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pH and feed-strength checks in one replaceable-probe pen. GroStar GS4 Gen II combines its model-specific sensor setup with a practical calibration routine and TruRead session summaries.
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Apera’s pocket-probe listing names GS4-E as the replacement pH/EC combo probe.
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APERA GROSTAR / GS4 / GEN II / AI104G
GS4 Gen II combines a double-junction pH sensor with titanium conductivity sensing in one replaceable probe. Switch between pH, EC and the 500/700 ppm scales, with temperature displayed and TruRead session statistics available.
Choose GS4 for liquid pH plus conductivity-based feed checks. Its standard GS4-E is a solution probe, not a soil spear; ORP needs a separately purchased GS5-E.
pH + EC
One replaceable combo probe
Double-junction pH and titanium EC
500 / 700
Two derived ppm scales
No extra nutrient sensors
TruRead
Stable-reading sessions
Average, maximum and minimum
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.
GS4 pen with GS4-E pH/EC combo probe and cap, 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: pH 10.01 and any optional ORP head are separate.
WHAT THIS MODEL IS BUILT TO ANSWER
Use pH and EC as separate pieces of information. EC reflects the combined ions; pH describes acidity. Neither alone identifies individual nutrients.
Use pH buffers for the glass sensor and 2.77 EC standard for conductivity. Do not substitute a pH buffer for EC calibration.
Match the scale on the feed chart. A higher number after switching from 500 to 700 is not evidence that the nutrient solution became stronger.
Supplied probe: GS4-E / AI804G. 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. The reviewed instructions do not establish retained history, timestamps or export. Use a suitable logger if those are requirements.
Complete pH 7.00 then 4.00 calibration without switching off, and perform the separate EC calibration at 2.77 EC. pH 10.01 is optional; a third recognized pH point is not an included bottle.
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.
For regular use, keep the combo probe in a tightly closed humid cap. For longer storage, follow GS4’s 3M KCl cap-fill instructions, never immersion storage in pure water. Rinse without rubbing the glass. Keep pH-only sample conditioning separate from EC measurements.
Allow stabilization and use the appropriate method. Do not add KCl to a reservoir or an aliquot you will then measure for EC; added salt changes conductivity.
Check whether the display is on the 500 or 700 scale. Use the same scale throughout a comparison, or compare EC directly.
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.
Alternative format: Hanna HI99300 portable liquid EC/TDS meter
Related workflow: Hanna HI98195 portable water pH/ORP/EC meter
The kit supplies GS4 with GS4-E combo probe, pH 4.00/7.00 buffers, 2.77 EC standard, KCl soaking solution, four installed AAA batteries, lanyard and instructions. The GS5-E ORP probe and pH 10.01 buffer are separate.
It counts pH, EC, 500ppm, 700ppm and temperature readouts. The two ppm scales derive from conductivity; they are not two independent nutrient-concentration sensors.
The base kit does not include an ORP head. Current Gen II instructions support the separately sold GS5-E; exchange the probe for ORP. GS4-P supplies that additional probe in the kit.
The standard GS4-E is a liquid pH/EC combo probe, not the GS2 soil spear. Choose a proper sample method and compatible soil configuration rather than forcing this probe into growing media.
pH uses pH 7.00 followed by 4.00, with optional 10.01. Conductivity uses a single 2.77 EC calibration point. EC calibration also underpins the calculated ppm scales.
Use the scale specified by the feed chart or compare EC. At 2.6 mS/cm the readings are 1300 ppm500 or 1820 ppm700 for the same solution; changing scale does not change the solution.
It reports pH electrode slope after calibration, not a guaranteed life percentage or the condition of the EC element. Cleaning, conditioning and recalibration checks come before assuming a replacement is needed.
Use the model’s humid-cap and longer-term KCl directions, rinse between samples and do not rub the glass. Do not store immersed in pure water or add KCl to an EC sample or nutrient reservoir as a general stabilization trick.
Review the GS4 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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