glass pH and titanium EC sensing; the two ppm scales are calculated from EC.
Replaceability gives the worn sensor a maintenance path; it is not an unlimited service-life guarantee. Install, condition and calibrate the correct head according to its instructions.
A SESSION SUMMARY, NOT A CLOUD LOGGER
In the documented Gen II TruRead workflow, a reading that has stayed stable for seven seconds is recorded for the session. Review the average, maximum and minimum after collecting your measurements. Seven seconds of stability is not the same as a fixed seven-second sampling interval.
6.2 pH
Illustrative minimum
6.3 pH
Illustrative average
6.4 pH
Illustrative maximum
The three illustrative values explain the summary; they do not prescribe a fixed sample count. This pH example does not establish TruRead behavior for the separate ORP setup. The reviewed instructions do not establish retained history, timestamps or export. Use a suitable logger if those are requirements.
Calibrate pH at 7.00 then 4.00 and conductivity at 2.77 EC. Check ORP with a suitable separate ORP standard at the bottle’s temperature reference; do not treat the supplied pH/EC standards as ORP calibration fluids.
Rinse between standards and samples to limit carryover. Remove excess water and bubbles, then wait for stability.
Use fresh, correct standards. A calibration reminder or electrode-condition display supports the routine; it does not replace checking the method.
Watch the manufacturer’s family tutorial alongside the current Gen II manual. The video is a practical family reference, not proof that every photographed control or accessory is identical across revisions.
The GS4-E combo head follows its own humid-cap/long-term KCl routine; do not store it immersed in pure water. Keep GS5-E checking and maintenance specific to the ORP head. The pH slope indicator does not describe platinum ORP-electrode health.
This is a probe-change workflow. The ORP head replaces the combo probe, so confirm the installed head and displayed unit before recording.
Match 500/700 to the feed chart or use EC. The same solution can legitimately produce different numerical ppm readouts.
The pH condition figure shown after calibration describes electrode response. It is not a battery indicator, soil-health score or guaranteed percentage of service life left. Use the manual’s cleaning, conditioning and calibration checks before deciding whether to replace the probe.
Calibration markers remind you about the routine. Their disappearance after the documented reminder period is not evidence that the pen has automatically erased calibration.
If the job is a cabled-probe water-testing workflow rather than a pocket grower pen, compare the alternatives below. These are separate instruments, not plug-in accessories or direct-soil replacements for GS4-P.
Alternative format: Hanna HI99300 portable liquid EC/TDS meter
Related workflow: Hanna HI98195 portable water pH/ORP/EC meter
GS4-P supplies the GS4 pen/combo probe plus a separate GS5-E ORP probe. The base GS4 kit supports the optional ORP head but does not include it.
No. It describes pH, EC, two conductivity-derived ppm scales, temperature and an ORP workflow using the separate head. The 500/700 ppm values are conversion scales, not extra chemical sensors.
No. Physically exchange the installed GS4-E for GS5-E to take ORP readings. Do not interpret a kit with two probes as simultaneous acquisition from both.
The supplied set includes GS4 with combo probe, GS5-E ORP probe, pH 4.00/7.00 buffers, 2.77 EC standard, KCl soaking solution, four installed AAA batteries, lanyard and instructions. ORP checking solution and pH 10.01 are separate.
Apera’s US family catalog lists GS4-P ORP range -1000 to +1000 mV, 1 mV resolution and ±2 mV accuracy. Those catalog terms are not the ±0.2% F.S. expression of a different tester, and no ORP ATC claim is added here.
The 500 and 700 scales calculate different numerical values from the same EC. At 2.6 mS/cm the values are 1300 and 1820 ppm respectively. Match the feed chart’s scale, not just its number.
The instructions establish session collection after seven seconds stable and summary average/max/min, not timestamps, retained history or export. Do not assume the same session function is established for every ORP-probe mode.
The combo head is GS4-E, AI804G, and the ORP head is GS5-E, AI805G. Follow each probe’s own checking and care instructions; do not apply pH-glass condition percentages to the ORP sensor.
Review the GS4-P kit and specifications, then return to the price and ordering controls. Blue Tiger Scientific can help you match the measurement, sensor and routine before ordering.
APERA INSTRUMENTS, LLC warrants this instrument to be free from defects in material and workmanship and agree to repair or replace free of charge, at the option of APERA INSTRUMENTS, LLC, any malfunctioned or damaged product attributable to the responsibility of APERA INSTRUMENTS, LLC for a period of TWO YEARS for the instrument and ONE YEAR for the probe from the delivery.
This limited warranty does NOT cover any issues due to:
Use the current Gen II guide for controls, sampling and care. The US catalog adds family/probe/kit comparison; it is not a replacement for the operator instructions. Each card opens a PDF in a new tab.
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