Quick Answer

Replacement double-junction probe for the Bluelab pH PenPlus. Fits nothing else.
Screws in by hand in under a minute, no tools, then recalibrate and you're going again.
Roughly half the cost of a new pen, which is the whole point of the PenPlus.
IP68 rated, supplied with a KCl-filled silicone storage cap.

Bluelab pH ProbePlus

pH probes wear out. The glass membrane ages through normal use, and once it does, readings drift and calibration stops holding. It isn't a fault, it's the nature of the thing. With most pens that means buying another pen. With the pH PenPlus it means fitting one of these.

When to Replace the Probe

A probe usually gives 12 to 18 months, though that varies a lot with how often it's used and whether it's been stored wet. The signs are consistent:

Readings drift between calibrations, so a pen calibrated this morning reads differently by the afternoon.
Calibration takes several goes, or the pen refuses to accept it.
Response goes sluggish, taking much longer to settle on a number than it used to.
Readings disagree with a second meter you trust.

Worth ruling out the cheap causes first. Old calibration solution and a dirty probe cause more bad readings than genuine probe failure. Our Probe Care Kit handles cleaning, and the inconsistent readings guide works through the whole checklist.

Fitting It

Unscrew the old probe, screw the new one in, calibrate. It takes about a minute and needs no tools. Calibrate straight after fitting, because a new probe won't match the calibration the old one was carrying.

Keep the KCl-filled cap on whenever the pen isn't in use. The internal sponge holds moisture against the probe, and a probe that dries out is permanently damaged rather than just needing a recalibration. That single habit does more for probe life than anything else.

Worth Keeping a Spare

A drifting pen mid-grow is worse than no pen, because you're making feed decisions on numbers you can't trust. Having a probe on the shelf turns that into a two-minute fix instead of a week waiting on delivery.

Technical Specifications

Fits. Bluelab pH PenPlus only. Not compatible with the older pH Pen, Combo Meter or Guardian Monitor.
Probe Type. Double-junction pH, gel filled, non-refillable
Measures. pH and temperature
Fitting. Screw-cap, hand tightened, no tools
Ingress Rating. IP68
Supplied With. KCl-filled silicone cap with internal sponge
Typical Service Life. 12 to 18 months with proper care
Calibration. Required immediately after fitting

Frequently Asked Questions

Will this fit my Bluelab pH Pen?

Only if it's the PenPlus. The older Bluelab pH Pen has a sealed, non-replaceable probe, so this won't fit it. It also doesn't suit the Combo Meter or Guardian Monitor, which use a different probe with a 2m cable.

How do I know whether the probe is worn out or just needs cleaning?

Clean it and recalibrate with fresh solution first. If it holds calibration afterwards, the probe was dirty rather than finished. If it still drifts or won't calibrate cleanly, the membrane has aged and it's time for a new one.

Do I need to calibrate after fitting a new probe?

Yes, straight away. The pen stores calibration values from the old probe and those don't carry across. Calibrate at pH 4.0 and 7.0 with fresh solution before taking any readings you intend to act on.

How long will the new probe last?

Generally 12 to 18 months. The biggest factor is storage. Kept capped and wet between uses, a probe lasts well. Left to dry out even once, it can be finished.

Can I extend the life of the probe?

Store it wet with the cap on, rinse with clean water after each use, and clean it monthly with a proper probe cleaner rather than wiping it with a cloth. Calibrate with fresh solution rather than a sachet that's been open for weeks.

Does this qualify for free shipping?

Not on its own. Free shipping starts at $150 for orders under 3kg to VIC, NSW, QLD, SA and ACT. Adding calibration solution or a Probe Care Kit will usually get you there, and both are worth having alongside a new probe anyway.